The measurement of the depth of a hole sounds straightforward and simple. In many cases it is. In others, there are design features of both the part and the gage that must be considered to insure an accurate measurement and a properly assembled or functioning part. In past Quality articles (January 2016 and February 2013), we have discussed the basics of using fixed limit cylindrical or threaded plug gages to accurately and quickly inspect the depth of a threaded or cylindrical hole. This time we will focus on some of the part and gage features that may cause problems or impede accurate depth measurement.
Depth measurement on parts is usually a measurement to a specific datum, plane or surface. Variations in the shapes of these features can cause issues with the depth measurement. If the surface is not flat, is radial or has complex geometries or angles, the depth measurement can vary based on the orientation to these features in relation to the gage or gaging surface. When this occurs, it is critical to know the exact surface or plane the depth is required to be measured from to qualify the depth. Figure 1 shows a sectioned or cut part view of a few of these challenges.