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GD & T question? Weak Definition??

GD & T question? Weak Definition??

GD & T question? Weak Definition??

(OP)
What is wrong with the following drawing view??

RE: GD & T question? Weak Definition??

I'm not sure what the intent is here, but I'll bite.

In this 2-dimensional view, it's difficult to tell what the features are that have position tolerances on them.  Are we looking at bosses of different heights, or are there some gaps in there too?

The 6.40 +/- 0.15 dimension is a classic case of applying a plus/minus tolerance to a "non feature of size".  Several features share the nominal 6.40 spacing, and some of them are non-opposed, so inspecting it would involve some guesswork and assumptions.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: GD & T question? Weak Definition??

ryandias,

   I assume that you have more information on the other views, so I am not willing to point out missing details.  We cannot see where datums D and E are.

   The 6.40 dimension looks like it should be 2X 6.40.

   The watermark looks like part of the word "CONFIDENTIAL".

   Your object lines should be thicker than your dimension lines.   

   It looks like an electrical connector.  Since we do not know what your requirements are, we cannot comment on the validity of your specifications.  If datums D and E are properly called up, I can interpret your drawing.

                       JHG

RE: GD & T question? Weak Definition??

Offhand, just about everything.

The min dims would seem inadequate.  Not seen positional tol applied to ext line.  Extension lines should not be interrupted with the FCF....  and more.

RE: GD & T question? Weak Definition??

(OP)
Damn you guys are Picky!!

Thanks for the replies.

I should have given more direction to my question.

I was told that the GD&T position call-outs are "a weak definition."

Unfortunately I was a bit embarassed, that I didn't know what he was saying explicitly.

Now can anyone mention why he deems this use to be "weak"?

RE: GD & T question? Weak Definition??

ryandias,

   You asked us to be picky!

   I would ask for a better definition than "weak".  Maybe he thinks your tolerances could be made tighter.  Maybe you used the wrong colour ink.  

                    JHG

RE: GD & T question? Weak Definition??

Where are "D" and "E"?

Maybe "soft" would be a better term than "weak".  Probably some yutz wants to see actual dimensions explicitly locating the features.  Giving benefit of doubt for what is not seen, it appears the features are adequately defined.

You can go ahead and post more of the drawing now.  Your secrets are not that impressive or important.

RE: GD & T question? Weak Definition??

(OP)
I'm pretty hesitant on posting a full page.  My initial conclusion as to why he said weak is because the positional tolerance is greater then the dimensional tolerance in one case.

 

RE: GD & T question? Weak Definition??

ryandias,

   Several of your prositional tolerances are tighter than your size tolerances.  So what?  There is nothing in ASME Y14.5M-1994 that says you cannot do this.

   What are you trying to do?  If your part is what I think it is, it must mate with another connector.  The mating features have to clear each other.  This is where I like to show nominal dimensions and zero positional error at MMC, although this is probably a bad idea for RP and any sort of casting or molding.

   You should be trying to call up the loosest tolerances possible.  

   Do a tolerance stack up with the mating part, and see if there are any problems.

                               JHG

RE: GD & T question? Weak Definition??

(OP)
Drawoh.

You are correct it is a connector.

It is a connector companies design.  I want no part in the design or responsiblity for its function.  The views shown are direct "copies" of the vendor drawing.  We have incorporated our own part with the connector spec'd from the customer.  

The supplier is now ripping appart the sections that show the vendor drawing details.  I am not sure of all the vendor drawing details, but have incorporated them as is (Neither I or my seniors want to undertake connector design responsibilities).

RE: GD & T question? Weak Definition??

I don't know where to start. I see a lot of problems... GD&T, drafting, etc.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

RE: GD & T question? Weak Definition??

Maybe a view, perpedicular to this one would show more correctly position of features, I guess -D- base is parallel with shown one. Features are square, aren'n they? Nasty...

RE: GD & T question? Weak Definition??

If datum D is the bottom or top surface of the connector and datum E is the OD (left to right), I don't think there is anything wrong with this in terms of how the features are dimensioned and toleranced but attaching the FCF to the extension line is not right and having the FCF cross over them is not right either as ringster said, but I don't think you were asking for a critique of your drafting technique. You were asking whether or not your dimensioning and tolerancing scheme was sound. In the absence of the rest of the drawing and making a few assumptions, I believe it is. There is nothing inherently wrong with having your positional tolerance looser than the feature tolerance unless it renders the part non-functional, but the practice itself is perfectly GD&T legal.

Evan,
I have a question; you mentioned the 6.40 dimension as being a classic example of a plus/minus tolerance on a "non feature of size" as if it were wrong to do. Is this what you meant or am I reading too much into your comment?

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD & T question? Weak Definition??

Powerhound,

You're not reading too much into it, I was trying to point it out as something that could be looked at as wrong.  "Wrong" is a strong word, but it's an example that exposes the limitations of plus/minus tolerancing.

For example, consider the round-ended feature at the far left of the part.  If I'm the inspector measuring the 6.40 dimension to the bottom end of this feature, how do I measure it?  There is no directly opposing feature.  I would have to line up on one (or more) of the 4 features that are nominally 6.40 away.  If those features are not all exactly parallel to each other and in line with each other, there is more than one way to line up on them.  The measurement could be done several ways, each with a different actual value.  Getting the "correct" actual value, if there is one, depends on "tribal knowledge" of what the designer intended or what the part function requires.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: GD & T question? Weak Definition??

