Wringability and Gage Blocks
Gage block length is defined as an interferometric measurement when the gage block is wrung to a flat platen. Gage block wring is incorporated into the lengths of the blocks themselves, and wringing film is included in the defined length of the gage block. When gage blocks are assembled in combinations, no additional correction factor for wringing films needs to be added to the length of the combination.
Wringability itself may be defined as the ability of two surfaces to adhere tightly to each other in the absence of external means--they are not magnetized or clamped together.
The source of the forces holding gage blocks together are thought to come from:
- Air pressure from the surrounding environment as the air is squeezed out when the blocks are slid together.
- Surface tension from oil that remains on the gage blocks or water vapor from the air that acts as an adhesive to hold them together.
- Molecular force caused by the interchange of electrons between the atoms of the separate blocks when two very flat surfaces are brought into such close contact with each other. This force will remain even in a vacuum or if no oil or water is present on the blocks.
Wringing requires two smooth, flat surfaces with surface finishes of 1 microinch AA or better. For gage blocks, it becomes difficult to wring surfaces once the flatness begins to exceed 5 microinches.
Remove all nicks and burrs before attempting to wring blocks together because a burr on one block may damage the surface of the other block. Blocks may be checked for burrs with a gage block stone before wringing. A gage block stone with serrated grooves is recommended because it gives a better "feel" for nicks and burrs that catch the edges of the serration. Badly nicked surfaces will click as a nick passes along the serration.
Foreign material em-bedded in the grain of the stone may scratch the blocks. Gage block stones should be conditioned before use. This may be done by lightly rubbing the surfaces of two stones together in a figure-eight pattern. Clean the stones with kerosene before use.
Will stoning a gage block change its size? It depends largely on the condition of a block. A block in good condition will not be affected by light stoning. Stoning removes portions of the block that have been raised above the true gage surface by nicks or scratches that can contribute to more variation or large readings during calibration. Stoning will remove this small amount of raised material. Repeatability of readings is improved, sizes appear to be truer to their original tolerances and blocks will wring together better.
- Stoning is to be performed only on used gage blocks where the surface finish may be degraded by scratches or small nicks.
- Make sure the stone is clean, dry and free from any dirt or abrasive compound. Abrasives on the stone may lap the block and significantly alter its size.
- With a light amount of pressure, stroke the block across the serration two or three times in a forward, back, forward motion. It is not recommended that anything more than light pressure be used unless necessary to remove nicks and burrs.
- Listen and feel for nicks and burrs. If the block glides easily across the stone without a scraping sound or clicking or jumping across the serration, then stop. Flip the block over, and repeat on the other side.
- If nicks and burrs are detected, repeat the procedure, but not more than twice. The pressure may be increased each time as needed to try to remove the nicks and burrs. Do not use more then seven strokes per session.
- If repeated attempts are unsuccessful at removing the burrs, examine the block for damage. In this condition it is not likely a block would be wringable.
Rusted gage blocks
Rusty blocks may be improved by stoning the blocks after the stone has been wetted with kerosene. While this may temporarily improve the utility of the block, it will not permanently remove corrosion or halt its advance.
- Dampen a cotton swab with kerosene.
- Wipe the stone with the dampened swab. A film of kerosene should be seen on the stone; the stone should not be dripping wet.
- Stone as before, using necessary pressure. It may be necessary to be aggressive to remove the corrosion.
- If the corrosion cannot be removed after repeated tries, then the block is considered damaged.
- Keep the stone dampened as the kerosene evaporates.
Recommended stoning pressures are: Light Stoning: 1 to 1.5 pounds pressure Medium Stoning: 2.5 to 3.5 pounds pressure Heavy Stoning: More than 6 pounds pressure
Wringing gage blocks
- Make sure that blocks are clean.
- Wipe the surfaces of the blocks to be wrung gently across the oiled wring pad.
- Wipe these surfaces on the dry pad, removing as much oil as possible.
- Slide the surfaces of the blocks together as shown in Figure 1. Apply pressure while sliding the blocks. The blocks should slide together without any feel of bumps or scratching and should adhere to each other after being rotated into place.
Wringability is an important property of gage blocks, and fortunately, it is a quality that can be controlled and monitored readily by the user of the blocks. With correct use and care, gage blocks will provide long, reliable and accurate service.
A wringability test may be done by the user of the blocks if a problem with a block is suspected.
To test gage blocks for wringability, a 2-inch diameter, Reference Grade (1 microinch flatness) quartz optical flat should be used. A double-sided flat is recommended if more than 40 blocks are to be tested. A double-sided flat does not add to the accuracy of measurement, but it provides a second wringing surface if the first surface becomes scratched during wringing.
As before, prepare the blocks for wringing.
- Wipe the surface of the optical flat gently across the oiled wring pad.
- Wipe the surface of the flat gently across the dry pad.
- Slide the test block onto the flat as shown in Figure 2. Apply pressure while sliding the block and flat together.
- Observe the surface of the gage block that is wrung to the optical flat from the opposite side of the flat. Repeat this step as necessary to ensure a valid result.
- Interpret results:For Federal Grades 0.5, 1 and 2, and ISO Grades K, 00 and 0, no color or oil should appear on the face of the flat. For Federal Grade 3 or ISO Grades 1 and 2, the surface shall not have less than 80% colorless wringing area.
- Repeat for the other surface of the test block.
The test may be difficult to apply to gage blocks that are not almost perfectly flat. This includes thinner gage blocks that are less than 0.1 inch (2.5 millimeters) thick, which are not usually flat in their free state, including most gage blocks in metric sets.