Material callouts - cross post from GD&T forum

[DReimer]

I originally posted this in the "Drafting Standards & GD&T" forum, but I figured I would crosspost it here as well. I know there are a lot more participants in this forum!
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I'd like to survey the participants in this forum regarding material callouts on part drawings. I'm especially interested in companies that design and fabricate their own parts (rather than design only).

Our company uses an MRP system, and as such every raw material we use has a part number. Our raw material part numbers are significant - e.g. ALF3/8X2 is, you guessed it, aluminum flat bar 3/8" thick by 2" wide.

Our tradition has been to call out the actual RM part number and quantity on the title block. If, for example, we design a part from aluminum and it is 3/8" thick and 2" wide and 7" long, then we would call out ALF3/8X2 (x7.00) in the title block (provided the standard mill tolerance on the material was acceptable for the application).

This aggravates me for several reasons:
- we've got a duplicate bill of material for every part: the MRP bill of material and the drawing bill. This is a significant source of error when you try to maintain parallel BOMs. Inevitably one gets changed without the other, and problems usually follow.

- it adds burden in engineering, because we are responsible for both BOMs. For example, if the planning department finds that we use a lot of 1/4" aluminum plate in certain widths they ask us to create a P/N for plate sheared to width and change the affected BOMs. With both MRP and drawings to change, this is a fair amount of (non value-added) work.

- we've been doing this long enough (on MRP for at least 12 years) that our material cutters aren't even taught how to read the drawing. As a result, if the title block says cut it 7" long, they'll cut it 7" long without regard for the length tolerance or machining allowance. We get an astounding number of requests to change the title block to tell the cutter to cut the part 1/16" too long so the machinist has some material to remove.

- while the RM part numbers may be obvious internally, it can cause problems with outside vendors. An example just came up today. We wanted a bronze-tinted polycarbonate plate with some holes in it, but the part we got was clear. The only material callout was: LEPBRZ1/4. Well I know that is our designation for bronze-tint polycarbonate, and the engineer that designed the part knows it, but obviously it wasn't clear to the vendor. That's a $150 mistake that directly affects our bottom line.

What I'd like to do is call out the material spec on the drawing (e.g. 6061-T6 Aluminum, C1018, Type 304 SS, Bronze-tinted polycarbonate) and let the planning department (or vendor) take ownership of the bill of material. If they're short of flat bar and want to substitute plate, it is their call as long as it meets our material spec.

What does your company do?

Cheers,
Dean

 

[handleman]

Sounds like you're in one-off machine design/automation as I am. I know my parenthetical says (Automotive), I'm in the in-house machine design division. Anyway, we just spec the type of material to use as you say in your last paragraph. There's no sense paying an engineer to order raw material. Your internal shop should see Engineering the same as an outside vendor would. That is, Eng. sends a drawing of a finished part, they send a part per that drawing. It's up to the shop what size material to use as long as the material type is correct.

There was a similar discussion to this in the SW forum about how/whether to phantom in the raw material on a part drawing - check out thread559-150538.

 

[MikeHalloran]

Given that you have a B/M already in the MRP system, maintaining a parallel copy on the field of drawings is, well, insane.

I could swear that many MRP systems can maintain a 'blank size' for individual part numbers, and can on demand produce a 'cutting list'. I think you are trying to carry too much information in the title field, and that prevents you from using the computer to advantage. If part records instead had individual fields for alloy, blank width, blank length, blank thickness, etc., you could maybe do better.




Mike Halloran
Pembroke Pines, FL, USA

 

[ajack1]

As a design house we have customers that require finished sizes, cut off sizes or both.

It really is hard to “guess” cut off sizes without knowing what they stock or buy, for example is mild steel hot rolled or cold rolled and tool steels black or pre-machined.

The way we approach cut offs is if the size is important we add .00 or if not just the stock size. So for example something that is just a spacer would be called out as say Dia 50 where as a shaft that requires machining would be Dia 50.00, this gives no indication of tolerance on the cut off but does denote that machining is required, both visually and as a note.

Things get even more difficult if there are more than one way to produce something, for example if you are going to CNC mill or wire erode the part then extra material is required all around for holding, or you may wire erode many parts from one block.

If someone who will not see the detailed drawings is cutting up or ordering there are many pit falls and nothing we have tried is perfect. If anyone has a perfect solution please let me know it will save many £’s and hours of work.

 

[handleman]

To indicate machining required/not required you could use surface finish marks. There's one for "no machining required" and plenty more for other levels of surface finish. Using 50.00 with no tolerance could be confusing to someone not familiar with that system, because the added precision seems to imply a tight tolerance (maybe ±4µm?).

 

[DReimer]

From MikeHalloran: "Given that you have a B/M already in the MRP system, maintaining a parallel copy on the field of drawings is, well, insane."

I agree. It is insane and it leads to frequent errors. On our legacy ACAD stuff the information is just text in a title block, so it isn't linked to anything which means it is very easy for the information to diverge from the MRP database over time.

Unfortunately our raw material cutters often don't even look at the part, and simply cut what the title block tells them. That's just putting too much faith in something that is so easy to get wrong.

Unfortunately the problem is compounded by the way our MRP system displays the material requirements on a work order. If, for example, a part requires 3.44" of material and the work order is for 7 pieces, the work order tells the guy to cut 24.08" of material (plus a scrap factor). And we can't configure it to display a length per part.

Of course, the guy could look at the drawing to see how much he needed, but our production department doesn't want to "slow them down" with such mundane things as actually looking at the part they're making.

 

[Tofflemire]

Hi All,

I would not call the material size or type in the title. Just dimension the part. Our machine shop then can use anything they have as long as it meets our dimensions. Use notes to state the material 6061-T6 Aluminum or 316 SS...

If I were to make a drawing using your way, I would do for a drawing of 3/8"thkx2"w would make the drawing title BAR,FLAT,Alum,.38THKx2.0W and drawing #ABC-01. Then would draw the part and put a length value (7"). The thickness and width would be a reference dimension. We also add a Note on the drawing to call up the material either as our internal part number or what ever you would purchase from the supplier. If we wanted another bar that is 12" long we would go to drawing ABC-01 and create ABC-02 on the same drawing make a table with a "L" variable. The drawing of the part would have "L" for the length and the table tells you what length to cut L based on the part you order ABC-01 or ABC-02.

Tofflemire

 

[drawoh]

I quote the material type, in as much detail as I require. For example, if I want stainless steel, I write STAINLESS STEEL in the material box. I only mark in the grade if it matters to me.

I have problems with people over-specifying things. Once purchasing and our fabricators learn that we do not really mean it, we are in trouble when we do really mean it.

ALL my dimensions are marked on the drawing itself, with tolerances, as per ASME Y14.5M-1994. If I can, I apply dimensions and tolerances that allow the fabricator to use standard thicknesses.

When you call up something like 3/8" plate as your material, your thickness tolerance is ambiguous. The last time I did this, I think our fabricator used 10mm plate. I have learned my lesson.

JHG