A Probing Look At Probes

The capability of a measuring instrument often comes down to how the contact point interacts with the part being measured—that is, the probe or contact. There are some things to keep in mind when using a contact tip or probe arm as part of your measurement.

Many contact points are made out of a single piece of material, but some 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 extreme. Not infrequently, these ruby or diamond inserts become loose or fall out. On a surface finish gage, a missing diamond usually results in the same value being seen no matter what is measured. A loose contact will typically cause what seem to be valid, but non-repeatable, readings from the gage. A loose contact is one of the first things to look for when this condition is seen.

The material of the contact can affect the reading. Certain materials don’t mix well. When measuring aluminum, for example, avoid carbide contacts. Carbide is porous, and aluminum can imbed itself into the contact. This builds up over time, and it can produce an offset in the readings. Even though the gage is mastered, the measuring loop has changed, and incorrect readings will result.

Contacts should be inspected for flats and scratches from continuous wear. Material can be removed from a contact. Flats on a spherical contact will produce offsets. Worn areas in a caliper will produce inconsistent readings. Scratches can raise high points on the measuring surface and cause errors. When gages go out for calibration, a complete inspection of the contact is required.

Gages are designed to be used with the correct probe for the application. Sometimes, to get a gage running again, contacts are substituted. This is usually not a good idea, but if you must, use flat contacts when measuring round parts and radius contacts when measuring flat parts. Using the wrong combination will make the measurement difficult, or—if the contacts are not parallel—incorrect.

Probe radius is also important. Some gages require a specific radius for their application to meet an industry specification. For millionth measurement, for example, contact penetration depends on the geometry of the radius. Changing this will affect gage performance and prevent correction factors from being applied correctly. Verify that the geometry of the probe meets specifications.

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 different results.

Lever probes on geometry gages or contour systems can be up to 10 inches long. Special designs must be used to make sure they are as stiff as possible. Otherwise, there is the chance of flexure or vibration in the probe becoming part of the measurement, which will produce noisy, unrepeatable readings.

Gage readings generated by lever probes may also need to be adjusted to compensate for their length. Ratios are used to make these calculations based on probe length. 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, the measuring system should be calibrated with the probe as part of the measuring loop, and any discrepancies should be calibrated out of the result.

Contact points react differently under pressure. Specifications for compressible materials show 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 reduces the ounces or pounds per square inch of force on the material and results in a different reading.