Linear Measurement

Objectives

To familiarize the student with the types, applications of calipers, micrometers and measurements.

At the completion of this experiment, the student will be able to:

1. Get familiar to the variety of the linear measurement tools, and know the type of a measurement tool needed to achieve a certain measurement.

2. Students will seek more efficient means of measure.

3. Take linear measurements with a certain accuracy depending on the instrument being used.

4. Clean, care for and store calipers, micrometers and dial indicators.

Apparatus

a. vernier caliper

b. dial caliper.

c. External micrometer.

d. 2-point inside micrometer.

Theory

  General specifications and functions of a caliper:

1. All stainless steel construction.

2. used to measure inside dimensions.

3. used to measure outside dimensions.

4. used to measure step dimension

5. convenient thumbscrew to lock a measurement in place.

6. accuracy equation:

  accuracy = 1 division main scale/no. of divisions on vernier scale

Block Gauges

OBJECTIVE:

1. To be familiarized with the types, applications of block gauges.
   
2. To calibrate some measurement devices.
   

APPARATUS:

a) Set of block gauges.

b) Granite surface plate.

Theory and Background:

Block gauges are made of stainless steel,each block has 6 faces, 2 of them are opposite and parallel and are highly polished. They must be used at standard conditions, which are:

a. Atmospheric pressure.

b. Temperature between (20 – 25°C).

c. Relative humadity (62 – 75)%.

d. Pure atmosphere (no dust). 

Uses of block gauges:

1. To measure lengths.

2. Calibration of measuring devices.

3. To measure angles.

4. To measure the pressure drop ( the head Δh in case of manometer).

There are 2 types of length standards:

1. End standard:

In which the unit of the length is defined as  being the distance between the end faces of the standard, these take the form of either slip.

2. Line standard or engraved scale:

In which the unit length is defined as being the distance suitably engraved lines.

End standards:

Gauge blocks are the practical length standards of industry. Amodern end standard consists fundementally of a block (slip) or bar of steel or cemented carbide, generally hardened. Whose end faces are lapped flat and parallel within a few tenth of a micrometer.

Gauge blocks are a good example of end standards. The name end standards rightly indicate that these consist of sets of standard blocks or bars, which are used to build up a required length.

1) End standards are highly accurate and are well suited to measurements of close tolerances.

2) They are time consuming in use and provide only one dimension at a time.

3) Dimensional variations as small as 0.0005 mm can be obtained.

4) They are subject to wear on their measuring faces.

5) Groups of blocks are (wrung) together to provide a given size, faulty wringing leads to damage.

6) End standards have a built-in datum because their measuring faces are flat and parallel and can be positively located on a datum surface.

7) The accuracy of end and line standards is affected by temperature change and both are originally calibrated at 20  °C.

8) They are made in high grade cast steel.

Criterea of the specifications for the block gauges:

• According to the number of block gauges inside the box:

a) 87 pieces.

b) 48 pieces.

• According to the roughness of the block gauges.

a) Grade 1

b) Grade 2

c) Grade 0

d) Grade 00

e) Grade calibration.

Grade 00 Block Gauge set:

This grade is accompained by a calibration chart which lists the departure from nominal size of each gauge in the set.

Calibration grade:

This grade provides the highest level of accuracy required in normal engineering practice and is intended for calibrating other blocks in conjunction with suitably accurate comparators. They are used where the tolerances are 2 micrometer or less, and are not intended for general gauge inspection. This grade is cheaper than grade 00 because of broader length tolerances. (flatness and parallelism tolerances are same).

The essential difference between 00 and calibration grade gauge blocks lies in their length tolerances. Calibration gauge blocks have broader tolerance than 00 grade gauge blocks, where as the geometrical tolerances, that is flatness and parallelism of measuring faces are the same in both.

A calibration chart is supplied with the calibration grade gauge blocks, because of their broader length calibration gauges are cheaper than 00 grade gauge blocks.

A theory is borne out by the face that if two highly finished gauges are firmly wrung together and left for a period of time (3 days) they may become very strongly attached, so that the faces are damaged when they are seperated with sufficient forces.

Thickness of the ringing film has been measured at the N.P.L. among various liquids as paraffin, heavy oil and vaseline and is found to ahve nearly 6 nanometers.

Instuctions for wringing together two slip gauges:

1) Surfaces must be clean and free from burrs. They should be washed in petrol, benzene, carbon tetrachloride or other DE-greasing agents and wiped dry on a clean cloth. Then be wiped with clean soft chamois leather. It is advised to take the following points into consideration.

In order to prevent damage to blocks:

• Protect from dust, dirt and moisture.

• Avoid magnetization.

• Handle lapped faces as little as posible to prevent etching from finger acid. Wipe all finger marks with chamois leather.

• Always wipe faces immediately before use even when use it continuous.

• Always replace clean gauges in their box and close it after use. If gauges are not in frequent use they should be coated to prevent corrosion.

• Do not handle gauges above the open box, they may cause damage to other gauges if dropped.

2) Slip gauges then should be held across one another at right angles and wring them with a rotary motion. This reduces the amount of surface rubbing necessary.

3) A minute amount of grease or moisture must be present between the surfaces for them to wring satisfactory. Unless a very firm wring is obtained there is always the possibility that the wringing film may be a micrometer thick.

Discussion :

A. Why do we always choose the minimum number of block gauges combination?

• The minimum number of blocks is used in order to decrease the error introduced by the error of each block (0.016mm) where increasing the number of block gauges the uncertainty of each piece adds to the total uncertainty of the overall assembly.

• To decrease the surface irregularities and dirts.

• To minimize errors due to gap wringing thick film.

• To minimize wear due to repeated usage of component.

• Using too many block gauges is a redundant and time consuming.

B. Suggest other applications for block gauges.

Gauge blocks are used as end standards, so the are useful in making standard sizes for various parts, since the error is minimum, another use is with the sinebar for angle measurement, also for calibration purposes of other measuring devices, and finally for measuring the pressure drop.

C. In the comparator measuring method what do we compare with?

In the comparator measuring method, the standard size of the blocks is compared with the measured size of the specified part by other measuring devices. This method is used when accurate reading is essentially to be found, if it is not the case then it is recommended to use direct reading methods.

Conclusions:

• In this experiment, the block gauges were identified and used for calibration for the micrometer.

• From the table, we conclude that as the number of blocks increase, the tolerance increases due to the accumulating error for each block (0.016 mm)

• In order to prevent damage to gauge blocks:

1. Protect from dust, dirt and moisture.

2. Avoid magnetization.

3. Always wipe the faces before use.

4. Always put the blocks in their box after use.

5. Do not drop the blocks.

• End standards are highly accurate and are well suited to measurement of close tolerances.

• They are time consuming in use and provide only one dimension at a time.

• Dimensional variations as small as 0.0005mm can be obtained.

• They are subject to wear on their measuring faces

• Groups of blocks are “WRUNG” together to provide a given size; faulty wringing leads to damage.

• End standards have a build-in datum because their measuring faces are flat and parallel and that can be positively located on a datum surface.

• The accuracy of the end and line standards is affected by temperature change and both are originally calibrated at 20 °c.

• They are made in high-grade cast steel.

