GD&T Question

[thepete]

http://i.imgur.com/VZW6KWA.jpg[/IMG]]
See picture above(which hopefully posts)

I'm trying to use GD&T to tolerance the flats of the D-hole. What I'm trying to describe is the flats centered on the 1.125 diameter hole.

I also have to do something similar with the shaft that fits through this hole.

Position tolerance I have doesn't feel quite right. What are your thoughts?

Thanks,
Pete

 

[KENAT]

forum1103 may be a better place for this type of question.

However, simplistically what you have appears to work.

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[Tmoose]

I think I could put the flats +/-0.125 along the 4.13 (foot?) long axis and even rotate them as long as they are 0.875 apart, since there is no dimension for a "true position" to datums D or B.
I'd say I would >need< to shift it the full 0.125 to create a D hole. Two flats is sometimes called a double D.

http://www.hartco-eng.com/shop.php?category=35

Will you be using the shaft/hole diameter tolerance to locate the shaft? Are the flats expected to allow the shaft to transmit torque to/from the link?

 

[KENAT]

Tmoose, the reference in the FCF to datum B controls the clocking issue. The reference to datum D gives you the location along the 4.13 length as centered on the dia. While there is occasional debate when invoking position tolerance like this my understanding of at lease ASME Y14.5M-1994 is that you don't need the 'centering' dimension when using appropriate GD&T controls.

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[thepete]

It has been suggested to me elsewhere that adding a basic dimension from the center to the flat. What do you think?

I guess I could add a parallel to B control, but it seems sufficiently implied and defined in the position control with B referenced as a third datum - and is frankly not that critical if the Double-D got rotated, so long as it is centered.

-Pete

 

[KENAT]

You don't say what drawing standard you're working to, I work to ASME Y14.5M-1994 so generally answer wrt that standard.

With normal +- dimensions you don't get to assume alignments or symmetry because there is no tolerance given on how alligned or symetric etc they need to be. See 2.7.3.

However, when you're using position or similar controls you can sometimes assume symmetry or alignment etc. (so long as it is clearly implied by center lines or similar) because the position control tells you how far off you can be. See fig 5-4 for a potential example.

So I would argue no you don't need the .438 basic.

You also don't need an additional control on the clocking as you already invoke B in the position fcf.

You would probably get much more input over in the GD&T forum.

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[thepete]

KENAT,

Thanks for your input. I saw the GD&T right after I posted, but I figured I'd get more flack for double posting than being in the wrong place.

I'm working off Y14.5-2009.

I see what you are saying about not assuming parallelism... If I had dimensioned one of the flats to datum B instead of the hole's center, then I'd have an implied parallelism with B (within the confines of the linear dimension). If I understand it right, yes? But since I'm more interested in the center, I'll leave it to the position tolerance.

 

[thepete]

(I reposted to the GD&T forum, thanks guys).

 

[KENAT]

I suggest then that any future posting be over in thread1103-351717

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[tiguy64]

Hello, I would suggest true position for the location, but a profile tolerance to control the size of the D hole flats to simplify.

T tg