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Machining holes on a PCD

Machining holes on a PCD

Machining holes on a PCD

Hello to anyone that's clicked this link!

As indicated above, I have a question concerning the machining of holes on a PCD. To be more specific, holes in a shaft flange (big flange).
The PCD of the holes (10 holes) is 900mm. Naturally there is a matching flange with the same number and PCD of holes. The holes are 90mm diameter. The PCD on the drawing has no tolerance. I am trying to establish what the maximum step between the two mating holes will be when the two shafts are brought together for assembly.One of the problems is that the two shafts may be manufactured at different sites and by different companies. This would rule out the use of a jig (if that were even possible), which I assume would offer quite a degree of consistency.
So, if anyone can shed some light on the question about the maximum step, I would be very grateful.

At the moment, I am considering that:-
Shaft 1 is machined to 900.25 (max if I assume a tol of +/- 0.25mm)
Shaft 2 is machined to a PCD of 899.75 (min)

That would leave a radial step of 0.25mm when they're brought together. Thing is, is it realistic to think +/-0.25 is possible?

Thanks in advance for any replies.

RE: Machining holes on a PCD

The holes you are machining have a true position tolerance not the Pitch Circle Diameter. The PCD is considered as a basic dimension.


RE: Machining holes on a PCD

Google "floating mating fastener", you will find plenty of info on how to specify this.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Machining holes on a PCD

Thanks to BillPSU and dgallup. I receive drawings all the time where the shaft manufacturers will do one of two things:-

Add a general PCD, machine the holes to say 3mm below the final intended size and disregard the misalignment. When assembled, line bore to create a true, straight hole without any step.

Apply a true position tolerance and relatively tight tolerance (say H7).

It is the 2nd scenario of the above two that I'm interested in. Can anyone advise what sort of step we might encounter without specifying a non standard/expensive/additional machining process? So lets say a 900mm PCD might have co-ordinates from the first hole of say X=180 and Y=-200. What would be a realistic tolerance to add to those two figures? +/-0.25? +/-0.10? Even better?

Sorry if it's all a bit 'woolly' at this stage but my gratitude to both the above and anyone else who can help.

RE: Machining holes on a PCD

What is the size and tolerance of the clearance holes in the flanges?

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
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SSG, U.S. Army
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