• Although a very firm wring is usually obtained, there is always the possibility that the wringing film maybe a micrometer thick.

Autocollimators

Introduction:

An autocollimator is an optical instrument that is used to measure small angles with very high sensitivity. As such, the autocollimator has a wide variety of applications including precision alignment, detection of angular movement, verification of angle standards, and angular monitoring over long periods.

Objectives:

1. To measure straightness of a beam with the use of Auto-Collimator.
   
2. To identify the principle of Auto-collimator device.
   
3. To be able to draw conclusions about straightness error using graphical methods and least square method.
   

Apparatus:

1. Auto collimator.
   
2. Straight edge with 100mm marked intervals.
   

Theory:

Autocollimators are sensitive and inherently very accurate optical instruments for the measurement of small angular deviations of a light reflecting flat surface. The autocollimator has its own target which is projected by collimated light beams on a remotely placed surface and the reflected target image is observed in the ocular of the instrument.

The autocollimator is stationed at the end of the bed with a rigid support base. The movement of the reflector along the bed will cause the reflected image of the target to deflect according to the angular error of the bed,

The autocollimator is a flat mirror mounted in a short tube made to fit a Newtonian telescope focuser, and set accurately perpendicular to the tube’s axis. Centered in it is a small peephole or pupil that you look through.

The autocollimator projects a beam of collimated “parallel” light. An external reflector reflects all or part of the beam back into the instrument where the beam is focused and detected by a photo detector.

The autocollimator measures the deviation between the emitted beam and the reflected beam. Because the autocollimator uses the light to measure angles, it never comes into contact with the test surface.

How to calculate tilt of 1 sec. of arc of the reflector:

 θ is the angle taken after adjusting the micrometer properly (sec)

 h is the vertical distance (to be found)

 L is the length of the reflector carriage is 0.1m in our case. 

 (These parameters are shown in the below figure)

tanθ = h/L

Assume θ = 1sec

tanθ = θ for small values.

1sec = (1/60*60)(π/180) = 4.8*10^-6rad

h = 4.8-10^-6m

i.e. in each 1sec indicates a vertical distance of approximately 0.5 micron “since L = 0.1m”

As we can see the relation is nearly linear. We connected the best fit line by connecting the first and last point by means of a straight line.

The vertical rise h as we discussed in the theory section was:

H = 4.8*10^-6 m = 0.5 μm

This rise is due to 1sec of reflection angle so 1 sec = 0.5 μm

As we can see from our results that the time needed for reflection don’t depend on the position of the carriage, this is shown by the curve above.

 

We have some human errors like the readings.

Also the beam o light may not been reflected one hundred percent.

Surface Texture

Objectives:

To study the effect of manufacturing processes on surface texture and to asses tools and procedures of measuring the surface texture of material (specimen). Also its classification, measurement and analysis. The surface texture produced by different production processes and the effect of this resulting surface texture on the performance of a product.

Surface finish, or texture, can be viewed from two very different perspectives. From the machinist's point of view, texture is a result of the manufacturing process. By altering the process, the texture can be changed. From the part designer's point of view, surface finish is a condition that affects the functionality of the part to which it applies. By changing the surface finish specification, the part's functionality can be altered—and hopefully, improved. 

Bridging the gap between these two perspectives is the manufacturing engineer, who must determine how the machinist is to produce the surface finish specified by the design engineer. The methods one chooses to measure surface finish, therefore, depend upon perspective, and upon what one hopes to achieve. 

Introduction and theory:

Every part’s surface includes some type of texture created by multiple factors. These include the material’s microstructure, the cutting tool’s action and instability, errors in tool guide ways and deformations caused by stress patterns in the component. 

The resulting texture, known as surface geometry, is actually a combination of three features (roughness, waviness and form) that can be likened to the characteristics comprising a desert’s surface.

Surface roughness, which is the dominant surface feature. In manufacturing, roughness is caused by a material’s microstructure and the cutting tool’s action on the material. This is where variables like tool shape, speed, feed and cutting fluid come into play.

Surface waviness, the second most prominent contributor to surface texture. Waviness is caused by cutting tool instability, such as a grinding wheel’s imbalance, as well as errors in machine tool guide ways.

Surface form, more commonly known as straightness error. This type of texture results from tool guide way errors and stress factor deformations.

In addition to their varying origins, roughness, waviness and form also differ in how they affect part performance. As a result, conventional surface analysis requires separating the three texture types in order to isolate the factor being measured or controlled.

Types of surfaces:

• Surface :A surface is a boundary that separates an object from another object or substance. 
  
• Nominal Surface : A nominal surface is the intended surface. The shape and extent of a nominal surface are usually shown and dimensioned on a drawing. The nominal surface does not include intended surface roughness. 
  
• Real Surface : A real surface is the actual boundary of an object. It deviates from the nominal surface as a result of the process that created the surface. The deviation also depends on the properties, composition, and structure of the material the object is made of.
  
• Measured Surface : A measured surface is a representation of the real surface obtained with some measuring instrument. This distinction is made because no measurement will give the exact real surface. Later portions of this manual describe many different types of measuring instruments.
  
• Surface Geometry: Surface geometry and geometric dimensioning and tolerancing are large subfields of metrology which parallel or exceed surface finish in scope and complexity. This is the realm of coordinate measuring machines and roundness measuring instruments and contouring instruments. However, there is an increasing overlap between geometric measurements and surface finish measurements, so it is helpful to be aware of some basic concepts in geometric measurement. 
  

Form: Form refers to the intentional shape of a surface which differs from a flat line. 

Dimension: Dimensions are the macroscopic sizes of a part, e.g. diameter or length. 

Tolerance: A tolerance is an allowable range for a dimension to take, a specified interval of dimensions where the part will still function acceptably.

Form Error: Form error encompasses the long wavelength deviations of a surface from the corresponding nominal surface. Form errors result from large scale problems in the manufacturing process such as errors in machine tool ways, guides, or spindles, insecure clamping, inaccurate alignment of a work piece, or uneven wear in machining equipment. Form error is on the dividing line in size scale between geometric errors and finish errors. 

Texture 

Surface texture is the combination of fairly short wavelength deviations of a surface from the nominal surface. Texture includes roughness, waviness, and lay, that is, all of the deviations that are shorter in wavelength than form error deviations.

Surface texture includes roughness and waviness. Many surfaces have lay: directional striations across the surface. 

Roughness 

Roughness includes the finest (shortest wavelength) irregularities of a surface. Roughness generally results from a particular production process or material condition. 

Waviness 

Waviness includes the more widely spaced (longer wavelength) deviations of a surface from its nominal shape. Waviness errors are intermediate in wavelength between roughness and form error. Note that the distinction between waviness and form error is not always made in practice, and it is not always clear how to make it. New standards are emerging that define this distinction more rigorously as developed in later sections. 

Lay 

Lay refers to the predominant direction of the surface texture. Ordinarily lay is determined by the particular production method and geometry used. 

Turning, milling, drilling, grinding, and other cutting tool machining processes usually produce a surface that has lay: striations or peaks and valleys in the direction that the tool was drawn across the surface. The shape of the lay can take one of several forms as shown below. Other processes produce surfaces with no characteristic direction: sand casting, peening, and grit blasting. Sometimes these surfaces are said to have a non-directional, particulate, or protuberant lay. 

