Page 34 - MMI-SeptOct2021 SM upload

Avoiding tolerance conflicts

Source: Nagel Precision Inc. The different ways we measure surface finish come with fine dis-
tinctions, says Keshavan. Roughness average (Ra) is a parameter com-
monly used to measure surface finish, but he argues that Ra does not
distinction between peak and valley parameters. For instance, a part
Superfinishing with stone. provide a full picture. In particular, the parameter does not make a
with five distinct valleys of the same depth could have the same Ra
ameters and faces with fixed as a part with five distinct peaks of the same height. Considering Ra
abrasives. It is considered a cold alone, the finishes of these parts are the same. That is why, in certain
process, and it also improves applications, other parameters will be more appropriate to call out for
the micro-contour accuracy. the surface finish measurement.
Polishing – Finishing of outside
features with loose abrasives.
Both brushing and vibratory
finishing are examples of a pol-
ishing process. This is not con-
sidered a cold process, and sig-
nificant heat can be generated
Both super- during the finishing operation.
finishing and Improvement of surface finish Keshavan recommends referencing the ANSI/ASME B 46.1 nation-
polishing is accompanied by loss of mi- al surface texture standard. It lists 100 or more parameters that can
are used cro-contour accuracy. be used to measure finish, just a few of which are Ra, reduced peak
to create a
fine surface height (Rpk), reduced valley height (Rvk), bearing area ratio (Tp), and
on outside Both superfinishing and polish- the average distance between the five highest peaks and the five low-
diameters. ing are used to create a fine sur- est valleys in a sampling length (Rz).
Polishing is face on outside diameters. Pol- “If these parameters are judiciously selected and toleranced correctly,”
a more flex- ishing is a more flexible process he says, “they permit manufacturing engineers to choose the most ef-
ible process
because because a brush, a lapping com- ficient way to process the parts and maintain the product’s integrity.”
a brush, pound or a vibratory process like He says problems arise when multiple parameters are specified with-
a lapping tumbling can all be used. Tum- out regard to their feasibility given the other specified parameters.
compound or bling makes it possible to polish Consider the following print callouts:
a vibratory many parts at once. In contrast,
process like
tumbling can superfinishing uses a fixed abra- Ra: 0.2-0.3 µm
all be used. sive to impart the finish, and only Rp ≤ 0.5 µm
one part is finished at a time. Ke- Rz ≤ 1.0 µm
shavan says superfinishing has
advanced beyond hard tooling. Empirically speaking, Keshavan says, Rz is equal to about 7 to 10
To add more flexibility to the times Ra. To target 0.2 µm Ra, he says Rz would be at least 1.4 µm
superfinishing process, his own and out of print.
company, Nagel, has developed Another common problem he sees is applying a blanket process capa-
what it calls ‘D-flex’ band tech- bility index (Cpk) without paying attention to tolerancing, particular-
nology that acts like a fixed abra- ly given upper and lower bounds on surface roughness. For example,
sive but can flex to compensate consider another print specification:
for as much as a 15 mm change in
diameter. Ra: 0.05-0.2 µm
Determining which finishing Rz: 1.0 µm
process to use depends on the
end use of the part. Keshavan “If a Cpk of 1.67 or better is demanded on all the parameters, we need
says, “Polishing is best suited to be at roughly 50% of the tolerance, so we shoot for 0.5 Rz,” he says.
for applications in which part “If we were to do that, Ra could be less than 0.05 µm, which is out of
geometry is not critical and aes- spec. If we were to target Ra to hit the Cpk, Rz would be pushed to-
thetics are the main concern; ward the higher end of the spec or out of spec altogether. Therefore, it Source: Nagel Precision Inc.
superfinishing is best suited for is extremely important to tolerance this correctly.”
mission-critical parts.”

34 | September-October 2021 Modern Manufacturing India