Flatness Implication

[jerry1423]

I have a steel shim cut to a certain shape.
It is symmetrical, so it can be flipped over and used that way.
The engineer wants me to put a flatness spec on the bottom face (ok, fine) . . . and he also wants me to make the bottom face datum A and add a parallelism (to datum A) spec to the top face.
How can I explain to him that the flatness spec on the bottom face is all that is needed, and it will apply to both the top and bottom faces?
Is this in a standard someplace? I rememeber reading it, but that was over 20 years ago. He needs more than just my word on this.

 

[powerhound]

That is rule #1. Per ASME a flatness tolerance equal to the total thickness tolerance automatically applies to each face of the shim. If the flatness tolerance needs to be tighter than your size tolerance then you do need to refine it with a flatness callout. If you are working to ISO then this has to be invoked by using the circle E.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
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SSG, U.S. Army
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[pmarc]

jerry1423,
No, the flatness applied to the bottom face of the part only (without any additional notations) does not apply to the top face. Flatness of the top face would be controlled by size dimension (acc. to Rule #1).

 

[KENAT]

Like others say, the flatness on the bottom does not explicitly apply to the top face as well. The top face may be limited by the thickness tolerance if rule 1 is in play, but it still may not be the same as on the bottom face.

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[jerry1423]

I should add that there is no machining to the top and bottom faces. It is the stock thickness
Does that make a difference ?

 

[powerhound]

Yes it does. Now you do need to control it.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MechNorth]

Rule #1 doesn't apply to stock material, so the flatness on the top and bottom do need to be controlled. If you have a toleranced thickness, consider applying a flatness tolerance at MMC to the FOS; this will allow you to indirectly control the flatness on both faces AND you can hard-gauge it quickly & easily; plus, you don't need to control individual surface flatnesses then.

Jim Sykes, P.Eng, GDTP-S
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[pmarc]

Does the print clearly specify it is a stock thickness?

 

[jerry1423]

Yes, the print clearly specifies it is a stock thickness

 

[powerhound]

Then rule #1 does not apply to this part. You will have to add a flatness callout in order to ensure it meets your requirement.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II