 

Several different types of lay are possible depending on the manufacturing and machining processes. 

Lay (or the lack thereof) is important for optical properties of a surface. A smooth finish will look rough if it has a strong lay. A rougher surface will look more uniform if it has no lay (it will have more of a matte look). 

Flaws 

Flaws are unintentional and unwanted problems with a surface. Usually the term flaw refers to individual and unusual features such a scratches, gouges, burrs, etc. According to the ANSI B46.1 standard a flaw is defined when agreed upon in advance by buyer and seller, leaving open all sorts of other types of surface problems. 

Surface Profiles:

• Types of Profiles
  

Profile 

A profile is, mathematically, the line of intersection of a surface with a sectioning plane which is (ordinarily) perpendicular to the surface. It is a two-dimensional slice of the three-dimensional surface. Almost always profiles are measured across the surface in a direction perpendicular to the lay of the surface. 

A profile is a two-dimensional picture of a three dimensional surface that may be thought of as the result of a sectioning place cutting the surface. Profiles are ordinarily taken perpendicular to the lay.

1. Nominal Profile : The nominal profile is the straight or smoothly curved line of intersection of the nominal surface with a plane which is (ordinarily) perpendicular to the surface. The nominal profile has a known mathematical shape for a known part (most often a straight line or a circle). 
2. Real Profile: A real profile is a profile of the real surface. It is the (idealized) shape of the intersection of a surface with a perpendicular sectioning plane. 
3. Measured Profile: A measured profile is a representation of the real profile obtained with some measuring instrument This distinction between "real" and "measured" is made because no measurement will give the exact real surface. Later portions of this manual describe many different types of measuring instruments, emphasizing profiling instruments. 

Profiling Methods

A profiling method is a means of measuring a profile of a surface. The result of the method is a two-dimensional graph of the shape of the surface in the sectioning plane created by the profiling instrument. 

The most common type of profiling instrument draws a diamond stylus across the surface and measures its vertical displacement as a function of position. Chapter 5 describes profiling instruments in detail. 

• Modified Profiles : A modified profile is a measured profile that has been modified by mechanical, electrical, optical, or digital filtering. The filtering is ordinarily done to minimize certain surface characteristics while emphasizing others. A modified profile differs from a measured profile in the sense that the real profile is intentionally modified as part of the measurement. The details of the modification are typically selectable by the user of an instrument. A measured profile is an unintentional modification of the real profile resulting from the limitations of the measuring instrument. 
  
• 
  
• Traced Profile : An instrument's raw trace of a surface is always relative to some reference plane. The traced profile is the raw measured profile with profile height measured relative to a zero line which is parallel to the instrument's reference plane. 
  
• Since an instrument's set-up will vary from measurement to measurement, the traced profile has little value except as the starting point for leveling or other form removal. 
  
• Form Profile : The form profile is the nominal profile in the coordinate system of the traced profile. That is, it is the nominal shape of the part relative to the reference line of the profiling instrument.  Ordinarily form will be a straight line or a circle. It is most often found by a least squares fit of the traced profile with a straight line or a circle. 
• Primary Profile : The primary profile is the traced profile alter subtracting the form. The primary profile is thus the sum of all the deviations of the measured profile from the nominal profile. The primary profile is the sum of the form error profile, the waviness profile, and the roughness profile. 

Often the primary profile is referred to as the "unfiltered profile" or the "total profile". In this case, it is the trace of the surface leveled and magnified, but otherwise unmodified. 

Wave length :

Wavelength (almost universally denoted X) refers to the repeat length of a periodic function. 

Wavelength is the distance between similar points of a repeating, periodic signal. 

A real profile can be thought of as the sum of many different individual functions, each with its own wavelength. 

Filter:

A filter (for purposes of surface finish measurement) is an electronic, mechanical, optical, or mathematical transformation of a profile to attenuate (remove) wavelength components of the surface outside the range of interest for a measurement. 

Form Error Profile 

The form error profile encompasses the very long wavelength deviations of the traced profile from the nominal profile. Form error is the modified profile obtained by filtering the measured profile to attenuate medium and short wavelength components associated with waviness and roughness. 

Texture Profile 

The texture profile is the sum of the waviness profile and the roughness profile, i.e. the remaining medium and short wavelength deviations of the measured profile from the nominal profile after form error has been subtracted from the primary profile. 

Measurement of texture is the primary domain of traditional surface finish analysis. 

 

An important concept in surface finish is the breaking of a surface profile into different components by wavelength. There is a hierarchy of components, as shown.

Waviness

 The waviness profile includes medium wavelength deviations of the measured profile from the nominal profile. The waviness is the modified profile obtained by filtering a measured profile to attenuate the longest and shortest wavelength components of the measured profile (i.e. the filter removes form error and roughness). 

Roughness Profile 

The roughness profile includes only the shortest wavelength deviations of the measured profile from the nominal profile. The roughness profile is the modified profile obtained by filtering a measured profile to attenuate the longer wavelengths associated with waviness and form error. Optionally, the roughness may also exclude (by filtering) the very shortest wavelengths of the measured profile which are considered noise or features smaller than those of interest. 

Roughness is of significant interest in manufacturing because it is the roughness of a surface (given reasonable waviness and form error) that determines its friction in contact with another surface. The roughness of a surface defines how that surfaces feels, how it looks, how it behaves in a contact with another surface, and how it behaves for coating or sealing. For moving parts the roughness determines how the surface will wear, how well it will retain lubricant, and how well it will hold a load. 

Reference Mean Lines:

• Mean Line 
  

A mean line is a reference line from which profile deviations are measured. It is the zero level for a total or modified profile. 

Least Squares Mean Line 

A least squares mean line is a line through a profile such that the sum of the squares of the deviations of the profile from the mean line is minimized. In practice, this is done with a digitized profile. 

 

A least squares mean line minimizes the sum of the squares of the deviations of a set of points from the line. This method approximates how your eye would fit a line through a set of points

The most common application of a least squares mean line is to "level" the raw traced profile. The traced profile is relative to the straight line reference of the profiling instrument. Unless the instrument is perfectly aligned with the part, that reference will be tilted with respect to the measured surface. A least squares line fit through the raw traced profile may be used as a reference line to remove the misalignment. 

More sophisticated instruments give greater control over this leveling process, either by providing for "releveling" or by providing alternatives to the least squares mean line. This is because a least squares mean line is distorted by flaws or unusually shaped profiles. 

• Filter Mean Line 
  

A filter mean line is the mean line implicit in a profile filter. (Filters are discussed at length in Chapter 7). For example, the waviness profile may be considered the mean line of the texture profile. Another name for the filter mean line in analog instruments is the "electrical mean line". 

Center Line 

The center line of a profile is the line drawn through a segment (usually a sample length) of the profile such that the total areas between the line and the profile are the same above and below the line. 

This concept is little used in modern instruments; it mainly served as a graphical method for drawing a mean line on the output of a profile recording instrument with no built-in parameter processing.

Profile Peaks and Valleys:

Profile Height 

The height of a profile at a particular point is the distance from the profile to its mean line. Profile height is considered positive above the mean line and negative below the mean line. 

