Eighteen-station assembly chassis optimized for test-centric assembly.
In-process testing minimizes rework and scrap costs.
Test-centric assembly is the upfront consideration of
real-world test requirements in test-intensive assembly operations that is
proven to lower production line inefficiencies. It is a best-practice approach
for any test-intensive assembly operation.
A common but less satisfactory
approach to machine building of test-intensive assemblies is to consider
testing as an afterthought to overall machine design. This often means that the
real testing requirements of an assembly and test application are
misunderstood. This approach typically compromises Gauge R&R. In many
cases, defective part problems are predominant and there is an inability to
diagnose whether testing processes are lacking and creating erroneous defect
readings or if real defects in the parts are at issue.
Hallmarks of test-centric assemblies are expert fixture
design and integration of both hardware and software at the system level.
Test-centric assembly refers to the entirety of the applications engineering
knowledge base that enables testing experts with a singular focus on testing to
improve production speed and yields in test-intensive operations.
This article reviews the general principles of test-centric
assembly for manufacturers and machine builders more accustomed to streamlining
assembly operations where testing is absent or plays a minimal part.
Fixture design is critical to test process integrity.
End of Line vs. In-Process Testing
In many manufacturing scenarios, there are considerable
advantages to focusing on in-process (as opposed to end-of-line) testing. As a
part or assembly is built, material and labor costs are constantly added. The
total value of a production component is usually a function of its percentage
completion, and includes both labor and material costs built into the item. If
testing is integrated throughout an assembly, the costs of adding material and
labor to defective production components can be avoided.
Following are some examples of how costs for rework and scrap can be avoided by in-process testing.
- Functional testing to verify electrical, pneumatic
and mechanical part characteristics before further assembly steps.
- Automatic resistance measurements in stations that
follow the addition of coils, heaters or resistors.
- Leak test following ultrasonic welding of two
- Leak testing following spin riveting of a cover to
a pump housing.
- Leak testing after the insertion of an “O” ring or
- Machine vision systems
or probes used to ensure that parts are positioned properly.
- Dimensional gauging
stations, especially after a crimping or staking operation.
- Electrical testing of continuity, voltage, current
and contact bounce.
When in-process testing becomes a centerpiece
of an assembly and test operation, a redesign of the chassis used in the assembly is recommended. The ease with which one
can relocate or remove work stations and accommodate test operations with
different times in each test station is key. A power-and-free conveyor, while
offering flexibility and asynchronous operation, has the disadvantages of
increased work in process and the need for fairly complex control systems that
become especially burdensome in test-intensive operations.
For better efficiency in test-centric assembly, testing applications
engineers designed a standardized heavy-duty in-line chassis that allows a
flexible workstation setup and retooling, including double dwell time at
selected stations without slowing cycle times throughout the machine. These are
typically small footprint machines, e.g., 40 inches (1,016 mm) x 90 inches
(2,286 mm) long for 18 stations or 61 inches (1,549.4 mm) long for 10 stations.
Assemblies can be made of multiple standard testing chassis, combining standard units end-to-end,
side-by-side, or end-to side for assembly and test operations requiring
additional stations. Servo drives allow one to optimize the transfer rate
depending on part inertia (weight) without requiring any machine component
replacements or mechanical adjustments. Individual station base plates are used
to simplify adding, removing or retooling any station without disturbing the
others. These standardized chasses for test-intensive operations accept a range
of standardized tooling and robotics or customized equipment needed for other
parts of the assembly process. What differentiates these test-intensive
assembly chasses from power and free systems is twofold: 1.) the use of quick
change fixturing and 2.) the ability to double up stations for greater
flexibility and to reduce work in process (fewer pallets).
Test-centric assemblies can always be used to track the contents of each
pallet and report to control systems on conditions of parts. Hence, the control
system can identify the presence or absence of parts on a pallet, whether or
not the part has passed or failed a test, and, in some cases, will identify if
the part was tested yet. Different signaling systems gather this information,
with the more sophisticated systems including a means of reading the pallet
number so the system can track information on a particular pallet, in addition
to identifying the status of the pallet contents.
Test-centric assemblies cannot only test and track parts, but handle
rejected pallets as well. This can be done by having rejected pallets continue
down the line but be ignored by succeeding operations until they can be
unloaded at an intermediate reject position or immediately after testing.
Rejects can then be sent to a parallel repair line. Whatever method is used,
its objective is to avoid any further work on a rejected or defective part,
which is the overriding advantage of in-process testing.
Test-centric assemblies have automatic defective parts removal.
Defective Parts or Defective Tests?
The inability to differentiate good parts from defective
parts may be due to faulty testing methods, which are usually the result of an
inadequate understanding of real testing requirements. An example is found in
test results with insufficient signal-to-noise ratios. In leak testing, there
are often problems with seal creep. Another example is in-process gauging
stations that are insufficiently rigid or do not have the correct reference.
