Position to a keyway

I have a base plate with a keyway and some holes. The design requires the holes to be located relative to the keyway, but the keyway does not need to be exactly centered on the plate itself. I am currently locating the keyway off a parallel edge of the plate with a fairly loose +/-.010 tolerance. The holes are positioned from the same edge with ordinate, basic dimensions. My datums are A=bottom surface of plate, B=centerplane of keyway, and C=one of the plate edges perpendicular to the keyway. My feature control frame for the holes is: [Position][Ø.002(m)][A][B][C]. So my problem now is that I think the basic dimensions for my holes are not giving me what I want. A basic dimension by rule - as I understand - is a theoretically perfect position from which GD&T applies. My hole dimensions are located from a plate edge, and my control frame is locating from the keyway. Unfortunately, dimensioning my holes from the keyway will result in a much busier print, and our milling department is accustomed to ordinate dimensions. My question is: am I overthinking this or am I correct to rethink my dimensions?[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1487&stc=1[/IMG]Note that I am trying not to overtolerance the position of the keyway from the edge of the part - otherwise I would make the edge of the plate datum-B and place another position tolerance on the keyway itself.

The required accuracy of your fixture (and part) depends upon a combined the MAX- MIN limts of this fixture and the mating part fixture. This will determine the required accuracy and therefore the required dimensioning method for this part. From my perspective all critical dimensioning should be between the affected features to allow the maximum allowable tolerances for a successful finshed part. So, unless your fxture edge is a critical feature then it should not be used as a primary reference.

Your response generally sounds like it follows my thinking. I realize I may have misrepresented my original intent. I didn't mean to ask for accuracy values (I am comfortable with that). I am questioning the method of communicating my design intent with GD&T.

I will try and clarify my concerns... Datum-B as drawn establishes a datum center-plane in the middle of the keyway. The feature control frame for Ø.3144 holes establishes that this is the secondary datum reference. I am not sure that it is acceptable to show basic dimensions to the holes from anything other than what the datum references establish.

For example: The keyway may be machined in-tolerance such that the centerline is 4.636" off the lower horizontal edge of part. Since this is offset .010" from the as-drawn location, the Ø.3144 holes need to move with the keyway feature. Therefore, inspection must establish the centerlines of the position tolerance zones at 3.514 and 5.286 rather than drawing values of [3.504] and [5.276] respectively. Is this design intent in agreement with how the GD&T is set up in the above drawing?

Note: I believe dimensioning the holes directly from the keyway may clarify the design intent, but the ordinate dimensions are nice for the CNC operator to enter in the locations into the machine. (I'm hoping for the best of both worlds)

One feature dimensioning point on the that concerns me is that on the left view you give basic (ideal) locations for the holes in the plate as sequential dimensions vertically from the bottom edge which is not a designated datum. The true vertical datum for the holes is datum B; and, as a result, the vertical dimensions on the drawing should be from that datum; and any dimensions from the bottom edge for those features should only be shown as reference dimensions. Alternatively, because the left edge is datum C the use of the basic dimensions laterally from that edge is correct. Beyond that, your current feature dimensioning appears to be appropriate.

So, here is my quick markup - simply identify the keyway a datum "D", make all dimensions basic and locate the holes from A | D | C As required.


[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1489&stc=1[/IMG]

Also, you need to clarify Datum B and indicate number of places...

I too always try to make my drawings so that they will not be misunderstood. But I also try to dimension in a way that matches the application of the part. (I've told young engineers that "holes generally don't care where the edge of the part is. They care where the other holes are".) It should only take a certain number of dimensions to define the geometry of the part, no matter where zero is. I don't see how ordinate dimensioning from zero at the keyway would make the print any busier. It would certainly make it more accurate to the application.

Thanks for the thoughts guys... I will talk it over with my team. I do feel that dimensioning from the key as (zero) is most appropriate in application.