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Perceptions of a Good Gage

The starting point: all gages are bad...

  1. Bob Doering
    Sometimes, getting a point across in a production setting is tough. Getting production folks to understand gaging issues can be one of those challenges. Here are a couple tips I use in my metrology class, that have also worked well on the shop floor.

    EVERY GAGE IS A BAD GAGE.

    No truer statement can be made - every gage is a bad gage. There is some type of physical limitation that will rear its ugly head when it gets to the limits of its usefulness. So, that being the case - why use any gage at all? Because sometimes it is good enough. The next question is how can you tell it is good enough? For most folks, they use the Gage R&R tool to provide insight to that question. Just because the gage has a specified resolution on its scale, does not mean that it can be used accurately to that scale. You can have a steel rule with a 1/50" scale, but you will find that using that scale over and over again will not get you the same results - probably because it is so darn hard to read. That is where Gage R&R provides an opportunity to sort out whether the gage is "good enough".

    DO NOT ASSUME A MICROMETER IS BETTER THAN A STEEL RULE.

    Some people just love micrometers. They claim they can read them very precisely. It almost sounds like they can tell fortunes with them! The fact is, it is easy to be biased with a micrometer, and the power of the toque of the screw can allow you to get any answer you want - gage force, is what it is typically called.

    But how can one really bring that point out?

    No problem! I lay out 3 gages - a steel rule, a caliper and a micrometer. I as the participants: "Which gage is best?" Most will jump to the conclusion that the micrometer is. Then the fun begins. I give them 10 marshmallows and have them perform a Gage R&R on each device. You might be able to guess which one wins that battle - the lowly steel rule! It has no gage pressure! It may be a little hokey - but it proves two points very nicely : gage pressure and micrometers are not always that great (especially because of gage pressure and bias!).