Two Cylinders as Datum Features for True Position
Two Cylinders as Datum Features for True Position
(OP)
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
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





RE: Two Cylinders as Datum Features for True Position
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.
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JHG
RE: Two Cylinders as Datum Features for True Position
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
RE: Two Cylinders as Datum Features for True Position
RE: Two Cylinders as Datum Features for True Position
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.
RE: Two Cylinders as Datum Features for True Position
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.
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JHG
RE: Two Cylinders as Datum Features for True Position
Please, please, please, use the correct terms!They're Datum Features NOT DATUMS!!! AAAAAHAHAHAHHAHAHAHHAAAAA" -- Don Day