Profile Peak 

A profile peak is a region of the profile that lies above the mean line and intersects the mean line at each end. In the figure below, each shaded region is a peak. The height of a peak is defined to be the point of maximum height within the region. 

 

Profile peaks are regions above the mean line. Local peaks are regions between two local minima. 

Profile Valley 

A profile valley analogous to a profile peak is a region of the profile that lies below the mean line and intersects it at each end. The depth of a valley is the depth of the lowest point within the valley. 

Profile Irregularity 

Sometimes it is convenient to speak of one profile peak together with one adjacent profile valley as a profile irregularity. 

 

Profile Valleys extend below the mean line. Local valleys lie between two maxima (above or below the mean line). 

Local Peak 

A local peak is a region of a profile between two successive local minima in the profile. 

Local Valley 

A local valley is a region of a profile between two successive "high points" (local maxima) in the profile. 

Few parameters say very much about local peaks or valleys, but very experienced surface finish experts can tell a great deal about a machining process by looking at the shape of local peaks and valleys within each larger peak or valley.

Spacing:

Spacing refers to the distance between features on a profile in the x direction, parallel to the nominal direction of the trace. The features that determine a spacing parameter usually relate to peaks and valleys or to average wavelengths, etc. 

 

a = roughness value Ra in micrometers

b = production method, treatment, coating, other text or note callout

c = roughness cutoff or sampling length in millimeters

d = direction of lay

e = minimum material removal requirement in millimeters

f = roughness value other than Ra in micrometers preceded by its parameter symbol (e.g. Rz 0.4)

Examples of Surface Texture Indication:

1. Basic Surface Texture Symbol. Surface may be produced by any method except when the bar or circle (Symbol b or d) is specified.
2. Material Removal By Machining Is Required. The horizontal bar indicates material removal by machining is required to produce the surface and material must be provided for that purpose.
3. Material Removal Allowance. Value in millimeters for "X" defines the minimum material removal requirement.
4. Material Removal Prohibited. The circle in the vee indicates the surface must be produced by processes such as casting, forging, hot finishing, cold finishing, die casting, powder metallurgy and injection molding without subsequent removal of material.
5. Surface Texture Symbol. To be used when any surface texture values, production method, treatment, coating or other text are specified above the horizontal line or to the right of the symbol. Surface may be produced by any method except when bar or circle (Symbol b or d) is specified or when the method is specified above the horizontal line.
   

Conclusion:

There is an inverse relation between the cutting speed and the arithmetic mean value (Ra).

So when we work on a certain operation (e.g. turning), and we need a low degree of roughness (and hence, high smoothness-quality), we have increase the cutting speed as much as we can.

The relation between roughness and depth of cut is direct relation, and it is also direct with feed rate, also roughness is relative so we cant say the surface is rough nor smooth.

Extrusion

Introduction:

Extrusion is a process that forces metal to flow through a shape-forming die. The metal is plastically deformed under compression in the die cavity. Extrusion processes can be carried on hot or cold. Extrusion differs from drawing in that the metal is pushed, rather than pulled under tension.

Forward extrusion

Forward extrusion is sometimes known as the Hooker process. In this process the confined metal is forced to flow downward in the direction of the punch travel. 

The process is generally used to produce thin-walled tubular parts with heavy flanges, straight tubular shapes, and extrusion of stepped; multiple diameters. 

Forward extrusion is best applied to parts having an outer diameter of 25.4 mm (1 in) or more. The production of rods and other solids shapes is also possible with forwards extrusion. 

In forward extrusion the work piece is placed in a close-fitting die. The punch is forced downward, displacing the metal through a restricted opening in the bottom of the die. The metal is forced to flow considerable distance beyond the end of the punch. Cupped or tubular parts of the punch extension serve as a mandrel. This controls the wall thickness and inner contour of the extruded parts.

Backward extrusion

Backward extrusion is a process that forces the metal confined in the cavity to flow in a direction opposite to that of the punch travel. 

The slug (workpiece) is contained in a closed die. The descending punch enters the slug. The pressure displaces the metal upward through the opening between the punch and die. This is generally used for extruding symmetrically shaped parts having a closed end.

Combined extrusion

Combined extrusion uses a combination of forward extrusion and backward extrusion. The metal is confined inside a matrix between the lower and upper punches. This forces the metal to flow both up and down. The extruded part is lifted from the die on the upward stroke of the slide by a lift out on the bed of the press. Some aspects of combined extrusion are: 

1. it is fast 
   
2. it can complete parts in few steps 
   
3. it can produce large quantities with low unit costs 
   
4. it wastes little material 
   
5. it can make parts with small radii 
   
6. it requires mirror tooling 
   

Design Considerations

Limit the irregularities of shape as much as the function of the part allows. Metal flows less readily into narrow and irregular die sections, making distortion and other quality problems more likely to occur. 

Many extrusion shops and metals suppliers provide standard shapes that might serve the designer. A good rule of thumb is to always use standard cross sections when possible. 

Tolerances are advised to be liberal enough to avoid secondary drawing operations, if possible. 

With all metals, and particularly with steel and less easily extruded metals, it is recommended to avoid extreme changes in section thickness. 

Sample Parts

The extrusion process can be used to manufacture building and automotive trim, window frame members, tubing, aircraft structural parts, railings, flashlight cases, aerosol cans, military projectiles, and fire extinguishers.

Technological advances have allowed extrusion companies to use this process in applications that were considered too difficult few years ago, as the figures below show.

Materials used in extrusion:

Metals and alloys: Brass, copper, lead, aluminum, steel, magnesium, tin, titanium, and zinc.

Thermoplastics: ABS, Acrylic, Butyrate, Flexible Vinyl, PETG Co-Polyester, Polycarbonate, Polyethylene- High & Low Densities, Polypropylene, Polystyrene, Polyurethane, Rigid Vinyl, Thermoplastic Elastomer.

Following is a table of materials and their ranking. The ranking indicates the material suitability for the extrusion process. 

Material Ranking

Cast Iron 50

Carbon Steel 80

Alloy Steel 80

Stainless Steel 80

Aluminum & Alloys 100

Copper & Alloys 100

Zinc & Alloys 80

Magnesium & Alloys 100

Titanium & Alloys 50

Nickel & Alloys 80

Refractory Metals 0

Thermoplastics 100

Thermosets 0

Ceramics 50

PhotoPolymers 0

Wood (dry) 0

A value of zero means that the corresponding material is never used with this process, a ranking of 100 means that it is excellent for use with this process.

Properties of extruded materials:

Deformation is greater in the outer zones of a bar than it is at the center, particularly when the extrusion ratio is low. The center receives only light deformation.

The improvement in tensile strength of Al 1100 resulting from extrusion with 3:1 ratio is 5000 psi (before extrusion) to 19000 psi (after extrusion). In general, the yield strength is increase about four times the initial strength.

In general, the inside surfaces of backward extruded parts are 5 to 25 Rockwell-B Hardness harder than the outside surfaces. The hardness of material alloys which undergo this backward extrusion process is decreased by 20 to 10 Rockwell-B Hardness. However in forward extrusion the outer surfaces are harder than the inner surfaces.

