Assembly
In dry air leak testing, another issue
In some instances assembly and test
is part damage during sealing. A simple
clamp and cylinder could eliminate
fixture designs must also take into
deflection of a part during testing but it
can also crush the part. Then there is
account and simulate the parts to
also the problem of masking leaks. If
you have a weld and the part is
which other parts are to be joined
operated under pressure, you want to
make sure that if the weld opens up you
have a leak to atmosphere. If you are
restraining the weld motion you are
restraining the leak. As another
example, if a fixture has threads and In some instances assembly and considerations (a Japanese term
you are trying to pull down to a certain test fixture designs must also take into meaning mistake proofing) are such that
vacuum level, air will escape as you pull account and simulate the parts to which often a fixture design will anticipate
the vacuum down if the threads aren’t parts are to be joined. This is very human errors of placing wrong parts in
slotted. This could appear as a gross predominant in leak testing medical the testing stations. A poka-yoke
leak or prevent the desired vacuum devices. For example, if the device has informed fixture design will make sure
level from being achieved and make it an O-ring you will generally want to that only parts with the correct
impossible to do reliable testing with mimic the mating surface. In the dimensions will be tested.
that fixture design. Thus, expert fixture automotive industry while leak testing a The above is only a brief discussion
designs for dry air leak testing have transmission casting you would try to of some of the factors that testing
several factors that they are managing: mimic the paper gaskets that will be in applications engineers take into
the part cannot have any creep or use in the field, as opposed to actually account. Since fixture design can make
motion, it must be stationery; the part using paper gaskets since these or break your assembly and test
cannot be crushed or damaged; and wouldn’t hold up during the testing solution it is generally advisable to
34
the fixture design cannot mask leaks. process. enlist only testing engineers with
From a fixturing perspective, Fixture design is often mediated by experience in building many turnkey
www
helium leak testing has yet another set how automated the overall assembly testing solutions. One needs a nuanced
.eur of challenges. In helium testing, you and test operation is and at the specific understanding of leak test and
oasiasemiconductor
need to ensure that you do not trap or testing stations. In manual systems the functional test requirements to bear on
retain any helium in seals. In helium or sealing and fixturing may be more fixture designs that allow superior test
any tracer gas application what happens difficult because of ergonomic issues. instruments to deliver their promised
to tracer gas at the end of the test is Clearance for loading and unloading performance. In fact, the growing use of
critical because if you have any trapped may be an issue. Poka-yoke off the shelf fixtures such as expandable
test gas staying around after the test seals is a reflection of how little the real
.com
sequence it contaminates the following requirements for fixture design during
test. This is not the case with dry air testing are understood. Reliable and
testing. On the other hand, deflection reproducible gauges almost always
square4
Issue III 2009
issues are less critical in helium leak require custom fixture designs given the
testing than in dry air testing. One unlimited variety of geometries in parts
needs to understand the different that are tested.
requirements of each test method vis-à-
vis fixture design and how improper Generic vs. customised
designs will compromise that specific software/instrumentation
type of testing. Testing experts know that there are no
In many assembly and test one size fits all generic testing
applications there is also a need to solutions. Taking short cuts by selecting
orient the part being tested in the same the lowest cost instrument from the
orientation that it will have when in use shelf and placing it into assemblies
in the field. The applications where this rarely achieves Gauge R&R in
is an overriding feature of fixture design acceptable ranges because the test
are quite diverse—from automotive sensors and software programming are
valves that are referred to as ‘true car not well matched to application
position’ when functionally tested to requirements.
respirator components in the medical Fixture design is critical to test process In fact, most of the assembly and
field. integrity test solutions that do not meet the test
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