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.