Tertiary Datum Needed?

[Cobalt_blue]

Hello,

I am new to GD&T, and I would like to know the proper way to dimension the three-hole pattern in the attached drawing. The holes have to be in the 12, 4, and 8 o'clock positions with respect to the flat. They can't be rotated, or this part won't align with the part it mates with. Is the drawing good as is, or should a tertiary datum be added? If so, what should be used for the tertiary datum?

Thank you.

 

[chez311]

What standard is this being drawn to?

As it is drawn, the pattern of 3x holes have no orientation requirement to the flat.

If ASME you can use simultaneous requirements by making the dimensions for the inner profile (semi-circle with flat) basic and applying a profile tolerance to |A|B|. This will clock the inner profile and pattern of 3x holes to each other. This would be the simplest solution.

 

[TheTick]

It would be difficult to fix the angle using the flat as a tertiary, thoguh CMM could do this no problem.. Though maybe a good place for composite FCF.

Alternatively you could use one of the holes as a datum, then control the center opening relative to the hole pattern.

 

[CheckerHater]

You can also control the flat (or the entire opening) with Profile wrt [A|B].
Simultaneous requirement will then align holes and flat together.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[drawoh]

Cobalt_blue,

You can define the hole pattern as a datum. Actually, the hole pattern makes an excellent secondary datum, since it appears your outline and your cutout are not particularly accurate.

--
JHG

 

[Cobalt_blue]

Thank you all for the help in completing this drawing correctly. Much appreciated.