datum setup of irregular holes on a flange

[bxbzq]

This is a typical starter on vehicles. When it starts to work, the pinion moves axially and engages with flywheel of the engine then drives the engine to rotate.

The starter is positioned by the pin hole and mounted and clocked by the two clearance holes. The edges of the flange are not important for function and assembly. I've been trying to figure out the datum setup for the starter. Datum feature A is the flange surface, should be fine. For datum B and C, I'm not sure. I'm thinking using pin hole as datum feature B and a plane established by the axis of two clearance holes as datum feature C.

I would like to see others' opinions.
Details see attached sketch.

Thanks.

 

[powerhound]

All you would need to arrest all degrees of freedom are datums A and the two clearance holes (call it B). You could then position the pin hole with respect to datums A and B(M).

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Belanger]

PH, that was my initial thought too, but it sounds like the far hole is the one that is meant to functionally position the part. Maybe it has a tighter fit with its mating pin. If so, then the two holes down below may only stop rotation.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[bxbzq]

Hi Powerhound, hi JP,

The pin hole is used to position the part before the part is mounted.
If use pin hole as secondary datum, would it be a better option to use only one of the clearance holes as tertiary datum?

Another basic question, if use the two clearance holes as a datum, is the datum the central axis of the two clearance holes or a plane established by the two axes?