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Machine probing for profits.

Measuring on the machine offers the ultimate in closed-loop control. If you know immediately that the last part your machine tool just made is wandering toward an out-of-spec condition, adjustments can be made to assure that that machine tool never makes a bad part.

A probe-equipped machining center is delivering on that promise for Bordson Machine & Tool Co, Dearfield, WI. The company is building a reputation for close-tolerance production with virtually no scrap. With an objective of "every part is a good part," the firm's Monarch vertical machining center with Renishaw probing system is helping it meet that objective. With the probing system, the machine can do its own setup calculations, postprocess inspection, and corrective cuts.

"On a batch of 1000 gear-pump housings," says Bob Bordson, president, "we can get 999 good parts, with our losses attributable to catastrophic tool failure or a bad casting. We're dealing with hole-location tolerances of [+ or -]0.0002", depth tolerances of [+ or -]0.0002" to [+ or -]0.0005", and diameter tolerances of [+ or -]0.0004". No shop using conventional setup and inspection can match our price, quality, or low scrap rate."

Recent work includes cast-iron gear cases for food-processing and hazardous-materials pumps, pre-hardened metals for hand-tool components, and special stainless and monel parts for compressed-gas cylinders. "It's just not practical to do [+ or -]0.0002" tolerance work without closed-loop machining," Bordson adds. "The Monarch is capable of the accuracies we need, and the Renishaw probes make it a 'smart' machine. The control takes probe data and establishes workpiece true position and critical feature position, calculates offsets, compares a cut with job requirements, and makes corrective cuts."

The probe system

The Monarch has a 18" x 30" work envelope and stores the Renishaw MP9 wireless probe in its 40-pocket tool magazine. After insertion in the spindle, the battery-operated probe is turned on by accelerating it to 500 rpm (at least one revolution) via a centrifugal switch. A timer switches the probe off automatically after 3 min of inaction. Probe signals are transmitted by infrared LEDS, around the probe's circumference, to a stationary optical receiving unit mounted on the machine and hardwired to the CNC. Probe repeatability is [+ or -]1 micron.

Bordson uses CAD/CAM software on an HP computer to create machining programs, adding probing and tool-recovery sequences with a postprocessor. "We use canned probing cycles built into the CNC, and all we need do is program a start location and initiate the cycle," says Bordson.

"Probing improves the accuracy and productivity of the process, starting with the first rough cut," he notes. "With the probe, it takes 15 to 20 sec to set up a part, and it is considerably more accurate than manual indicators subject to human touch and interpretation. After probing the upper bore-used to center the program-the probe moves over a set distance and probes the second bore. We then use a feature called 'axis alignment' to rotate the entire program around the exact number of degrees the part is twisted in the fixture. We can locate the program on the exact center of the part, without worrying that some flash on the outside of the casting will throw a bore off target."

Finishing up

After machining, Bordson uses the probe to measure the finished part while it's still fixtured. The CNC is programmed to pause and call for a tool-size change if a bore is off . It also automatically updates the cutter comp file for the next part, and can make corrective cuts on the current part if programmed to do so.

"It takes just a few additional seconds to measure the part while it's on the machine," points out Bordson. "If corrective work is needed, it's quicker and easier to do it immediately. It ensures we have a salable part coming off the machine every time."

Printouts of inspection data have proved very valuable to customers. These include measurements for each part, probe datuming, and reliability and repeatability. These printouts add extra value to the product we sell,' he adds. "Every time we look at a new or used machine, a prime consideration is whether it includes or can be retrofitted with Renishaw probes."
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Jun 1, 1991
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