Datum Tolerance using sheet stock
Datum Tolerance using sheet stock
(OP)
I have a question on datum tolerances using stock material (in this case .100” aluminum sheet). My drafting department says we must qualify all datum surfaces. However, my manufacturing department says sheet material this thin will be difficult (impossible?) to maintain a certain flatness (I agree here).
Typically for a machined part like this we would have:
* A - .010” flatness
* B - .005” perpendicular to A
* C - .005” perpendicular to A B

Now, flatness does not matter so much here because this aluminum plate will be screwed into a frame and any warpage will become flat by default. So I am comfortable leaving off the .010 flatness. The .005” perpendicularity of B to A seems impossible to measure, so I am also wanting to leave that off. What about datum C tolerance? Is it no implied to be 90 degrees to B?
Any tips of tolerance of sheet stock would be helpful.
Typically for a machined part like this we would have:
* A - .010” flatness
* B - .005” perpendicular to A
* C - .005” perpendicular to A B

Now, flatness does not matter so much here because this aluminum plate will be screwed into a frame and any warpage will become flat by default. So I am comfortable leaving off the .010 flatness. The .005” perpendicularity of B to A seems impossible to measure, so I am also wanting to leave that off. What about datum C tolerance? Is it no implied to be 90 degrees to B?
Any tips of tolerance of sheet stock would be helpful.





RE: Datum Tolerance using sheet stock
You have two choices here. You can specify flatness that is within the capabilities of your material, and make your design work with it. You can specify that datum A applies when the face is clamped to a flat reference surface. There are all sorts of other things you can do, but I think one of these two should work.
In ASME Y14.5-2009, look at section 4.20 Restrained Condition.
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JHG
RE: Datum Tolerance using sheet stock
There is an 'assumed square' rule in ASME, for edges drawn visibly close to 90. That is if you specify a blanket angular tolerance.
RE: Datum Tolerance using sheet stock
No need for the datums, unless maybe there are holes in the part? Maybe only a 90 deg ref dim.
FYI,
Over the years I have learnded that anything thinner than .062 is difficult to control flatness.
I found these links for ref:
Link
Link
Link
Chris, CSWP
SolidWorks '16
ctophers home
SolidWorks Legion
RE: Datum Tolerance using sheet stock
Is there a good reason that "Typically for a machined part like this..." the tolerances are as they are? If they are not required for the design of your particular part, you should be able to loosen them as long as the design parameters are not negatively affected.
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV