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Henry Stier

Automatic Measuring During Or After Finish Turning On A CNC Lathe

With fully automatic machining, especially in small or medium lots, close part tolerances pose a technical measuring problem. Without an operator it is often desirable to measure the part while it is still in the machine. This process has been done for a long time during grinding operations. Turning poses greater problems. No chips or cutting tools can be in the path of the measuring stylus. The surfaces to be measured have to be totally free of coolant and small particles. This can be achieved by an air blast clearing the location to be measured. The gaging can be executed by one or a multiple of styli in the tool turret. The machine's CNC has to be able to position the measuring stylus, make the measurement and then correct any deviation from the required part dimension with tool compensation. Despite temperature variation and tool wear, the system is adequate in most cases where tolerances of [+ or -] 0.002 inch are allowed.

The system can be used to measure outer diameters and bores as well as groove width, depth and distances between two faces. No gaging fixtures or masters are necessary for this system. It is mostly applied for the production of bigger parts in small quantities. Since the parts stay chucked during the gaging, corrections on oversized portions of the part can be made. A disadvantage is the increase in production time, since the cleaning and measuring adds to the actual cycle time. In-process gaging is valid mainly on bigger parts with higher material costs and longer cycle times.

Post-process gaging is better applied on high-production runs of smaller parts. The gaging station has to be adopted to hold the part. Several dimensions are measured at once. Are the dimensions within the given tolerance zone? If so, no correction is signaled to the CNC system. Is the measured value above the warning zone? A correction is then made through the CNC to the next part. All measured dimensions can be statistically held for an evaluation of the process capability of the machine. With this system a high quality standard of the output can be achieved. With post-process gaging a higher productivity is achieved since the gaging is not part of the machining cycle.

The average extra cost for automatic quality surveillance is approximately ten percent of the total production cost. A reduction in defects, rework and less customer complaints more than makes up for this added cost. In the future both in-process and post-process gaging will become necessary to stay competitive in the marketplace to satisfy the quality requirement of customers who expect more from their suppliers.

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