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Dimensional integrity of machined parts
produced in a high-volume setting, such as a manufacturing cell
or transfer line, is very important for obvious economic
reasons. Post-process verification of dimensions can result in a
pile of bad parts before corrective action can be taken.
Dimensional feedback to the machine-tool
control has in the past been accomplished mostly by
single-purpose gage heads stationed within or next to the
machine. Such gages usually access only a few dimensions because
of physical constraints and high cost. Coordinate measuring
machines (CMMs) have become quite reliable and offer true
integration into the manufacturing process to overcome this
shortcoming.
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CMMs can provide quality and process control in
high-volume machining operations. |
The number of features a CMM can check is
limited only by cycle time. Even more features can be checked by
distributing them over two or more cycles; for example, a
particular dimension might be checked every third cycle.
If a dimension changes, the CMM program can
be quickly modified, rather than acquiring a new gage head and
facing extended down-time. Savings become even more pronounced
if a different part is to be produced. The CMM requires only a
new program, and maybe some probe styli, as opposed to expensive
new hardware.
Error-proof feedback to the machine control
for generating a desired response is an area requiring proper
attention. Close cooperation with the control supplier is
important, and a CMM also has to communicate with a robot when
employed in a cell application.
An example of CMM integration in a
high-volume manufacturing setting is provided by a cell that
produces disc brake rotors. It consists of two vertical lathes
to produce the ID on OD, and a multispindle drill for the bolt
hole pattern. A robot advances the parts between machines and
puts finished parts on an outbound belt.
Before use of an in-line CMM, operators
performed only manual spot-checks of important dimensions down
the line; this approach often resulted in many unusable machined
parts. Engineers decided to integrate a CMM into the cell to
qualify every part and, just as important, keep the process
under control.
The CMM had to communicate with the cell's
Fanuc robot for loading and unloading of rotors at the CMM.
Of course, it had to accurately measure all
machined dimensions of a brake rotor. It had to send the
appropriate tool offsets to the machine control to enable it to
automatically maintain machined dimensions within tolerance. It
had to recognize out-of-tolerance parts and send a signal to the
robot and the machine control designating defective parts. It
had to accumulate measurement results over time, and use the
data to generate SPC information pertaining to the cell
production. And it had to do all this in a maximum of 90 sec,
and with a minimum level of 90% machine uptime.
The CMM integration was implemented without
altering the machining sequence, but the robot required
additional instructions to remove finished parts from the drill
and notify the CMM that it has a part for inspection. At this
point the CMM retracts and signals the robot it is ready to
accept a part. The robot then loads the part into a fixture,
initiates clamping, and exits the measuring cube. The inspection
cycle starts when the robot has cleared the measuring area.
The CMM checks all specified dimensions and
compares the results against a defined table to identify tool
wear or breakage. Corrective action is taken once a dimension
has reached a predefined percentage of the tolerance band, say
80%. The proper tool offset is sent to the lathe, which will
bring the finished part back to the target size.
The CMM keeps track of the parts count.
When it checks the first part machined after a tool-offset
command, it decides if additional adjustments are necessary. If
a part is out of tolerance, the CMM shuts the cell down.
Once the measuring cycle is complete, the
CMM returns to the retracted position and notifies the robot to
put the part either on the outbound belt if accepted or in a bin
for review.
Transfer line applications are simpler,
since no robot is involved. Part presence is verified by
switches or other sensors. Locating and clamping mechanisms can
be supplied by the CMM manufacturer.
Regardless of application specifics,
integrated CMMs can provide both quality control and process
control in high-volume machining operations.
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