Advantages:

1. The tooling cost is low, as well as the cost due to material waste ( it has high material utilization). 
   
2. Intricate cross sectional shapes, hollows and with undercuts can be produced. 
   
3. The hardness and the yield strength of the material are increased. 
   
4. In most applications, no further machining is necessary. 
   

Disadvantages:

1. High tolerances are difficult to achieve. 
   
2. The process is limited to ductile materials. 
   
3. Extruded products might suffer from surface cracking. It might occur when the surface temperature rise significantly due to high extrusion temperature, friction, or extrusion speed. Surface cracking might also occur at low speeds due to periodic sticking of the extruded product along the die land. 
   
4. Internal cracking might also occur. These cracks are attributed to a state of secondary tensile stresses at the centerline of the deformation zone in the die. 
   

Drawing of Sheet and Plates

Drawing is a plastic deformation in which a flat sheet or plate is formed into a recessed, three-dimensional part with a depth several times the thickness of the metal. 

As a punch descends into a die or the die moves upward over a punch, the metal assumes the configuration of the mating punch and die tooling. 

We distinguish hot drawing and cold drawing. 

Hot drawing is used for forming relatively thick-walled parts of simple geometry, usually cylindrical. The thickness of the material reduces considerably as it moves through the dies. This process is used in forming components such as oxygen tanks and large artillery shells. 

Cold drawing uses relatively thin metal, changes the thickness very little or not at all, and produces parts in various shapes. 

  

Method of cup drawing or hot forming

The ram force F, or the input force exerted on the ram, during a forward extrusion process can be calculated. The value of the calculated force depends on the model used. 

The input power is the power supplied by the ram force as it moves with a velocity uo:

Pinput = F uo 

This total input power is transformed into:

1. Ideal power consumed by the plastic deformation. 
   
2. Frictional power dissipated due to friction along the die angle. 
   
3. Redundant work due to inhomogeneous deformation. 
   

In general, the total ram force depends on the above three components. There is a die angle, see figure below, for which the ram force is minimum. However, unless each component of the powers is know as a function of the die angle, it is very hard to determine the optimum angle.

 

Ideal Deformation

Let Ao be the cross-sectional area of the billet (material before being extruded), and let Af be the cross-sectional area of the extruded piece. An extrusion ratio is defined as:

R = Ao / Af 

Then, the absolute value of the true stain,  , is given by:

x = ln(R) 

If Y denotes the yield stress of the perfectly plastic material, the energy dissipated in plastic deformation per unit volume is:

E = Ydef

The power due to plastic work of deformation is:

Pplastic = uo Ao E = uo Ao Y  

In the ideal case, we assume that the total power input is equal to the power due to plastic work of deformation. Recall that the input power is:

Pinput = F uo = p Ao uo 

Where p is the extrusion pressure at the ram. Equating Pplastic and Pinput, we find that:

p = Y ln(R)

Note that the value of extrusion pressure is equal to the area under the true stress / true strain curve for the material.

Ideal Deformation and Friction

When friction at the die-billet interface is accounted for, the power input is equal to the sum of the plastic deformation and the frictional power.

Because the billet is forced through a die with a substantial reduction in its cross-section, a dead zone in the metal flow pattern develops at the die exit region. 

 

We assume that the material flow in that region takes place at a 45 degrees, this is an "effective die angle", and that the friction stress is equal to the shear yield stress k = Y/2 of the material. The power dissipated due to friction along the die angle is:

Pfriction = (uo/cos(45)) Ao (Y/2)  

Equating the power input to the sum of the power of plastic deformation and the power of friction force:

p Ao uo = uo Ao Y  + (uo/cos(45)) Ao (Y/2)  

It follows immediately that the extrusion pressure is:

p = 1.7 Y ln(R)

In this analysis, the force required to overcome friction at the billet-container interface was neglected. It can be easily calculated if we assume again that the frictional stress is equal to the shear yield stress of the material, k, and we let AL denotes the lateral surface of the billet remaining in the die, then an additional ram pressure, pf, due to wall friction is given by:

pf Ao = k AL = (Y/2) AL 

Thus, the total extrusion pressure becomes:

p = Y( 1.7 ln(R) + AL / 2Ao )

 

Actual Forces

The derivation of analytical expressions, including friction, die angle, and redundant work due to inhomogeneous deformation of the material, can be difficult. Consequently, a convenient empirical formula has been developed:

p = Y(a + b ln(R))

where a and b are constants determined experimentally. Approximate values for a and b are 0.8 and 1.2 to 1.5, respectively.

For strain hardening materials, Y in the above expressions should be replaced by the average flow stress.

Flow of metal during the process of extrusion

When a billet of material is forced through a die, with a substantial reduction in its cross-sectional area, the metal flow pattern in extrusion is important. Typically, three different metal flow patterns have been observed during the process of extrusion depending upon the prevailing conditions. The conditions under which the different flow patterns are obtained are as follows.

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The most homogeneous flow pattern is obtained when there is no friction at the billet-container-die interfaces. This type of flow occurs when the lubricant is very effective or with direct extrusion. 

When friction along all interfaces is high, a dead-metal zone develops. As a result a high-shear area appears as the material flows into the die exit, somewhat like a funnel. This configuration may indicate that the billet surfaces could enter the high shear zone and be extruded, causing defects in the extruded product.

The high shear zone extends farther back. This extension can result from high container wall friction, which retards the flow of the billet or materials in which the flow stress drops rapidly with increasing temperature. In hot working, the material near the container walls cools rapidly, subsequently increasing the strength. Thus, the material in the central regions flows toward the die more easily than that at the outer regions. As a result, a large dead metal zone forms and the flow is inhomogeneous. This flow pattern leads to a defect known as a pipe or extrusion defect.

Thus the two factors that greatly influence metal flow in extrusion are the frictional conditions at the billet-container-die interfaces and thermal gradients in the billet.

Materials property change during the extrusion process

After the extrusion process the properties of materials will change, for example hardness, strength and grain size.

Results :

Initial diameter      25 mm

Final diameter        10 mm

Initial area = Ao = pi*Do^2 /4 = 490.874 mm^2

Final area  = Af = pi*Df^2 /4 = 78.54 mm^2

Extrusion ratio = Ao/Af = 6.25

Strain = ε = ln(Ao/Af) = 1.83

Extrusion reduction in area = (Ao-Af)/Ao*100% = 84%

The Performance of A Radial Flow Fan

Objective:
The performance of a radial flow rotor in air is to be determined over a wide range of operating conditions and for interchangeable impellers with forward, backward and radial blades.

Apparatus:
Single stage radial flow fan equipped with interchangeable impellers, with forward-curved, backward-curved and radial blades, variable speed D.C. electric motor with swinging field dynamometer, a counter for speed measurements, three single column manometers, standard 75 mm nozzle.

The fan draws air from the atmosphere by way of the measuring nozzle, a flow straighter, and a diffuser, while the fan discharge into the atmosphere is regulated by a throttle valve.

Procedures:
The fan is to run at a series of constant speeds not exceeding 3000 rev/min, and the flow rate is to be varied in each test by means of the throttle. Measure the speed N; torque T by noting the balancing force, the pressure rise H generated by the fan and measurable by a manometer across the machine graduated in cm of water. For each run N is kept constant and the throttle is altered from open to fully shut, thus changing T, H and Q.

