When a cylinder is a datum used to clock?

[JoeBlock]

When a cylinder is used as a datum to clock (Example: Rack and pinion housing yoke bore)do you just use the center point of a circle or slice of the cylinder or use a line constructed from two separate circles of the cylinder?

 

[ewh]

You would use the axis of the cylinder (or, as you put it, a line constructed from two separate circles of the cylinder).

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[KENAT]

Though to follow in from ewh your 'datum feature' would be the diameter of the cylinder. If I'm understanding your situation that is.

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[drawoh]

JoeBlock,

I do not think we understand the context of your question. Read my reply carefully and see if I understand your intent.

I have a part with a base, a big round thing, and a small cylinder, tagged as datums_A, B and C, respectively. In this configuration, the small cylinder, datum_C, is the clocking feature.

For datum_C, I would apply my datum indicator to the OD of the cylinder, on the top view if at all possible. My second choice would be to apply it to the diameter callup. It could be applied to a side view, but this would be less clear.

Any dimensions to or from datum_C would be to the centre of the cylinder.

My manufacturing and inspection fixture would have a slot that would pick up the cylinder, and which would be aligned to the big round thing, datum_B. Note that your cylinder is a feature of size. If it does not have an accurate diameter, you are going to have to think about and call up the cylinder's MMC.

If I wanted my datum to be tangential to the cylinder, I would apply datum targets. This is more complicated and somewhat weird, but it would get you around the MMC condition noted above.

JHG