Metrology
Metrology
Tired Of Bickering Over Part Specs?
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TIRED OF BICKERING OVER PART SPECS?
STANDARDIZE THE MEASUREMENT PROCESS
George Schuetz, Mahr Federal Inc.

  How many times have you heard an assembly operator complain that incoming parts are consistently out-of-spec?  How many times have you heard the parts people trash assembly folks for not knowing how to use their measurement tools? They were shipped good parts and now they measure bad.  What's going on here?  Where is the problem?

  During the manufacturing cycle for a part or product, 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" long x 5" diameter having a tolerance of ±0.0005".  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 w/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 they contact 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 the results be comparable? Not if the part has an odd number of lobes.  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 parts diameter. Another disparity between inspection systems is about to happen. 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? As you 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 the acceptance of the part.

  If this tune is all too familiar and you don't want to hear it 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.