Profile with no datums
Profile with no datums
(OP)
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
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





RE: Profile with no datums
Lack of datums in profile FCF means no instructions on how the part has to be set up for profile measurement. In your case, if it was profile of a surface callout, you would simply measure flatness of the surface.
RE: Profile with no datums
that was my original understanding but what MECHNORTH seems to be saying is that isn't the case. The profile still controls the size because the profile tolerance is centered on the basic dimension in the drawing. This is what is confusing me as he seems to be saying, you need to find where the surface is supposed to be (by the basic dimensions) and then you can use the 6 DOFs to make the surface fit within the profile.
RE: Profile with no datums
If there is no datum in profile FCF, the basic dimension between datum feature B and toleranced surface does not apply = the distance (size) is not controlled.
RE: Profile with no datums
Peter Truitt
Minnesota
RE: Profile with no datums
Dave D.
www.qmsi.ca
RE: Profile with no datums
Frank
RE: Profile with no datums
Not sure if that is the case, look what MECHNORTH says here:
Getting to the use without DRF is an exercise. 6.5.1 says that it is used to control form or combinations of size, form, location and orientation. Form is not related to datums, therefore in the absence of a drf, all you get is form ... the next extension then is to recognize that "form" for an enclosed boundary is essentially a size control as well. 6.5.4 Indicates that in most cases, Profile requires a drf. (i.e. not always required).
He is saying it controls the size, hopefully we can hear it straight from the man himself.
RE: Profile with no datums
Imagine OP's example of a simple block for a moment. Upper segment of a composite profile of a surface callout with datums A, B, C would control location and orientation of the surface relative to referenced datums, and the lower segment with no datums would simply refine form of a surface - in this case, surface flatness.
Some could ask, why not to apply single segment profile of a surface relative to A, B, C and flatness callout separately? I would answer there are sometimes many ways to express the same intent in GD&T we know.
RE: Profile with no datums
I agree, it would be the best to wait for Jim's clarification on that.
My vote is that profile without datums does not control size as long as it is not applied to a surface of a circular/cylindrical feature and the diameter is speciifed as a basic dimension. In this case the size (diameter) is obviously controlled.
RE: Profile with no datums
Frank
RE: Profile with no datums
Peter Truitt
Minnesota
RE: Profile with no datums
So, what I was referring to was a net body, rather than individual feature(s) controlled by a profile control without a DRF. Indeed, a single feature with a profile sans DRF would just be controlled for form (flatness, straightness, cylindricity, circularity, or mathematically defined).
That's not to say that it's ideal in most cases; I have seen it properly used a couple of times, though it could as easily/effectively have been done with datums referenced.
On Pete's OP then, if he had a simple block part, datum A is a face, Datum B the longer edge and C the short edge and he had a profile callout on say the edge opposite of datum B without referencing any datums, then the location of the surface is not controlled, just the form. The product specification would then be incomplete. If it had a +/- size tolerance to locate the face wrt a visibly opposed face, then the profile control would again only be controlling form, but at least the face would have been located.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile with no datums
Dave D.
www.qmsi.ca
RE: Profile with no datums
Depending on the systems (hardware & software), datums aren't useful or practical in the inspection process. Basic function contact-CMMs typically are used with datums in reasonably-skilled environments, but I've seen a lot of situations where the operators didn't use them at all; it took a lot more data collection, and I'm skeptical of their results, but they were doing it. There's a lot of different data acquisition systems out there beyond the basic CMM. We both have to be careful when suggesting things to people that we consider the entirety of the metrology industry, and not just one specific technology.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile with no datums
Just so I can fully understand; if I had that same block I referred to earlier, but instead had a surface profile of the edges all around with no reference to datums. And it also had two basic dimensions giving the length and width of the part, then that would control form and size correct?
RE: Profile with no datums
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile with no datums
Frank
RE: Profile with no datums
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile with no datums
Frank
RE: Profile with no datums
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile with no datums
Subtle compared to Don, anyway. ;^)
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Profile with no datums
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile with no datums
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?
RE: Profile with no datums
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Profile with no datums
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.
RE: Profile with no datums
Frank
RE: Profile with no datums
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
RE: Profile with no datums
I agree with you, I was actually thinking in terms of compound angle surface definition cases.
Frank
RE: Profile with no datums
For only 2 days of focusing on Profile, you're doing well.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Profile with no datums
Peter Truitt
Minnesota
RE: Profile with no datums
RE: Profile with no datums
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com