Tolerance strategy for precision molded parts

Interesting. Trying to picture this -- you say you want it controlled within a specified area, but 3 dimensions are given. Is it meant to be a volume and we're controlling the portion of the contour that passes through that volume?

Good stuff, though. I like these proposed scenarios that stretch our brains!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

At first I had it 40X40 but then I realized that surface profile is 3 dimensional. Think of is as a narrower zone wraped around the basic contour that is permitted to translate (not rotate) within the larger overall zone (effectively controlling smaller relative local sizes, offsets, or other geometry.

Paul

I think you have one 40 too many.

If you imagine tolerance zone translating along the vertical axis, it (kind of) becomes irrelevant.

• http://files.engineering.com/getfile.aspx?folder=4741f4e8-fe5c-4202-a883-4d

Certainly the "All Surfaces [Profile...] is controlling the 3D contour of a part... I think that a subset of that area would have to be a 3D volume (cube like or spherical) considering contour detail that has size, offsets, or other geometrey relative to depth don't you think?
Paul

Paul,
Could you post a sketch or something showing application of such callout?

The profile control is 3-D (just as flatness is), however it can only be applied per unit area of the SURFACE (just as flatness can be refined per unit area), not per unit of volume. You are not controlling the tolerance on a volume.
Otherwise, yes, the refinement seems fine to me. Your refined zone floats within the primary zone, and is not required to be centered on the nominal, as the primary zone is. The refinement prevents sudden changes in the surface. I like it.

Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com

pmarc,
The specification is intended to be a title block tolerance for precision moldings or castings akin to general tolerance allowances for categorical lengths. Its drawbacks are that it doesn’t have as many defined levels but its benefits are that it is defined relative to the datum reference frame and the tolerance value does not depend on how far a feature is from the chosen origin but rather how it conforms to the upper segment zone ALL OVER and how it conforms to the refined lower segment(s) zone(s) for contour oriented to the datum features (as necessary).

MechNorth,
ALL SURFACES or ALL OVER controls contour in planes XY, XZ, and YZ... don't you think?

Paul,

This is an interesting idea. You're taking the concept of Flatness applied on a unit basis for planar surfaces, and extending it to apply to Surface Profile on curved surfaces. So any given "patch" of a certain size must conform to the 0.2 Profile tolerance. Is that right?

I agree with CH and Jim that it should be a 40 x 40 area and not a 40 x 40 x 40 volume. I understand that the surface exists in 3D space, but the surface itself is still a 2D entity.

Having said that, defining the 40 x 40 patches on a curved surface will be very complicated. The patches would all have the same area, but each would have a unique shape. It would be kind of like cutting out a 40 x 40 piece of tape, and then sticking it on the casting in different places. The math for this would be pretty far out, because you're taking the idea of unit-based Flatness on a planar surface (Euclidean flat 2D space) and extending it to unit-based Surface Profile on a curved surface (non-Euclidean curved 2D space). I think we might need to use Gaussian coordinates, which is a type non-Euclidean geometry used to deal with curved spaces. I wouldn't want to have to explain this to the guy at the casting supplier ;^).

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

Paul,
I am with others saying that one 40 should be removed from the FCF.
I also like Evan's comment about difficulties in application of profile per unit area concept to curved surfaces. I suspect that none of ASME standards clarifies this in a way that the concept could be safely transferred to a real world application.

OK... I think I see the objection the refinement area boundary is always planar but it can be viewed cross-eectionally from various perspectives xy,xz,yz,... Is that correct?

So if we drop the 40... do you think this has the potential to constrain basic geometry to a smaller refined profile tolerance in a specified area while constraining the over all to its specified boundary?

Paul

A quick side note -- when identifying a datum target on a countoured surface, the datum simulator is supposed to be made to follow the basic contour, not merely be a flat surface. So just extrapolate that idea to this profile concept, and still use 2-D area dimensions for a contour (thus keeping only two of the 40 dims).

I think that summarizes much of our reaction to the OP, although it is interesting to imagine trying to describe the area of a really crazy contour with only two dimensions.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com

Rather than 40 x 40, why not 40DIA ? Is that better, worse, or the same?

Peter Truitt
Minnesota

Spherical diameter?

So this is the corrected concept.

• http://files.engineering.com/getfile.aspx?folder=b27754a7-3052-4c81-80d2-ab

Paul,
As long as a reader of this note knows exactly how the smaller areas (40x40, 200x200, 20x20) are established on surfaces other than just nominally parallel & perpendicular to A|B|C DRF - which I think may be an issue in case of molded parts due to drafts - I think both options are fine.

Checkerhater:

I'm thinking of the surface patch where it is limited by a basic sphere.