One thought as to why they might consider it weak, you don't have a third datum in the FCF for orientation.

You say it's drawings of a vendor part if I understand correctly, in this case do you really need to show it, and if so unless it's a source control or something, should they all be reference dimensions?

I agree with a lot of the other comments about poor drafting practices etc, it's difficult to see the wood for the trees as it were.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: GD & T question? Weak Definition??

I think the part that jumps out at me as being wrong is the fact that the word "confidential" appears on the drawing and this is a publically viewable form.

RE: GD & T question? Weak Definition??

Might a centerline added to the datum feature G help any?  And I just noticed that a positional tolerance is used to relate G to itsself.  Hmm the weakness is getting stronger..   

RE: GD & T question? Weak Definition??

ringster,
I don't think a centerline won't help at all. How do you think a centerline will help?
As far as relating G to itself, I don't see what you're seeing. Can you elaborate more?

KENAT,
  If datum D is the top or bottom surface (which it probably is), and datum E is the outside of the part, relation to datum G provides orientation.

Evan,
  Did you mean to use the obround feature as your example? It is certainly a FOS but you then referred to the 6.4 dimension which is used on something that is certainly not a FOS. Maybe now I'm not reading enough into what you're saying...:)

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD & T question? Weak Definition??

Powerhound,

The datum feature G has a FCF attached, improperly, to the extension line on the right side.  

The addition of the centerline is in reference to the lates tip of the month on Tec-Ease.  Have you seen it?

RE: GD & T question? Weak Definition??

(OP)
Isn't the centerline sort of voided by the coding feature on the left??
 

RE: GD & T question? Weak Definition??

Centerplane flatness - more mental exercise...

The Edge... there is no honest way to explain it because the only people who really know where it is are the ones who have gone over. - Hunter S. Thompson
 

RE: GD & T question? Weak Definition??

Powerhound, orientation may have been the wrong word looking at it again, however there are only 2 datums specified in any of the FCF visible, which leaves one unconstrained degree of freedom.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: GD & T question? Weak Definition??

ringster,

  I agree that the FCF is improperly attached, but your comment stated that datum G was referencing itself and I wasn't seeing that. I also don't see the correlation of the centerline comment and the latest tip of the tec-ease website.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: GD & T question? Weak Definition??

Powerhound,

To be quite honest, I did not see the need for the centerline in the Tip Of the Month.  But somehow things like this get disseminated by some people and it becomes the way to do it.

The statement was intended to be thought provoking.  

RE: GD & T question? Weak Definition??

Ryandias

This is a lot of guesswork, mostly to try and figure out why someone would have an opinion about the relative strength of a GD&T callout.  I am a newbie to GD&T, and I have discovered that there are about a thousand and one different ways to call anything out.  

Some people like positional tols, some like surface tols, if you know what you are doing, you can make just about anything say what you want.

To make matters worse, we here are all going to pick on different things to nitpick as well.

My advice, go back and ask the original critic why they say that it is weak and see if you agree.  It is entirely possible that your drawing is fine and the other person is wrong.  You are not doing yourself, the critic or your company any favors by not following up directly.  There is no reason to be embarassed either, if the critic is unwilling to help you, find someone else there who will.

GD&T is NOT an exact science, more of an art form that will take years of practice to master.

If you need a good text on GD&T, I recently bought "Dimensioning and Tolerancing Handbook" by Paul Drake and I have found it to be a very good reference.

Good Luck
 

RE: GD & T question? Weak Definition??

(OP)
After discussing with them during a meeting.  My understanding of why he called it "weak" is that;

They position tolerances are assuming we want everything symetrical.  BUT the keying feature on the left side is non symetrical.

A centerline would have made the implicit locations a bit more explicit, but the keying feature would violate that.

RE: GD & T question? Weak Definition??

And just how does adding a centerline 'strengthen' the drawing.   

RE: GD & T question? Weak Definition??

(OP)
I think the line would be constructed of carbon fiber and would add to the ridgidity of the paper.

I have no clue.

RE: GD & T question? Weak Definition??

ryandias,

   The drawing looks symmetrical to me.  I cannot come up with any interpretation other than that.  A centreline would make it look a little more symmetrical, which does not change the fact that there are no other possible interpretations.

   One asymmetric detail does not affect any of this.

   Perhaps the carbon fibre centrelines could run from corner to corner, creating a truss!

                             JHG

RE: GD & T question? Weak Definition??

Why there could be a centerline. By attaching a datum in-line with the dimension arrow it indicates that the feature is a datum (also, the fact that Datum G is accompanied by MMC in all the FCF). And that feature creates a datum plane at the center of the feature, represented on the drawing by a centerline.

Thus, Datum G is the 16.05±0.05 feature, constrained by Datums D and E (another feature of size, indicated by the MMC symbol), and the centerline is what all the other features are positioned to in a 0.25 tolerance zone. The only symetrical relationships are those that are related to Datum G.

The little thingus/doohickey on the left isn't an issue.

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