How many angles are in a circular pattern? 6

[cwdaniel]

See attached drawing.

Is fig. A or B correct?

 

[vc66]

KimBellingrath-

Exactly as you said. The 360° is understood; therefore dimensioning the 5 angles and having the 360° implicitly understood is over-dimensioning, period.

I also agree with KENAT and ewh. This argument is moot, because if those angular dimensions are that important--use GD&T.

V

 

[ewh]

I am not measuring a part in it's final configuration, I am defining what the part will be. 4x 72° fully locates the hole pattern, regardless of how long you have been doing this. Using your argument, we should be dimensioning every gap, linear and angular, because it is there, double dimensioned or not.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[vc66]

KimBellingrath-

http://www.despair.com/tradition.html

V

 

[MintJulep]

5 x 72 actually limits the fifth space as well

How?

Example of how it DOESN'T:

5 x 72, with angular tolerance of +/-1

Hole 1: Perfect at 0
Hole 2: Conforming at 0 + 72-1 = 71
Hole 3: Conforming at 71 + 72-1 = 142
Hole 4: Conforming at 142 +72-1 = 213
Hole 5: Working counter-clockwise conforming at 0 (360) - 72-1 = 289

Angle between holes 4 and 5 = 76 degrees non-conforming.

Five individually conforming features result in a non-conforming and non-functional part.

 

[ewh]

Now, now... KimBellingrath's method may have a good, logical reason behind it. Heaven knows, I've seen that method used very often. I'm just waiting for the logic (or standard) which explains why it is a better way to do it that way, and why double dimensioning doesn't apply in this situation. "Because that's the way that I've always done it" is not a sufficient reason.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[drawoh]

KimBellingrath,

...You guys can analyze this to death but for as many years as I have been doing this, I have always counted ALL of the gaps in a pattern like this not how many 'moves' it takes to make the part. As a former machinist, I can tell you that nobody ever drills the first hole twice!

Just for the hell of it, let's punch in some actual numbers.

[ul]
[li]1/4 inch bolts with nuts[/li]
[li]9/32 inch clearance holes[/li]
[li]8 inch pitch circle[/li]
[/ul]

What are the required tolerances?

Clearance C = Hole-Bolt = 9/32in - 1/4in = 1/32in = .031in.

Assume that the bolt is located precisely at the nominal position. If either (any?) holes shift C/2, the bolt will interfere with the side of the hole.

Assume that the maximum error occurs at 45[°] from the normal or from the tangent to the pitch circle.

At 45[°], the maximum error in the normal or tangential direction is C/2sqrt(2), or .031in/2.828 = 0.011in.

Double this to get your pitch circle tolerance of 2[×]0.011in = 0.022in.

So far, so good.

My angular error is approximately linear error divided by half the pitch circle.

It is exactly arctan(2[×]0.011in/8in) = 0.16[°] (to two decimal accuracy

I do not know how accurately you can rotate a part on a fixture in a drill press, but I think this is getting pretty accurate. It reinforces your point that you cannot accumulate tolerances as you go around. The fifth angle imposes another tolerance control on the fifth hole. I am not so sure about the third hole.

The real point here is that it is not very practical to control holes using angular tolerances around a pitch circle. Either you must slot the holes, or you should plan on an X/Y positioning system such as a milling machine. For quite some time, I did not specify pitch circles on machined plates. I worked out the Cartesian coordinates, and I relied on [±] tolerances.

Today, I assume that part will be fabricated on a CNC machine, programmed from some sort of CAD. I specify pitch circles again, and I use GD&T positional tolerances.

You might as well follow the ASME standard and say 5[×]72[°].

JHG

 

[KENAT]

Kim, I'm not measuring anything per se, nor am I telling the machininist how to do his job. In preparing the drawing I'm defining where the holes need to be. I'm not counting the number of moves either, that was an example someone else gave.

If you put the 5X without using GD&T then you have multiple conflicting interpretations of tolerance. To the best of my knowledge there should only be one.

It looks like one of these arguments that will be difficult to get everyone to agree on. For instance Drowoh says:

You might as well follow the ASME standard and say 5×72°.

I, in my infinite wisdom, believe this is not what the ASME explicitly says for all cases, so I disagree. 5X 72 is what it shows for all the examples using positional/GD&T, I haven't yet found a similar full examaple using +- dims in the standard, which makes sense as it says positional is preferable.

