Datum Tolerance using sheet stock 1

[XaeroR35]

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.

 

[drawoh]

XaeroR35,

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.

--
JHG

 

[JNieman]

Have you considered whether or not Datums B & C should be the edge of the sheet at all? You specify that it's going to be screwed into a frame well enough to pull the part flat. Are the edges doing anything other than covering something? How is the part being located in the assembly? By the screws? If so, maybe the periphery should be controlled with a profile tolerance with the secondary and tertiary datums being locating features. If your edges really are locating features, then it makes sense to keep it as you stated, though. Even if the edges are just lined up 'by eye' with the edge of the frame, before installing screws.

There is an 'assumed square' rule in ASME, for edges drawn visibly close to 90. That is if you specify a blanket angular tolerance.

2.1.1.3 Implied 90° Angle.
By convention, where center lines and surfaces of features are depicted on 2D orthographic engineering drawings intersecting at right angles, a 90° angle is not specified. Implied 90° angles are understood to apply. The tolerance on these implied 90° angles is the same as for all other angular features shown on the field of the drawing governed by general angular tolerance notes or general tolerance block values. See para 1.4(i).

 

[ctopher]

I would make the stock thickness a ref dim, unless you plan on controlling the thickness tolerance...which would make it a more expensive part.
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

 

[ewh]

I think adding a 90° reference dimension is unnecessary and could set a bad precedent. Next someone will be asking for 0.000 dimensions. ;-)
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