Inspection Requirements & Engineering Drawings - Measuring Shaft Dia.

somashekar

Staff member
Super Moderator
#11
I have since checked the part and if you measure it in say 2 places around it's circumference you are likely to record it as OK. If you check in say 6 places, or if you look for the lowest / highest measurement for that dimension then you will see if falls below min tolerance.
I have no idea how long this shaft is, and certainly there can be several several places where measurement can be taken.
Are the measurement being taken for some machine capability analysis ? or measurement is made to decide the parts good for use ?
If it is to check its fitness for use, where exactly does the diameter measurement matter. Measurement just for the sake of it is meaningless. Check the diameter at such location(s) where it matters based on fit, form, function requirement.
Decide your CTQ measurements. This includes in the inspection plan the location, the number of measurements and averaging. Make a risk based decision.
CTQ = Critical To Quality
If roundness is a concern, we expect a separate callout. That's what we have been doing for years.
Superb input from Golfman
 
Last edited:
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David-D

Involved In Discussions
#12
Fundamentally, the requirement for the shaft according to ASME Y14.5 is that it shouldn't exceeds the maximum dimension (MMC) envelope and in no individual location should it be less than the the minimum (LMC). As someone above correctly identified, the proper way would to be with a ring gage (really a long tube to be 100% accurate) to establish that it doesn't exceed the maximum dimension and it should be measured along the length and around the shaft w/ a 2 point measurement system (micrometer, etc) to make sure that at no point it is less than the minimum requirement.

That being said, often in reality one would take some sort of approximation by only taking one or several measurements. What it comes down to is how much variation you expect/fear in form of the part. If you expect the diameter to be uniform along the length, perfectly round, and perfectly straight, than a single diameter measurement would be sufficient. The more types of form error you're trying to inspect for, the more you need to broaden how many measurements (and what locations) you take and how you make them. Perhaps on initial production parts, you could do the full gammut of inspections, and once you have better understanding on the form variation and the nature of the manufacturing process, you can simplify the measurement.

As an aside, someone be above mentioned averaging of values (and using CMMs); id caution against doing that unless the print calls explicitly for an average diameter. We've been horribly bitten by this method in the past as it overlooks the true requirements. In our case, we were using a CMM and had (unbeknownst to us) oval shaped holes that in one direction were larger than the maximum hole size while in the opposite direction were smaller than the minimum size. The CMM took the points and curve fit a best fit circle to the group coming up with an average circle that fell within requirements but was really nonconforming. We ended up showing this by being unable to fit in MMC pin gages into the hole and also making multiple measurements w/ a 2 point bore gage. We learned the hard way to the importance of not using the CMM's average. I've pretty much tried to eliminate averaging, it rarely represents what we really care about in the parts - it is much more important to report the maximum and minimum. The only time i still even consider it is when the measurement uncertainty is particularly large (usually a chemical/analytical test) and I'm trying to minimize the gage error.

David
 

Golfman25

Trusted Information Resource
#13
The problem with having engineering decide measurement methods is that in most cases they are not metrology experts (or even good amateurs). They'll specify CMM's when calipers are needed and calipers when CMM's are needed. They may not even know what kind of equipment is available int the inspection area.

If one was to get technical about it, The inspector should be doing a MMC check using a ring gage (Full form gage) and a LMC check using a two point measuring instrument such as a micrometer. Reality is that most inspectors will only use one or the other method to do the check.
Likewise the quality guys are not experts in form, fit and function. So somewhere between the two they have to decide what is needed and how it is to be measured. Unfortunately, that usually doesn't happen.
 

Golfman25

Trusted Information Resource
#14
As an aside, someone be above mentioned averaging of values (and using CMMs); id caution against doing that unless the print calls explicitly for an average diameter. We've been horribly bitten by this method in the past as it overlooks the true requirements. In our case, we were using a CMM and had (unbeknownst to us) oval shaped holes that in one direction were larger than the maximum hole size while in the opposite direction were smaller than the minimum size. The CMM took the points and curve fit a best fit circle to the group coming up with an average circle that fell within requirements but was really nonconforming. We ended up showing this by being unable to fit in MMC pin gages into the hole and also making multiple measurements w/ a 2 point bore gage. We learned the hard way to the importance of not using the CMM's average. I've pretty much tried to eliminate averaging, it rarely represents what we really care about in the parts - it is much more important to report the maximum and minimum. The only time i still even consider it is when the measurement uncertainty is particularly large (usually a chemical/analytical test) and I'm trying to minimize the gage error.

David
That makes perfect sense in your situation. But in others it does not. And people need to realize this. We deal with "flexible" parts that are typically out of round beyond the stated diameter tolerance (which tends to be unreasonably tight). They will "round up" when they are assembled and used. So for us and average works well. As I have stated before, it take all parities to sit down and figure out what is actually needed and go from there.
 

David-D

Involved In Discussions
#15
Sorry, perhaps I was a bit too overzealous on my crusade against averaging. A very valid point - flexible parts cause unique issues of their own. Averaging is one way to address the problem, as what you really care about in the end is the circumference if you're trying to fit it over another part. We've seen issues with IDs in thin wall metal tubes and contact gaging (calipers) where we ended up going to 3 point bore gages but had also looked at air-gaging.

The best guidance, which has been reiterated many times in this chain is to have communication between the designers and inspectors (and everyone else).

David
 

optomist1

A Sea of Statistics
Trusted Information Resource
#16
Appears as though the shaft diameter was incorrectly or incompletely specified regarding GD & T......more time/consideration should have been given up front....
 
N

Nick-C

#17
Thanks everyone this has been very helpful.

This has been resolved by review of how the drawings can be better specified and by communicating specific inspection requirements via an Inspection Plan.

It seems we have gotten out of the habit of doing these things over the years...... :bonk:


Thanks

Nick
 
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