Enforcement of inspecting general profile tolerance 5

[prdave00]

In thread1103-276931 the merits of having a general profile on a print controlling features not critical to the part's function were discussed.

Our company is starting to adopt the practice of specifying a general profile tolerance and add a note to the print stating: ALL UNDIMENSIONED FEATURES SHALL BE MANUFACTURED BASED ON THE 3-D CAD MODEL FILE "_____". ALL UNSPECIFIED DIMENSIONS ARE BASIC ASSUMING [SURFACE PROFILE SYMBOL|.010|A|B|C]. Furthermore on some prints for milled parts stipulate conformance to be checked by a CMM scan at the start/end of the batch and at a tool change. I would argue that the absence of this stipulation makes it only an inspection point at the final QC check.

Our parts are complex requiring 5-axis CNC milling / surfacing and we are also transitioning to injection molding. I'd say 50% of the features are .01 to .125" internal and external radii primarily for cosmetic purposes.

My question has many layers...

(1) How does one ensure the same algorithm / methodology is being used to inspect to the general profile across multiple suppliers? Is there a "rule of thumb" about the minimum number of points to probe on curved versus flat face?

(2) The cosmetic radii and such aren't critical enough in my opinion to need to be checked (and am beginning to doubt they even are in lieu of the supplier focusing in on the large features / surfaces), but I can't say this about all the radii. We could waive the general profile tolerance on some of these radii but (a) the print would be messy where there's a bunch of notation flagging features that can be ignored and (b) it seems to defeat the purpose of a general tolerance if it doesn't imply across the board for non-critical features. Any thoughts?

(3) We're transitioning one milled part to molding, but the injection molder we are using says they won't accept a general tolerance on the print. Their argument is that by having the tolerance they are legally obligated to inspect it and certify to it. They say this gives the customer latitude to reject a batch of parts because there is a flaw (void, sink, etc.) in some parts that would make them nonconforming. Anyone else encountered this? I'm actually surprised our machine shops haven't thrown this argument at us.

I'm starting to convince myself that having a general profile tolerance specification is no better than stating "best effort" based on the CAD model. First off I'm doubtful that our fabricators, as reputable as they are, are actually checking against this specification instead of just relying on the accuracy of their equipment / tooling. Secondly, my experience with our injection molder has made me realize how many loop holes a general tolerance might provide for both the fabricator and customer to get out of a contract. Am I being too cynical here?

My apologies if this topic is covered in past post. If that's the case, I'd appreciate a redirect. The term "profile" in the search engine gets plenty of hits.

 

[dingy2]

JP:

If one was inclined to place a default profile tolerance on non-functional dimensions, it should be different from the ones shown in the feature control frames, usually larger.
Theoretically, I agree that if one had a larger tolerance shown in a FCF (highly unlikely), I would not increase the default profile tolerances.

Here is what I have found from a Quality perspective. Default profile of a surface tolerances are as tight as possible and include both functional and non-functional features. I have never seen a FCF with a larger tolerance than the default but, I imagine, there must be some around.

There is a definite blur in the area of default GD&T and personnel in process & quality planning better realize this fact. They should not expect that functional features have stand alone FCFs and should begin, as we did so many years ago, by asking how the part fits and functions so that we can determine which features require ongoing control. Don't ask which dimensions are important because I already know the answer to that one.

I will rescind any statement I made regarding default positional tolerances. The more I think about this, the more inappropriate the use of this practice. You are correct, the term is MMB from the 2009 standard.



Dave D.

http://www.qmsi.ca

 

[prdave00]

Jim: I read through Y14.8 this weekend. It's a pretty quick read (especially compared to Y14.5) & gave me much to consider regarding tolerancing of molded parts especially as dimensions apply with respect to the parting line(s). Plus it was a good refresher on molding terminology. Now I'm curious if our CAD package supports the symbols introduced.

Just for the record I don't think GD&T = critical, but I may be guilty of implying that an explicit dimension (whether linear or using a FCF) = critical & therefore worthwhile to verify on an ongoing basis. I'm also trying to figure out if I was arguing in support or against my colleague's attitude toward this subject.

I appreciate the examples you and KENAT gave, but also considered that without a general surface profile tolerance a machine shop might consider any surface/feature not dimensioned as superficial and alter how they would fabricate the part (e.g. use faster speeds/feeds, forego finishing cuts, etc.) regardless of how capable their equipment is. Same thing can be said about having a general surface finish call out. I wouldn't expect a shop to break out the profilometer to check every surface every time they make a batch of parts, but I would expect them to keep an eye on gross changes in appearance. After all, if they can get away with making something faster then the customer may have some to gain in terms of price. I can also see this as a good reason why thought needs to go into the general surface profile, roughness, etc. specifications since you could be driving up the price of the part without gaining anything in terms of function or aesthetics.

Can someone recommend a forum to discuss whether or not my wife is correct that I should stop chatting with strangers online about drafting?

 

[MechNorth]

PRDave, I don't anticipate the CAD packages being up-to-date for Y14.8 and Y14.5 any time soon. You may need to manually pop them onto your drawings and pray to the gods of associativity that they stay in the right place. As you are now thinking through it, so did many of us on this forum; our "classic" assumptions and beliefs about drafting weren't always correct or helpful, and sometimes were self-destructive. As people get further into GD&T, they tend to find that it needs to be an all-or-nothing perspective on their in-house and supplier practices. As for how your suppliers will interpret the drawing, that's always a kind of crap-shoot. You can mitigate some of the risk by validating their GD&T (Y14.5) credentials, and by actually sitting down over a cup-o-joe and making sure that they understand how the general tolerance works. I know that seems like a step backward, actually having to confirm the suppliers' understanding, but it is a first step and should only need to be done once whereas explaining what you want on each drawing is a cyclic process.

I've dealt with customers and suppliers who didn't like GD&T or various aspects of it and wanted to customize it for their convenience (i.e. make it a hybrid of old practices/folklore and GD&T). One recently was adamant that there was no value in adhering to GD&T, that the "traditional" ways were perfectly adequate. I asked him how he liked driving his old Model-T Ford. If everything stayed as status-quo, then we'd never have the comparatively safe, economical, efficient vehicles of today. I'm sure that Henry Ford didn't find it "easier" to do the assembly line, but it sure improved things.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

Jim:

Is figure 3-29 in the 2009 ASME Y14.5 reflecting a mix of both linear and geometrical tolerances the "hybrid" that you mentioned above?

Here is what you stated about this figure - "Fig 3-29, yup, I think this is a good mix for this part's functionality."

Maybe you can explain in more detail what you mean by the word "hybrid".

Thanks



Dave D.

http://www.qmsi.ca