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.