By using effective probe techniques when inspecting a workpiece you can eliminate many common causes of measurement error.

For example, probe measurements should be taken perpendicular to the workpiece surface whenever possible (Figure 17). Touch trigger probes used on coordinate measuring machines are designed to give optimal results when the probe tip touches the workpiece perpendicular to the probe body. Ideally, you should take hits within ±20° of perpendicular to avoid skidding the probe tip. Skidding produces inconsistent, non-repeatable results.

Approach of the probe should be within ±20° of the perpendicular to probe path point to minimize skidding error. Approach vectors are perpendicular to the surface of the sphere.

Probe hits taken parallel to the probe body, that is, along the axis of the stylus, are not as repeatable as those taken perpendicular to the axis (Figure 18).

Probe hits that are neither perpendicular nor parallel to the probe body (Figure 19) produce results that are even less repeatable than those taken parallel to the probe body. You should avoid taking probe hits parallel to the stylus and at an angle to the probe body, since they will produce large errors.

Shanking is another cause of measurement error (Figure 20). When the probe contacts the workpiece with the shank of the stylus and not the tip, the measuring system assumes the hit was taken in a normal manner and large errors will occur.

You can reduce the likelihood of shanking by using a larger diameter tip to increase the clearance between the ball/stem and the workpiece surface. Generally, the larger the tip diameter, the deeper the stylus can go before it touches the workpiece feature. This is called the effective working length of the probe (Figure 21).  Also, the larger the tip, the less effect it has on the surface finish of the workpiece since the contact point is spread over a larger area of feature being measured. However, the largest tip that can be used is limited by the size of the smallest holes to be measured.

Measurement points taken with an electronic probe are recorded when the stylus is deflected enough to either break mechanical contacts or generate enough force to trigger pressure-sensitive circuitry. The physical arrangement of the contacts causes slight errors in accuracy, although these are reduced during probe qualification. However, the longer the probe tip extension, the larger the pre-travel error and the more residual error is left after probe qualification. Longer probes are not as stiff as shorter ones. The more the stylus bends or deflects, the lower the accuracy. You should avoid using probes with very long stylus/extension combinations.