Profile with no datums 1

[PRuggiero]

Guys,

I've read a couple posts here about surface profiles with no datums but I'm still a little confused. I found this post by Jim (MECHNORTH):

By "best fitting" one can shift the workpiece about its 6dof to find the best fit of the workpiece within the maximum & minimum boundaries (i.e. offset geometries from the basic dimensions) to qualify the part.

So if I had a simple block part, datum A is a face, Datum B the longer edge and C the short edge and I had a profile callout on say the edge opposite of datum B without referencing any datums how would it get inspected?

From the post above it seems that if I placed the surface representing datum B down on a granite table and measured the distance that the basic dimension gives to the surface I am profiling, this would give me the "line" that my profile tolerance would be centered on, correct? Then I am allowed to move the part however I want to get that surface to fall within the profile tolerance.


Thanks,
Pete

 

[MechNorth]

I really like profile. Bet nobody ever guessed! I'm subtle that way.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

Jim,

Subtle compared to Don, anyway. ;^)

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

Who...? Oh...him.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Dawg2012]

I've spent the better part of two days trying to understand these seemingly subtle differences in surface profile tolerancing. Glad to find this forum! Seems a lot of practical knowledge here.

My situation is a block similar to that described above - however I have two holes in datum surface A that are datums B and C (the edges of the part are functionless in the design).

I have a suface opposite and parallel to A that I want to control with a surface profile tolerance for form and location (the surface needs to be a certain distance from A and relatively flat). My thinking is that I put a basic dimension from A to this surface, then a surface profile tolerance referencing only datum A. What I'm not sure about is referencing datums B and C in the FCF - I don't see what additional control it would provide so I don't think I need them. Right or wrong?

 

[Belanger]

Right. You don't need them.

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

http://www.gdtseminars.com

 

[Dawg2012]

Okay. Thanks.

To take this understanding one step further - if the surface in question was NOT parallel to A, and other dimensions were required to locate it, say a basic angle from A and a basic dimension from, say, the datum B hole, then I would reference A and B in the FCF?

If so it would then follow that if datum C were required to help locate said surface, it would be referenced in the FCF? Is this the general idea for datum referencing?

Btw, to the OP's question, we do fairly frequently use a surface profile tolerance without any datum reference. We do this when we are establishing datum A and it is comprised of more than one coplanar surfaces.

 

[fsincox]

I might reference all three on an angled surface located by profile.
Frank

 

[Belanger]

Yeah, I guess if you think the angle needs another datum to reference its vertex or vanishing point, then use the other datum(s).

Sometimes a datum is referenced in a FCF even though it doesn't directly, quantitatively impact the tolerance. But it makes it common with other FCFs to imply that they are gaged simultaneously. Or it helps maintain a certain datum precedence to imitate the function of the part.

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

http://www.gdtseminars.com

 

[fsincox]

JP,
I agree with you, I was actually thinking in terms of compound angle surface definition cases.
Frank

 

[MechNorth]

Yo Dawg (sorry, had to go there),
For only 2 days of focusing on Profile, you're doing well.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ptruitt]

It seems to me that coming to GD&T 'green' can be a blessing. There is a hot topic being debated on another thread trying to justify or condemn what seems to me to be an unnecessarily complex method of defining a common mechanical connection. ASME Y14.5-Next should take away some of the redundant symbology and hand out dutums judiciously, in my opinion.

Peter Truitt
Minnesota

 

[Dawg2012]

Woof! (Thanks)

 

[MechNorth]

Peter, maybe start a new thread for that tangent. Could (ok, will) be an active topic.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com