Position and Align

[Creech]

I have an extruded shape - round on the outside and round except two "key-ways" 180 degrees apart on the inside. The OD is Datum A and the keyways are Datum B. I have to machine a slot in the end; Looking down at the end (the cross section if you will) I will endmill a slot central to a depth leaving two "ears". I have a geometrical callout to that slot width to be True-Position of .004 to A and B. No diametral symbol. My question is; If my slot is slightly off center say .001 (TP of .002) and there is angular misalignment how do I calculate TP total (needed for process capability reasons). Do I have TP to A then another TP to B? or do I lose angular misalignment tolerance as my centrality gets larger? Or if anyone knows where a good example is I would really appreciate it. Thanks in advance.

 

[Heckler]

Parts with cylindrical datum features – The datum established by a cylindrical surface is always associated with two theoretical planes intersecting at right angles on the datum axis. These planes indicate the direction of measurements made from the datum axis. Don’t forget datum –A- is a feature of size so you would need to take it at MMC or leave blank for RFS. Do you have a copy of ASME Y14.5M? if not, it would be in your best interest to get a copy.

Best Regards,

Heckler
Sr. Mechanical Engineer
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Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[ctopher]

ASTM-Y14.5 has all the examples.
Maybe use semmetry?


Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
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ctopher's home (updated 06-21-05)

 

[wgchere]

Only one of the keyways is needed as a datum to clock the planes created by datum A. But you need a datum which is perpendicular to both planes of datum A to fully define your framework.

 

[TheTick]

But you need a datum which is perpendicular to both planes of datum A to fully define your framework.

Not true. Not necessary. Setting orientation with the keyway (Datum B) infers direction parallel and perpendicular to that datum. The frame of reference looking down the part axis is now fully defined. Center of "A" provides position of center, orientation of "B" defines "clock position" of part from which dimension directions are drawn.

 

[Creech]

The true position callout is assumed RFS so the example in the book ASME Y14.5M-1994 does't quite match my situation. So do I lose "clocking" tolerance to B for every .001 used (off-center) on Datum A?

 

[wgchere]

Tick,

The keyway feature will define a center plane of the part as a datum. This plane is parallel/coincident to one plane of datum A and perpendicular to the other plane, but not mutually perpendicular to them. The minimum requirement is to have "3 mutually perpendicular planes." I'd guess that would be one of the end surfaces of the extruded shape, defined with a control related to datum A.

 

[MechNorth]

Tick has the right idea. The cylindrical FOS (feature of size) generates a Datum Axis-A, which has two mutually perpendicular planes (not Datum Planes) associated with it; this fixes the cylinder from moving in any direction normal to its Datum Axis-A. The part can still rotate about, and translate along Axis-A. Datum-B, generated by the midplane of the keyway, stops the rotation of the part, but doesn't stop the tanslation along Axis-A. To stop that translation, you need a third datum. You don't always have to completely fix a cylindrical object in space to inspect it, however. In this case, a third datum isn't needed.

You didn't actually tolerance the position of the slot width; what is toleranced is the mid-plane of the slot. Inspection of that feature (the slot) means verifying the size first, then establishing the mid-plane of the slot feature. The tolerance zone emenates from the basic (theoretically perfect) position wrt the datums. The actual Positional Error of the physical part corresponds to the total difference (one end to the other) of the displacement from Datum-B, which includes the angular error you are concerned with. I'm not sure if that's clear.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

 

[ewh]

Good explanation, Jim. It's good to see you back!

 

[MechNorth]

Tks ewh. It's hard to find time before 11pm!

Jim Sykes, P.Eng, GDTP-S
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CAD-Documentation-GD&T-Product Development