Technology Update
CMMs can be calibrated and fixtured with ease


Getting the most out of your measuring process.

When you talk to Eugene Gleason, president of the Bal-tec Div. of Micro Surface Engineering, Los Angeles, CA, you get the feeling that he is on a mission to eradicate error from coordinate measuring machines wherever it occurs before the fact. As he explains it, errors can result from any number of causes. The two most prominent ways of eliminating these causes: 1) Calibrate machines properly, even daily, using a ballbar; 2) Use fixtures that meet the specific needs of CMMs.

"You know there are eight orthogonal positions and 21 possible errors in a CMM, 18 of which can be readily checked," Mr. Gleason says. The secret, he believes, is to check the CMM for accuracy, almost on a daily basis with one of the most economical of artifacts, the ballbar.

A ballbar consists of two very round spheres of exactly the same size firmly attached to opposite ends of a rather long, rigid bar at a fixed and known distance. The ballbar is located on a robust vertical stand that can be located in various positions throughout the volume of the CMM that is being checked.

The accuracy of the ballbar, however, does have a single, yet major, limitation, according to Mr. Gleason. "A conventional freestanding ballbar will bend when the contact force of the measuring probe is applied to it. The longer the ballbar, the greater the bending. With all the automatic error-correcting computer power, it seems as if it should be easy enough to compensate for these ballbar deflections. However, these corrections turn out to be a good deal more complex than they first appear. The resulting deflection of the ball is a rather complex vector function of gravitational sag, some simple bending, and large twisting moment.

"This is all complicated by the fact that there is no deflection at all on the very ends of the ballbar. The end result of this complexity is that the CMM and its software see two much smaller diameter spheres with the distance between their centers much farther apart than the true dimension."

Mr. Gleason's company has introduced an advance in Ball Bar (Dumbbell) technology that is aimed at correcting this source of error by providing two additional spheres that are rigidly attached to the ballbar shaft just behind the master spheres for better support.

The additional spheres are used to kinematically support the ballbar in a very rigid manner while leaving the full surfaces of the master spheres exposed for measurement. Called the "Way Out Ball Bar (Dumbbell) Support," the new device prevents any bending or twisting of the ballbar caused by the contact force of the CMM measuring probe.

The two kinematic couplings are mounted near the very ends of a robust aluminum supporting structure that resists all bending. The very rigid aluminum structure is only used as a support mechanism for the ballbar. Nor does its high rate of thermal expansion in any way influence the interball dimension of the steel ball- bar itself.

One coupling consists of three precision spheres rigidly mounted to the aluminum structure forming a nest that holds the first support sphere. The other coupling consists of two precision-lapped cylinders rigidly mounted to the aluminum structure. There is a powerful rare earth magnet just below each of the couplings that pull the support spheres solidly down against the kinematic couplings. The ballbar has been accepted as the major tool for functional "performance evaluation of CMMs" according to ANSI-B 89.1.12M, the governing standard.

Basics of fixturing

Proper part fixturing for CMMs can be broken down into three important elements, says Mr. Gleason. The first is test-part orientation; the second is test-part location; and the third is test-part clamping. If possible, the test part should be oriented so that all of the features to be measured are at right angles to the measuring probe axis. If these conditions cannot be met, the test part may have to be reoriented several times in different positions.

Here's how Mr. Gleason explains the fixturing process:

Location points or stops must be provided to restrict any movement of the test part. Every test part has six degrees of freedom.

The number one fundamental of fixturing is to provide location points to constrain each and every one of the six degrees of freedom. To simplify the process of fixture design, it is good practice to list and then check off each of the degrees of freedom as positive location points are provided to constrain them, Mr. Gleason says.

Clamping methods for good CMM fixturing are much different from those used for machining in as much as you don't need brute-force clamping for measuring, Mr. Gleason explains. Clamping force must be sufficient to restrict any movement of the part during probing and yet hold the part without distorting its features.

Basic clamps include the hold-down bar. "High-tensile aluminum will give the same strength as a steel type, at a fraction of the weight. The hold-down clamp must be positioned parallel to the test-part surface. To facilitate perfect alignment, choose a model with a leveling screw at the rear of the clamp instead of a conventional step block," he notes.

Other types of clamps for test-part clamping for CMMs include the screw clamp, which can work vertically, horizontally, or at any angle; toggle clamps for production work where absolute positioning is not critical; vacuum clamping, which is fairly expensive; and magnetic clamping for appropriately magnetically permeable materials. Pneumatic and hydraulic systems are used only sparingly in CMM fixturing.

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