SLIP GAUGES

Gauge blocks (also known as gage blocks, Johansson gauges, slip gauges, or Jo blocks) are a system for producing precision lengths. The individual gauge block is a metal or ceramic block that has been precision ground and lapped to a specific thickness.

 

An important feature of gauge blocks is that they can be joined together with very little dimensional uncertainty. The blocks are joined by a sliding process called wringing, which causes their ultra-flat surfaces to cling together. A small number of gauge blocks can be used to create accurate lengths within a wide range. 

Manufacturing of slip gauges :- 

Gauge blocks are usually made either from hardened alloy tool steels, ceramics or cemented carbides (such as tungsten carbide or tantalum carbide). Often the carbide has a hardness of 1500 Vickers hardness. Long series blocks are made from high-quality steel having cross section (35 × 9 mm) with holes for clamping two slips together. These are also available in carbon steel material. Steel blocks are hardened and tempered. The hardness is important because it slows down the gauge's rate of wear during use (this is why other kinds of gauges, such as pins, thread plugs, and rings, are also hardened.) 

The cutting of the blocks to size is accomplished with grinding followed by lapping. Usually no plating or other coating is involved. Blocks are kept very lightly oiled, and are stored and used in dry climate-controlled conditions; unplated, uncoated steel gauge blocks can last for decades without rusting. 

(Method) Following steps gives a brief of method of manufacturing of slip gauges :- 

1. The high grade steel gauge blanks are taken with appropriate size.

2. They are subjected to hardened and rough grinding process.

3. Then they are subjected to a cyclic low temperature heat treatment, to provide stability of dimensions and to relieve the internal stress.

4. A batch of 8 blanks of similar nominal size is mounted on eight Co-planner Faces of a magnetic chuck.

5. Their one set of Faces is lapped truly flat by lapping process.

6. By changing the lapped faces on magnetic chuck, opposite Faces also lapped truly flat.

7. Now, the required Parallelism and equality of size is achieved by interchanging four of the eight gauges as shown in figure 1.8 (e). They are interchanged diagonally and turned end for end. Thus any errors in Parallelism are equalized.

8. Now, to determine whether the gauges are the required size they are removed from the chuck, wrong together in combination, and their aggregate size compared with an appropriate size master in a suitable comparator. A magnification of 8 is obtained in this calibration since, as each of the 8 gauges must be identical in size, the difference between combination and master may be divided by eight and this difference is distributed to each gauge.

9. If necessary, individual block can be mounted on chuck to bring their individual lengths within required accuracy.

How gauge blocks are measured ? 

A gauge block is a block of metal or ceramic with two opposing faces ground precisely flat and parallel, a precise distance apart. Standard grade blocks are made of a hardened steel alloy, while calibration grade blocks are often made of tungsten carbide, chromium carbide or ceramic because they are harder and wear less.The length of each block is actually slightly shorter than the nominal length stamped on it, because the stamped length includes the length of one wring film, a film of lubricant which separates adjacent block faces in normal use. The thickness of the wring film is about 25 nanometers. The gauge's nominal length also known as the interferometric length.

Wringing :- 

Wringing is the process of sliding two blocks together so that their faces lightly bond. Because of their ultraflat surfaces, when wrung, gauge blocks adhere to each other tightly. 

While the exact mechanism that causes wringing is unknown, it is believed to be a combination of - 

• Air pressure applies pressure between the blocks because the air is squeezed out of the joint
• Surface tension from oil and watervapor that is present between the blocks
• Molecular attraction that occurs when two very flat surfaces are brought into contact; this force causes gauge blocks to adhere even without surface lubricants, and in a vacuum. 

The process of wringing involves four steps :- 

1. Wiping a clean gauge block across an oiled pad.
2. Wiping any extra oil off the gauge block using a dry pad.
3. The block is then slid perpendicularly across the other block while applying moderate pressure until they form a cruciform.
4. Finally, the block is rotated until it is inline with the other block.

After use the blocks are re-oiled or greased to protect against corrosion. The ability for a given gauge block to wring is called wringability; it is officially defined as "the ability of two surfaces to adhere tightly to each other in the absence of external means." The minimum conditions for wringability are a surface finish of 1 microinch (0.025 μm) AA or better, and a flatness of at least 5 μin (0.13 μm).

Grades :-  

Gauge blocks are available in various grades, depending on their intended use. The grading criterion is tightness of tolerance on their sizes; thus higher grades are made to tighter tolerances and have higher accuracy and precision. 

Tolerances will vary within the same grade as the thickness of the material increases.

• reference (AAA): small tolerance (±0.05 μm) used to establish standards.
• calibration (AA): (tolerance +0.10 μm to −0.05 μm) used to calibrate inspection blocks and very high precision gauging.
• inspection (A): (tolerance +0.15 μm to −0.05 μm) used as toolroom standards for setting other gauging tools.
• workshop (B): large tolerance (tolerance +0.25 μm to −0.15 μm) used as shop standards for precision measurement.

Uses of Slip Gauges:-

Slip gauges are important means of measurement in industries and laboratories.

Their uses are:

1. They universally accepted as a “Standard of length”.

2. They used for direct precise measurement where accuracy of work piece being measure is high.

3. They used with high-magnification comparators, to establish the size of the gauge blocks.

4. They are used for checking the accuracy of measuring instruments.

5. They are used to setting up a comparator to specific dimension.

6. They are used to check a batch of components quickly and accurately.

Care of Slip Gauges :-

Since the initial cost of slip gauges in high, so to maintain their accuracy, they require great care.

Following points should be kept in mind regarding the care of slip gauges:

1. When not in use, the slip gauges should be kept in their respective positions in the gauge box.

2. Before wringing the blocks together, ensure that their faces are perfectly clean.

3. Measuring faces should not be fingered.

4. Gauges should not be wrong together over an open gauge box, due to the possibility of accidently drop of any gauge on several gauges placed in the box and could be damaged.

5. Gauges should not be wrung together for a long time.

6. After use, does not break the pile but slide one gauge over the other to separate them.

7. After use, a thin layer of good quality grease should be applied on their faces, before they are kept in their case.

8. As far as possible, slip gauges should be used in air-conditioned rooms, free from dust and maintained constant temperature.

9. During the use, their working faces should never be placed on the surface plate etc.

10. Check accuracy at appropriate intervals.

11. Use minimum number of gauges for a combination.

12. Wring together in correct manner.

13. Use 2.5mm protector slips whenever possible.