Theory:
The fan total pressure rise is defined as the difference between the total pressures at fan outlet and fan inlet i.e. it is a measure of the total pressure difference imposed on air by the fan. In the apparatus (fan) the cross-sectional area at inlet and exit are equal.

It follows that the velocity pressure at inlet and exit are equal and the fan total pressure rise is equal to the difference between the corresponding static pressure.

If Q = volumetric rate of flow (m3/s) and is calculated by:

Where:
T = air temperature in oK
h1 = drop in head at the standard nozzle in cm H2O
Pa = atmospheric pressure in N/m2

The total air power of the fan or the useful work done is equal to the product of fan total pressure and volumetric rate of flow:

Note h3> 0 and h2 < t =" Torque" f =" Load" n =" Angular" r =" 17.9" cm =" 0.179" power =" Powershaft">

Results & Tables:

The data taken in the lab are shown in here (xls file 40.0 kb)

Refrigeration System

Objective:

To study the variation of the operating characteristics of the system (C.O.P., refrigeration effect and compressor work) with compressor speed.

Theory:

1. Refrigerating effect QL = mr (h1-h4)
   
2. Compressor work Wc = mr (h2-h1)      {assuming adiabatic}
   
3. Condenser duty QH = mr (h2-h3)
   
4. C.O.P. = QL/Wc = (h1-h4)/(h2-h1)
   
5. Compressor work = torque*speed
   
6. Condenser duty = mw*Cpw*∆Tw
   
7. Refrigerating effect = VI
   

Conclusion:

1. The effect on C.O.P. is decreasing when increasing the speed and causing the enthalpy of the compressor to increase.
   
2. The refrigeration capacity, the compressor power and the heat rejected in the condenser will increase by increasing the speed of the compressor.
   
3. The maximum enthalpy is the one of the compressor, when the heat transfer to the surrounding is maximum.
   
4. The heat rejected in the condenser is maximum.
   
5. The assumption of ideal cycle neglects the heat transfer to and from the surroundings, which affect the enthalpy of the system and it neglects the irreversibility.
   
6. The C.O.P. also indicates the efficiency of the machine where it relates the output power in the condenser to the input power in the compressor.
   

Losses In Pipe Bends

Objectives:
To determine the variation of friction factor with Reynolds Number.
To determine the relationship between total head loss and flow rate for pipe bends and other common pipe fittings.
To determine the loss coefficient for each fitting.

Apparatus:
The apparatus shown in figure (2), consists of two separate hydraulic circuits, each one containing a number of pipe system components. Both circuits are supplied with water from the same hydraulic bench. The components in each of the circuits are as follows:

• Dark Blue Circuit: 
  

1. Gate Valve (D)
   
2. Standard Elbow Bend (C) with radius 12.7 mm 
   
3. 90o Miter Bend (B)
   
4. Straight Pipe, length 914.4 mm, small diameter = 13.6mm, large diameter = 26.2mm, pipe material is copper (e=0.0015mm)
   

• Light Blue Circuit:
  

1. Globe valve (K)
   
2. Sudden Expansion (E).
   
3. Sudden contraction (F).
   
4. 150 mm 90 radius bend (J).
   
5. 100 mm 90 radius bend (H).
   
6. 50 mm 90 radius bend (G).
   

In all cases (except the gate and globe valves) the pressure change across each of the components is measured by a pair of pressurized piezometer tubes. In the case of the valves pressure measurement is made by U-tube containing mercury


Procedure:

1. Close the globe valve K and open the gate valve D, see figure2. Switch on the bench pump and open the bench supply valve to admit water to dark blue circuit. Allow water to flow for 2 to 3 minutes.
   
2. Close the gate valve D and bleed all of the trapped air into the top of the manometers tubes. Check that all the manometers show zero pressure difference.
   
3. Open the gate valve and then, by carefully opening the bled screws at the top of the mercury U tube, fill each limb with water. Make sure that all air bubbles have been expelled, then close the bleed screws.
   
4. Close the gate valve, open the globe valve, and repeat the procedure for the light blue circuit.
   
5. Open fully the bench supply valve. Then close the globe valve and open fully the gate valve to obtain the maximum flow rate through the dark blue circuit.
   
6. If necessary, adjust the water levels in manometers by pumping air into, or releasing air from the bleed valves at the tops of the manometers.
   
7. Record the readings of each of the manometers in the dark blue circuit. Note the reference of each manometer and also record the type of filling next to each pair of results. Also read the levels in the mercury U tube connected between the inlet and outlet of the gate valve D.
   
8. Measure the flow rate by timing the collection of water in the bench weighing tank.
   
9. Measure the water temperature by holding a mercury thermometer in the flow at exit-from the outlet hose.
   
10. Close the gate valve to reduce the differential manometer reading by about 10%. Again read the manometers and the U tube, then measure the flow rate.
    
11. Repeat the procedure until you have about 10sets of readings over the whole range of flow.
    

Results and Discussions:

• The loss coefficient does not vary with the flow rate, it depends only on the fitting or shape
  
• My calculations for K are close to those of the standard data.
  
• The calculation of head loss for flow through a pipe with known conditions is generally carried out as follows. If the fluid velocity and the pipe diameter are known, the Reynolds number can be calculated. The Reynolds number and the pipe roughness are used to determine the friction factor, f, from the Moody plot using the appropriate curve. Once, the friction factor is known, the major head loss can be calculated. The head loss can then be used to determine the pressure drop between two sections.
  

Impact Of a Water Jet

Introduction:

    Over the years, engineers have found many ways to utilize the force that can be imparted by a jet of fluid on a surface diverting the flow.

For example, the pelt on wheel has been used to make flour. Further more, the impulse turbine is still used in the first and sometimes in the second stages of steam turbine. Firemen make use of the kinetic energy stored in a jet to deliver water above the level in the nozzle to extinguish fires in high-rise buildings. Fluid jets are also used in industry for cutting metals and debarring.

Many other applications of fluid jets can be cited which reveals their technological importance.

This experiment aims at assessing the different forces exerted by the same water jet on a variety of geometrical different plates. The results obtained experimentally are to be compared with the ones inferred from theory through utilizing the applicable versions of the Bernoulli and momentum equations.


Objectives:

1. To determine the force produced by a water jet when it strikes a flat vane and a hemispherical cup.
   
2. To compare the results measured with the theoretical values calculated from the momentum flux in the jet.
   

Theory:
    For the general case shown in figure (1) the momentum flux in the jet is (muo)
Where:

• m is the mass flow rate
  
• Uo is the jet velocity just upstream of the vane.
  

