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Multiple Angle Dimensions (Equally Spaced)

Multiple Angle Dimensions (Equally Spaced)

Multiple Angle Dimensions (Equally Spaced)

(OP)
We have a customer that dimensions gussets welded to circular plates on a regular basis and we have come to an argument on their dimensioning scheme.

I have attached an example and am wondering if the 6X 60 Deg dimension for the 6 gussets is an accepted practice as the stack up doesn't make sense.

If a welder welded all of the gussets going around the plate at 60.5 Deg apart, he would be way out of tolerance when he welds the final gusset on. The dimension would be 57.5 Deg between the last gusset and the first.

-or-

Does this mean that the welder has to find all of the angles theoretically perfectly and he has +/- 30' off that?
 

RE: Multiple Angle Dimensions (Equally Spaced)

This comes up frequently. In my opinion, the drawing is incorrect as is for exactly the reason you state. The correct GD&T way to do it would be to make the 60 deg. dimension basic and position the gussetts with a tolerance of position callout. However, this is a welded assembly and for the most part welding is not precise, especially for gussetts. What I have done in the past in this situation is go 5X if it's okay that the last and first gussetts are 57.5 deg. apart. If the tolerance cannot stack up then more specific definition through the use of GD&T is probably in order.

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

RE: Multiple Angle Dimensions (Equally Spaced)

While 6X does result in one of the 60 degrees being extra or reference, I have seen and used 6X for this. I have also seen and used 5X. I would consider either ok for this situation. Or leave the angle dimension off and just state "EQUALLY SPACED".

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net

RE: Multiple Angle Dimensions (Equally Spaced)

This subject has been beat to death in the other thread, and I agree with the others here.  What including all angles can do is indirectly indicate non-cumulative tolerances on the angles.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Multiple Angle Dimensions (Equally Spaced)

Peter,

  I have seen 6X used too; that doesn't make it okay. It is incorrect to use it in this situation but like a lot of drawings that are illegally specified, the part can still be made and could possibly still work. Again, that doesn't mean the drawing is correct, because a second maufacturing run will likely yield a different result -but still be compliant to the print- and that may NOT work. Using EQUALLY SPACED is also incorrect because they will NOT be equally spaced, every gussett spacing will vary, the question is how much can they vary and still work. How can you justify both 5X and 6X being okay? One is incorrect and one is not. When you start getting into the "they know what we mean" mentality, you are setting yourself up for a big disappointment eventually.

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

RE: Multiple Angle Dimensions (Equally Spaced)

EidenC,

   I agree that the 60° angle tolerances are cumulative, however, there are six of them.  If the specification was 5X60°, the last angle could be out 2.5°.  The "6X" means that all six angles must be within specification.

   My point is a pendantic one, and I agree that basic dimensions and geometric position controls would make for a better drawing.  

               JHG

RE: Multiple Angle Dimensions (Equally Spaced)

ahh whats the point....lol lol You can interrupt that drawing any way you want. Heck if our shop built it they would still screw up...lol lol

Solid Edge V20

RE: Multiple Angle Dimensions (Equally Spaced)

Ya'no, drawoh's comments are both correct.  This general scenario is covered in ASME Y14.5M.  Check out page 22 of ASME Y14.5M-2009.  The pattern would (in my mind) preclude the scenario you suggest because of the reason you state, the last angle would be out of tolerance, so it would not comply with the print.  All angles much comply with the 6X dim, so the print prevents cumulation at a point. I think you are OK as is, though positional tols would give you what you are looking for as well.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

RE: Multiple Angle Dimensions (Equally Spaced)

deadhorse

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Multiple Angle Dimensions (Equally Spaced)

fcsuper,

   I was just reading through the discussion that KENAT linked to.  I must have taken some time on my responses there.  

   While cumulative tolerances do not affect the last hole in your series, they do affect holes in the middle.  Adding two or three tolerances is not as bad as adding five or six tolerances, but you still are adding them.

