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The need for interim checks for CMMs (Special Supplement)(Coordinate Measuring Machines)

During the last 20 years, coordinate measuring machines (CMMs) have gone from an interesting concept that was an oddity in most quality-assurance departments to a mainstay. Companies of all sizes now use this method of inspection.

Even though the reliance placed on CMMs has increased dramatically and the results are used to accept or reject all types of products, many users still have no formal method of checking the validity of the results from their CMM other than prescribed annual reviews. This is in contrast to other computer-assisted production equipment that is constantly monitored with independent first-piece inspections, inspections of parts at ratable intervals, and, in many cases, statistical-process-control (SPC) procedures.

Why should the CMM be treated differently?

There are several important elements of an interim check. Some of these factors include:

* Mechanical operation of the machine

* Accuracy of probe being used

* Environmental factors on

* measurement results

* Operator practices

* Type of parts being checked

* Ease of performing interim check

Mechanical operation of the machine

Here it's important to verity the standard measurements being performed by the CMM. These would include linear length, diameter, squareness, and volumetric accuracy. They should be performed using all elements of the CMM as is done in normal operating conditions. Our investigation indicates it is helpful for the individual individual making thecheck to have a standard that can be used over and over, comparing actual results to the know values.

Accuracy of probe being used

Probes used on CMMS are miniaturized versions of sophisticated machines. Our illvestigation shows some probes are more accurate than others, and some are very susceptible to damage or misalignment during operation. Whatever item is being used to perform the interim check should also be able to check this accuracy, because it is of paramount importance to the overall result.

Operator practices

Probably the single greatest variable in the operation of a CMM is operator involvement. Questions such as these arise: Wits the ball size of the probe correctly input? Was the program checking the part written correctly? Was the part positioned correctly on the machine and the plance correctly established in the CMM? What is the difference in the touch of one operator vs another?

Our experience indicates that using a highly calibrated item like the Glastonbury Gage Quikchek allows the operator to double-check his procedures to make sure they are being performed correctly. It also allows the quality-assurance director to make sure results are consistent from operator to operator.

Environmental factors on results

Humidity, temperature, and foreign-particle counts in the atmosphere can affect measurements. These factors should be looked at very closely when ail area is first established as a quality-assurance environment. Unfortunately, we have seen instances where these factors are changed without considering the possible effects oil CMM measurements.

For example, one customer wanted to increase the size of an environmentally controlled area to allow room for a second CMM. When he moved the first CMM to the opposite side of the room and installed the second CMM, he didn't think about the proximity of each machine to the air-conditioning duct. One machine sat immediately under the duct, and the other was in the corner of the room. For about six months, inspectors struggled with the fact that parts were sometimes accepted or rejected using the same production methods, depending on which CMM was used for inspection. The problem was aggravated during warm weather-and was finally detected.

Type of parts being checked

We have found that some CMMs will measure cylinders more accurately than spheres. Thus, when performing interim checks, it would make sense to use an item that mirrors what the user is actually checking the most-cylinders or spheres. Since our experience shows cylinders are tile most prevalent item checked, we used that in the design of the Quikchek.

The number of reference points used in making the measurement is also a factor when using spheres or cylinders as a reference point.

Ease of performing check

When performing an interim check. sure it's complete enough to discover any problems in the minimum amount of time. Concentrate the check on the area and functiona od the machine most used.

We recommends use of the Quikchek each week. Record the results of each check. By keeping it simple, the operator and supervisor won't mind doing it. We have talked to numerous companies who have made large, cumbersome masters at great expense that were used religiously for a while, but now do nothing but collect dust because the check took so long. In the case of the Quikchek. after becoming familiar-with the item and establishing the routine, it takes the average user 15 to 20 min to perform the check.

It's imperative that interim checks be performed on your CMM, as is done on all other types of critical production equipment. Once you've decided to perform interim checks, it's important that the discipline remains intact: record the data and continue the process on a regular interval. Don't let interim checks become another New Year's resolution put on the shelf ! For more information from Glastonbury Gage, circle 898.
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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Author:Watts, William A.; Prout, Stephen L.
Publication:Tooling & Production
Date:Sep 1, 1991
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