Two Cylinders as Datum Features for True Position 1

[ModulusCT]

Hi guys,

Pretty simple stuff here, but I'm drawing a blank and could use some advice. I have a part (pictured below) and I'm trying to apply True Position to the 8, small cylindrical protrusions... My Datum A is a surface normal to the axis of these protrusions, and Datum B is a cylinder below the face you can see in the picture (indicated by the dimmed hidden lines). Now, my understanding is that Datum A & B will constrain the part, but still allow it to rotate about the axis of Datum B. So, the cylinder on the other side of the part, I thought, should be datum C and well, you can see my thoughts on how to fully constrain the part. Is this "B-C" legal in this situation?

If I were to say TP to A|B|C, I don't think that would make very much sense, as I would have to duplicate the x and y dimensions from Datum Feature C.... Maybe TP to A & B-C only would be best? Well, I'm confused. Which is the correct method? The datum features I've called out in the pic below are just for reference... They do not show the actual datum feature... Though, they lie on the same axis. Thanks for your help!

Mod

 

[drawoh]

ModulusCT,

As you note, datum[ ]B controls your position in[ ]X and[ ]Y. All your tertiary datum controls is rotation. I don't see why datum[ ]B should be involved. If you were to call up the datums as A|B|C, datum[ ]C would be a clocking feature, and you would constrain it in a slot. Probably, this is what I would do.

You could call up A|B-C. Datum[ ]B-C would be a slot that would accurately contain the two features. I don't know why you would do this.

--
JHG

 

[ModulusCT]

Hi drawoh, thanks for the response.

Would you mind explaining a little more about how one might go about using Datum C as the clocking feature? Specifically, what kind of equipment would one need to use a tapered cylinder to control rotation? Would it literally just be a slot in some material? I only ask because.... well, because I'm learning. Thanks for the help.

Mod

Please, please, please, use the correct terms!They're Datum Features NOT DATUMS!!! AAAAAHAHAHAHHAHAHAHHAAAAA" -- Don Day

 

[greenimi]

Could someone explain how this part gets assembled? Which feature first ?

 

[ModulusCT]

The 8 circular bosses are threaded (internal) and used to mount the thing to a bracket. My goal with my dimensioning scheme is to provide as much tolerance as is allowed by the design of the bracket.

There is no functional relationship between the bosses in question and the datum features called out (except in a rather loose sense - simplification of the drawing, perpendicuarity control to A, etc.)... they were chosen simply because they are critical for other parts of the design not shown.

So, I suppose, to answer your queston, the 8 bosses will first be assembled to a mounting bracket and then threads on Datum B and C are used to assemble the other components. Can't specify much more than that. I apologize for the incompleteness of this question if it's making things difficult.

 

[drawoh]

ModulusCT,

If you use primary, secondary and tertiary datums[ ]A, B and[ ]C, datum[ ]C keeps the part from rotating. Since the datum[ ]C feature is not located perfectly from the datum[ ]B feature, you cannot use a round hole in your fixture. You need a slot that controls only rotation.

Greenimi is asking a good question. You are showing how your part is to be fixtured for fabrication and inspection. This is how it ought to be mounted when used. If your eight bosses are your mounting system, they make good datum features.

If your datum[ ]C feature is tapered, how are you going to fixture to it? This is all the more reason to use your eight bosses.

--
JHG

 

[ModulusCT]

I thought for a bit about using the 8 bosses as a datum feature... However, they're tapered too (I believe there are chucks out there that can adjust to accept a tapered part yes?). They are for mounting, however, the slop in the clearance holes on the bracket is generous, and can be because the overall position of this device is not critical. Still, perhaps it's best to use them the way you suggest. Not entirely sure how I would go about doing that. Phantom lines connecting all of the highest points of the bosses and then a flag note to an extension line? I'll look it up. Thanks.

Please, please, please, use the correct terms!They're Datum Features NOT DATUMS!!! AAAAAHAHAHAHHAHAHAHHAAAAA" -- Don Day