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.

 

[SpecialtyMFG]

I think I need to clarify. Datum A is the face of the short leg...not an edge.

 

[ewh]

It seems to be only controlling perpendicularity to A.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[drawoh]

SpecialtyMFG,

Does the specification apply to one hole, or to all three?

--
JHG

 

[SpecialtyMFG]

Ive tried attaching the print.

 

[powerhound]

I get the same idea as drawoh. If the face of the short leg is datum feature A and the holes through that leg are positioned to datum A then all you're getting is a perpendicularity control.

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

 

[powerhound]

You need to get rid of the comma in your link.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[SpecialtyMFG]

Anyone else having trouble viewing it? When I click on it it seems to work.

 

[SpecialtyMFG]

Ill try again with no comma

 

[powerhound]

That worked.

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

 

[powerhound]

That goes for the .250 basic dimension from the left edge too.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

OK, so the position FCF of the holes is controlling perpendicularity to -A- and their relative location to each other.

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.

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[powerhound]

If that were the case then there should be a profile tolerance to locate the edge of the part from the holes. What am I not thinking about?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[SpecialtyMFG]

Thanks. Now I don't know what to do. This is a hot job from a customer and its through production and waiting in quality and this particular customer doesn't like to hear they have bad prints.

 

[powerhound]

No customer likes to hear that.

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

 

[KENAT]

powerhound - yeah you could go either way keep the dims basic & add profile control or I was thinking just lose the basic and invoke block tols however I have no idea of functionality etc. so who knows.

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).

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[Belanger]

While this is controlling perpendicularity, that's not the ONLY thing this position tolerance does. It is also controlling the position of the three holes to each other!

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

 

[powerhound]

I initially refrained from saying that because I hadn't seen the print and wasn't sure if the 3X was there or not. Once I saw the print, I got sidetracked on the other issues and forgot to address this one. Thanks for clarifying it. It's definitely an important point.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

Hey, I already said as much in my 29 May 13 11:46.

Just 'cause I don't have GDTPS after my name don't mean I'm never right.;-)

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[powerhound]

Actually Ken, you're usually right. A star for you my friend.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II