A system for quick change

Modern Machine Shop, May, 1995 by Tom Beard

Control

Certainly, all of this scrutiny of tool position has been aimed at achieving meaningful setup reduction. But another benefit has accrued with experience. The more Danville Metal's engineers studied the sources of variability in their tooling, the more they came to understand about their metalcutting processes in general. In combination with the shop's SPC program, which Mr. Minnick says was a valuable tool in understanding the nature of the machines, there is little question that knowledge has resulted in overall quality improvements in their work.

For example, because they have quantified standards for the major areas of process variability, when they do create a bad part, it is much easier to track down the cause. The shop has a formal procedure by which all non-conforming processes are analyzed. First they check to make sure all process parameters were followed as planned, which is the clear responsibility of the operator, and that the presetting task was executed correctly, the responsibility of the crib. If all is in order, they examine the equipment, checking for spindle runout, axis backlash and so on.

Of course, that procedure implies a high degree of confidence in the shop's planning functions, which they have. A great deal of thought and effort have gone into making sure the shop is always working with accurate, tested and up-to-date information. Consider, for example, how they handle part programs. New programs have to be accepted by the operator, who reports back to Mr. Minnick how the job ran. Mr. Minnick then has to certify the program before it can be returned to the DNC file server for repeat runs of the job. Also, should edits on the shop floor be required, they too must be documented and certified. And rather than just saving the edited postprocessed files, all changes are made to the original CAM file. The rule is to never store two versions of the same program.

A similar rule is applied to shop documentation. They keep close track of how jobs run relative to their process plans, which include estimates of setup and run times. Should they run into problems in the shop, they first examine how faithfully the plan was executed. If all is on track, they then re-evaluate the plan itself, looking either for improvements or at least a more realistic set of expectations. In any case, any and all modifications are incorporated into the electronic documentation for the job. All paper is purposefully discarded in order to the eliminate the possibility of out-of-date information getting back to the shop floor.

But the essential element in quality control is that they have refined their processes to the point they can hold far better tolerances than ever before, and that's really helped their conformity rate. "SPC took fifty percent of the tolerance out of engineering," says Mr. Minnick, which at first seemed a curse, but ultimately proved a blessing. It forced them to do a better job controlling their turning processes, and on some parts, they've been so successful at it that they've been able to eliminate subsequent grinding operations.