CMMs in the Manufacturing Cell

March 2002 Vol. 128 No 3

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.

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.