GD&T Call Out - True Postion Call Out on Datums

CarolX

Super Moderator
Super Moderator
#1
Hi folks - need some help - and I will try to describe the issue -

I am reviewing a preliminary drawing for a sheet metal part. There is a hole that is identified as Datum "B". And I have a callout for the diameter of that hole to be 4.80, and a true position requirement of .5 at MMC with respect to datum A,B,C.

My question is - how can you have a true postition call out on a datum?

I haope this is enough info for some response.

Thanks!
 
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Stijloor

Staff member
Super Moderator
#2
Re: GD&T call out - true postion and datums

Hi folks - need some help - and I will try to describe the issue -

I am reviewing a preliminary drawing for a sheet metal part. There is a hole that is identified as Datum "B". And I have a callout for the diameter of that hole to be 4.80, and a true position requirement of .5 at MMC with respect to datum A,B,C.

My question is - how can you have a true postition call out on a datum?

I haope this is enough info for some response.

Thanks!
Carol,

Any chance you can attach a basic sketch?

Stijloor.
 
T

True Position

#3
Re: GD&T call out - true postion and datums

Normally you'll see datums called back to other datums, very commonly I'll see datum targets for casting, then different datums for first machining operation and so on through the process.

If I am given a datum referencing itself I normally ask the engineer in question if they are asking for the position of the datum relative to the other datums.

In your ABC alignment with B dimensioned with respect to ABC, generally A is your perpendicular plane and what they are asking is B @ MMC to AC.

Good luck.
 

CarolX

Super Moderator
Super Moderator
#4
Re: GD&T call out - true postion and datums

Here is a waterdowned version of the print. Part is made from 16 ga (.059) galvanized sheet steel.
 

Attachments

Stijloor

Staff member
Super Moderator
#5
Re: GD&T call out - true postion and datums

Here is a waterdowned version of the print. Part is made from 16 ga (.059) galvanized sheet steel.
Carol,

The datum feature symbols indicate that this is an older version of the GD&T Standard. (ANSI Y14.5M-1982). Not a show stopper, just to keep in mind.

A datum feature of size (the slot width and the hole) can be controlled with a positional tolerance. What's unusual however is that the datum feature callout is incorrect. The "C" and "B" datum feature can not be included in the feature control frame that controls itself. That's a contradiction.

The position control on the open slot can not have the diameter symbol included because only the width of that slot is a feature of size.

You need to talk to the customer and point that out to them.

I hope this makes (a little bit of) sense.

Stijloor.
 

CarolX

Super Moderator
Super Moderator
#6
Re: GD&T call out - true postion and datums

A datum feature of size (the slot width and the hole) can be controlled with a positional tolerance. What's unusual however is that the datum feature callout is incorrect. The "C" and "B" datum feature can not be included in the feature control frame that controls itself. That's a contradiction.
That is exactly what I thought - but I am no GD&T expert - I just know how to find the answers in the book.

Thanks!!!!!!!!
 
D

David DeLong

#7
Re: GD&T call out - true postion and datums

When a hole is a secondary datum, all dimensions come from this hole with the tertiary datum only for orienting the part.

The positional tolerance on the hole that is datum B in this case does not make sense. The hole only has 1 relationship and that is to datum A - perpendicularity but it is not relevant on a stamping.

I would suggest that the designer take off the positional tolerance qualified on this hole and just designate it as datum B.
 

Miner

Forum Moderator
Staff member
Admin
#8
A better datum scheme would be to draw a line through the centers of the hole and the slot width, and call that Datum B. The center of the hole could then be called Datum C. The relation of this hole to the perimeter of the part could be controlled with a Profile callout
 
U

Umang Vidyarthi

#9
Re: GD&T call out - true postion and datums

That is exactly what I thought - but I am no GD&T expert - I just know how to find the answers in the book.

Thanks!!!!!!!!
Hello Carlox,

In addition to expert guidance given here,there is an enlightening article,entitled 'All Those Datum Things' on the subject,by William Tandler;founder of Multi Metrics.:

http://qualitydigest.com/IQedit/QDarticle_text.lasso?articleid=12583

And another article by the same author,entitled 'Tolerance Zone "Size" and "Mobility" Modifiers' is here :

http://qualitydigest.com/IQedit/QDarticle_text.lasso?articleid=12521

/Umang :D
 
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