How to Measure a Diameter - Average or Maximum/Minimum?

k3nny

Periodic Visitor
#21
We have acquired a Zeiss Contura G2 that allows for evaluation options. Average is there, Maximum Inscribed Circle, Minimum Inscribed Circle, ISO XXX etc...

I agree with the comments made above. As a custom molder we treat it this way. Product should be all inclusive of print tolerance. If the product through first article or capability proves to not meet this requirement - then we ask the customer for clarification. Transfer tools will be different than a new tool. One lesson I have learned is get in early - ask questions about application and do your homework with the numbers. If you are clearly not capable, but the original designer was cautious, work with the customer on what you can do - recommend tolerances.

Back to the Contura. I use maximum inscribed and minimum circumscribed with scanning technology with no less than 50 points. The idea is to duplicate a pin measurement, but to capture min and max along the way.

Ken:agree1:
 
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bobdoering

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#22
My preference for measurement of a circular feature is min/max. Min/max is best for control of circular features, also. The average is really irrelevant. How many measurements can be a matter of more is better, although the tolerance and the relationship of roundness to tolerance for the process may provide some measure of reasonableness. If not scanning, more than 10 points on a CMM rarely provides significantly improved data for most applications. However, it is darn good thing to know the lobing!!! 3 point measure on 3 lobe and 2 point on 2 lobe, when using hard gages. Pins are nice, but only detect MMC. Pins and CMMs should not agree: pins require "fit", CMMs do not. :cool:
 
Q

qualitytrec

#23
It all is summed up in function. The measurement must account for function. For example we manufacture and distribute bottles and caps. The diameters of the neck of the bottle and the cap have to be able to function together with proper thread interface and with out undue interferance. So while there is no ovality call out it is still pertainent for us to control most of our products min cap ID and Max bottle ID in addition to this we test function by if it meets torque recomendations without either leaking or stripping. One size (measurement technique) does not fit all. The measurement technique has to account for functional use. If the diameter is a hole and a pin is to slip through it a go no/go will likely work. If the hole is for venting then the average function from a CMM measurement should work. If a pin is for locating another part a ring gage should work. If it is a stand off or spacer that needs certain paramaters for support the average on the CMM may work. If function is unknown then in my opinion it should be recorded as a worse case scenario which may require multiple outputs for the same feature. For example it may require the min, max and average to be output on the report until better definition of function is given.

Just my thoughts,
Mark
 

bobdoering

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#24
The use of go/no go pins or rings are OK verification, but when the question is measure a diameter, then I do not think they apply. Pin sets may be used for variable data - if one understands their physical limitations. They provide minimum only (MMC), and have issues with fit. That may be suitable for quality level TCE.

However, either min/max or average/roundness (a little tougher for some to visualize) are the best to fully understand the nature of the circular feature (and are the representation of the GD&T zone for a round feature). Again, if you are trying to control processing a round feature, min/max is by far the best, and average is useless, if not dangerous. :cool:
 
Q

qualitytrec

#25
I do not disagree.
I guess the question is, is it being measured for control of function? Control by itself is useless if it is done for its own sake it becomes dead and muda. And function of a batch can be uncertain if there is no control. What is being controlled should be tied directly to functionin this case if I understand it correctly. Therefore function needs to be defined and the measurment system needs to be analysed to see if it is going to catch non functional features with the techniques employed.

Mark
 
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M

mboucher

#26
Where this diameter discussion gets interesting is when the cylinder is a datum.

For a datum being the axis of a cylinder (see figure 1) it is defined in ISO 5459:1981 [2]:​
.The datum is the axis of the largest inscribed cylinder of a hole or the smallest circumscribed cylinder of a shaft, so located that any possible movement of the cylinder in any direction is equalised..

The diameter must be the largest inscribe but I wonder how many are still using LSQ for this measurement.

Mark​
 
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