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GD@T true position. 2
- Thread starter rwild
- Start date
powerhound
Mechanical
With the exception of having no locating dimensions, I see no problem with what you show. There is nothing angular about the datum structure. Those holes will be directly located from 3 mutually perpendicular planes per your drawing. The rest of the part is skewed from the datum frame but the holes are not. You need no formula for this one. It's pretty straight forward.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
-
1
- #3
Powerhound, I think the hole is through the back wall of the front view, so it does seem to be at a nontraditional angle with respect to datums B and C.
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
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
- Thread starter
- #4
Powerhound and Belanger thanks for your responses.
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
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
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1
- #5
pmarc
Mechanical
- Sep 2, 2008
- 3,248
rwild,
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.
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.
powerhound
Mechanical
Thanks JP, I got hung up on the 2 hole pattern in the top left view for no other reason than I wasn't paying close enough attention.
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
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
- Thread starter
- #8
pmarc
Mechanical
- Sep 2, 2008
- 3,248
After re-reading my first post, I would rather suggest not to use the formula I gave, because it only gives a possibility of calculating 3D distance between two points. It could work if the tolerance zone was a sphere and the toleranced feature was a point, but in your case tolerance zone is a cylinder within which the axis (so line) of the hole must fall.
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.
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.
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