Controlling angular orientation on a shaft

[Larkspur415]

I have what seems like a pretty basic problem, but I cant find the answer I'm looking for.

I have an assembly of two components that are pressed into a shaft, and I want to control the angular orientation of them relative to each other. I'm not concerned about clocking to any features on the shaft, I just need them to be aligned to eachother. This isn't exactly my scenario, but you can think of this essentially the same as aligning gear teeth on a shaft.

I was thinking I would set the shaft up as my primary datum, use the feature that I'm concerned about aligning on one of my components as a datum, and having a control on the matching feature of the other component, I'm just not sure what control I'm looking for. Basically I'd like to control position, but with an angular tolerance zone rather than Cartesian

 

[KENAT]

Angularity?

Or are you better of just placing a simple angular dimension and tolerance?

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[Larkspur415]

Doesn't angularity still just create a linear tolerance zone at the specified angle?

I asked myself the same question about just using a normal angle spec after posting, and I'm thinking that's probably the way to go. Still I'm interested to know of there's a way to do the same thing in gd&t.

 

[KENAT]

That's my concern with Angularity that for an assembly relationship like yours it may not be appropriate.

Angular dimension is GD&T as it's dimensioning & tolerancing of geometry but no it's not using 'fancy' FCF's.

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[3DDave]

You can control the position of the width of a feature. For small amounts the difference between an angle and a linear width is very little (sin(alpha)~= alpha). There's even an example in the 1994 version of the standard.

What a directly toleranced dimension doesn't do is guarantee the orientation of the part when the measurement is made. In this case it's worse because it's not definite which 'center' will be the right one for the vertex of the angle.

 

[CheckerHater]

Position feature A to the shaft.

Position feature B to the shaft.

Simultaneous requirement will keep them aligned to each other (see ASME Y14.5-2009 Fig. 4-40).

"For every expert there is an equal and opposite expert"
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[dtmbiz]

OP describes component relationships to each other. Good

OP considers making a “component” a datum. Bad (What does that mean?)

“Features” of a component are used to define & simulate datums.

Considering the “features” of the component (e.g. shaft) that interface with
the press fit components, some features are apparently oriented to each other per OP.

What are those features? surfaces? center planes? axis?

An angularity geometric control is defined by resultant parallel planes or cylindrical tolerance zones .
What is meant by “linear tolerance zone” ?

“fancy GDT” = more specific geometric characteristic control. Good

Does OP want specific geometric control ?

Simple +/- degree tolerance has built in misinterpretation problems.
(inspection: who measures what from where ?)

Definitely “ a way” to accomplish with “GD&T”.

Need to be more specific about the considered features; relationships, functionality, & mating interfaces.

 

[Larkspur415]

Thanks for the reply dtmbiz. Like I said in my original post, I'm using a feature of a component as a datum. In my case that feature is a slot. Each of the components I am assembling has a slot and I need those slots to be aligned.

What I mean by 'linear tolerance zone' is that the tolerance zone is in a Cartesian frame of reference; like you say, angularity creates parallel planes that define a tolerance zone. The issue is that the tolerance zone I'm trying to create is best described with a polar frame of reference in which I want to control theta. My tolerance zone needs to look like a slice of pie rather than parallel planes. If I were to use angularity or position I'd create a situation where I'm putting a specification on an assembly drawing that can't be 100% controlled by assembly

 

[dtmbiz]

Larkspur415

OP: "....aligning on one of my components as a datum..."
This was the reason for my statement... confusing to me...

Are the slots cut across the shaft circumference vs. along the shaft axis?

Difficult to visualize your configuration of features from OP

Not sure why you would need a "pie shape" tolerance.
A number of misinterpretations can arise.
You do realize that your +/- dimension will not be produced or measured for mfg or inspection using a specified DRF. In my experience that can be can of worms, especially if there are
concerns that it seems you are expressing.

Not seeing how a +/- polar shaped tolerance zone is better than geometric controls
related to datums and GD&T that conveys feature / part relationships & functionality.

From Foreward of ASME Y14.5 2009
"Since many major industries are becoming more global, resulting in the decentralization of design and manufacturing, it is even more important that the design more precisely state the functional requirements. To accomplish this it is becoming increasingly important that the use of geometric and dimensioning (GD&T) replace the former limit dimensioning for form, orientation, location, and profile of part features"

Could make one wonder why all of these professionals & committees & corporations investing so much time, energy and money into GD&T if the old cartesian and "pie shape tolerances" zones are sufficient ?

