PCD tolerance wrt true position

[dogbural]

Hi,
I have question on tolerance of PCD Dia 225 with profile tolerance of 0.2mm.
When drawing specifies the true position tolerance of Dia 6.2 holes at 0.1mm (+0.05), do we still need to measure PCD Dia 225? If so, do we still need to apply 0.2mm tolerance?

Thanks

 

[Burunduk]

The PCD diameter is basic because it is part of the definition of the true position axes of the holes. This diameter is not measured. Only the holes are measured for conformance to the dia. 0.1 position zones that are constructed around those true positions axes. Profile of a surface is irrelevant to all that.

 

[3DDave]

Since it appears to be unclear to dogbural how true position works, I suggest starting here:

https://www.gdandtbasics.com/true-position/

 

[drawoh]

dogbural,

Your holes must be located inside a diameter of 0.1(mm?) with respect to the exact nominal position defined by the pitch circle and the angle position. You need to inspect the true position of each hole.

How are you going to measure pitch circle diameter? Will you arbitrarily select three holes? Will you measure them all and take an average? These all are bad ideas.

Positional tolerances actually are a good thing when you specify pitch circles. If you specify [±][ ]tolerances on the circle and on the angles, things get inaccurate as the diameter increases.

--
JHG

 

[drawoh]

dogbural,

Is this your drawing?

I recommend that you leave out the text "PCD". Not everybody knows what it means. The intent of the dimension is pretty clear without the text.

--
JHG