Datums order

[pmarc]

Guys,

I have a situation where I must decide which features should be assigned as primary and secondary datum feature for two components of an assembly. Under the link you can find very simplified drawing of the case.

I would like to know your opinion about the issue:
Should the cylindrical features be assinged as primary datum features or flat surfaces which both parts contact on? Maybe any other features should be taken into consideration?

 

[PeterStock]

Going by your posting and the drawing, for the top part I would choose the bottom of the hole and the hole dia as datums. As you show a loose fit, I would make the bottom of the hole the primary. For the bottom part I would use the top and diameter of the post with the top being the primary. These choices are base only on the mating situation that you present. I suspect I would choose other datums base on what other parts mounted to what.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[fsincox]

The standard attempts to tackle this issue directly. I feel the "do not bother with perpendicularity of pilots", crowd obscures just this issue. That has always been one of my problems with their way of thinking. In my admittedly idealistic view it is the designer's job to do just this, make that decision and "design" each joint so it will work the way it is intended. In the simplified example you show your diameter is significantly long enough to warrant controls by anyone's definition (I would assume???) I agree with Peter on strategy for the shown example.
Frank

 

[dtmbiz]

Since I dont see any dimensional values nor the scale of the drawing, I would caution using the internal diameter and particuarly the bottom of the hole as datums. The 1994 standard does mention that the selection of datums should include "practicality". Depending on if the size of the diameter is very small, then it and the hole bottom may be disqualified as being "practical" datums. Even though the they may not be selected as datums it does not disqualify them from having the required geometric controls to assure proper fit.

Something to think about.