I believe I got this information from Matt some time ago but the actual formulas are: x(dev)^2+y(dev)^2+z(dev)^2 or(x(dev)*x(vector))^2+(y(dev)*y(vector))^2+(z(dev)* z(vector)^2)
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Xcel 15-20-10 - PFXcel 7-6-5 - Merlin 11-11-7 - Romer Absolute 7525SI
PCDMIS 2012
Windows Office XP
This will give the TP in 3 axis, but not TP using 3 'datums', which was the original question, and if the datums are with MMC, then you will need about 17 pages to print out all the math!
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Originally posted by AndersI
I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.
The basic formula for True Position is 2 x the square root of the deviation in one axis squared + the deviation in the other axis squared.
True Position = 2√ Deviation in XÂ² + Deviation in YÂ²
It's hard to convey a good example of the formula with the font limitation of this forum. However, the X & Y axis’ are for a feature measured in the Z axis plane. If you measure in the X axis plane, you would substitute the XÂ² with ZÂ². If you measure in the Y axis you would substitute the YÂ² with ZÂ².
Subsequently, if there is a MMC modifier in the call out you would be allowed bonus tolerance.
As long as you do NOT have any modifiers (meaning the tolerance, as well as the datums are at RFS (Regardless of Feataure Size)), you can do the math by hand. Even a modifier on the tolerance is still quite easy to calculate.
The moment you at a modifier on the datums though, things get complicated immediately. I agree with Mat, it can take pages to figure it out by hand.
I have done many experiements using modifiers on datums using PC-DMIS V4.1, comparing it to hand estimates. Generally, I think that they calculate it correctly. You have to use the new Xactmeasure GD&T to get it right. I believe it does not work using the legacy dimensioning.
Jan.
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PC-DMIS/NC 2010MR3; 15 December 2010; running on 18 machine tools.
Romer Infinite; PC-DMIS 2010 MR3; 15 December 2010.
The basic formula for True Position is 2 x the square root of the deviation in one axis squared + the deviation in the other axis squared.
True Position = 2√ Deviation in XÂ² + Deviation in YÂ²
It's hard to convey a good example of the formula with the font limitation of this forum. However, the X & Y axisâ€™ are for a feature measured in the Z axis plane. If you measure in the X axis plane, you would substitute the XÂ² with ZÂ². If you measure in the Y axis you would substitute the YÂ² with ZÂ².
Subsequently, if there is a MMC modifier in the call out you would be allowed bonus tolerance.
Thanks for all the good replies guys. I screwed my knee up last Thursday and just got back to work today.
Wish I could post a pic but can't. I'll try it this way:
the call out is: t/p dia. .015 to f d & e
We make landing gear for large airplanes so everything is round and cyclindrical(sp).
The part is shaped like a cyclinder, open on one end (the face on that end happens to be datum -e-), closed on the other end with a lug w/an approx. 4.5" hole running perpendicular to the cyclinder. Clear as mud?
The C/L of the cyclinder is datum -d- with the face of the lug being designated as datum -f-. All datums are listed as RFS.
It was my understanding that you simply added the third datum to the equation as Matt has shown. My partner in crime here on the 2nd shift was under the impression that there was a lot more to it than that.
Thanks again. I can always count on you guys for the help.
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