Measuring Diameters in Coordinate Measuring Machine (CMM)

H

haripachan

Hello,

Which is the better method of measuring diameter in coordinate measuring machine & reporting min/max values? (for diameter greater than 1m)

1---Measuring the feature as circle.& .finding the min/max value from circularity error.

2---Measuring points.&. finding the min/max values by simply adding diametrical opposite points?i.e, 0degree+90degrees, 45degrees+225degrees..etc

Please guide me....


 
T

True Position

Re: Measuring diameters in Coordinate Measuring Machine

Hello,

Which is the better method of measuring diameter in coordinate measuring machine & reporting min/max values? (for diameter greater than 1m)

1---Measuring the feature as circle.& .finding the min/max value from circularity error.

2---Measuring points.&. finding the min/max values by simply adding diametrical opposite points?i.e, 0degree+90degrees, 45degrees+225degrees..etc

Please guide me....


Most CMM software has options to output both the normal Least Square results and the two functional fit type calcuations. Your #2 option is definitely questionable and is the reason micrometers will measure odd number lobed parts incorrectly. Your #1 option is better if you are not familiar with changing the evaluation settings.

What CMM software are you running? If you're running Calypso I'm more helpful.
 
H

haripachan

Re: Measuring diameters in Coordinate Measuring Machine

Thanks for your reply....

I am LK Studio.....

User is asking for the diameter measured in the option 2...I am not sure whether this is right...Since we havent done it for any job.We usually measure circle, find its least square diameter and accordingly give the min/max based on circularity. Kindly help me whether the option 2 holds good for measuring diameter greater than 1.5m.
Also note that the number of points we are using to measuring diameter is 8 in both the cases...Whether there is any standard that mentions to take points more than 8 depending on size being measured...

Kindly help
 
T

True Position

Re: Measuring diameters in Coordinate Measuring Machine

Your #2 option is definitely incorrect if you are concerned about diameters being out of round. You will rarely see the form error using 2 points, especially on parts with an odd number of lobes.

For now I would keep using your Least Squares then add/subtract form error method if you do not wish to change the evaluation settings.

As to number of points, I have not ever heard of a standard suggesting number of points to measure. When I used a touch trigger type machine I believe I used around 12 points per circle but I think it's a matter of preference.
 
H

haripachan

Re: Measuring diameters in Coordinate Measuring Machine

thanks for your reply...
 

bobdoering

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I either report highest and lowest diameter - handy for correct SPC if precision machining - or average diameter with roundness. Reporting one diameter - whether with CMM or hand gages - is only meaningful when roundness is always less than 5% of the tolerance. Reporting two seems to show an interest in the roundness, so I would not recommend that approach at all.

The studies I have performed on a computer controlled CMM showed that 8 points is typically a good number, and over 10 points generally shows little significant improvement on a turned diameter. This may be different for manual CMMs.
 
H

haripachan

So shall I adhere to option 1? Measuring circle and reporting the min/max diameter from Least square fit diameter +/- circularity error??
 

Stijloor

Leader
Super Moderator
Re: Measuring diameters in Coordinate Measuring Machine

Most CMM software has options to output both the normal Least Square results and the two functional fit type calcuations. Your #2 option is definitely questionable and is the reason micrometers will measure odd number lobed parts incorrectly. Your #1 option is better if you are not familiar with changing the evaluation settings.

What CMM software are you running? If you're running Calypso I'm more helpful.

I am not a CMM expert, but how about reporting the diameter of the Actual Mating Envelope (AME)? Re:ASME Y14.5–2009.

Stijloor.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
Re: Measuring diameters in Coordinate Measuring Machine

I am not a CMM expert, but how about reporting the diameter of the Actual Mating Envelope (AME)? Re:ASME Y14.5–2009.

That would be appropriate if it was a fit function and - even more so - if it was called out as true position at MMC. Very practical for cylindrical features, too - but may not be necessary for a circular feature. Good point, though - 10 points are good for a circular feature, but cylindrical feature need to be checked at least 3 separate circular sections - for a total of 30 points as rule of thumb - to capture taper, skew, etc.
 
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