GD&T - How to measure these Positional Tolerances on a CMM

H

hamstar89

#1
Hi,

Need help with how to measure these positional tolerances on a CMM. Our quality department are a bit new to this and the customer requires a capability study on the major features. Any help on how to do this would be appreciated.
Ive had a quick look on the forums but couldn't couldn't find any thing similar to this.
Pos Ø0.51 M A B
Pos Ø0.25 M A B
See attached PDF

James
 

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T

True Position

#3
Hi,

Need help with how to measure these positional tolerances on a CMM. Our quality department are a bit new to this and the customer requires a capability study on the major features. Any help on how to do this would be appreciated.
Ive had a quick look on the forums but couldn't couldn't find any thing similar to this.
Pos Ø0.51 M A B
Pos Ø0.25 M A B
See attached PDF

James
What problems specifically are you having? Those are fairly straightforward position tolerances that any CMM should have no trouble working on. The only notable issues are applying MMC to a thread. Also per ANSI standards the position is that of the PD.

To accurately locate the PD a number of companies make threaded locators, for example:
http://www.flexbar.com/shop/pc/Hole-Location-Gages-c37.htm

Calculating capability for position tolerances is complicated, similar to the problems of any one sided tolerance.
 
D

David DeLong

#4
James:

I think that you need to take a seminar or too in GD&T and then you could probably answer the questions yourself. Both of your examples, the positional tolerance is in MMC (M with a circle around it) and in the ASME Y14.5 -2009, it means the virtual condition diameter. In the 94 standard, you could calculate the actual tolerance and then use your CMM to find the center and figure out how far it is off true position (theoretical center). I have listed below three of my web sites that, hopefully, might answer your question.

http://www.qmsi.ca/positional.html

http://www.qmsi.ca/reportingpositional.html

http://www.qmsi.ca/capabilityindex.html

Good luck!
 
M

Mr.Happy

#5
Hi James,
If you can switch on MMC in your CMM then you will get a bonus on your position tolerance. :D
The biggest problem :( is to make an accurate measurement on the thread holes, you should take enough measuring points (depends on the pitch) and also use the biggest probe possible.
To make sure you have reliable results you should do an MSA study on these thread holes.

Good luck,

Mr.Happy
 

Danny Hoover

Involved In Discussions
#6
Capability on a True position
lets say the deviations are:

x=1 y=1
x=-1 y=1
x=1 y=-1
x=-1 y=-1
These values will return the same positional deviation, but the center is in 4 different locations.

In turn, you could use the x and y deviation themselves to evaluate the center, but what tolerance do you use? Remember the true position formula is based on a mathematical formula that is much less than ±1/2 of the tolerance.

Is that a meaningful study? Yes.
Is this a capability study. No.

This information has been discussed many times in this forum:
http://cmmguys.com/forum/

I hope this helps you on your quest for useful data.

DH
 
V

vimalm

#7
I have the same doubt in CMM in MMC. I got reading from CMM with MMC condition.
So the bonus points are added automatically. Then how to find the actual deviation in the reading?
True position nominal value - 0.15mm
For Diameter-10mm (+/- 0.1mm)
This is i got reading:
upper tolerance- 0.2627mm
Deviation - 0.2153mm
How to i calculate for 0.15mm?
 
V

vimalm

#9
No,its not possible. You just say what is the actual calculation for the true position of 0.15mm.
 
T

True Position

#10
The calculation using 0.15mm is the same thing but with MMC turned off. The MMC modifier changes the 0.15mm tolerance, that's what MMC bonus tolerance is.
 
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