quality lever and adaptive control, The

Manufacturing Engineering, Jul 1998 by Stewart, Tom

The concept of the quality lever is a crucial one. In a nutshell, it states that the earlier in the process you implement an improvement, the more net value you gain from it-sort of a bean-counter's justification for investment in factory process improvement.

For machining operations, adaptive (or closed-loop) process control is one of the most effective means of process improvement. It occurs in the early stages of the manufacturing process, so the payback in terms of net improvement surpasses that achievable by traditional, inspection-based quality control methods. Adaptive control is workpiece monitoring with one crucial difference: the adaptive control automatically feeds the resulting data back to the machine's control. The control can then adjust machining parameters on a near real-time basis to maintain quality.

Given the availability of high-tech CNC machine tools, cutting tools, and coolants, why should you concern yourself with adaptive control? The answer is that despite tremendous advances in metalworking technology, manufacturers still battle the same process variables they've struggled against in the past: differences in material, thermal instability, differing batch characteristics of tools and abrasives, and the vagaries of operators. In this imperfect world, these factors are just a few of the old haunts that keep expensive systems from operating at or near their expected capability.

Closed-loop process control can almost eliminate the ability of such variables to affect part quality by using feedback of gaged data before, during, or after the machining cycle. Preprocess control performs best when used for cast parts, gang fixtures, and pallet machines; inprocess control works well for all types of machines and parts; and postprocess control is most effective on CNC lathes.

Manufacturing engineers can make part measurements for closed-loop process control with a device as simple as a hand-held gage, or as complex as a multidimensional gaging fixture. Yet the most cost-effective solution can often be a machine-mounted touch probe. Quality control managers frequently reject this measurement technology out of hand because it relies on the coordinate feedback system of the machine tool to produce its measurement reading. Many QC people reject the concept of measuring a part on the machine that made it. If the purpose is simply to control the trend of the machining process, however, the touch probe can meet your needs at far lower cost than alternative gaging systems.

Adaptive control systems can also employ postprocess gaging. Tool compensations made with postprocess gaging systems depend on an algorithmically determined running average of gaged dimensions. Users generally set upper and lower compensation limits for each dimension at 50-70% of the part print tolerance. If a running average of gaged measurements falls outside these limits, the system instantly sends a tool compensation signal to the machine's CNC, which actuates the required tool offsets in time for machining the next part. Usually the system makes these adjustments prior to finishing operations, which virtually eliminates production of scrap parts.

Whether in-process or postprocess, all adaptive control systems are based on the belief that it is more efficient and costeffective to build quality into the process upstream than to correct for quality defects downstream.

Tom Stewart

Manager of Standard Products Marposs Corp. Auburn Hills, MI