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True Position

True Position

True Position

Hello everyone
I am wondering about the true position diamter value. In my case I have two circle parts which has several hole and the coaxiality tolerance is very important between the holes. Please se the attach image. The hole is marked with black color.
I have add true position value for upper part (the green one) 0.2 and for the lower part (yellow one) I add the true position value 0.1. Is it correct to do that or shall I keep the same value for both part?
Between the parts pins and shaft will be placed.

RE: True Position

Quote (OP)

coaxiality tolerance is very important between the holes.

Aren't those clearance holes in the green part? Why you care so much about their alignment with mating holes in the bottom part?

RE: True Position


Your true position tolerance is a function primarily of your screw configuration and clearance.

There are instructions for calculating this in the ASME Y14.5 standard. I too have worked them out, and I have provided pictures — HTML PDF.


RE: True Position

Thank you for your inputs. Acctually I am not aiming for the bolt holes. I have 3 smaller hole with diamter 1.98 mm in both part.
In the lower part (Yellow one) I used 0.05 as true position diameter and in the upper part (green) I devide the true position as 0.05/2 and used 0.025 as true position diamter.
What I am looking for is, if there is a rule of choosing or calvulating the true position diameter, when it comes parts that will make an assembly.


RE: True Position


It does not matter if your holes clear bolts, shafts, cotter pins or dowels. The calculations are the same. ⌀0.05mm is a tight positional tolerance. Standard practise with dowels is to make a hole to provide XY, and a slot to control θ.


RE: True Position

We can't talk about position tolerances until we know the datums. Are the position tolerances for each part tied back to common datums on each part?

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