KENAT, probably the least qualified checker you'll ever meet...

 

[KimBellingrath]

mint julep - Have you ever played the shell game?
ALL dimensions must be met simultaneously. You state that the angle between 4 and 5 is not conforming...that's a show-stopper right there. It does not matter if some of the dimensions are conforming some of the time - ALL dimensions must conform and as you stated - the angle between 4 and 5 is nonconforming. What part of that statement is causing a problem? It seems to be a mixing of time and space here. The drawing must be met in it's entirety; all at the same time.

Example of how it DOESN'T:

5 x 72, with angular tolerance of +/-1

Hole 1: Perfect at 0
Hole 2: Conforming at 0 + 72-1 = 71
Hole 3: Conforming at 71 + 72-1 = 142
Hole 4: Conforming at 142 +72-1 = 213
Hole 5: Working counter-clockwise conforming at 0 (360) - 72-1 = 289

Angle between holes 4 and 5 = 76 degrees non-conforming.
The whole idea behind non-accumulating is to verify all of the increments, including the 5th which would not allow each one to be '72-1' would it? By tolerancing all of the spaces, you keep a 'creep' in incremental moves from happening.


ewh - what kind of double-talk is this?
"I am not measuring a part in it's final configuration, I am defining what the part will be."

Do you measure 'will' or 'is'? You've gotten distracted here. Time is not a factor and you cannot measure and verify a part's conformance until it has been made. I assume that the final goal of a drawing is a conforming, finished part and NOT a process!

 

[KimBellingrath]

vc66 - that was just so lame - I bet you make faces at people behind their backs, too. Do you consider change just for the sake of change a good thing. I bet I can name quite a few things that YOU do the same way as you always did that should NOT be changed.

 

[CorBlimeyLimey]

WOW ... talk about much ado about nothing!

How on earth did machinists make parts correctly before GD&T came along?

 

[KENAT]

Kim, I'm pretty sure you're arguement goes against at least the 'spirit' of 1.7.7 and figure 1-17. You can't have the overal and all the intermediates applying. You're double dimensioning and with tolerances the requirements conflict.

KENAT, probably the least qualified checker you'll ever meet...

 

[vc66]

I apologize if you took it the wrong way... By the I meant that it was a joke.

I'm changing the way I do things ALL the time, because there is usually a better way to do them. I certainly don't reject things just because I've always done them a different way.

V

 

[ewh]

Double talk? The purpose of a drawing is to define a part, not to measure it. I don't measure, I define. It is up to the machinist and the quality department to measure.
I've yet to see an explanation or source stating that double dimensioning does not apply. SHOW ME THE STANDARD.
If tolerance accumulation is an issue, dimension it differently, or add a local note stating that tolerances are non-cumulative. I've have done that before. Again, as MintJulep posts, any problems arise "from the poor decision to use chained dimensions".

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[MintJulep]

The whole idea behind non-accumulating is to verify all of the increments.

No, it's not. "Non-accumulating" in essence causes the 72 to become equivalent to a BASIC dimension. The "perfect" hole locations are calculated as 1 x 72, 2 x 72, 3 x 72 and 4 x 72.

The original question was not "Which is right, '4 x 72 non-accumulating', or '5 x 72 non-accumulating'". You are assuming design intent not in evidence in the drawing.

Laying out the part with that ASSUMPTION will make conforming parts that meet the designer's probable design intent. However that assumption IS NOT what the drawings posted originally call for.

 

[btrueblood]

"Five individually conforming features result in a non-conforming and non-functional part. "

Uh, hope you are being facetious here, Mint? The part is NOT in compliance, because one of the angle gaps between adjacent holes is out of tolerance. The drawing tells you to measure 5 angles, in your example 4 angles are in tolerance, and one is not. Thus, the part is not compliant. Let's take a closer look at the shell game maneuver you tried to pull, my comments in italics next to your words:

"Example of how it DOESN'T:

5 x 72, with angular tolerance of +/-1 this callout tells you to measure the angle between adjacent holes, in 5 independent places, and verify them to be within +/-1 degree

