Sheet stock tolerances 1

[QAOne]

Have a requirement for .105 steel stock on a drawing. The question has come up about whether the normal drawing tolerance applies to this (+/- .010 for example) or does the ASTM requirement for sheet tolerancing (in this case ASTM-A568, +.006/- .000) apply? And should the ASTM tolerance be put on the drawing?

 

[KENAT]

How is the sheet called out, & what standards are you working to?

1.4 (a) Each dimension shall have a tolerance, except for those dimensions specifically identified as reference, maximum, minimum or stock (commercial stock size).

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[QAOne]

The material required is steel sheet to ASTM A1008 which requires ASTM A568 for sheet tolerances as stated. The drawing now just states .105 stock for the thickness. Should the standard drawing tolerance apply (in this case +/- .010 or should the ASTM A568 tolerance of +.006/-.000 be stated for this?

 

[KENAT]

When I said standards I meant drawing & documentation standards, sorry if I wasn't clear.

If the drawing says 'STOCK' or 'STK' then you have to reference the relevant material spec (ASTM-A568 in your case) for actual tolerances - general drawing 'block' tolerances do not apply. You wouldn't normally copy the tolerances from the standard onto the drawing.

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[WKTaylor]

QAone...

Be careful. Aerospace wrought materials [like sheet, plates, extrusions, etc] have specs that control tolerances for each form AND alloy type [aluminum, steel, CRES, titanium, etc].

ASTM materials are a much different animal. What bothers me is the thickness You indicated: 0.105-inch... which is a highly non-standard. In this case there would have to be a unique purchase agreement [contract] in-place that specifies sheet thickness, width and length tolerances.

Regards, Wil Taylor

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[berkshire]

It sounds like somebody has called out a wire gauge sheet thickness , 12gauge steel is .1046" nominal thickness. Ok for the general sheet metal trades, not for the aircraft industry.
B.E.

You are judged not by what you know, but by what you can do.

 

[tbuelna]

If the drawing shows a dimension followed by "STK" that means the inspected dimension shall be accepted as whatever condition the raw material comes with. The dimension does not normally require any inspection other than that performed during receiving inspection of the raw material. The thickness of steel sheet stock can change due to forming operations. For example, sheet stock will be thinned in the bend area of a brake formed part. Possibly enough to reduce the thickness in the bend below the spec tolerance.

 

[GregLocock]

As a matter of experience, if you pay the material supplier by weight it'll tend to be surprisingly close to top tolerance, and if you pay by area, it'll be surprisingly close to bottom tolerance. But if you ask them to hold tighter tolerances you'll be told it can't be done. They really do think we are that stupid.

Cheers

Greg Locock


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[rb1957]

i think berkshire has the answer ... some engineer tried to be too smart, or maybe some CAD program, or maybe the rest of us (maybe only me??) are too lazy.

we'll often refer to 0.1" thk Al sheet, which might be referred to in the materials world (as it truly might be) as 12swg. i guess the inherent advantage to this is buying 12 swg thick sheet satisfies the drwg call-out (for 0.1" thk).

in any case, i don't think standard drwg tolerances (+-0.010) apply to thickness of stock sheets.

i thnk another clarification comes from how the sheet is annotated in the BoM. if the engineer wanted a non-standard thickness then the detail view would show the finished thickness (say 0.115") as a hard dim'n, and the BoM would call out "0.125" thick"

Quando Omni Flunkus Moritati

 

[3DDave]

It depends on the drawing standard that is used in preparing the drawing.

According to 'Y14.5 the tolerances on stock material is that given by the stock material specification. Unless there is a note to override this, that is what would generally be used.

The ugly part is that the tolerance given in material specifications varies depending on the width the material is originally rolled to. It is a spec that tells what the buyer of material direct from the rolling mill can expect. After the material is cut to smaller sizes, it isn't easy to tell what the original width was. Side-by-side one can have pieces where one would have failed the thickness tolerance of the other.

If it is important, I'd create or look for an in-house material spec that is used to constrain material to a more desirable range.