Not directed at you personally, I just dont understand why engineers and designers would rather not use GD&T considering the greater communication with GD&T.

 

[powerhound]

I have to agree with dtmbiz's concern about why a pie shaped toerlance zone is preferable to a rectangular one. If a single rectangular feature fits inside both of them simultaneously, you will most likely need a rectangular zone. What are we missing?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[3DDave]

"I just dont understand why engineers and designers would rather not use GD&T"
Because it costs money to train people, and in many cases is limiting variations that aren't present within the manufacturing process.

It's not a bad idea to be precise in the description, but if the supplier is doing a better job than is required by the drawing why spend time making the drawing more precise? Example: Typical angle tolerance is 1 or 2 degrees. Most directly toleranced parts, if made to the limits of that angle tolerance, would not assemble at the next level, and yet they do, because fabricators hold .05 degrees (or some other small number.) I would know the value except QA/QC never checks the implied 90degree tolerances. On a simple rectangular block that would mean 12 different setups (one for each edge,)

ASME has also done no one favors by steadily increasing the difficulty of understanding what they mean, introducing concepts without clear interpretations and not eliminating errors introduced in the rush to alter the names of concepts in previous versions.

 

[dtmbiz]

"It's not a bad idea to be precise in the description, "...


Wow, Engineering right? (rhetorical, no need to answer)

How times have changed...

true: Quality = time = money (corp / eng world)

Actually fascinating that so many have missed the entire point of ASME Y14.5.
It is ultimately about money ... saving money.

IMO:
Personally its not my position to convice peoplpe to believe in the concepts or even use GD&T.
Just like to contribute and continue to learn with people that do see its value and use it without duress.

 

[CheckerHater]

It is not "Quality = time = money"

It's "Quality, time, money - pick one."

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[3DDave]

ONE?? Not even two of three? Dark times indeed.

Still, what could better save money than having a manufacturing process that is sufficiently precise there is no need to worry about its deviations from nominal?

The two ways Y14.5 -should- save money is by accepting more parts that are marginal (the round vs square positional zone, typically argues this) and by allowing a more precise description to let manufacturing loosen degrees of freedom that they would otherwise be unable to.

The first is mostly offset by widespread access to CNC equipment. A decent setup will make parts with little variation and is no longer as monstrously expensive as it was in the 1960s. By eliminating the majority of per-person hand work the need to allow for it has decreased.

The second is offset by the overall narrowing of range of typical precision, so the cost of precision is not as spread out as it once was. For the majority of production it doesn't seem to me that reducing variation below the level at which people can see a difference increases the cost over sloppier work, and whatever decrease in part price is made up for in increased difficulty at assembly.

For no noticeable extra cost I designed (not uniquely) a cabinet with slot and tab self-aligning construction for welding. Only small variations could be allowed, but it more than paid for not having to fixture, not having to straighten, and went together quite quickly. Increasing tolerances would only have allowed for the potential it would not go together; instead it was done based on typical CNC waterjet/puch press variations to set clearances.

Where geometric characteristics control is valuable is when the variation of a process is nearly the same as the allowable variation. Then it is important to orient the part according to the desired datum features and inspect the feature accordingly.

If most of the drawings I have come across did not have terrible choices for datums, where only the precision of the manufacture made the parts workable, I would have more confidence. But when I see a 1/2 inch flange on a 6 foot panel set as the primary datum for holes in the flange, but the default 2 degree tolerance for the implied angle between them, I have to doubt that anyone at any part of the manufacturing or inspection process knows or cares. And, it turns out, it rarely matters as the precision of the part is a tiny fraction of the allowable.

 

[KENAT]

3DDave, that's great until for some reason the normal machine isn't available or the normal vendor is too busy and the parts get shipped to a different tool/vendor with different process capability etc.

Or there was a mistake in the set up of the machine and they want to salvage parts...

Having been hit with cases like this, I prefer to set my tolerances fairly loose driven primarily by function with secondary consideration to potential manufacturing processes. Rarely do I leave everything at default 3 place decimal of +-.005 based on the idea that 'a CNC machine will hit +-.001 nearly all the time in a single set up so +-.005 is plenty'.

However, I probably spend more time on my drawings than many colleagues doing things like refining tolerances from the default. Perhaps my employer would get better value from my half assing the drawing but getting more out.

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