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Goal R.002 *Pulls hair out!* 5

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ModulusCT

ModulusCT

Mechanical
Nov 13, 2006
212
US
OK, so I was told to do the following by my supervisor today. I really hate having to tell him that he has no idea what he's doing and that he doesn't know the applicable drafting standard, so I thought that it would be better to simply offer a solution that does jive with the spec.

He wants to indicate R.002 +.003/-.000, but with an emphasis on trying to produce the radius at .002 rather than taking the total permissible error and shooting for the middle. His solution to this problem is to say .005 MAX; GOAL R.002, but I hate this personally because the word GOAL is not mentioned in the spec as an acceptable descriptor for tolerancing a part. In fact, I'm pretty sure there are parts of the spec that say not to do things like this.

Any suggestions? Is there a way that you can say, what my boss wants the drawing to say by using acceptable symbology and terminology?
 
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powerhound,

I'm not thinking that you're trying to say if the target dimension is not met then the part is not good. I'm just saying that there is nothing enforceable in the implied target. If the designer specifies 0/-0.5 and gets a batch of parts that are all at -0.499, and the implied target was completely ignored, the designer has no basis for objection. I believe that you would agree with this, but I have met designers who would have a different opinion.

I'm not inferring that the designer might specify a unilateral tolerance when they really meant equal bilateral, just because they didn't know how else to express it. But I have seen instances where they would specify an unequal bilateral or unilateral tolerance for other CAD-related reasons. One was that a fit requirement had changed and they didn't want to change the model. Another was that the assembly was modeled with zero clearance, and so the tolerances were all unilateral +0/-.XXX. I'm not saying that these are good design practices, just examples that I have seen.

I would agree with you that the +/+ and -/- specs are poor practices, and often generate confusion and scrap parts. I have seen similar problems with unilateral tolerances, where the dimension was programmed at nominal and a batch of borderline parts was made.

As you can probably tell, I'm coming at this from a QA/inspection perspective. We don't care what the designer wanted, all that matters is what they'll accept ;^). I don't doubt that most machinists would infer the designer's preference and try to make the dimension as close to the nominal as possible.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
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As Kenat posted and others agreed, there is no standard that tells anyone reading a print how to interpret the intent of a particular tolerancing scheme other than "make it in this range". Unless you provide a different distribution requirement, there is nothing to guide the machinist away from your baseline normal distribution.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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...nor is there anything to guide the machinist towards your "normal" baseline distribution.

By the way, what is "normal" and what criteria is used to classify it as such?

This must just be a Texas thing. It looks like everyone else is from the north. I guess we just do and think about manufacturing differently.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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SSG, U.S. Army
Taji, Iraq OIF II
 
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KENAT,

"Sorry Powerhound & Belanger, you still haven't convinced me. It seems you're still trying to take advantage of aspects of the human psyche or something rather than what the standard explicitly says."

This is a negative spin. Using the phrase "taking advantage of" implies something bad. Why not say we're relying on the machinist to use common sense and realize that if the designer wanted to use a +/- dimension then he would have, but since he didn't there must be a reason. I'm not arguing that my interpretation is in any standard or that it can even be legally enforced, I'm only saying that from a machining standpoint 0/-0.5 tells me something different than +/-0.25. QC just checks to see if the part is in tolerance. The part is already made by that point so it doesn't matter what the goal of the dimension is. That has to be addressed at machining.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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SSG, U.S. Army
Taji, Iraq OIF II
 
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Ah come on Powerhound, I would like to think we've fought alongside each other in these virtual GD&T trenches long enough not to let this point come between us.

I wasn't intentionally negatively spinning it, after all we say things like 'take advantage of MMC' all the time and I don't think that's a negative connotation is it?

For part drawings I tend to have to work toward the 'level 3' type category where they can be sent to any more or less competent shop and we'll get good parts. I have little input in vendor selection for prototypes, and even less for production especially a few years down the line. So I tend not to take advantage of things like you're alluding too with your take on unilateral tolerances.

I'm sure some, perhaps many, machinists will think like you're saying but I'm far from convinced it's all or even most and as I think we both believe it's not explicitly in the standard either way so not enforceable.

Have a good day Powerhound.

Oh, and I'm not from the North;-)

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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I heard of a study many years ago where non-symetrically toleranced parts were measured coming from many different suppiers. It was said that the results showed that nearly every suppier aimed for the middle of the tolerance limits. If you are generating a 3D solid model, where should the surfaces lie? If you are fitting a point cloud to a CAD model, what will you get for results?

This makes me wonder about bonus tolerances on holes. Where is the middle of the tolerance zone? What should a supplier aim for? If the designer wants Cpk, what are the implications?(Just ignore this since it is off topic.)

Peter Truitt
Minnesota
 
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From the point of view of a former production machinist, precision sheet metal worker, and welder (and son of a career uber machinist (yes,that matters)), and design and GD&T guy, and now learning more about inspection person, same limits means same effect..

Stating same limits in some way other than equal bilateral often means either a constrained or lazy designer who is not allowed to, or doesn't want to, move the feature on the 3D model when it is discovered that it is not in the center of the optimal tolerance limits. Once in a while a situation will be encountered for which an unequal tolerance should be specified, but most of the time equal bilateral is better. While the standard allows a variety of equal, unequal, and unilateral specifications (& should continue to do so), it's notable that no standard describes that the effect being different unless the limits actually differ.