   For a given linear accuracy, angle tolerances to each of your features must get more accurate as the pitch circle increases.  This gets very accurate very fast, even if all your tolerances are from the same reference point.  This is the best reason to use GD&T positional or profile tolerances.  

               JHG

RE: Multiple Angle Dimensions (Equally Spaced)

Typically I would use a basic dimension on the angle and position tolerance on the holes.  This way the angle dimension isn't cumulative and you end up with a clear understanding on the drawing.

RE: Multiple Angle Dimensions (Equally Spaced)

With 6 gussets and 6 spaces, you've double dimensioned the part.  That's not permitted by ASME y14.5 because as you've found, the reality depends on how you look at it, and Y14.5 gives you no real guidance.

Why not box the 6X 60 degree angles to make them basic, attach some datums, and use a feature control frame to tolerize location in the tangential direction?  That way, your gussets will be controlled for both location and orientation.  No stack up at all!

RE: Multiple Angle Dimensions (Equally Spaced)

Actually, Martin, with patterns that's not true about double dimensioning.  In fact, the standard states to call out the circular pattern in just that way.  It's only double dimensioning if it is the same spec twice.  In the case of a circular pattern, this does not happen.  

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

RE: Multiple Angle Dimensions (Equally Spaced)

fcsuper, can you remind me where in 14.5-94 it says or shows that for non basic dimensions.  I'm pretty sure it came up before and I thought it was being taken out of context or something.

Second thoughts forget it, we did this to death on the previous thread.  To me if you have a circular pattern where you dimension all the angles with +- dimensions you end up with conflicting/ambiguous requirements.

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?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Multiple Angle Dimensions (Equally Spaced)

5X on the angles could be OK, but it still doesn't address orientation.

I've done tons of drawings for a fabrication (weld) shop.  True, there are AISC standards that leave drawing interpretation "loose".  This is a matter of philosophy, and there is no "right" and "wrong" answer.

I would rather err on the side of being specific.  Just because tolerances are loose, it's better to say just how loose.  If it's rough, put a locational tolerance of .25 inch or so.  Or .50.  Then if there are problems, everyone is clear about what the standards for success are.

RE: Multiple Angle Dimensions (Equally Spaced)

I looked this issue up in Genium's good ol' Drafting zone book.  They address this point as an issue of concern and they do recommend PosTol's.  They example they discuss is linear, but I think it still applies here.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

RE: Multiple Angle Dimensions (Equally Spaced)

fcsuper,

   A linear array does not wrap around the way a pitch circle does.  If I show six holes positioned around a pitch circle by 6X the angle, the final dimension value may be redundant, but not the tolerance.   

               JHG

RE: Multiple Angle Dimensions (Equally Spaced)

fcsuper,

   This is sort of like discussing what sort of hammer to hit ourselves over the head with.  We can have all sorts of opinions on the subject, but really, we shouldn't hit ourselves over our heads with hammers.  

               JHG

RE: Multiple Angle Dimensions (Equally Spaced)

2thumbsup

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Multiple Angle Dimensions (Equally Spaced)

I know that this was all discussed in that other thread, but maybe here's a better way to summarize. There are (at least) three ways to dimension this:

  1. Dimension it with GD&T: Six dimensions, all basic, e.g. [6X 60°] with some geometric tolerance on the holes.
  2. Dimension it with a conventional tolerance, 6 angles: Six dimensions, e.g. 6X 60.0°±.5°
  3. Dimension it with a conventional tolerance, 5 angles: Five dimensions, e.g. 5X 60.0°±.5°, and one undimensioned gap
With the first method, nothing weird happens. Each hole is located according to the tolerance zone specified by the geometric tolerance.

With the second method, because you've specified six angles, you have to measure six angles—you can't assume that if five are good, the part is good. For example, with actual angles of 60.1°, 60.1°, 60.1°, 60.1°, 60.1° & 59.5°, every angle conforms to the ±.5 tolerance and the part passes. But with 60.2°, 60.2°, 60.2°, 60.2°, 60.2° & 59.0°, only five of the six angles are valid, and the sixth is invalid. In effect, by calling out ±.5°, you're saying that you'll accept that deviation on any given angle, so long as the entire system is still self-consistent. Therefore, on average, an angle's actual deviation from nominal will be much less than .5° in a conforming part.

