An Inside Look At Special Diameters

George Schuetz, Mahr Federal Inc.


Sometimes we are faced with making critical inside diameter checks on parts that do not present themselves in a straightforward fashion.  Usually these checks are on the inside of some type of bearing and they can be almost any size.  Examples include measuring the diameter of an internal surface behind a shoulder, where the entry diameter is smaller than the diameter being measured; or measuring a ring groove in a bore, the pitch diameter of an internal thread, the effective diameter of a barrel roller bearing, or the included angle of a tapered bore.

Measuring this type of ID requires that the gage have some unique characteristics.  These include the high repeatability of a comparative gage, long-range jaw retraction to allow entry into the part, and the ability to set the depth of the contact to make the measurement at the proper point.

Many people immediately think of a shallow bore gage for this type of application.  As we have discussed in the past, this type of gage rests on the face of the part and measures the inside diameter at a certain depth.  These are fairly accurate comparative gages that allow the depth of the contact to be set to a specific location.  However, these gages are typically limited by the amount of travel found in the indicating device used for the readout.  As a simple diameter gage they work well, but they do not meet the need of long range to get behind a shoulder, or to have the contact measure a groove diameter (Fig. 1). 


Two types of gages do meet the needs of these applications: a gage very similar to a shallow, comparative bore gage, but with a long-range slide; and a long-range slide coupled with a high-resolution, long-range digital indicator.

The first example uses the precision slide purely as a transfer mechanism, or a way to retract the comparative indicator away from the surface to get past the obstruction.  When released, it slides back against its stop and the comparative indicator does its thing.  This sounds like a simple solution until one considers the mechanics involved.  The contact has to be long enough to get down and around the smaller diameter.  This is where the design of the slide and contact become critical.  If the slide has any looseness in it, it will cause the gage to be unrepeatable.  Or, if the contacts are too flimsy, they will bend and twist and become sensitive to placement and gaging pressure.  Either of these design flaws in the precision slide can make even the best comparative indicator look bad.

A better way of achieving the same result is to use a long-range digital indicator as part of the measuring frame.  This has the benefit of actually referencing the measurement based on the accuracy built into the gage's own long-range slide.  Of course, the contacts must be designed to handle the depth and gaging pressures, but the beauty of this type of gage is that its long range offers the ability to measure any number of depths within its range.  So, besides measuring a depth at a particular location, it can measure two diameters at different depths, thus becoming a taper gage measuring the included angle of the tapered bore (Fig. 2).


The combination of the long-range, high-resolution digital indicator on a soundly designed mechanical frame can provide a universal ID (or OD) gage that has endless applications for those hard-to-reach inside measurement applications.