Standardize the Measurement Process

How many times have you heard assembly personnel complain that incoming parts are consistently out of spec or parts that were shipped good now measure bad?

During the manufacturing cycle, many people will look at the part to determine whether it meets the specification. Typically, these could include the machinist producing the part, a QC person, an incoming inspector at the company using the part, and finally another inspector who may be responsible for evaluating the manufactured part’s performance within an assembly.

With this many inspection processes, it’s very unlikely that they will all be similar, let alone use the same gaging equipment. Even if skilled craftsmen at each inspection point follow their particular measurement processes to the letter, there will, at times, be unsettling disparities in measurement results.

Let’s look at a very simple part, a cylinder 1 inch long by 5 inch diameter having a tolerance of ±0.0005 inch. How many ways could we measure this part? Here are some of the most popular: micrometer, digital caliper, snap gage, bench stand with anvil and dial indicator, air fork, light curtain, optical comparator, special fixture with two opposing gage heads and electronic amplifier or, even, a vee with a digital indicator.

Add just a little form or surface finish variation in the part, and it’s very likely that each one of these inspection systems mentioned above would produce a different result. Even with gages in top condition, there will be slight, if not major, differences. Simply the type of gaging being used and how it contacts the part may cause this.
Suppose one company is using an air gage to measure the roundness of a part, and another company is using a snap gage. In addition, let’s suppose that the part has a very coarse surface finish greater than 50 RA. In this case there will rarely be a correlation between measurement results. This is because the air jet tends to average the peaks and valleys in the finish, while the hard contacts of the snap gage will ride on the peaks. This situation is a disagreement waiting to happen.

What if one company is using a two-point contact gage and the other is using a gage with three contact points? Will roundness measurement results be comparable? If the part has an odd number of lobes, another disparity between inspection systems is about to happen. It is an interesting quirk of geometry that a two-contact point gage, as it is rotated around an odd-lobed part, will always see the part’s average diameter, making it impossible to determine roundness. On the other hand, if a part has an even number of lobes, both gages will deliver comparable results.

One more: What if the part isn’t quite straight, and one inspector measures with two direct contact points and the other uses a snap gage which makes two “line” contacts with the part? Asyou rotate the contact point gage, it follows accurately around the part’s diameter, but the line contact tool will interpret the part’s out-of-straightness as out-of-roundness.

There are certainly dozens, if not hundreds, of variations on this theme, but you should get the point by now. While a variety of tools may be used to measure a given dimension, a disparity in measurement processes up and downstream from your inspection point will, sooner or later, cause unneeded rejection and delays in acceptance of the part.

If you don’t want to hear that tune any more, get together with your suppliers and customers and standardize your measurement tools and processes, particularly for critical dimensions. It may seem like a little extra work, but in the long run, everybody will benefit.