Reprinted from Modern Machine Shop

SCANNING THE HORIZON   

Corrections Made In-Process With Post-Process Closed-Loop Automatic Feedback Gaging

One of the notable trends in quality control for machine tool users is post-process closed-loop feedback gaging. Post-process feedback gaging is a method of effectively controlling the quality of parts by controlling the size of the parts. It is a measurement technique that enhances part quality by automatically regulating machine tool cutter positioning. According to Edmunds Gages (Farmington, Connecticut), a developer of these systems and a leading proponent of the concept, most post-process feedback systems work in this way:

At the exit end of the machining cycle, the workpiece is located for measurement, away from the environment in which it was machined. The gaging system measures the part, calculates corrective data statistically, and feeds the information to the machine controller prior to the next machining cycle. Characteristics such as diameters, lengths, and geometric forms that correspond with each axis of the machine tool's path are measured. In a truly closed-loop system, the company says, the gage is the brain. Every part is measured for conformance while the process is controlled to produce similar target or nominal-sized parts – preprogrammed from within the gage. Product continuity is assured because machining defects are effectively prevented from occurring.

One of the key differences between post-process gaging and in-process gaging is that workpieces are measured out of the machine, away from heat and cleaned of debris. Many in-progress gages measure workpieces while the machine tool is cutting the part. Edmunds Gages claims that these types of gages do not necessarily assure the quality of the workpiece; they simply prove that each part is being manufactured the same way.

Edmunds Gages is one manufacturer who custom designs and builds post-process automatic closed feedback gages. A gage of this type was recently engineered to measure tappet rollers. It combines one of the company's Micro CAG (Computer-Aided Gaging) microprocessors with air tooling. The gage automatically checks five dimensions of each part as it exits from the lathe. It measures the ID at three planes, bellmouth, and taper.

In operation, all workpieces enter on a gravity feed track from the machine tool and are stopped at a metering station. Upon detecting the workpiece, a verification measurement is performed on the master to ensure gage validity. If verification is successful, a three-circuit air plug advances into the workpiece's ID, where its characteristics are obtained. These results are compared to programmed part tolerances and an "accept" or "reject" status is issued. A flip gate, just past the meter, directs the good and bad part traffic.

The system's microprocessor includes an SPC (statistical process control) package and software to conduct repeatability and reproducibility studies of the gaging system. Although closed-loop feedback gages are most commonly utilized on longer production runs, the gage can be retooled to measure other types of cylindrical parts.