Kremer CMM Hole Location Gauge (Jo Plug)

Why use the hole location gauge (Jo Plug)?

All product features are designed with tolerances which are set to provide optimum functionality of the end product. A threaded hole may have to be concentric to another feature. A threaded hole may need to be perpendicular to another feature. A threaded hole may need to be positioned in relation to other holes or part features. This sounds like it would be a simple thing to test, just use a Coordinate Measuring Machine (CMM), but it is easier said than done.

Internal threaded hole centerlines are not the easiest product feature to quantify. There are actually two centerlines directly related to in an internal threaded hole. There is the centerline of the internal thread pitch diameter cylinder and the center of the internal thread minor diameter cylinder. The main feature that establishes the threaded hole centerline is the thread pitch diameter cylinder. The thread pitch diameter cylinder is a theoretical cylindrical plane that passes through the thread at the point where the width of the thread tooth ridge is equal to the width of the thread valley. The centerline of the thread pitch diameter cylinder is the actual functional centerline of the thread. There may also be other centerlines present for counter-bore, relief, bearing surfaces or other required diameters.

The internal thread pitch diameter cylinder centerline may be independent of, and different from, all other hole centerlines, except the internal thread major diameter cylinder centerline. The internal thread major diameter cylinder will always be concentric with the thread pitch diameter cylinder because they are both cut with the same tool at the same time; however, the internal thread major diameter is very difficult to measure.

The internal thread minor diameter cylinder centerline is much easier to locate than the internal thread pitch diameter cylinder centerline, but the centerline of the thread pitch diameter cylinder can be different from the centerline of the internal thread minor diameter cylinder in any aspect. This is because the internal thread minor diameter may have been manufactured with a separate cutting tool and even in a separate piece set-up. Typically, before the thread is cut, the internal thread minor diameter is machined along with other diameters located on the same center. Then the threading tool is inserted to cut the thread form. If the threading tool is a single point cutter or a threaded insert being machined in the same set-up as the minor diameter, the chance of the thread pitch diameter cylinder being concentric with the other diameters is high. If the thread was cut with a tap, a thread mill, or in a separate set-up the thread pitch diameter cylinder may be different from the other diameters.

Regardless of how the thread was manufactured, the best way to locate the thread centerline is to locate the internal thread pitch diameter cylinder centerline. This is done with the help of a CMM hole location gauge (Jo Plug). The Hole Location Gage is screwed into the threaded hole and then the hole center is located with a CMM, or some other measurement tool. The end of the Threaded Hole Centerline Location Gage that protrudes out of the threaded hole has a ‘known cylinder’ which is the measurement reference.