Surface Profile and True Position Question

M

markro

#1
We are working on a new part that has a surface profile callout of .040mm "between K and L". "K" and "L" are not datums but rather just a couple different spots on the part. Within this "K and L" area there is a 4mm radius (basic dimension). Our customer sent us a part that they have machined to show us what the part should look like. We have checked that radius in particular and it is much closer to 6mm rather than 4mm. I talked to the CMM programmer at the customer and he said that because the 4mm is a basic dimension, as long as the surface profile of that area falls within the 0.40mm, then the size of that radius does not matter.

I've seen this as far as true position is concerned where the basic dimensions give me an idea where my x and y should be, and usually they have to be pretty close to the callout or the true position goes out of tolerance. It seems this would hold true of surface profile as well, but a 6mm radius rather than 4mm seems like a big jump. I'd rather not nod in agreement with his assessment and move on. Is there a way to mathmatically figure out how far this radius would have to be out of tolerance before the surface profile would be out of tolerance? Can anyone add anything that may shed a little more light on the subject? Thanks!
 
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#2
Re: Surface Profile Question

We are working on a new part that has a surface profile callout of .040mm "between K and L". "K" and "L" are not datums but rather just a couple different spots on the part. Within this "K and L" area there is a 4mm radius (basic dimension). Our customer sent us a part that they have machined to show us what the part should look like. We have checked that radius in particular and it is much closer to 6mm rather than 4mm. I talked to the CMM programmer at the customer and he said that because the 4mm is a basic dimension, as long as the surface profile of that area falls within the 0.40mm, then the size of that radius does not matter.

I've seen this as far as true position is concerned where the basic dimensions give me an idea where my x and y should be, and usually they have to be pretty close to the callout or the true position goes out of tolerance. It seems this would hold true of surface profile as well, but a 6mm radius rather than 4mm seems like a big jump. I'd rather not nod in agreement with his assessment and move on. Is there a way to mathematically figure out how far this radius would have to be out of tolerance before the surface profile would be out of tolerance? Can anyone add anything that may shed a little more light on the subject? Thanks!
Welcome to The Cove Forums!! :bigwave: :bigwave:

6 MM is out of tolerance! The 0.4 mm profile tolerance zone specifies a zone within which the actual profile must be located. The default tolerance zone for profile is bilateral, meaning 0.2 up and 0.2 down from true profile. Flats and reversals are permitted within the tolerance zone.


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S

s529f

#3
Re: Surface Profile Question

how large of a section of radius is it? It is usually a rule of thumb to get an accurate radius measurement with a CMM, you need at least 110 degrees of the circle in question. If you dont have that much or more, the math kindof breaks down in the software and you can get vastly varying results. In my experience a profile callout is usually more favorable in those situations. So if you do a profile on that section and find it to be within the bands of the tolerance, you're usually in good shape.
 
M

markro

#4
Re: Surface Profile Question

Welcome to The Cove Forums!! :bigwave: :bigwave:

6 MM is out of tolerance! The 0.4 mm profile tolerance zone specifies a zone within which the actual profile must be located. The default tolerance zone for profile is bilateral, meaning 0.2 up and 0.2 down from true profile. Flats and reversals are permitted within the tolerance zone.
Thank you. That was my understanding.

Thanks as well, S259f. it sounds as if you're agreeing with the programmer.


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S

s529f

#5
Re: Surface Profile Question

In all fairness I do tend to agree with programmers. I interned at Hexagon Metrology as a CMM programmer right before I graduated and know pretty well the different issues that come up with short arc lengths like that. Like I said, it is very hard to get a reliable/repeatable measurement when you use a short arc length, there are ways to get around it, though. How are you measuring the part/what software?
 
M

markro

#6
We are using a Contura G2 Zeiss CMM running Calypso. We have used a radius gage for this area and it seems quite obvious that the radius is not 4 and is much closer to 6. According to printout of their CMM report (not necessarily of this part in particular), the radius checks at 4mm ? .100 and they are giving it the correct tolerance of ?.200, so it is well within tolerance.
 
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