Hole 1: Perfect at 0 this is not a hole-to-hole angle measurement, and is meaningless, as you can start measuring from any hole you want to...
Hole 2: Conforming at 0 + 72-1 = 71 angle measure #1, or between holes 1 and 2
Hole 3: Conforming at 71 + 72-1 = 142 angle measure #2, holes 2-3
Hole 4: Conforming at 142 +72-1 = 213 angle measure #3, holes 3-4
Hole 5: Working counter-clockwise conforming at 0 (360) - 72-1 = 289 angle measure #4, holes 4-5

Angle between holes 4 and 5 = 76 degrees non-conforming.
This last measurement would make angle measurement #5, and the fact that it is not conforming means the part is rejected. What Kim and I are pointing out is that in closed-figure angle measuring, you are dealing with a non-Cartesian coordinate system, and the "RULES" that you think apply don't. By closing the loop, and specifying ALL of the angular gaps, you ensure that the entire pattern is located to within whatever the angular tolerance is. My comment about land surveying holds, despite the snide comments from the unwashed. George Washington was smarter than most of you, since he knew that by measuring the last angle (called the closure angle) in his surveying triangles, he had a built-in error estimate for every triangle he surveyed, more importantly - he knew which angles he would need to re-measure in order to bring his total error under a given figure (e.g. the other guy's surveyor's error), which is everything in land disputes.

...and I'll let that last statement soak in, give it about 10 minutes and somebody is going to challenge my non Cartesian assertion...

 

[KimBellingrath]

All right - one last time - please pay attention. This is why we dimension ALL of the spaces in a hole pattern:

If you have five 72 degree increments and they are all controlled by the same general tolerance they CAN'T accumulate! Get it? Somewhere at least one of them would have to be out of tolerance to allow that to happen. THAT'S why you say "5X" and not "4X". That forces non-accumulation simply because you must measure every one and every one must be in tolerance.

If you don't understand this, please, please, please: read it over and over until you do before making another half-baked comment. This is such a basic idea and it's getting clouded with over-complicated ideas and statements.

 

[ctopher]

All good responses. But, the original drawing does not indicate GD&T or any type of tolerances. Based solely on what the drawing shows, I vote for Fig A.
But, depending how cwdaniel wants to use the part and dimension it, it could be either figure. I can not suggest otherwise unless I see the mating parts. It is up to him.

cwdaniel,
I strongly suggest a copy of ASME-Y14.5.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 02, 2008)

 

[ewh]

I not sure a copy is going to help him much with this issue, Chris.

Regardless of what standard is followed in surveying, machine design IS a different animal and follows different standards. The insults hurled really don't help to clarify this situation.
If a drawing is to be interpreted by the standard, then that is what we have to go by, regardless of better methods. I agree with your logic, KimBellingrath, but it does not address the issue of double dimensioning per the standard. Revisiting the standard, it states in para 1.9.5.2 "Equal spacing of features in a series or pattern may be specified by giving the required number of spaces and an X, followed by the applicable dimension." That still leaves room for argument as to what is actually required, as we have seen. The figures referenced do call out ALL of the equal angles, but the intent of the figures is to denote the proper use of "X", and go against para 1.4(a) "Each dimension shall have a tolerance..." and para 1.4(c) "No more dimensions than those necessary for complete definition shall be given."

I feel that both sides of this issue have valid arguments, and if I weren't so stubborn, I'd concede and be done with it. As it is, I would have a hard time marking either method as incorrect on a drawing (though the tolerancing would still have to be addressed), so I'm going to do as others have suggested and use GD&T.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[cwdaniel]

ctopher, Thanks for the suggestion. I happen to have a copy of the standard right here on my bookshelf. Or is it keeping the conference table from wobbling?

No matter. I've looked it over several times since posting this to review the references of others.

My original question was more of a drafting question than an analysis of tolerancing implications. Obviously I had no idea of the implications and the amount of fomenting of opinions this would generate. That being said, I think I've got my answer and learned a few things along the way. I'm leaning towards KENAT's camp. Positional tolerance gets 5x otherwise 4x. To avoid the risk of igniting some sort of international incident, I'll abstain from any further comment or opinions.

This has all been quite exciting though!

But, back to the drafting aspect of all of this. Earlier I conceded to be swayed from my current thinking. I posted another drawing that has been completely ignored in lieu of grand posturing, slander, and condescension.

Does anyone have any rational comments about my post of
7 Jul 08 13:08?

 

[vc66]

Please explain to me why the figure on the right is considered double dimensioning, but the figure on the right is not. I'm not trying to argue the point, I want to learn.



V