Gaging Diameter Variation In Machined Holes

As tolerances for machined holes keep getting tighter, we are learning that no hole possesses a geometrically perfect, symmetrically round shape. No hole is likely to take on a perfect oval (two-lobed) form, either. In reality, the form of a machined hole or cylinder has a large degree of variation, from something with a fairly symmetrical outline to some completely distorted and irregular shape. Much of that shape is the result of the machining process, with the typical three lobe shape of centerless ground parts being the most common.

This can complicate even basic diameter checks. Plus, while roundness variations—including lobing and its counterparts, waviness and roughness—may only be a small portion of hole size, their presence is the major cause of vibration, noise, wear and added friction in rotating assemblies.

Plain, elliptical form error (also known as two point lobing) is easily detected by rotating the part in a two point gage or gaging fixture. Assuming the gage has the resolution necessary to discriminate the variation (an important consideration), any type of indicating snap gage, air or mechanical fixed plug gage—or even rotating the part under a test or dial indicator in a fixture gage—will be able to pick up the variation. With a two lobe condition, the max. and min. diameters are located at 90 degrees from each other (Figure A). Because the contacts are diametrically opposed, a two point gage can easily show these variations. In fact, a two point contact system will be able to measure the variation of diameter whenever there are an even number of lobes, regardless of their number.

Odd numbers of lobes are a different animal. It is not uncommon for operators to “mike” a diameter, or inspect a part with some high resolution equipment, only to have the parts not fit into the assembly for which they were designed. The reason is that gages having diametrically opposed contacts are not capable of measuring the complete circumference or envelope of a part and therefore cannot detect an odd number of lobes. This can be seen in Figure B. With a three-lobed hole (or part), the two contact points are riding on opposing high and low points. As the part is rotated in the gage, one contact sees a high spot while the other sees a low point. Moving through the rotation, the contacts move up and down these opposing points, and the variation is cancelled out. The result is a seemingly consistent diameter reading for a decidedly inconsistent form.

To detect lobing and to count the number of lobes, a basic geometry form gage should be used. This can help understand the nature of the form being produced by the machining process and help determine the best gaging method for the application. (When measuring outside diameters, this can sometimes be accomplished with a vee-block method and a formula. But because the formula requires you to know the number of lobes, it’s not much use when that’s what you are trying to determine.) If your geometry check shows a three point or other odd-numbered lobing condition, two point gaging will not suffice. A gage with an odd number of sensing contacts (usually three) is required. Just as two contacts can find most variation in a part with an even number of lobes, a three point gage is better at finding variation in holes and parts with an odd number of lobes.

While there are a number of three point contact gages available for checking this condition, the most common is air gaging. Because air gaging has the resolution needed to measure tolerances where roundness variation is critical, it best suits the diameter variation measuring requirements.

Three jet plugs, with the jets set at 120-degree intervals, are sufficient to measure diameter variation in most odd-lobed holes, regardless of whether there are three, five, or even seven lobes. The operator simply inserts the probe into the hole, rotates it through a minimum of 120 degrees and either notes the max/min variation on the gage indicator, or in the case of some electronic probes, gets an automatic read-out of TIR.