True position 1

[SpecialtyMFG]

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.

 

[KENAT]

Wouldn't go that far but cheers.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Belanger]

My bad Kenat. I skimmed too fast over things

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[CheckerHater]

To me it looks like one of the set-ups that may be legal, but not very practical.
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?

 

[powerhound]

The 4 hole pattern must be centered on B. The .218 is irrelevant to the vertical location of the pattern. The horizontal location looks okay to me.

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

 

[fsincox]

JP,
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

 

[pmarc]

Again, Frank, I would like to hear a clarification on what you want to recant. What J-P said is absolutely correct - I mean his comment about position callout controlling not only perpendicularity of holes to primary datum but also their mutual spacing.

 

[fsincox]

JP and pmarc and all,
I MUST APOLOGIZE!, it thougHt it was the perpendicularity controls position debate, again
Frank