True Position Formula

[tenndoc]

Some discussion here about how to calculate t/p using 3 datums.

Any and all help is appreciated.

thanks

[John Kingston]

This site has some good examples.

[tenndoc]

Thanks for the quick reply CJ but IT has us locked out of most of the internet. I can't follow the link.

[Rob_K]

Maybe an example?
The site that John pointed out is really good, you must get there

[rangerboat72]

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)

[Matthew D. Hoedeman]

You forgot the sqrt of the formula

SQRT(xdev^2+ydev^2+zdev^2)

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!

[rangerboat72]

Ooops! Sorry to misquote you Matt. I thought I was missing something there.

[mdobb829]

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 X² + Deviation in Y²

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 X² with Z². If you measure in the Y axis you would substitute the Y² with Z².

Subsequently, if there is a MMC modifier in the call out you would be allowed bonus tolerance.

[Underspec]

On the wilcox site there is a document written by Rob Jensen about true position and others.

[Jan d.]

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.

[rangerboat72]

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 X² + Deviation in Y²

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 X² with Z². If you measure in the Y axis you would substitute the Y² with Z².

Subsequently, if there is a MMC modifier in the call out you would be allowed bonus tolerance.

This is a good 2 dimensional true position calculator: http://www.efunda.com/DesignStandard...ion_single.gif

[tenndoc]

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.

[Paul Sarrach]

This is a good 2 dimensional true position calculator: http://www.efunda.com/DesignStandard...ion_single.gif

I have had a need for this many times. It makes life simplies.

[jleond]

This is an excel file with a tp calc also, downloaded from:
http://www.geomeasure.com/documents.htm

HTH

[Jesus Ramirez]

Ese leon...! Do you have the formula for True position with polar coordinates?