The first considerations in test-centric assembly are: developing a
thorough and detailed understanding of required testing accuracies; finding the
best methods to achieve the required results; and obtaining a detailed
understanding of how the tests actually proceed so that one does not
unwittingly undermine the test process.
For example, in leak testing, the choice between methods is driven by a
compromise between a leak detector’s performance and cost of purchasing a
highly specified piece of equipment. Several methods could be used - simple
pressure decay testing, differential pressure decay testing, mass flow leak
testing, and helium mass spectrometer testing. The lowest cost method - pressure decay testing - actually has lower initial
costs for fixtures, etc., but may be more costly due to longer test cycle
times. In addition, it is unsatisfactory for testing for small leaks under 5
sccm. Differential pressure decay testing methods are superior to simple
pressure decay testing for small leak testing or for testing at high pressures,
but are similarly inadequate for testing parts where temperature compensation
is an issue. Both pressure decay and differential pressure decay test methods
involve increased cycle times compared to other dry air test methods.
(Note: The most common mistake made by those without testing expertise
is to select the least expensive instrument. However, this does not imply that
higher cost test methods are directly correlated to the best test technology
choices. High-cost helium testing, especially costly in the current context of
rapidly rising helium prices worldwide, is rarely justified except for
aerospace and similar applications where potentially hazardous gas leaks less
than 0.01 sccm need to be detected. In most applications requiring detection of
low leak rates, the lower-cost mass flow testing method using customized
sensors is preferable, since it has the shortest possible testing times, can
readily compensate for temperature, and it delivers the best accuracy for applications
where testing is required at 0.05 sccm or larger.)
The details of how a test is performed can be
all important in determining if the testing process is adequate. Often, this
means having the ability to test the test process as testing proceeds. In
downstream mass flow testing, for example, where there is zero pressure from
atmosphere, it is not possible
to know if a valve used in testing is working or not unless extra steps are
taken. A zero-leak measurement could otherwise be due to a dysfunctional valve
or a line being plugged. For that reason, accurate downstream mass flow test
methods will always use a bias leak that verifies that the entire system is
working and that the test circuit’s integrity is not compromised. A bias leak
is simply the introduction of a known leak as a self-check of the testing
system. If valves are not operating properly and test seals are insufficient
the bias leak will not measure as a leak, indicating that remedial actions on
the test system need to be taken.
Similarly, test systems should be frequently and accurately calibrated
and validated. Mistakes often occur due to reliance on antiquated mechanical
calibration methods that have inherent limitations (such as pneumatic valves
and other moving parts that can stick or wear, and/or small orifices that can
clog). Today’s test-centric assemblies rarely use human-error-prone mechanical
calibration methods; instead, they rely on electronic technology for
calibration and validation of testing. These electronic test methods are not
only more reliable but are fast, which is one of the reasons why test-centric
assemblies generally have significantly higher yields and shorter test times.
Custom software for test-intensive assembly helps cut costs.
Differences in Fixtures for Assembly vs. Testing
It often comes as a surprise to those familiar with building
standard assemblies that the fixtures they take pains to create for fastest
assembly throughput often have quite the opposite characteristics of what one
would want for fail-safe testing in most assembly and test operations. This is
why one is best advised to consult with applications laboratories dedicated to
developing best-match test processes to help with test fixture designs.
For example, if you have a part that is driving a screw you
generally want a lot of float. However, to leak test that part, you would have
to seal it on dead nuts so the sealing location would interfere with other
steps of the assembly operation. Hence, it’s best not to use the same pallets
during leak testing as in other phases of assembly and testing.
In dry air leak testing, another issue is part damage during
sealing. A simple clamp and cylinder could eliminate deflection of a part
during testing - but can also crush the part. In addition, if you have a weld
and the part is operated under pressure, you must ensure 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 you are
trying to pull down to a certain vacuum level, air will escape as you pull the
vacuum down if the threads aren’t slotted. This could appear as a gross leak or
prevent the desired vacuum level from being achieved and make it impossible to
do reliable testing with that fixture design. Thus, expert fixture designs for
dry air leak testing must manage several factors: the part must be stationery;
the part cannot be crushed or damaged; and the fixture design cannot mask
From a fixturing perspective,
helium leak testing has another set of challenges. In helium testing, no helium
can be trapped or retained in seals. In helium or any tracer gas application,
what happens to tracer gas at the end of the test is critical; if you have any
trapped test gas staying around after the test sequence, it contaminates the
following test. This is not the case with dry air testing. On the other hand, deflection issues are less
critical in helium leak testing than in dry air testing. One needs to
understand the different requirements of each test method in relation to
fixture design and how improper designs will compromise that specific type of
In many assembly and test applications, there is also a need
to orient the part being tested in the same orientation that it will have when
in use in the field. The applications where this is an overriding feature of
fixture design are quite diverse - from automotive valves that are referred to
as ‘true car position’ when functionally tested to respirator components in the
In some instances, assembly and test fixture designs must
also take into account and simulate the parts to which parts are to be joined.