After being deflected through the angle , the momentum flux is (mu1cos) in the x-direction. The force on the fluid is therefore (mu1cos-mu) in the x-direction. Thus the force F in the x-direction on the vane is:
F = m (uo-u1cos) (1)
Now in the case of the plate figure 3a =90 so cos=0 and equation (1) reduces to:
F=muo
For a hemispherical cup, figure 2b =180 so cos = -1 and equation (1) reduces to:
F=m (uo-u1)
Furthermore if there is a negligible reduction of a speed so that (u0=u1) then
F=2muo

In the experiment it is not possible to measure directly just upstream of the vane. However the velocity u at the exit of the nozzle can be determined. The velocity uo is somewhat less this due to declamation caused by gravity and can be calculated from the Bernoulli equation that is:
(U^2/2g)+(Z)+(P/g) = (u0^2/2g)+(Zo)+(Po/g)
Now from figure 2a Z=0 Po=P Zo=s and this yields to:
u^2= uo-2gs
which can be written as:
uo^2=u^2-2gs

where s is the distance between the nozzle exit and the surface of the valve. In order to calculate the force on the valve due to the jet we take the moment about the pivot of the weighing beam figure 3 and substitute known values into the equation we get:
F*0.1525 = 0.610*g*X
Or
F = 4gX

Apparatus:
    Hydraulic bench, water jet apparatus, stopwatch.
The water is supplied to the jet apparatus in a closed loop by a pump. The flow rate is determined with the use of a weighing tank, and a stopwatch. The water issues vertically upwards into the air, through a nozzle. Two objects are available:

1. A flat plate
   
2. Hemispherical cup
   

Each object can be mounted on a horizontal lever above the water jet and receive its impact. The force on the object can be determined with the use of weights that can be hung at different positions on the lever, see figure 4.

Procedure:

1. Stand the apparatus on the hydraulic bench, with the drainpipe immediately above the hole leading to the weighing tank, see figure 4. Connect the bench supply hose to the inlet pipe on the apparatus, using a hose-clip to secure the connection.
   
2. Fit the flat plate to the apparatus. If the cup is fitted, remove it by undoing the retaining screw and lifting it out, complete with the loose cover plate. Take care not to drop the cup in the plastic cylinder.
   
3. Fit the cover plate over the stem of the flat plate and hold it in position below the beam. Screw in the retaining screw and tighten it.
   
4. Set the weigh-beam to its datum position. First set the jockey weight on the beam so that the datum groove is at zero on the scale, figure 5. Turn the adjusting nut, above the spring, until the grooves on the tally are in line with the top plate as shown in figure 6. This indicates the datum position to which the beam must be returned, during the experiment, to measure the force produced by the jet.
   
5. Switch on the bench pump and open the bench supply valve to admit water to the apparatus. Check that the drainpipe is over the hole leading to the weighing tank.
   
6. Fully open the supply valve and slide the jockey weight along the beam until the tally returns to record the reading on the scale corresponding to the groove on the jockey weight.
   
7. Measure the flow rate by limiting the collection of 30Kg of water in the bench-weighing bank.
   
8. Move the jockey weight inwards by 10 to 15mm and reduce the flow rate until the beam is approximately level. Set the beam to exactly the correct position (as indicated by the tally) by moving the jockey weight, and record the scale reading. Measure the flow rate.
   
9. Repeat step 8 until you have about 6 sets of readings over the range flow. For the last set, the jockey should be set at about 10mm from the zero position. At the lower flow rates you can reduce the mass of water collected in the weighing tank to 15Kg.
   
10. Switch off the bench pump and fit the hemispherical cup to the apparatus using the method in steps 2 and 3. Repeat step 4 to check the datum setting.
    
11. Repeat steps 5 to 9, but this time move the jockey in steps of about 25mm and take the last set of readings at about 20mm.
    
12. Switch of the bench pump and record the mass m of the jockey weight, the diameter d of the nozzle, and the distance s of the vanes from the outlet of the nozzle.
    

Sample Of Calculations:
1. m = mw/T [ in table 1: 7.5/16.52=0.454 Kg/S]
[ in table 2: 7.5/17.29=0.434 Kg/S]


2. m = ρuA ⇒u= m/ρA ρA= 1000*/4*0.001=0.0785 [in table 1: 0.454/0.0785=5.78m/s] [in table 2: 0.434/0.0785=5.53m/s]


3. From Bernoulli’s equation: (u^2/2g)+(Z1)+(P1/ρg)= (uo^2/2g)+(Z2)+(P2/ρg) Z1=0 Po=P Z2=s=40mm ⇒ uo^2= u^2-2gs [in table 1: (5.78^2)-(2*9.81*.04)]^2=5.71m/s [in table 2: (5.53^2)-(2*9.81*.04)]^2=5.46m/s



4. By summing the moment around the pivot in figure (3) we obtain:

Fw*0.1525-m1gX=0 ⇒ Fw=4gX

5. For flat plate:

F = m (uo-u1cos)
Where =90

⇒ F = muo [so F for X = 0.06m = 2.59N]

6. For the hemispherical vane:

F = m (uo- u1 cos)
Where =180

⇒ F = m (uo+ u1) ⇒ F = 2muo [so F for X = 0.012m
= 2*2.37=4.74N]

Source of Errors:

1. Turning the adjusting nut above the spring until the grooves on the tally are in the line with the top plate as shown in figure 6.
   
2. Recording the reading on the scale corresponding to the groove on the jockey weight.
   
3. Starting timer and adding weights when beam moves to horizontal.
   
4. Stopping timer when beam moves to horizontal again.
   

The values of F theoretical (calculated from 4gx) are close to those found experimentally. So we connect these points with a straight line.

Also from this graph we see that the calculated F (4gx) is equal to the double of mu  2mu

Conclusions:
The results obtained theoretically are close to those obtained experimentally.
Accuracy = (muo-4gX /4gX) *100%
For flat plate:

(2.59-2.35/2.35)=10.2%
(2.10-1.96/1.96)=7.14%
(1.73-1.57/1.57)=10.2%
(1.35-1.18/1.18)=14.4%
(0.9-0.78/0.78)=15.4%



For hemispherical cup:

(4.74-4.71/4.71)=0.64%
(4.08-3.92/3.92)=4.08%
(3.6-3.14/3.14)=14.6%
(2.7-2.35/2.35)=14.9%
(1.90-1.57/1.57)=21.0%
(0.94-0.78/0.78)=20.5%



3. If the line didn’t pass through the origin that means that there is an error, because if the force is zero ( the jet doesn’t touch the vane) the should be placed at the origin which means X=0 so F=0

4. F = m (uo = u) u  uo because we neglect reduction of speed so that u=uo fo = 2muo but the force on the hemispherical cup less than twice that on the flat plate.

5. to find the thickness of the water film at the circumference of the hemispherical case:
mo = .uo.A
m1 for the water film is equal to .u1.A
where A = 2rt and u0 = u1
r = 0.03m and the mass flow is constant then
t = (*0.01*0.01)/(8**0.03)= 0.4167mm

6. The effect on the calculated force on the flat plate if the jet was assumed to leave the plate at 1 upward will be a moment in the x-direction which will decrease the moment in the y-direction F=m (1.99uo) and it won’t effect the results too much.

7. a. The effect will be:
F = [(0.610  0.001) gX]/0.1525

b. The effect will be:
F = (0.610*gX)/(015250.001)

c. The effect will be:
F = m [{(m)/( ¼(D0.0001)²)}² -2gs]^1/2

8. If the velocity in the jet wasn’t uniform we will divide the jet into a two separate regions in which we solve for each one separately.


♣ in our experiment we have learned lots of things, we have had a close look on how the hydraulic bench and how a water jet work. We also have learned to calculate the force caused by a water jet experimentally using different sets of data and also learned how to calculate the mass flow rate and speed of water using the mass of weights and their X using time intervals measured experimentally.

