Measuring machine tools with ball bars

Modern Machine Shop, March, 1994 by George Schuetz

Every time we detect a part that is out of tolerance, the implication is that something went wrong in the machining process. A lot of the time, problems with the machine get blamed on the operator. Rather than expect the operator to compensate for every machine tool problem--at the expense of increased setup time and scrap it makes more sense to quantitatively assess machine error through the process of characterization. Then the operator can either compensate more efficiently, or better yet, address the problem directly by adjusting the errors out of the machine. Rather than gaging parts to detect problems, we gage the machine to prevent them. It's particularly important with CNC machines, where we expect the machine to do much of the work for us.

The telescoping ball bar is one of the most useful and economical devices to characterize CNC machine tools. A simple ten-minute check with a ball bar can often provide much of the information needed to verify a machine's performance. If the machine is out of spec, the same tool can provide the data to diagnose many errors. It's particularly useful in acceptance testing of new machines. A ball bar is a fairly simple device. It consists of a telescoping shaft with precision balls at both ends and a transducer in the middle to measure changes in shaft length. The balls at both ends and a transducer in the middle to measure changes in shaft length. The balls fit into sockets (usually magnetic) placed on the machine itself: one is fastened to the table; the other in the spindle or toolholder. The transducer is wired to gage a readout or computer.

The machine is set up at a zero, or starting position, and then cycled in 180- and 360-degree motions, as shown in Figure 1. Normally, this is done in the middle of the machine's work area or wherever most of the machining is performed. The ball bar detects deviations from the programmed circular or semi-circular paths, and the computer generates a polar chart readout of the deviations.

By checking accuracy in the XY, XZ, and YZ planes, the machine's overall contouring accuracy can be assessed, and the nature and sources of error can bc determined based on characteristic patterns of the polar charts. As shown in Figure 2, linear and squareness errors appear as oval elongations of the trace. Reversal error--often caused by worn ballscrews shows up as a sharp displacement along the axes. Servo adjustment problems are indicated by smooth-shaped deviations near the zero-, 90-, 180-, and 270-degree positions, and loose gibs produce a saw-toothed pattern all around the trace. Naturally, several errors may be present simultaneously, which can make interpretation more difficult. Note that the ball is primarily a test of contouring performance. Other tests can be run to check repeatability and Hysteresis. However, because of its short measure-meat range (about 0.080"), it is not an effective tool to check linear or volumetric accuracy over the machine's range of travel. For these jobs, a laser interferometer may be required.

One of the ball bar's strengths is its ease of use on all kinds of machine tools. This makes it an economical aid when seeking ISO 9000 certification. It is also part of a new standard, ANSI B5.54, which establishes methods for evaluating machine center performance. By measuring machines to a consistent standard, it is possible to establish an "error budget" for all of the processes that contribute to a certain part. And this allows companies to target quality improvement efforts more effectively.