【转帖】gdt question for 2 keyways on shaf

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gd&t question for 2 keyways on shaft
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.
potrero,
   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
potrero,
  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:
   
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
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thanks to both of you.  i've incorporated the a-b method, as well as the simultaneous requirements rule to handle alignment.
new rev is attached.  what do you think?
thanks.
the only thing i would do differently is to change the dimension on the slot length from the ends instead of centerlines. as you now have it, the 20mm dimension is not a feature of size and thus, should not be located using position. go ahead and make it 32.7 and put a 2x r on the end.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
powerhound,
got it.  on the 2x r... should it be 4x r in this case because there are 2 slots?
thank you.
since this part is symmetrical, why is the shoulder on the left more important than the one on the right?  this will create an r&r issue when inspecting. i don't believe it is, so a secondary mid-plane datum is preferred here.  i also would consider use of composite position on the width of the key slots if you truly want to allow some amount of deviation between the two features.
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.  re  
xplicator,
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?
potrero,
   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
jhg,
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.