Geometric control on "stock" 1

[GordyRC]

All,

I am a new member here and was reading some of the posts with great interest.

One topic I encounter often involves including a form control on material which is designated "stock". The example I will recite would be a 1/2 inch thick stock plate, but then there is a flatness tolerance applied. The stock, as delivered may or may not meet the flatness requirement. The material standard tolerances are often hard to find, and usually not within the engineering requirement. Additionally the plate might be included in a weldment which might deform the flatness even more. Unfortunately there is nothing that allows a machinist to make a clean-up cut to achieve the flatness requirement. If a clean-up cut is taken to achieve the flatness then the 1/2 stock requirement is violated.

Here is my solution; Bill of Materials may list the detail as 1/2 inch stock plate. However in the field of the drawing I include the flatness requirement and dimension the plate thickness with something less than the 1/2 inch stock, say, .45 minimum. This allows up to (in this case) .050 material for clean-up to achieve the flatness requirement.

Regards,

GordyRC

 

[drawoh]

GordyRC,

I have learned the hard way to not specify stock. I may design around and specify tolerances that will allow stock, but I do not call up stock. When you specify stock, you are telling the fabricator how to do his job. You should not do this.

Call up the dimension, tolerance and feature controls of that part that you are willing to accept. Let the fabricator figure out how to do it.



--
JHG

 

[KENAT]

I don't know, completely excluding 'stock' for say sheet metal parts may not make sense Drawoh. However, for machined parts etc. it does often cause issues.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[GordyRC]

At my company we are required to call out the stock thickness in the BOM. This is required to facilitate planning and purchasing material.

 

[drawoh]

KENAT,

ON sheet metal parts, I call up the thickness and tolerance. Obviously, I know darn well they will be using stock material, but consider this specification for aluminium sheet...

.081"±.015

I have called up 12[ ]gauge, however, 11[ ] and 13[ ]gauge material are within specification. If I am designing something out of sheet metal, I try very hard to use the same thickness material on everything.

--
JHG