♣ there is for sure a difference between our results and the theoretical ones because of different sources of errors that have been discussed earlier in this report.

Flow through a nozzle

Objectives:

1. To study the pressure distribution through a nozzle at different ratios across it.
   
2. To learn, study and understand how a nozzle works.

Apparatus:
    Air is admitted to a cast iron pressure chest by way of adjustable values. A nozzle of highly finished brass is screwed into seating in the base of the chest and the air expands through the nozzle.

To enable the pressure distribution through the nozzle to be plotted, a search tube or probe of stainless steel may be traversed along the axis of the nozzle. A small cross-hole in the search tube connects with a high-grade pressure gauge, which registers the pressure at any point in the nozzle. The search tube is traversed by rotating a calibrated dial and pressures are usually recorded at intervals of 2.5mm. A pointer moves with the search tube past a replica of the nozzle profile in order to indicate the point in the nozzle at which the pressure is being measured. The nozzle discharges into a vertical pipe of large bore fitted with the throttling valve for controlling the downstream pressure. Other instruments include a second pressure gauge for recording the pressure in the chest and a thermometer for indicating the temperature of the air.

Procedure:

1. Open the back pressure valve (inlet valve) whilst keeping the probe in position no. 1.
   
2. Set the inlet pressure to 300KPa and check that the inlet pressure and the chest pressure are to be equal.
   
3. Chest pressure is to be observed throughout the experiment and re-adjusted to initial setting when necessary.
   
4. We start recording the probe pressure at position no. 1 and then we go throughout all the other positions till reaching final position (38) measuring the pressure at each position.
   
5. Finally we repeat our procedures different back pressure values (400KPa and 600 KPa)
   

Sample of calculations:

1. Pressure ratio when P₀ = 300KPa 220/300 = 0.733
   
2. Vt when P₀ = 400KPa = (2*1.4*287*(18+273))/1.4-1=584619 584619*(1-(0.75)^0.4/1.4)= 46130.8584 46130.8584^1/2= 214.781
   
3. Mt when P₀ = 500KPa= 9.16(500)(0.76)^(1/1.4)*[2*1.4/(1.4-1)(287)(291)*(1-(.76)^(0.4/1.4))]^1/2= 9.47
   

Discussions & Conclusions:

1. The pressure decreases with the nozzle’s diameter decreasing
   
2. A nozzle increases velocity
   
3. Velocity keeps increasing till it chocks, if reaches velocity of sound (A) it cant increase anymore
   
4. If VA which is the nozzle V increases. If VA then V decreases
   
5. The negative value declares that the probe is out of the nozzle 
   
6. The relation between the pressure ratio and the mass flow is direct
   
7. The relation between the pressure ratio and the velocity of the throat is inverse.
   

Surface Hardening by Carburising

Introduction
    The service conditions of many steel components such as cams, gears, and shafts make it necessary for them to posses both hard, wear resistant surface and at the same time, tough, shock resistant cores. This situation can be best dealt with by introducing low carbon steel with suitable core properties. In this case the carbon penetrates the surface to a regulated depth (case depth) causing the material to be harder.

Basic Theory
    Surface hardening by carburizing can be classified into three kinds; solid carburizing, liquid carburizing and gas carburizing. In each of these three processes, certain chemicals and methods are used, for the solid carburizing we use charcoal, and in liquid carburizing we use Cyanide (CN) but in gas carburizing we use carbon monoxide gas (CO). The best way among these three ways is characterized according to depth, which goes from gas to solid. But in our case, we’ll be using solid carburizing because chemicals used in liquid and gas carburizing are poisonous and apparatus are of high cost.
Carburizing is solid media involves packing the work into heat resisting boxes by which the distance between them is 50mm. The boxes are heated to a temperature equal the carburizing temperature and maintained at that temperature for a period of time according to the case depth required.
The entrapped air between carbon molecules reacts with carbon to form CO gas according to this equation:
2C + O2 -> 2CO

The actual carburizing process depends on carbon monoxide gas to carry carbon atoms to the surface of the work piece.
And at the surface of the work piece, carbon is released according to this equation:
2CO -> CO2 + C

These carbon atoms are dissolved on the surface of the steel.

The carburizing process is affected by three factors:

1. Temperature
   
2. Time
   
3. Energizers: such as BaCO3, CaCO3 and NaOH
   

These energizers speed up or aid the process of carburizing by adding CO2, which later reacts with charcoal producing CO, where it carries C atoms to the surface of steel as explained above, according to these reaction:
BaCO3 -> BaO + CO2 & CO2 + C -> 2CO

Experimental Apparatus & Methods
    The materials needed in this experiment are 0.15% C steel specimens (the specimens are divided into three categories; 870ºC, 900ºC and 925ºC, and each specimen is left in the furnace for five different timings, 4, 16 ,22,24 and 30 hours), Stainless steel box containing solid carbon powder (Charcoal). As for the apparatus, we need a heating furnace, timing equipment, a microscope supplied with a monitor.

The procedure of this experiment is easy to follow, and it goes as:

1. Put each steel specimen in the stainless steel box containing the charcoal.
   
2. Put the box containing the specimen in the heating furnace at the specified temperature and leave it for the specified time (at 950ºC, steel changes from BCC at room temperature to FCC).
   
3. Turn off the furnace and take out the specimen out of the box and put it into another furnace at a temperature of 850ºC for half and hour.
   
4. Take out the specimen and cool it with water.
   
5. After cooling, each specimen is cut in half, grinded (with 120, 180, 240, 320, 400, 600 paper) and polished.
   
6. Each specimen is then put under the microscope and the case depth is calculated.
   
7. The last step in this experiment is to do the hardness test for each specimen with the Rockwell C scale (HRC) where we use the diamond cone as the indenter.
   

For the calculation of the case depth, the length of the carbon layer is measured on the monitor, and then it is divided over the magnification to give the true depth.
Another method may be applied is to follow a simple and easy rule: case depth = K(t), where K is a constant and t is the time.

In heat treatment, sometimes specimens are treated at two different temperatures each for half an hour (760ºC and 880ºC). One half hour is for case hardening and the other is for refining. Moreover, refining is done to get a high toughness from inside and increase the fatigue life.

Discussion
    As we can see, for the case depth, i.e. increasing each time and temperature, the case depth increases for each step more than the previous one. Thus, if we do the hardness test, as tabulated, we can see that hardness increases as we go down the table for the 5mm, 10mm,15mm (the center) positions but stays almost constant for the case (or surface). The case reaches a max value at 920ºC 6hrs and then goes back to the constant value.
Moreover, at the three other positions, the strength increases as the temperature and time increases, and this is because the concentration of carbon going from the case to the center decreases, thus causes the strength to decrease.

Conclusion
    As the concentration of carbon increases, the strength increases. As we saw, the strength is higher for the specimen at the case than at the 10 mm, and at 10 mm higher than at 5 mm, and at 5 mm higher than the centre. Thus, as a conclusion, the strength increases wherever there is a higher accumulation of carbon.

Also, surface hardening makes steel components hard, with a wear resistant surface and at the same time, tough and with shock resistant cores. Whereas time, temperature, and case produced by surface hardening are directly proportional, and as time and temperature increase, the case produced increase in depth.