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GD&T doc 2

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Nice find. Too bad it is uncredited.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?
 
Oops, spoke too soon. This was a repent from Tec-Ease (Don Day).

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?
 
There may be. I have not looked through them all.
This doc was sent to me a while back. I was going through my docs on my laptop (getting ready for upgrade) and found it.

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion
 
Thanks Chris, I sent it to all the drafters as a refresher for those who have taken our GD&T class and as a preview for those who need to.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
 
It's almost as if Don has a bias toward Y14.5 over ISO. Did anyone else pick up on that? ;^)

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
Yes, axym, I thought it was kind of a cheap shot. We are all more comfortable with the devil we know.
That is a nice site and while I can not agree with every statement I think he should be commended.
 
There is good & bad in both systems. It's too bad neither is completely willing and motivated to form a single truly international standard that embodies the best of both and addresses the deficiencies of both.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
I don't know their system well enough to be real critical. I am sorry to hear that that things are not going well, though. I am very happy with our 2009 version except for the continuation of this feature of size issue. I see no logical prohibition on being able to reference a cone as a feature of size and invoke MMC. Nor do not see how a cone is not a feature of size. At least now I can with a profile-position combination. seems more complex than it needs to be.
 
fsincox.

How would you define the size of a cone, and what potential advantages do you see in referencing a cone as a feature of size and invoking MMC? I'm not sure that I follow the logic on this one.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
Axym,
My current application is on mixed flow fans. The shroud and impeller are shaped like 2 tapered ice cream cones, one inserted inside the other. The fans are designed to cool military stuff, these are ITAR applications so no pictures.
The impeller is the part that has the fan blades and spins at a high rate of speed. Balance is a consideration, here. The shroud is just the shell around, no spinning. The datum axis on both parts is established on the associated bearing mounting features.
I am maintaining the shroud inner conical wall can benefit from MMC, I will assume the benefits of MMC are known, here? I want to create a VC boundary which no element will violate. Tolerances are tight and manufacturing complains. I am trying to give every benefit I can. Larger taper on the shroud means it can be off location more, simple as that.
I can really relate to your comment from another thread: “The way I like to think of it is that we don't really choose the functional datum features, we have to discover them.”
You don’t have to like the way things work and may not be able to redesign them, it just is what it is. My feeling is it IS our job to document it.
The dimensioning scheme we use currently is at 2 opposing points (really, circles) on the taper and either the depth or diameter is basic.
2009 seems to be heading to profile-position combination for these applications, we are on 1982. I am 53 now, I love what I have seen in 2009, maybe before I retire I will be able to work at a place that actually uses it.
 
A cone is not a feature of size? Says who? Does it not have apposing elements? Does it not have an actual mating envelope? Can the derived median line not be derived?

A cone cannot be referenced at a datum feature? Oh man have I been making a lot of mistakes involving injection mold design. :)
With regard to MMC, lots of drafted pins may fit into drafted holes with clearance or not. In fact, I have used MMC for the interference fit of "plastic" drafted pins into drafted holes.

You can tell that I don't subscribe to the theory. I am stuck on the reality. :) But I do go along with cones not being a "classic" feature of size and the dimensioning can sometimes be creative. Most fundamentally, I like to dimension cones with BASIC and ler profile control size and form. That does not permit MMC, but that is a lot to get into here.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
 
Norm, is there one single size dimension perpendicular to the axis? To me, it's more like an infinite number of local features of size perpendicular to the axis ... or even skewed to the axis maybe ... ? It would definitely fit within the '09 definition of an irregular feature of size at least. And now it's "legal" to use MMC on irregular fos too. yipee!

As for using cones as datums features, I was once told by an instructor that cones could not under any circumstances be used as datum features..."it can't be done. nobody does it" was what I heard. We were using 0.0025mm profile tolerance on our cones (molding industry) and he said we had to change to a different alignment feature. Tks, but we'll find another training supplier. The molding industry relies on tapers. Tooling industries rely on tapers. I've had "creative" tolerancing schemes thrown my way too ... particularly entertaining when dealing with ISO types.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
I hear yah Jim! I bet that trainer has a real problem with tapered bearing races too. lol

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
 
fsincox,

You said that "larger taper on the shroud means it can be off location more". This sounds like you want to treat the cones as "features of angular size", for lack of a better term. So the location tolerance of the conical shroud would be a function of its included angle. Hmmm. Some sort of maximum material boundary would be defined, based on the smallest included angle. Bonus tolerance would be added on as the shroud's angular size increases. This is an interesting concept, but I'm not sure that it will hold together if we think through the details. There are complicating factors that come in, like how to define the included angle of a real (i.e. imperfect) cone.

Norm,

A cone is not a feature of size, says me. Does it have opposing elements? No, they're only partly opposed and never fully opposed so an actual local size is not defined. Does it have an actual mating envelope? No, not a rigorously defined one. Can the derived median line be derived? This is debatable, but it's a moot point because having a DML is not part of the criteria for being a feature of size. I'm glad that you qualified (corrected) your statement by saying that cones are not "classic" features of size.

Regarding cones as datum features, of course there are many parts and assemblies where conical features constrain degrees of freedom and are therefore functional datum features. So for Jim's instructor to make those statements is rather silly.

What is true, though, is that the way that cones are used as datum features contains a lot of implicit assumptions and approximations. You'r right that the dimensioning can sometimes be creative, Norm. I would restate that as the dimensioning often goes outside of what Y14.5 allows.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
Can you pick up a cone with your opposing thumb and fore finger?
Local size of a cone is very often defined. Again, people do cones different ways but the nominal or perfect cone can certainly be defined and the allowable variation from that true cone can be defined.
The only complication is depending on choice of dimensioning, there is stack-up to consider.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
 
Norm,

Can I pick up a cone with my opposing thumb and fore finger? That depends on the size of the cone :^). If I can, it's only because of friction. My opposing thumb and finger will always tend to push the cone away along its axis.

It's true that local size of a cone is very often defined, but not in the same way that Actual Local Size is defined for features of size. The nominal or perfect cone can certainly be defined, no problem there. The allowable variation from that true cone can be defined, but not by a size tolerance. The circular elements on a cone resemble features of size, but that doesn't make the cone itself a feature of size.

If directly toleranced dimensions are used (such as a combination of an angle tolerance and a size at one end of the cone), I don't think that stackup is the only complication. The allowable variation isn't rigorously defined. The only reason that these methods work is that cones are generally manufactured with very little form error (such as the tapers used in the molding and tooling industries). Because of this, the results of the ambiguities are reduced to an insignificant magnitude. This makes the simplified dimensioning work in practice, but let's not pretend that it's rigorous.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
 
Good point Evan on the friction thing with cones. But again, I deal with the reality of things.

Lets just say that I am glad a shot glass works my way. :)

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
 
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