Inspection of Diameters on a CMM (Coordinate Measuring Machine)

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rbriggs5145

#1
My company makes large bushings, collars and inserts for the construction industry. One of our customers has a third party receiving inspection that insist on using a CMM to check both ID and OD's of these parts. The tolerance are between ±.0005" - ±.001" or 0.013 - 0.025. They are not creating a cylinder and then checking the diameter perpendicular to the center line. I am looking for any and all articles, websites and/or references that can assist us in convincing them there is a better way to check diameters. Thanks!
 
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EdZachary

#3
Re: Inspection of Diameters on a CMM

I would suggest a simple approach ... use of Go / No Go pins for the bushing ID and a machined Go/ No Go attribute gage to check the OD or a mock-up of the end item use. If the tolerances are not enough and an out of round condition exists, then the Customer should specify roundness (See GD&T guidelines). Hope this helps!
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#4
Re: Inspection of Diameters on a CMM

I would suggest a simple approach ... use of Go / No Go pins for the bushing ID and a machined Go/ No Go attribute gage to check the OD or a mock-up of the end item use. If the tolerances are not enough and an out of round condition exists, then the Customer should specify roundness (See GD&T guidelines).
Actually, a circular feature should be a basic diameter with a roundess zone at a minimum. For longer round feature cylindricity is a good call. For gage-pin stule with CMM, use MMC. Sounds like the feature is too big for a real pin gage. Besides, CMM will give you variable data-which will aid in control.
 

Stijloor

Staff member
Super Moderator
#5
Re: Inspection of Diameters on a CMM

My company makes large bushings, collars and inserts for the construction industry. One of our customers has a third party receiving inspection that insist on using a CMM to check both ID and OD's of these parts. The tolerance are between ±.0005" - ±.001" or 0.013 - 0.025. They are not creating a cylinder and then checking the diameter perpendicular to the center line. I am looking for any and all articles, websites and/or references that can assist us in convincing them there is a better way to check diameters. Thanks!
There are a number of characteristics to consider:
  • Dimension (and its associated tolerance)
  • Form (roundness, cylindricity, straightness)
  • Surface finish (RMS, RA, etc.)
For example, are there any "Form" callouts on the print? (GD&T?)

Each characteristic must be considered and the appropriate equipment and methods determined and agreed upon between customer and supplier.

Many arguments stem from the fact that there is/was no concurrence on methods.

Just a few thoughts...

Stijloor.
 

Paul F. Jackson

Quite Involved in Discussions
#6
Re: Inspection of Diameters on a CMM

My company makes large bushings, collars and inserts for the construction industry. One of our customers has a third party receiving inspection that insist on using a CMM to check both ID and OD's of these parts.
What is your company's practice of inspecting them and how does the customer's third party inspection results differ from yours... more or less percieved defectives... more or less observed variation... differences in means?

They are not creating a cylinder and then checking the diameter perpendicular to the center line. I am looking for any and all articles, websites and/or references that can assist us in convincing them there is a better way to check diameters. Thanks!
A long long time ago when I worked prototype inspection at Ford Motor Company (prior to electronic calculators) we had a special measuring device that was used to check bushings, collars, and the like. Since these parts were pressed (forced) into their mating component applications and were therefore marginally malleable their "free" form and diameter measurements were not good predictors of their functional acceptance.

Long before me a bushing manufacturer and an auto company had agreed on a method of inspecting the bushing ODs. The spec on the print went something like this... "OD measurement +.012" / +.007" in a 3.6500" diameter split block with 50lb load". Both the supplier and the customer had a library of split blocks with strategic master bore size increments, a pnumatic ram, and a mastered dial indicator to accomplish the measurement. I was working at Ford... the supplier "I think" was Federal Mogul... but the measuring device was labeled "General Motors".

Chances are there are bushing suppliers that still coordinate acceptable methods of bushing measurement with their customers like this... whether it is just the notation "AVG" adjacent to the size specification as illustrated in ASME Y14.5M-1994 or something more elaborate that defines feature restraints as was done in the old days.

Paul
 
Last edited:

Kales Veggie

People: The Vital Few
#7
Re: Inspection of Diameters on a CMM

My company makes large bushings, collars and inserts for the construction industry. One of our customers has a third party receiving inspection that insist on using a CMM to check both ID and OD's of these parts. The tolerance are between ±.0005" - ±.001" or 0.013 - 0.025. They are not creating a cylinder and then checking the diameter perpendicular to the center line. I am looking for any and all articles, websites and/or references that can assist us in convincing them there is a better way to check diameters. Thanks!
I read most of the responses. We are making it way too complicated (pulling in other characteristics and features).

Why do you want to change their method? Are there false rejects?

(Using go/nogo plugs gages is a lot faster and cheaper solution than a CMM)
 
R

rbriggs5145

#8
Re: Inspection of Diameters on a CMM

If you try to check a circle that is part of a cylinder, you have to be perpendicular to the cylinder to get a true diameter reading. With ±.001" tolerance on a ID that is over 9" wide, it is easy to get a false reading. In addition, these parts change size over .001" in 10° temperature change. There is no form tolerance on these parts. There is a true position tolerance but we are meeting that as long as they are reading the print correctly. We have tried doing correlations but without getting the temperature just right (we send them the temperature that the parts were checked at after the parts have soaked) and the measurement, the error is not set. This company asks for control plans before parts are sent. They approve the PPAP based upon that control plan but then change their method of gaging by what the 3rd party receiving inspector decides to use. I have asked many times that if I am to submit a control plan that it become the record of gaging criteria for the dimensions but they do not want to invest in the gages required to do this. From what I have heard, this is an issue with many of their suppliers not just us. However, without some form of documentation stating why they cannot check a diameter with a CMM accurately, I do not believe they will change. Any help will be greatly appreciated!
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#9
Re: Inspection of Diameters on a CMM

Wow. You have a lot of issues.

First, you have recognized you have to control the temperature. Is there any commitment to control the measurement lab temperature? The temperature that the measurement is taken should be reported for a material that you describe. If you can not control it, you need to have a temperature factor - Coefficient of Thermal Expansion is nice on straight pieces, but circular features may not expand perfectly. So, you may need a dimension to temperature study if the labs are not committed to measuring at a tight temperature tolerance.

Then, there is the control plan measurement problem. Yes, they signed off on your control plan by approving your PPAP, and really it should be their responsibility to verify other labs are using equivalent measurement techniques, but it does fall in the cracks from time to time. Your situation sound like more of a routine than accident, though. Sounds like the customer is not committed to lab-to-lab variation issues. That is going to be a tough one to resolve without some irritation.

If it was not for all of that, a CMM should be applicable, if they take enough points in enough places to ensure an adequate measurement and report it in the correct manner (MMC? Average Dia?)

Without some agreement on all of the issues, you are going to be fighting this forever.
 
R

rbriggs5145

#10
Re: Inspection of Diameters on a CMM

You are correct, without some agreement, this will go on forever. This customer is not normally one that is difficult to deal with as so many are. That is why I am hoping to get some ideas from other places to assist me in reasoning with them. Thanks!
 
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