How do I call out tight parallelism but loose flatness on two surfaces?

[Figbash]

I have a part roughly 3" x 5" with two parallel surfaces. The surfaces only need to be flat within .008" but parallel with each other within .0002". Can I apply a flatness tolerance of .008" to one surface and call it datum feature A, then apply a parallelism tolerance of .0002" to the other surface referencing it to datum feature A? My goal is to tightly control parallelism between the two surfaces but loosely control flatness.

Thanks,

Tom

 

[waqasmalik]

Yes you can.
Remember,variation in datum reference does not affect the parallelism tolerance value.It can as small as design requirements allow u.Datum feature A flatness will be be controlled in 0.008 of an inche and parallelism of considered feature will vary within .0002 of inch but remember one thing as well.For Planar feature whenever an orientation tolerance is applied then it controls its flatness also.So flatness of considered feature is limited to 0.0002 on an inch UNLESS U SPECIFY FLATNESS WHICH MUST BE LESS THAN 0.0002

 

[Figbash]

Thanks for the response. I found an example of a dimensioned part similar to ours that used a profile of a surface tolerance so I thought that might be more appropriate.

Here is a pic of my drawing:

Tom

 

[waqasmalik]

Advantage of adding profile is that it also locates your considered feature wrt Datum A and locating dimension has been made basic .426.So profile is controlling 3 things.
Location
Orientation
Form

 

[drawoh]

Figbash,

Parallelism is not a workable concept for what you are doing. Parallelism is measured with respect to a datum that is determined by picking up two or three points of your reference surface. Your reference surface is allowed to be very sloppy, even with a tight parallelism tolerance.

I think you are going to need a note on your drawing.

--
JHG