Profile and MMC 1

[jerry1423]

Would it be acceptable to have the profile of a surface as "zero" at MMC?
Then as the size devates from the MMC the profile tolerance would increase.

 

[drawoh]

jerry1423,

In a copy of the standard or your textbook, look up unilateral profile tolerances. You do not call up MMC, but you accomplish the same thing.

JHG

 

[pmarc]

Using M modifier in tolerance portion of profile of surface feature control frame is not allowed according to Y14.5 standard.
M can be only applied to feature-of-size elements while profile of surface as it is called controls surfaces only.
However M can be put in a datum portion of profile of surface feature control frame as long as referenced datum feature is a feature-of-size.

 

[PaulJackson]

Since profile is capable of controlling size and location simultaneously while constraining form within its boundaries... for a circular feature you can think of it as a control that is both Zero at MMC and Zero at LMC for form and location. When the feature is at its median size with perfect form it has its greatest liberty to translate.

 

[joedal]

Bottom line is: NO! You cannot use the MMC modifier in the tolerance compartment of the feature control frame. When a profile tolerance is applied with no datum references it is solely a form control. The actual size of the feature must be specified in some other manner, which then bounds the overall size. (Could be a plus/minus dim for example.) If the surface to which the profile control is applied is part of a "feature of size" then "Rule #1" would apply to that surface. Any part size departure from MMC would allow for form error on the surface up to the amount limited by the profile control. If, on the other hand, the part size dim is BASIC and the profile control references the opposing surface as a datum, then the profile tolerance will control BOTH size and form. In this case "Rule #1" still applies. It is when a profile control is applied to a surface that is not part of a "feature of size" that "Rule #1" does not apply. A surface, to itself, has no size therefore has no MMC condition.

 

[MechNorth]

joedal, surface profile without a drf can control location of a surface (or size of a boundary) as well as form.

Also, in Y14.5'09, a surface (planar, or irregular boundary) can now be used as a datum feature, and the resulting datum can be referenced in a drf at MMC. Of course, the tolerance within the profile fcf still cannot have an MMC or LMC modifier There are some good figures in the book.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[prdave00]

Jim,

Where does specifying a surface profile tolerance without a DRF leave us in terms of inspection of a complex surface?

Also is this explicitly stated somewhere in the '94 version of the standard? I'm on my 20+ time perusing section 6 and I only see section 6.5.6.1, Fig 6-20, & 6-21 where a surface profile without a DRF is applied to planer surfaces (similar to how a flatness tolerance my be applied).

I'm also aware of no DRFs being specified in the 2009 version when it comes to composite profile and combined tolerances, but I'm getting ahead of myself.

 

[MechNorth]

PRDave, the profile control is the allowable deviation from the basic geometry. Whether the surface is complex or simple becomes irrelevant. I don't recall the standard indicating inspection methodology for a profile control, but the graphics support an interpretation that the inspection should be normal to the design (basic) surface as opposed to a constantly vertical probing or probing normal to the actual surface. Now the trick is that the inspection essentially needs to be done as a point cloud so that all facets are inspected, establishing an envelope and best-fitting it to the basic geometry. It's easiest with a CMM though it can be done with more difficulty using bench-top methods (though I haven't done it myself). 2009 does a better job of the profile control, but it's still not complete ... another story for another day.

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).

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[prdave00]

Thanks Jim. My colleague is trying to save parts that failed a profile tolerance check using a CMM. She got the notion somewhere that deleting the DRF from the FCF would increase the amount of form error allowable. I don't understand her logic but wanted to make sure my story was straight. I advised her that the DRF instructs the inspector how to immobilize the part (whether with hard gauging or soft gauging as is the case with a CMM) so that they know where the surface is in space with respect the the simulated datums and from there they can link points in space back to the true surfaces on the CAD file. You seem to be suggesting the opposite, i.e. the surface is probed to generate a point cloud and then the 3D model is moved around until it fits. Di I have this correct? This kind of stuff drives me crazy - just when I think I have it down, my confidence gets shaken.