With the third method, five angles are specified, so you don't care what the sixth is (for purposes of inspection). As long as they all pass, the part passes. This means that 60.5°, 60.5°, 60.5°, 60.5° & 60.5° is valid, but if you measure the remaining angle, it's 57.5°. If that situation is unacceptable for your application, don't use this dimensioning method!

Quote (EidenC):

If a welder welded all of the gussets going around the plate at 60.5 Deg apart, he would be way out of tolerance when he welds the final gusset on. The dimension would be 57.5 Deg between the last gusset and the first.
It looks like your drawing follows method 2, so that implies that the welder should definitely avoid placing the gussets at 60.5° from the previous one, because he'll quickly discover that he can't make all of the tolerances work. The assumption that because the tolerance is ±.5°, he could err by .5° on each dimension, is faulty. He can err by .5°, but only if he makes up for it in the other dimensions.

Quote (EidenC):

Does this mean that the welder has to find all of the angles theoretically perfectly and he has +/- 30' off that?
No. If he does that, he might put the first at -0.5° and the second at 60.5° (relative to the origin). The angle between them is 61.0° > 60.5° (out of tolerance).

So, the bottom line is that any of the three methods are potentially valid, but they're not equivalent. In particular, method 2 isn't overdimensioned, but it may be confusing (because the six dimensions cannot simultaneously and individually take advantage of a full .5° in tolerance).

Quote (PeterStock):

Or leave the angle dimension off and just state "EQUALLY SPACED".
With regard to the notation "EQUALLY SPACED", I think that would be unclear. How do the tolerances apply? Does that imply method 1, method 2, or EidenC's example (where you determine the theoretical lines 60° apart and apply tolerance from those points)? Or worse, does it override the tolerances (only perfect 60° angles are allowed, within the ability of the inspection gauges)?

RE: Multiple Angle Dimensions (Equally Spaced)

My interpetation of "equally spaced" would be that each element (hole, rib, gusset ect) would be positioned within the default tolerance in the title block (or company equivalent, our client has its own addition to ASME Y14.5 that covers this). Yes it can be confusing (ie with reference to what?), but it does work in a lot of cases.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net

RE: Multiple Angle Dimensions (Equally Spaced)

You can't fix everything with a size callout.  If you want to know what's "in-tolerance" and what's "out-of-tolerance", you need to consider form, orientation and location.

Put another way:  You can't properly control part geometry with only a ruler (or in this case a protractor).

Doing it right is simple.  Box the angles to make them basic, and put a feature control frame on the drawing that describes the gusset tolerance zone.  If a quarter inch is OK, then quarter inch it is.

Leave off proper geometric controls, and all you have is a down payment on a dispute about what the drawing really means -- after the part is made.

RE: Multiple Angle Dimensions (Equally Spaced)

And I disagree, but we didn't get anywhere in the 90 or so posts last time and I doubt we will now.

As if this poor equine carcass didn't get beaten enough then now it's been resurrected and the stench is terrible.

deadhorse

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?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Multiple Angle Dimensions (Equally Spaced)

The way the drawing shows, the "6X 60.0" should be "3X 60.0". There would not be "6X" after tolerances unless they were 'very' loose.
I'm ok with the angle dim followed with an equally spaced note.
The "17.99" is OK.

Chris
SolidWorks 09, CATIA V5
ctopher's home
SolidWorks Legion

RE: Multiple Angle Dimensions (Equally Spaced)

"3X 60°" would only be acceptable if there are extension or centerlines connecting the oppposite sides, and even then it could be brought into question.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Multiple Angle Dimensions (Equally Spaced)

I agree. EidenC's drawing shows the CLs extended.

Chris
SolidWorks 09, CATIA V5
ctopher's home
SolidWorks Legion

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