Peter Truitt
Minnesota

Peter,

I understood you; I am just not sure each and every shop out there will do.

I am always suspicious of forcing some mathematical trickery on people who are trying to make a living using limited set of tools.
Same with very vague or broad requirements like “profile of everything all-over” or “perfect part required at MMC”.

Yes, our drawings are supposed to be as process-independent as possible, but if you don’t know how your requirement will be measured, don’t put it on the drawing…

Thanks, Checkerhater,

I guess I am not sure how to check either Paul Jackson's refined approach or the variation I asked about other than employing analysis software that would analyze a point cloud. I keep thinking about the benefits of round vs. square tolerance zones, but I know that we are not talking 2D, here, such as the location of a hole on a flat surface… Nevertheless, it seems that a sphere will give more tolerance than a cube while maintaining the functional design requirements.

Peter Truitt
Minnesota

Maybe in few more years when CMMs will be as common as calipers…

Right now I am not sure if you can program one to check “round” spots.

With the pace of product development these days where prototype cast or molded structures are printed on a 3D plotter, patterns are grown and generated from the solid model, casting or molding prototypes are delivered before the layout stacks are completed and the profile inspection comes in the form of whisker plot comparisons to the solid model… we could really use a simple yet robust method to constrain the cast or molded geometry, that recognizes the inherent variability of cast and molding processes in the contour extremities, but also recognizes limitations on local contour detail for function and material weight savings.

Simply using overall profile tolerance means that if one has those additional concerns… then additional profile constraints need to be detailed on the drawing. If those details conflict with the predicted variability in curing profile change… then the contour may need post processing.Historical cast tolerance methods “shrink equation allowances” have their unique problems in that they are origin dependent, i.e. if two features have functionally similar requirements yet one only one is found non-conforming due to its proximity to the measurement origin… the strategy often fails to support function. The ISO categorical tolerance strategies are sensitive to dimension magnitudes regardless of their proximity to the origin… but like “shrink equation allowances” they fail to establish the fundamental coordinate system necessary to constrain measurement orientations and translations to a common origin.

The potential advantage of this composite profile strategy is the establishment of that datum reference frame, a uniform tolerance relative to the component extremity, and window refinements “as necessary” to limit subordinate sizes and contour detail… one caveat is that those refinements need to be complementary to the predictable variation for the size of that window. Another is that readers of the drawing need to understand composite feature control frames to understand the translation liberties in the refinements.

Many designs position datum feature targets on a cast or molded structure so that the coordinate system X0, Y0, Z0 reference is on the extremity of the structure and… if the requirement for lowest functional variability is nearest that origin and increasing as it departs from that origin I would say that is good… but from what I have seen in castings over the years that is seldom the case. I typically try to define targets positioned on the extremities to stabilize the structure but then position and equalize translational targets on the structure so that the origin nearest the structure center. By doing so the effects shrink variation relative to the origin are minimized. Sometimes I put all of the targets in the one cast section corresponding with the most critical cast attributes to minimize mismatch and closure variability and sometimes I put them spanning sections to equalize that variability as function requires.

I envision that this profile strategy will be inspected just as it is written where there is a global or over all tolerance wrapped bilaterally about the basic contour that all points of the surface must reside within, and then a narrower bilateral zone of given window boundaries… that is oriented to the composite DRF as specified but able to translate within the global zone… that all points of the surface within the window must reside. In effect it can be like using a cross-sectional overlay with overall boundaries, oriented and located to the DRF… and another cross-sectional overlay with narrower boundaries oriented to the DRF but able to translate within the larger zone.
Paul

You may wish to invoke the ALL OVER requirement instead of using "ALL SURFACES". It is covered in ASME Y14.5-2009

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion

I am still thinking about cubical vs. spherical tolerance zones. One of the attached illustrations shows a cube, with a basic surface and its boundaries, similar to Paul’s. The other illustration (same PDF) shows the same basic surface and boundaries, but clipped by a circumscribed sphere. My contention is that the boundaries of each are likely to work equally well, functionally, in the design. (The same argument as square vs. circular tolerance zones.) The table below the illustrations show how the surface area of the two boundaries, as clipped by the sphere, have a more linear variation from the basic surface than the cube. Don’t ask me to explain how one might measure either of these in practical applications, because I don’t know. Does this make sense to anyone else?

Peter Truitt
Minnesota

• http://files.engineering.com/getfile.aspx?folder=127fd4a6-e9cc-41f6-929a-a9

J-P, my automatic assumption (perhaps erroneous, but there it is) is that the areas (40x40) would be based on xy, yz, xz planar projections as established by the DRF (a more evolved concept in '09). Topographically, as viewed from one of these planes, the area would be equal, however the actual surface area would vary with the topography.

Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com