 

[MechNorth]

I understand what you're going thru; we've all been there. In the absence of a DRF, the CAD geometry essentially floats within the data point cloud. With a DRF, it's locked down in space which makes it much easier to set up & repeat the inspection. If there are specific functional relationships (and there should be if a DRF is established) then arbitrarily dropping the DRF is invalid and yields different results. Hope that helps.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ptruitt]

I took a GD&T class recently from a PH.D guy here in Minnesota who is pushing profile tolerancing. So many of the GD&T symbols boil down to profile of a surface.

 

[axym]

ptruitt,

I believe I know the fellow in Minnesota that taught your class. He certainly does have some strong opinions about profile.

It's true that many characteristics in Y14.5 can be thought of as special cases of profile. The tolerance zones for flatness, perpendicularity/parallelism/angularity of a planar surface, cylindricity, and total runout can be duplicated using permutations of surface profile.

Some people find the idea of a reduced set of characteristics to be simpler, others find it more complicated.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

ptruitt,

I like profile tolerances a lot, however, your comment is extremely general. We would have to write out some problems, and see how everybody would solve them.

JHG

 

[MechNorth]

When I teach, I explain and relate profile tolerances to other controls, establishing along the way that profile is an encompassing control. Yes, it can be substituted for the majority of applications where other controls are applied, however it requires an extremely high level of understanding to do it successfully. It's like pointing at something and saying "blue" ... do you mean the blue water, blue sky, blue gatorade, blue eyes, chemical persian blue powder, a heat treat process ... ? The contest is critical and you have to know and appreciate/consider the surrounding factors.
As Evan indicated, many of us on this forum will know the instructor you indicate. He presents some very interesting questions about profile in particular that are glossed over in the committee and standards so far. As more people and companies evolve in their use of profile, things change and improve wrt the definition of the control.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ptruitt]

It seems as though I could take a typical GD&T drawing and simply replace all the flatness and perpendicularity symbols (maybe others, as well) with the profile symbol. What am I missing?

 

[axym]

ptruitt,

I suppose that you could replace all of the Flatness and Perpendicularity symbols with Surface Profile and achieve the same tolerance zones and constraint behaviors. I'm not saying that I recommend doing that, but you could.

There are other characteristics that can be converted into a variant of Surface Profile, but it gets more complicated. Parallelism and Angularity for planar surfaces can be expressed as Surface Profile, but you need to make sure to relieve the location constraint. This can be achieved by using a composite FCF (the lower tier only orients the tolerance zone to the DRF and does not locate it) or by combining the Surface Profile FCF with a plus/minus tolerance.

It is also possible to express Cylindricity or Total Runout as variants of Surface Profile, but this is even more obscure and controversial. The Surface Profile FCF must be combined with a plus/minus tolerance.

When characteristics are "simplified" into variants of Profile, the drawing reader must have a much deeper understanding of degree of freedom constraint concepts to derive the proper meanings.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

It seems as though I could take a typical GD&T drawing and simply replace all the flatness and perpendicularity symbols (maybe others, as well) with the profile symbol. What am I missing?

Your design requirements.

I have frequently done a composite FCF with a profile tolerance and either a parallel, perpendularity or angle specification. Typically, the profile tolerance is sloppy. The second feature is what I am really concerned with.

Don't forget that the profile controls more stuff than flatness and perpendicularity. The whole point of GD&T is to control the stuff you need, and to not control the stuff you do not need.

JHG

 

[axym]

drawoh,

Combining Profile with another characteristic is a great idea, that allows the largest functional tolerances. I hate to be the terminology police but that's never stopped me before ;^). If you combine a Profile FCF with a Perpendicularity, Parallelism or Angularity FCF, then it's called a multiple single segment FCF. Not a composite FCF.

You're right that Profile is capable of controlling more than just the form (e.g. Flatness) and orientation (e.g. Perpendicularity) of a planar surface. It is also uniquely capable of controlling the location of the surface. But things can be manipulated to make Profile control form only or form and orientation only. If no datum features are referenced, then Profile will control form (Flatness) only. If the only datum feature referenced is nominally square to the considered feature, then Profile will control form and orientation only (Flatness and Perpendicularity).

Again, I don't recommend replacing Flatness and Perpendicularity callouts with Profile, just because we can.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca