Datum selection based on manufacturing or upper level assembly

[theedudenator]

Just wanted an opinion on this one...

This is my thought on the subject.

Plate with a hole in the center.
The center hole is created by holding the bottom surface and 2 sides.
The plate is assembled by placing a bolt to the hole and clamping against the large bottom surface

I would do the following:
Bottom of the plate has primary
The hole as secondary
Possibly one edge has tertiary (if needed)

Some people follow the manufacturing method.
Bottom as primary
Side as secondary
Side as tertiary

Any thoughts on this??

 

[KENAT]

If the hole is effectively the critical feature I'd probably use it.

We have cassettes that are located by a pin through a hole. On the drawings of the cassettes we use the hole as the secondary datum if I recall correctly.

Function/Inspection of the part is more important in the dimensioning scheme than manufacture.

You could perhaps make the face and sides Datums A, B & C then make the hole datum D and relate other features to it. Depends on the function/design of the specific part though.

 

[theedudenator]

But.... the datums should be defined by part functionality.
Not in an attempt to mimic the manufacturing method.

 

[KENAT]

That was the point I was trying to make and didn't mean to imply otherwise.

Hierarchy in dimensioning is:

Function
Inspection
Manufacture

Of course as there's no point putting a requirement you can't inspect or otherwise verify it almost ranks equal to Function.

 

[dingy2]

You are correct by using the hole as a secondary and the mounting face as the primary datum. If the part is assemetrical, then a tertiary is a must and probably another hole should be used rather than a side.

Manufacturing must somehow follow your datum set and not change it.

Dave D.

http://www.qmsi.ca

 

[ctopher]

I use the datums based on how the parts are assembled to each other, not per manufacturing.

Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[dingy2]

Datums should always be developed by the function of the part. That is it!!

Dave D.

http://www.qmsi.ca

 

[ctopher]

Of course!
But you need to look at how the part is assembled in relation to a mating part ... not to a machine.
It is all I was trying to say.

Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)

 

[ringman]

theedudenator,

What is it that you are looking for thoughts on?