Where the Contact Meets the Part
Understanding aspects of the contact point should boost gaging accuracy.
The capability of a measuring instrument often comes down to how the contact point interacts with the parts being measured. The term “contact point” is pretty generic, so we will focus on contact points that screw onto a dial indicator, lever probes as used on a test indicator and lever probes on contour systems. These are really the first point of measurement contact, so there are some things to keep in mind when using a probe arm as part of a measurement.
Many contact probes are made out of a single piece of material. Some, however, consist of ruby or diamond inserts swaged or glued into the tip. Rubies are often used on surface finish probes, while diamond contacts may be used on millionth-class gages or where wear may be deemed extreme. Not infrequently, these ruby or diamond inserts become loose or even fall out. On a surface finish gage, a missing diamond usually will result in identical values being seen no matter what is measured, while a loose contact will typically produce apparently valid, but non-repeatable readings from the gage. A loose contact should be one of the first things checked when this occurs.
The material of the contact can itself affect the reading in a number of ways. Certain materials don’t mix well. When measuring aluminum, for example, use of carbide contacts should be avoided. Carbide is porous, and bits of aluminum can become imbedded in the contact. Over time, this builds up and can produce an offset in readings. Even though the gage is mastered, the measuring loop has been changed, and incorrect readings will result.
Contacts also should be inspected for flats and scratches caused by continuous wear. Just as material can build up on a contact, contact material also can be removed or wear away. Flats on a spherical contact will produce offsets. Worn areas in a caliper will produce inconsistent readings, while scratches can raise high points on the measuring surface and cause errors. A complete inspection of the contact should occur when a gage is sent out for calibration.
Gages are designed to be used with the correct probe for the application. Sometimes, in an effort to get a gage up and running quickly, different contacts are substituted. This is usually not a good idea, but if it must be done, flat contacts should be used when measuring round parts and radius contacts when measuring flat parts. Using the wrong combination will make the measurement very difficult or, if the contacts are not parallel, incorrect.
Contact-point radius is also important. Some gages require a specific radius for their applications in order to meet an industry-wide specification. For millionth measurements, for example, contact penetration is dependent on the geometry of the radius. Changing this will affect the performance of the gage and prevent correction factors from being applied accurately.
The same is true with surface finish probes. There are specific contact-point radii called out as part of the surface finish parameter. Using a 0.0004-inch instead of a 0.0002-inch radius probe will provide completely different results.
Lever probes on geometry gages or contour systems can be very long—sometimes as long as 10 inches. Special designs must be used to make sure contacts this long are as stiff as possible. Otherwise, there is a chance that flexure or vibration in the probe will become part of the measurement. This will demonstrate itself as noisy, unrepeatable readings.
With long contact points used on form, contour or test indicators, it’s important to understand the design of the gage head. Test Indicators, lever gage heads and some contour systems use a pivoting arm. The original design includes a ratio between the contact-point end touching the part and the sensing system in the gage that is based on probe length, say 2 to 1. However, while the contact may be designed properly, the actual contact itself may not be quite to print, and the ratio may have to be adjusted accordingly. To avoid this problem, the measuring system should be calibrated with the probe as part of the measuring loop and any discrepancies calibrated out of the result.
Contact points react differently under pressure. Specifications for compressible materials show very well-defined characteristics for size, shape and finish. For example, using a 0.5-inch flat contact in place of a 0.125-inch contact greatly reduces the ounces or pounds per square inch of force on the material and will result in two completely different readings.
Getting closer to your probes and knowing how they are used will definitely improve your gaging performance.
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