GD&T question for 2 keyways on shaft
GD&T question for 2 keyways on shaft
(OP)
Please see attached drawing for details - comments are enabled in the PDF...
I've got a shaft with 2 keyways, which should be inline with each other along the shaft. My question relates to how to properly spec the drawing; is it sufficient to use "2X" and only detail one keyway, or is there a better way to detail this drawing? What is the appropriate way to indicate that I want the centerplanes of each keyway to be coplanar with each other (with a bit of tolerance)?
Thank you.
I've got a shaft with 2 keyways, which should be inline with each other along the shaft. My question relates to how to properly spec the drawing; is it sufficient to use "2X" and only detail one keyway, or is there a better way to detail this drawing? What is the appropriate way to indicate that I want the centerplanes of each keyway to be coplanar with each other (with a bit of tolerance)?
Thank you.





RE: GD&T question for 2 keyways on shaft
You called up one of your 1.75" <grin> diameters as datum_A. I do not think this datum scheme can be fixtured with any accuracy, especially at LMC.
I would have called up the other diameter as Datum_B, and used two datums to define your centre. The perpendular face you have as datum_B would become datum_C, and your slot side would be datum_D.
Your tolerance boxes would show datums |A-B|C|D|. In ASME Y14.5M-1994, look at figure 4-19. This is clearer than anything I can do with the text editor.
You can specify 2X on the features of size for your keyway. I would specify location tolerances separately on the length of your first keyway, and on the width and length of your second keyway.
JHG
RE: GD&T question for 2 keyways on shaft
Regarding the alignment of the slots, there is a "simultaneous requirements" rule in GD&T. You can use this tip for a similar example to what you are trying to do:
http://www.tec-ease.com/tips/january-00.htm
The important thing to remember is that both features must use the same datum reference frame. This does not mean they have to have the same tolerance in the instance that you invoke the rule on features of different size or features that can have different tolerances.
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD&T question for 2 keyways on shaft
New rev is attached. What do you think?
Thanks.
RE: GD&T question for 2 keyways on shaft
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD&T question for 2 keyways on shaft
got it. on the 2X R... should it be 4X R in this case because there are 2 slots?
thank you.
RE: GD&T question for 2 keyways on shaft
Also, be very sure you understand the effects of zero tolerancing as well as including bonus and allowable datum shift before you so liberally apply them. Perhaps a tol stack is in need to justify.
This next suggestion is for all who will listen and comes from Lowell W. Foster himself, never, ever use dual unit dimensioning it just opens the door. Pick one and go with it. Remember it is a legal binding document. Lowell told me has found himself in many court cases testifying as an expert and this particular issue never bodes well for the supplier of the specification.
RE: GD&T question for 2 keyways on shaft
Not sure I follow on your comment about the left shoulder being more imporant than the right. If I'm interpreting you correctly, then you are correct - what's important is that the key is centered between the shoulder "C" and the retaining ring groove. I dimensioned from "B" just to give a way of locating the keyway. How do you suggest dimensioning the keyway location?
Also, can you explain which feature would get the secondary midplane datum? (I think you mean that the dimension in zone E2 would get a midplane datum similar to how datum D is defined.)
Regarding zero tolerancing at MMC:
My goal is to have axes A and B, be coaxial. Then runout on the other diameters to be as specified. I want to have a spec which is not burdensome to inspect or produce; this is a machine shaft which will have sprockets mounted on the 50.8mm dia and bearings on the 44.45mm dia. Would it be better to spec the 44.45mm dia's with a small tolerance at MMC?
RE: GD&T question for 2 keyways on shaft
You do not need the run-out specifications on the 44.45mm diameters. These two features are in exactly the correct position. You have arbitrarily specified this by setting them as datums A and B. The only possible thing you might specify is roundness, and this only if your diameter specification does not control it enough.
JHG
RE: GD&T question for 2 keyways on shaft
right... I don't think I have spec'd runout on the 44.45mm dia's...only on the 50.8mm dia's and the 57.15mm dia. I don't see a need for roundness spec. Not sure how to interpret your comment?
Thank you.
RE: GD&T question for 2 keyways on shaft
RE: GD&T question for 2 keyways on shaft
RE: GD&T question for 2 keyways on shaft
A mid-plane cannot be a datum. You can select either one of the two shoulders to use as a datum, or you can select the dimension across the two shoulders and specify the datum at MMC. I prefer it the way potrero has done it.
weavedreamer,
Datum_D is needed to locate the second slot.
potrero,
I said you could use the roundness specification if you wanted. It would allow you to control roundness more accurately than your diameter tolerance. Your diameter tolerances are tight, so your parts are quite round without an additional specification.
JHG
RE: GD&T question for 2 keyways on shaft
Are you kidding me? Better check again!
RE: GD&T question for 2 keyways on shaft
RE: GD&T question for 2 keyways on shaft
There's another good tip this month on www.tec-ease.com about simultaneous requirements. I think it's February's tip.
One more suggestion is to add the standard to the note section to actually invoke it as the standard of interpretation.
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD&T question for 2 keyways on shaft
Is it really more economical to specify position on the two 50.8 DIA and the 57.15 DIA? According to one of the tec-ease tips ( http://www.tec-ease.com/tips/november-99.htm ) runout can be more economical to specify...
RE: GD&T question for 2 keyways on shaft
RE: GD&T question for 2 keyways on shaft
(a) the 44.45 dia's with position tolerance and assign datums A MMC - B MMC
(b) the 50.8 dia's with position tolerance, same as above, possibly with some amount of tolerance at MMC
(c) the 57.15 dia with runout w.r.t. A-B
?
RE: GD&T question for 2 keyways on shaft
I guess it depends on who's perception you subscribe to on the economical part on position vs total runout. Alex Krulikowski seems to believe that it is and so do I.
Powerhound,
The invoking positional boundary is for slots that is why it was put in the standard see 5.10.1(c) regardless of simultaneous requirement. Again it is only a suggestion for consideration here as another method for a slot.
RE: GD&T question for 2 keyways on shaft
I thought that by your statement - "And if you use position invoking BOUNDARY on the pattern of key slots, a tertiary is not necessary unless you need control of other features to the key slot." - that you were saying that BOUNDARY was required to invoke simultaneous requirements. Sorry for the misunderstanding.
Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD&T question for 2 keyways on shaft
You originally said that the datum should be the mid-plane between the two shoulders. It sounds like your original assumption was correct, however, your terminology was bad. Applying datums to centre lines is a common error on drawings that (are supposed to) conform to ASME Y14.5M-1994. I suggested the option of using the dimension across the shoulders as the datum, which I believe is what you actually meant. Since the two shoulders are a feature of size, you have to at least consider calling the datum up at MMC, although it is an accurate tolerance, and I think all you are doing is locating slots lengthwise. You would get away with it in this case.
When you call up a feature of size as a datum, you need to think about the fixturing.
Your are right about datum_D. This is defined by datum_A-B. Datum_D is not needed unless location to the first slot is more important than location to the shaft ends.
JHG