wringability and gage blocks by dave friedel, general manager of starretts webber gage division wringability is an important proper


Wringability and Gage Blocks
By Dave Friedel, General Manager of Starrett's Webber Gage Division
Wringability is an important property of gage blocks. In fact, gage
block specifications recommend replacing blocks that have lost their
ability to wring. Not everyone uses gage blocks in wrung combinations,
but wringability is a test on the integrity of the surface condition
of the gage block. Gage blocks that don’t wring may give erratic and
unreliable results.
T he fact that gage blocks wring is incorporated into the
lengths of the blocks themselves. Gage block length is defined as an
interferometric measurement when the gage block is wrung to a flat
platen. This includes one wringing film in the defined length of the
gage block.
This definition is very useful. 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.)
Wringing requires two smooth, flat surfaces with surface finishes of 1
microinch AA or better. For gage blocks, it becomes difficult to wring
surfaces if the flatness starts to exceed 5 microinches. The sources
of the forces holding gage blocks together are thought to come from:
1) Air pressure from the surrounding environment as the air is
squeezed out when the blocks are slid together.
2) Surface tension from oil that remains on the gage blocks or water
vapor from the air acts as a glue to hold them together.
3) When two very flat surfaces are brought into such close contact
with each other, this allows an interchange of electrons between the
atoms of the separate blocks, which creates an attractive molecular
force. (This force will remain even in a vacuum or if no oil or water
is present on the blocks.)The last two sources are thought to be the
most significant.
Preparation of Gage Blocks Prior to Wringing
Make sure that the blocks to be wrung are free from nicks and burrs.
It is important that all nicks and burrs be removed BEFORE attempting
to wring blocks together because a burr on one block may damage the
surface of the other block.
B locks 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 serrations. Badly nicked surfaces will click as a nick
passes along the serrations.
Helpful Hint: Gage block stones may pick up foreign material that may
embed in the grain of the stone. These foreign particles 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.
Stoning Gage Blocks
Stoning Gage Blocks Will stoning a gage block change its size? The
answer to that question is No . . . and Yes. It depends largely on the
condition of a block. A block in good condition will not be affected
by light stoning. The purpose of stoning is to remove portions of the
block that have been raised above the true gage surface by nicks or
scratches which can contribute to more variation during calibration or
large readings. Stoning will remove this small amount of raised
material. Repeatability of readings is improved, and sizes appear to
be truer to their original tolerances. Blocks will wring together
better.
1) Stoning is to be performed only on used gage blocks where the
surface finish may be degraded by scratches or small nicks.
2) Make sure the stone is clean and dry--free from any dirt or
abrasive compound. Abrasives on the stone may lap the block and
significantly alter its size.
3) With a light amount of pressure, stroke the block across the
serration two or three times. (Forward, back, and forward.) It is not
recommended that more than light pressure be used unless necessary to
remove nicks and burrs.
4) Listen and feel for nicks and burrs that might be present. 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.
5) If nicks and burrs are detected, repeat the procedure but not more
than twice more. The pressure may be increased each time as needed to
try to remove the nicks and burrs. Use not more then seven strokes per
session.
6) If repeated attempts are unsuccessful at removing the burrs,
examine the block for damage. It is not likely a block would be
wringable in this condition.
Rusted Gage Blocks
The condition of rusty blocks may be greatly improved by stoning the
blocks after the stone has been wetted with kerosene. While this may
temporarily improve the utility of the block, this will not
permanently remove the corrosion or halt its advance.
1) Take a cotton swab and dampen it with kerosene.
2) Wipe the stone with the dampened swab. A film of kerosene should be
seen on the stone, but the stone should not be dripping wet.
3) Stone as before, using whatever pressure is necessary. It may be
necessary to be quite aggressive to remove the corrosion.
Stoning Pressures
Light Stoning: 1 – 1.5 lbs
Medium Stoning: 2.5 – 3.5 lbs
Heavy Stoning: > 6 lbs
4) If the corrosion cannot be removed after repeated tries, then the
block is considered to be damaged.
5) Keep the stone dampened as the kerosene evaporates.
Wringing Gage Blocks
1) Make sure that blocks are clean.
2) Wipe the surfaces of the blocks to be wrung gently across the oiled
Wring Pad. (See figures below.)
3) Wipe these surfaces on the dry pad, removing as much oil as
possible.
4 ) Slide the surfaces of the blocks together as shown. Apply
pressure while sliding the blocks. The blocks should slide together
without any feel of bumps or scratching, and should adhere to each
other strongly after being rotated into place.


Testing Wringability of Gage Blocks
There is a formal test for wringability as described below. This test
may be done by the user of the blocks, and does not require a
laboratory to perform the test. This test is only usually done 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. It provides a second wringing surface if the first
surface becomes scratched while wringing blocks to the flat.)
1) As before, prepare the blocks for wringing.
2) Wipe the surface of the Optical Flat gently across the oiled Wring
Pad.
3) Wipe the surface of the flat gently across the dry pad.
4) Slide the test block onto the flat as shown. Apply pressure while
sliding the block and flat together.
5) 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.
6) Interpret results:
a) 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.
b) For Federal Grade 3 or ISO Grades 1 and 2, the surface shall not
have less than 80% colorless wringing area.
7) Repeat for the other surface of the test block.
T his test (at right) may be difficult to apply to gage blocks
that are not almost perfectly flat. This includes thinner gage blocks
that are less than .100” or 2.5 mm thick, which are not usually, flat
in their free state. This includes most gage blocks in metric sets.
Wringability is an important property of gage blocks, but is
fortunately a quality that can be controlled and monitored readily by
the user of the blocks. With proper use and care, gage blocks will
provide long, reliable, accurate service.

David Friedel is General Manager and Quality Assurance Manager of the
Webber Gage Division of the L. S. Starrett Company. The Webber Gage
Division specializes in the manufacture and calibration of gage
blocks. Mr. Friedel has been active with the American Society of
Mechanical Engineer’s B89 Codes and Standards Committee for twenty
years, and is the current chairman of the B89.1.2 Working Group for
Measuring Gage Blocks by Comparison Methods.

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