GD@T true position.
GD@T true position.
(OP)
Can true position be measured from 3 angular datums, and if so what is the formula. Please see attached drawing.
Thanks.
Thanks.
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RE: GD@T true position.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD@T true position.
Such a measurement can be done, but it really depends on the inspection method. Here's the basic concept: Somehow, on the drawing or in the CAD database, there is already a known angular relationship between the hole and the three mutually perpendicular datums. The GD&T simply constructs a .060 diameter imaginary cylinder around the perfect axis, and the actual axis must fall within that cylinder. So the tolerance sits at the same angle as the intended hole.
It sounds like you're asking for a breakdown in the direction or the datums (kind of like an x-y-z measurement report), and then you might be looking at sines and cosines and the Pythagorean Theorem (CMMs can do this pretty easily). Using these trig formulas you or the computer could determine the portion of the .060 diameter tolerance that applies in each direction.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: GD@T true position.
Belanger,I understand that these measurments can be done. What I do not understand is the formula you would use to calculate the errors of measurement to 3 datums. Generally I am used to seeing 1 of the datums located perpindicular to the hole which then gives you 2 datums to measure from at which point you would take the error of your measurements and use the equation of 2 times the square root of X squared plus Y squared to give you your true position. How would the 3rd datum come into play with this equation?
Thanks again
RE: GD@T true position.
First you need to find coordinates of hole's true position defined as a reference for measurements. This could be properly done only if the true position of the hole is correctly dimensioned from the origin of measurements with the use of basic dimensions. In your case the origin of measurement would be at the intersection of planes A, B & C. This will give you basic [X,Y,Z] coordinates of true center of the hole. Then after you get coordinates of actual center of the hole [x,y,z] you can calculate position deviation using the formula:
sqrt[(x-X)^2+(y-Y)^2+(z-Z)^2)]
Side note: primary datum feature does not have to be always perpendicular to the axis of the hole. It is just the most common situation, but it is not the rule.
RE: GD@T true position.
rwild, please disregard my previous post if you haven't already.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD@T true position.
Frank
RE: GD@T true position.
Thanks again.
RE: GD@T true position.
I am not so sure there is a simple formula describing what you are looking for in this particular case. Like J-P (Belanger) said GD&T is able to construct cylindrical position tolerance zone of particular size, orientation and location relative to specified datums. So this is the first thing you have to do - make sure that the GD&T properly defines orientation and location of the tolerance zone and then check whether the actual axis of the hole lies within this zone.
RE: GD@T true position.
SeasonLee