If you wish to think that two specs that state the same limits in different ways actually have differing effects, then of course you can continue to wish that this be so... In some cases with some people (who think like you do), there will be a difference. A difference that cannot be counted on is not a difference I want to acknowledge though.

If you really want a difference then ask Don Day about his proposed "delta modifier"... Now that would be a way to make those wishes be true. Until then, I like equal bilateral unless I run into one of those very rare cases that unequal happens to be the better choice. Don hasn't brought up the delta modifier for quite a few years now, so it doesn't appear that it will be adopted any time soon. I would probably be in favor of adopting it, but for now I think there are bigger issues that need improvement.

Dean
 
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I'm pretty sure I never said that stating the limits in different ways has a different effect. In fact, I know I never said it. I said it communicates a different message.

Man, why do I keep saying one thing and everyone hears something else? I feel like I've had to state and restate the same thing over and over again because I keep reading rebuttals to arguments I'm not making.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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SSG, U.S. Army
Taji, Iraq OIF II
 
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Oops, in that case we may have fallen into some illogical religious battle a bit like "right to Lift" V "right to choose" situation.

In which case apologies if I've misinterpreted you and how about we call it a day before it gets ugly.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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"I'm pretty sure I never said that stating the limits in different ways has a different effect. I said it communicates a different message."

what's the difference?
By 'communicates a different message', I assume you mean the machinist should look at the 0/+X tolerance in a different way as he would look at a +X/-X tolerance. Hence, you expect a different result(not aiming for the center of the tolerance), or a different effect.
This may be a semantics discussion though. If I'm wrong, please educate me.


On a sidenote:
To everyone who says they rarely encounter +/+ or -/- dimensions: they are used quite a lot in fittings (bearings, axles, etc.)
At least if you work according to ISO standards. I don't know about ASME.

NX 7.5
Teamcenter 8
 
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Sorry KENAT I wasn't referring to you. I was referring to Dean's comment about a different effect.

When I see the word "effect" in this context, I'm taking it as a different outcome. And from that perspective, parts that are in tolerance have one outcome, parts not in tolerance have another outcome. I thought he was saying that I was arguing a good part/bad part argument. Upon reading Waltereke's response I think I overthought that too much.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
 
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poerhound,
I'm in your camp here. The dimension is what is being asked for, the tolerance what will be accepted.
Simple communication and in no way should be confused with lazy design. Some designs may perform better at a dimensional extreme than others.

Technically, the glass is always full.
 
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This information is disappointing. So essentially, my tolerancing method is irrelevant. The machinist will use the total tolerance and shoot for the middle. Yuck.

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!
 
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I believe your tolerancing method is fine, as does a single other person on this thread. Tell you what, send your parts to Texas to get made and they'll be made by folks who care what you're trying to say.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
 
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They only make BIG parts in Texas. Try Rhode island for .002-.005.

Peter Truitt
Minnesota
 
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Hold on, everyone ... back up to Dean's post:
"Once in a while a situation will be encountered for which an unequal tolerance should be specified, but most of the time equal bilateral is better."

Well, what if I have one of those "situations" where an unequal tolerance makes sense? You're all telling me (and a minority of others) that it ain't gonna happen, because we all know -- wink, wink -- that the manufacturer will simply aim for the mid-point.

In other words, then, there really is no such situation "for which an unequal tolerance should be specified"!!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
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Well, perhaps if it's based around standard tooling - classical example being the typical drill hole tolerances.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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Powerhound,
I wasn't replying to anything you said in your posts. I was commenting about what I think of unequal bilateral and unilateral tolerance zones.

John-Paul,
The cases I'm referring to for which an unequal tolerance would be OK are those such as when a long narrow plastic part may have a slight curve in one direction which is acceptable since another part in the assembly will tend to straighten out the plastic part, but if the plastic part were to bow the other way that would not be acceptable... There may be a feature towards the middle of the part which is best handled with an unequal tolerance to provide more tolerance in the "acceptable bow" direction and less in the other direction. This would be done because it may not make sense to model the part bowed in the acceptable direction, but it's OK to allow the tolerance zone boundary to be unequally disposed about the straight ideal geometry. Once that unequal spec is there it is very likely that manufacturing will make the middle of the tolerance zone their target. They may do that not by moving steel within the tool, but by tuning the cooling in the mold tool to make parts that bow slightly in the acceptable direction. This is an example of what I meant by saying "Once in a while a situation will be encountered for which an unequal tolerance should be specified".

Manufacturing will tend to do things in a way that is better for them... If the tolerance is tight relative to their capabilities then they'll shoot for the middle of the zone, or possibly stay towards a metal safe region as was mentioned in another post, to help ensure an acceptable part. If their process is capable enough to given them more confidence then they may shift to one side of the tolerance zone to allow a cutting tool or mold tool to wear, bringing the feature through the middle of the tolerance zone, then eventually to the other side of the tolerance zone, in order get more useful life from their tools. To hope that manufacturing will target a stated value that is not centered within the limits may be OK, but it's not a hope that I hold out much hope for.

Dean
 
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