This is very predominant in leak testing medical devices. For example, if the
device has an O-ring, you will generally want to mimic the mating surface.
While leak testing a transmission casting for an automotive assembly, you would
try to mimic the paper gaskets that will be in use in the field, as opposed to
actually using paper gaskets since these wouldn’t hold up during the testing
Fixture design is often mediated
by how automated the overall assembly and test operation is and at the specific
testing stations. In manual systems, the sealing and fixturing may be more
difficult because of ergonomic issues. Clearance for loading and unloading may
be an issue. Poka-yoke (a Japanese term meaning mistake-proofing)
considerations are such that often a fixture design will anticipate human
errors of placing wrong parts in the testing stations. A poka-yoke informed
fixture design will make sure that only parts with the correct dimensions will
This is only a brief discussion of some of the factors that
testing applications engineers must consider. Since fixture design can make or
break your assembly and test solution, it is generally advisable to enlist only
testing engineers with experience in building many turnkey testing solutions.
One needs a nuanced understanding of leak test and functional test requirements
to bear on fixture designs that allow superior test instruments to deliver
their promised performance. In fact, the growing use of off-the-shelf fixtures
such as expandable seals is a reflection of how little the real requirements
for fixture design during testing
are understood. Reliable and reproduceable gauges almost always require custom
fixture designs given the unlimited variety of geometries in parts that are
Seventy percent reduction in test cycle times is standard in test-centric assemblies.
Generic vs. Customized Software/Instrumentation
There are no one-size-fits-all generic testing solutions.
Taking short cuts by
selecting the lowest cost instrument from the shelf and placing it into
assemblies rarely achieves Gauge R&R in acceptable ranges because the test
sensors and software programming are not well-matched to application requirements.
In fact, most of the assembly and
test solutions that do not meet the test-centric assembly standard are easily
identified by their use of one or another generic off-the-shelf software
packages. This probably reflects the fact that most assemblies have been
configured by machine builders with little training or inclination to master
software engineering. In fact, test-centric assemblies require expertise in
both mechanical engineering and software engineering.
In most cases, custom software
for test-intensive assemblies is less expensive than a generic off-the-shelf
application because it is designed to meet real business needs and does not
include superfluous program features and functionality. Moreover, many general
applications are not especially easy to use, or do not reflect the real data
handling requirements of the application.
Most test applications require software that works in real
time, for example, which is usually not the case with off-the-shelf packages.
It is important to have the means to view real-time traces of test
instrumentation transducers’ performance, as opposed to simple test cycle
times. Slower and less detailed applications do not process data quickly enough
to provide for meaningful test control and/or test method calibrations. If the
required data analysis is more complicated than generating a simple line graph,
the off-the-shelf software packages that are commonly used similarly fall
short. The best-in-class custom software for test-centric assemblies will also
automatically calculate R&R percentages based on the number of trials
How networks are built is also important in fully
integrating testing into assembly operations. When there are steep requirements
for testing, there is a lot of data that typically has to be processed and also
sent to a plant-wide network. BUS compatibilities need to be considered.
State-of-art test-centric assemblies have Ethernet
capabilities that allow the huge datasets from intensive testing to be
distributed to plant-wide networks for analysis. Internet-based remote
diagnostics are another feature of the best-in-class test-centric assemblies.
Custom software development for test-centric solutions
starts with identifying business goals as to how the data and information
garnered during testing will be used not just in initial production but also
during the entire lifecycle of the product being produced in the assembly and
test operation. This is of growing importance to the many industries that now
want processes to attach data to products from cradle-to-grave to assist with
recalls or reducing time-to-market in new product development cycles.
Multiple Cost Savings
A less-than-ideal assembly and test operation is one where
the machine builder defines testing gauge R&R in terms of those proffered
by the manufacturers of the test instruments. Gauge R&R of the entirely
assembly and test solution is what counts, and this is what must be guaranteed
to users of the test-centric assembly and test solution. This explains why the
most savvy machine builders around the world are now incorporating applications
engineers with a sole focus on testing into their project development teams.
The improvements to be expected from a test-centric assembly
approach are not trivial. Leak test and functional test cycle times can be
decreased 25-70% in the best-in-class examples, and yields increased
There are numerous cost-savings from implementing a
test-centric assembly approach. First, greater returns from higher yields in
test-centric assemblies can be achieved by using better fixture designs, and
customized test instruments, and related software and high throughput chasses
designed for test-intensive operations. Second, in-process testing cuts the
added costs for finishing products that will ultimately be deemed defective.
Third, defective parts are not misclassified as “good” parts and returns are
minimized. Finally, time-to-market for new product development can be decreased
by as much as 10% by mining extensive test data from assembly and test
For more information on InterTech instruments, Applications Lab, and
consulting services, contact Gerald Sim, e-mail firstname.lastname@example.org
, phone (847) 679-3377 or fax (847)