BOM/PL Question 1

[Qrazy]

Hi Guys,
Long time lurker deciding to join. I have a strange question, I was hoping I could get opinions on. I was looking for everyones thoughts on material called out in the BOM/PL.
For example: If the BOM/PL calls out 2.500 thick aluminum material for an item to be made from. Does the drawing imply that it can ONLY be made from 2.500 thick aluminum, or is it a suggestion that the material be made from that in order to meet the minimum requirements made by the draftsman/designer/engineer? Does the drawing imply the item could be made from 3.000 thick aluminum or any other size of the same material, as long as it allows the item to be made?
There is a discussion going on here regading this topic, and I was looking for your thoughts...

 

[MintJulep]

Unless there is a valid technical reason to make something from a very specific precursor part the drawing should only describe the necessary finished part.

Process information generally should not be included on part drawings.

 

[jjengel]

2.500 in what units? Inches? Millimeters? Is it sheet metal to be formed or a machined part? If it's a machined part, is there a final thickness dimension?

My opinion is that if there's a final thickness dimension then it wouldn't matter. It makes me think that 2.500 may be a stock size that keeps machine time and material costs down to a minimum.

If it was a 2.5mm sheet metal part, then I'd say that you'd be best to stay with that thickness for several reasons.

But that's just my novice opinion...

Jason

 

[GregTirevold]

The only time I've put dimensional information in a B.O.M. was in a description for a raw material "buy item". In my case it was a part from McMaster Carr, a block of 6061 aluminum, that was sufficiently sized to allow the machining I wanted done to it. Basically I was not telling the machine shop they HAD to use the part from McMaster as the raw material, just offering it up to them as a quick way to get the material if they didn't have stock as they primarily machine steel parts for us.

As Mint mentioned, there would have to be a very specific reason to insist on the raw stock size and anyone who was that concerned would go out of their way to ensure you knew it. Plus there would almost certainly be other material handling requirements or surface finish requirements above and beyond the norm noted on the drawings as well.

Maybe a former vendor had issues with their tooling or equipment, like a 3" thick piece would fit in their equipment, that mandated they get certain size raw stock and it was spec'd on the drawing for that reason.

Can't you just ask the customer as well?

http://www.linkedin.com/in/gregtirevold

 

[Qrazy]

Hi,
Thanks for the quick responses. I should have elaborated more in my first post. Greg's first response is more what I am asking about.
It is in inches. If we have an item that will be machined from an aluminum plate and it has to be 2.250 thick (dimensioned on the drawing) to the tolerances specified on the drawing. We have in the past put 2.500 thick on the BOM/PL as a material for that item. In this case that would leave enough material to clean the part and machine to final size. With that being said, does the drawing imply you have to use 2.500 thick aluminum plate to make the part from, or is it there to tell you that is the minumum material the drawing recommends you to make it from. In this case, a piece of 2.250 aluminum plate in it's stock form may come in too small from it's manufacturing tolerances to allow it to be considered as the material for the part. Does the drawing imply you could make it out of 3.000 material because it is more readily available, or because it clearly says 2.500, does it have to be made from 2.500?
Sorry, I know this is a strange question, but it has been a philosophical debate here. Looking for other opinions.

Thanks again

 

[MintJulep]

If you take a 5 thick piece of stock and cut it in half (with a kerfless saw) you get a 2.5 thick piece of stock. Would that comply with your "requirement"?

If it is not important to the form and function of the finished part then it does not belong on the drawing.

If the "make from" material is critical to the finished part then the "make from" description must be complete and unambiguous.

 

[Qrazy]

Interesting MintJulep...So accordingly, just putting Plate as the description and the material specification (5052-H32 for example) is all that should be put in the BOM/PL?
Could you then make the argument if you take a cylinder of aluminum and cut/mill it into a plate it would suffice?
My thoughts are, the material listed is the material recommended by the person creating the drawing. As long as the material meets the specifications for that material the only size and shape that really matters is the final one.

Thanks for the thoughts, I appreacite them

 

[MintJulep]

Is it important that the finished part be made from plate stock?

If, using your example, you took a cylindrical piece of stock, cut or milled a "plate" from it, then made your finished part, would anyone ever be able to look at the finished part and know what is was made from? Would the finished part perform any differently?

If it is not important to the form and function of the finished part then it does not belong on the drawing.

If the "make from" material is critical to the finished part then the "make from" description must be complete and unambiguous.

 

[KENAT]

As Mint has been telling you, it's the finished part that matters in your situation. Per ASME Y14.5M-1994 section 1.4, only give 'process' type information if it effects end function/performance.

Specify the grade of material etc. but don't spec the start stock dimensions. Heck even this can get tricky because the heat treat may vary for different thicknesses e.g. T6 V T651 although for most intents and purposes they are interchangeable, or different sizes (or other condition) are to different specs etc..

Specking start size sounds like a good idea and purchasing love you for it (assuming internal machine shop) etc., however one day you can't get 2.5" stock, you have a critical deadline, you can only get 3" stock, your contract doesn't allow any production permits/concessions/waivers/deviations from drawings... and you are royally screwed.

Technically, if you say it has to be made from 2.5 inch stock plate then it has to be made from 2.5 inch stock plate. Not making it from such material is not in compliance with the drawing and from a paperwork point of view cause for rejection, even though in most situations it wouldn't' be obvious from examination of the part.

Oh, and and unless it's a detail or inseparable assy or something, why does a machined part have a parts list/BOM?

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[KENAT]

Oh, there is sometimes justify for a suggested make from note.

For example "MAY BE MADE FROM MCMASTER-CARR PN XXXXXX AVAILABLE FROM..."

However, not the wording this is not a mandatory requirement and I tend to avoid doing this unless there's a real benefit.

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[Qrazy]

Thanks again for the responses.....

The machined part in these cases usually have other components with them but not always. Item/Find number 1 is the material the item is to be made from, in this case normally we would list plate, 2.500 thick, then list the material in the material specification block to the right, 5052-H32, ASTMB209. Item/Find number 2 may be something like a helical coil insert, etc...
I guess my thoughts were if you called out 2.500 thick material, and you could only get 3.000 material, how would you ever know in the end the part was made from 3.000 and not 2.500? In this case I am not talking about a "stock" size where the material is used in the size it comes from the manufacturer in. I was refering to calling out an oversized material to allow machining of the material to bring the part into the form, fit and function the drawing dictates. I guess I didn't realize the drawing was a binding document saying the bulk material called out to make the part from has to be exactly that, and not another size of the smae material that can be used to greate the same end result.
Again, strange question, interesting responses...Thanks