Gear center distance question

[Robi]

I have a print from a customer when he only has the straight line distance from one hole (a datum) to the other holes

I know he doesn't care about the gear position per se, just the center distance so that the gear meshes correctly.

I have a CAD model so I can get the XY distance from hole to hole, but can you use TP with just one dimension?

If not what would be the best way to GD&T it without over constraining it?

Thanks,
Robi

 

[Robi]

Hi everyone,

It seems the discussion has continued without me...

I have created a simplified sketch showing the pertinent info and how it is currently called out by my customer.

Datum B is indeed a gear center. there are three gears in the train. The center distance is what needs to be controlled to +/-.0005 inches. The angular position is acceptable at +/- .005 inches (aprox 2 degs)

The original question is how do use GD&T to accomplish this?

My customer says he is correct but is willing to see what I can come up with. He says having a TP callout with one basic controls the center distance and the hole can float within the title block tolerance of +/-.005, this would give me an elongated tolerance zone.

I hope this clarifies what I am looking for.

Thanks,
Robi

ps I wonder why the standard does not address gear centers?

 

[KENAT]

For the way you want to do it I think bidirectional tolerance like I suggested works.

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[PaulJackson]

I suppose this could work for you.

Paul

 

[TheTick]

Dual GDT frames control center-to-center w/ tight tolerance but allow loose tolerance for spot position. See attached.

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[PaulJackson]

TheTick,

It's good but there wold need to be the note "SEP REQT" under the refinement tolerances otherwise the pesky simultaneous requirements rule would restrict the rotation of the features within the diameter .001 as well.

Paul

 

[KENAT]

Paul, why are you saying not to use Position and instead use +- tols?

This seems like an ideal candidate for polar coordinate bidirectional position tolerance or am I missing something?

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[PaulJackson]

Paul, why are you saying not to use Position and instead use +- tols?

I didn't say that.

This seems like an ideal candidate for polar coordinate bidirectional position tolerance or am I missing something?

Your suggestion would work fine! TheTicks would as well! Your suggestion is subject to the sim reqt rules just like The Tick's but it is marginally insensitive to it because of the slab rather than cylindrical tolerance requirements... but when sep reqt notes are added to TheTick's his becomes an advantage because the zones are free to rotate and not locked at the included angle between the centers.

There are many ways to do it but the best mirrors the functional constraints and liberties... We don't know them! We know only what Robi tells us! Most of us recognize what is missing in the DRF construction, form and orientation controls of the datum features, basics for the features in question, etc... but those details do seem to be important to the discussion here so... there is nothing wrong with the option I suggested last and one doesn't need I depth knowledge of GD&T to understand it.

Paul

 

[KENAT]

Well, except that preferably features of size are located using position tol.;-)

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[PaulJackson]

I say it is alway's function that drives preferences not rules that are made up by designer's or inspector's or machinest's, those that want everything checked with an attribute gage, etc.

If you nail the datum constraints exactly the way the degrees-of-freedom are removed in function (assembly or otherwise) and apply tolerances according to the function liberties and constraints required for the item to perform as predicted... that is the goal.

Paul