Multiple Angle Dimensions (Equally Spaced) 1

[EidenC]

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?

 

[drawoh]

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

 

[ewh]

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

 

[TheFeds]

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:

[ol][li]Dimension it with GD&T: Six dimensions, all basic, e.g. [6X 60°] with some geometric tolerance on the holes.[/li]
[li]Dimension it with a conventional tolerance, 6 angles: Six dimensions, e.g. 6X 60.0°±.5°[/li]
[li]Dimension it with a conventional tolerance, 5 angles: Five dimensions, e.g. 5X 60.0°±.5°, and one undimensioned gap[/li][/ol]

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!

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.

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).

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)?

 

[PeterStock]

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

http://www.infotechpr.net

 

[MartinSr00]

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.

 

[fcsuper]

Well, I think ASME looks at patterns on the whole, which is why #2 to correct. Particularly on a radial pattern, where there is no beginning or end to which accummulated tols can occur.

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

 

[KENAT]

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.



KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[ctopher]

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

 

[ewh]

"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." - [small]Robert Hunter[/small]

 

[ctopher]

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

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