Speeding And Gaging Don't Mix

Manufacturing is under constant pressure to get more productivity out of its process. Machine tool speeds, feed rates and part positioning all keep getting faster, yielding more parts in less time.

Columns From: 6/8/2005 Modern Machine Shop,

Manufacturing is under constant pressure to get more productivity out of its process. Machine tool speeds, feed rates and part positioning all keep getting faster, yielding more parts in less time. On the inspection side, speed is also critical, especially for gaging systems designed for use inside the manufacturing process. A slow gaging system can become a bottleneck if it can't keep up with the parts being spun out of a machine tool.

However, there are physical characteristics inherent in some measuring processes that limit how fast a measurement can be made. There are also characteristics of gaging equipment that limit the speed of inspection.

The most common place we see these limitations is in the area of dynamic measurements such as Total Indicator Runout (TIR), minimum or maximum dimensions. Other areas include checks for form error, such as roundness or surface finish. To perform these types of inspections, the part must be rotated against a sensing probe, or the probe is dragged across the part surface.

Runout and roundness measurements are similar in that they both involve rotating either the part or the gage and inspecting for a dimensional variation in the surface.

Most electronic transducers have the capability of responding very quickly to changes in variation. However, if the measurements are taken too quickly, the contact point (which has some mass) may tend to bounce when it goes over high spots on the part's surface. You can test this yourself. If you have a lever-style gage head on a round part, mounted either on a precision rotary table or in a set of vees, and you rotate this against the transducer, you will see the readout device follow the part's variation. If you turn the part slowly, you will see every peak and valley as they go by the transducer. If the part is rotated faster, the probe will tend to rise up the peaks. Because of its momentum, keep going a little bit while the part surface starts its downward slope. If you speed up the rotation fast enough, the probe may miss some valleys and bypass some peaks. Most form systems bring the part rotation up to a fixed speed prior to measurement. This serves to best match the response of the probe to the surface speed of the part and provides a constant surface speed for all recorded values.

On surface finish machines, the probes are different. These delicate and sensitive probes are designed for speed. They have very little mass, which helps eliminate tip bounce, and they have fast electronics to process data quickly. However, the drive units still travel quite slowly to minimize the loss of data.

Gaging systems employing digital sensing technology have become more common on the shop floor. These are important tools because they often offer long measuring ranges with high resolution, which increases their versatility. However, the issue with many digital sensing technologies is how fast the gaging processor reads the probe or probes. Single probe applications yield the best performance because the processor has to monitor only one probe. Add multiple probes and the shared data handling sacrifices the number of readings on the part. If the part is rotating under the probe, there are going to be times when spots on the part do not get measured. On a part rotating very quickly, or if the part has a large diameter and high surface speed, there is apt to be more of the part not measured than measured. This will affect the ultimate quality of the measurement.

Air gaging also limits the speed of both static and dynamic measurements. Air is a compressible medium and needs time to stabilize. To measure a part with air, two things must happen: if the air jet is open to the atmosphere and then brought against the part, the air lines need to pressurize. This takes about 1 second per foot of air line. Then, as minor changes in the part go by the jet, the air pressure in the line has to restabilize. Thus, if the part is not in position long enough—or if the surface of the part is moving past the open jet too fast—the pressure in the line can't keep up, and important part data can be lost. The longer the air line, the more time is required for the air reading to stabilize.

Simple trial and error will help determine the best measuring speed. Run the gaging station unbearably slow and record your results. Then speed it up a bit and record those readings. Repeat the process until you start to see a degradation in results.

If the process can't wait for these time limiters, however, you have to work around them. One approach is to increase the number of gaging stations, but be wary of faster gaging. If you get better results than others who are using a slower process, you might be missing something.

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