True position
True position
(OP)
I need help interpreting a true position tolerence. It is a laser cut and formed part It is formed in the shape of an L with one leg being shorter than the other. It calls for 3 holes thru the short leg.( I hope you can picture this.) Now my question. The shorter leg is labeled as datum A and the tolerance on the hole says I have .014 true position to datum A. How do I measure true position of a hole if it is thru the datum? It doens't reference any other datums in the tolerance.





RE: True position
RE: True position
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV
RE: True position
Does the specification apply to one hole, or to all three?
--
JHG
RE: True position
RE: True position
In order for the control to apply to all three holes, the feature control frame has to be placed underneath the hole size callout AND the hole size has to be preceded by 3X or something indicating the size applies three times. There has to be some way that those holes are located from edges or from a center plane. How do you know where to put the holes on the short leg?
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
RE: True position
RE: True position
This print is not correct in how it is calling out the hole diameters. The holes should be located from datum features using basic dimensions and they are not. If you want to locate the holes from the lower edge (1.6 basic dimension) then it needs to be specified as a datum feature. The feature control frame should then reference the holes with respect to A and whatever datum letter you assign to that edge.
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
The pattern of 3 holes then forms datum B.
However, unless I'm missing something I don't believe the 1.600 & .551 dimensions should be basic. The edge of the part is located from the hole pattern, not vice versa. This general concept applies elsewhere too.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: True position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
RE: True position
Production probably made the part as it was intended to be made but you checked the part per print, and you have found the problem. There is no reference to horizontal or vertical location via the feature control frame. Position callouts must be referenced back to datum features. All is this clearly stated in the standard so they really don't have a legitimate argument other than "We've never had a problem before." and that's not a legitimate argument.
An alternative is one that KENAT alluded to. You can reference the edges from the holes instead but that would require something else.
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
SpecialtyMFG, sorry this probably isn't much help to you. On the bright side a bad print can make it difficult for them to reject the parts too (at least from a strict legal point of view).
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: True position
Since the 1.600 and .250 are basic, they should have other datums on there too. But just don't forget that a position tolerance -- when applied to two or more like features -- does not control only perpendicularity.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: True position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
Just 'cause I don't have GDTPS after my name don't mean I'm never right.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: True position
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: True position
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: True position
Lay part down on flat A. This is primary.
Add datum B; the part is going nowhere, it is fully constrained.
Basic .551 locates 4 holes pattern to datum A (left-right)
Basic 1.600 and basic .218 create a chain locating 4 hole pattern to datum B (up-down)
Everything is dimensioned and constrained in space.
It is possible to create a check fixture, it is possible to CMM 4 hole pattern in relation to A and B (correct me if I am wrong)
It is quite possible, that it is the correct “functional” requirement where positioning holes to each other is more important than how edges are related to the rest of the part.
Essentially only basic .25 looks out of place, it could be regular dimension.
The real question still (as usual) is it REALLY what the customer wants?
RE: True position
The 3 hole pattern is shown with a basic 1.600 from the bottom edge but that edge is not defined as a datum feature. The position callout only references datum A. There is no correct location of the 3 hole pattern, vertically or horizontally. The basic 1.600 means nothing per this print. In order for it to be relevant the bottom edge needs to be a datum feature and then the position callout needs to reference it. That goes for the left edge and the .250 basic as well.
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: True position
Sorry please, specifically recant that statement we need to be very clear on this as we have all been though this before!
The ISO made a similar statement and then we hear it was recanted!
Frank
RE: True position
RE: True position
I MUST APOLOGIZE!, it thougHt it was the perpendicularity controls position debate, again
Frank