Material callouts

[DReimer]

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

 

[Heckler]

Dean - We do the same thing and it's so frustrating. I just recently had a design taht called out cast iron. But had to create a part number for the new material. I told the designer I don't care what size material the machine shop starts with but care about finished dimensions. The very second I call out the start size planning/purchasing will come back with our supplier can't get that size. It's extremely nonvalue added and I could use my time in other areas.

Best Regards,

Heckler
Sr. Mechanical Engineer
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[ctopher]

Raw mtl size should never be called out on dwgs. Mtl specs can be used if desired, military projects require it.
On the part dwg, call out mtl, and let the planners and machine shop take care of the raw mtl ordered.
If you call out on the dwg "aluminum flat bar 3/8" thick by 2" wide" and the shop does not have it in stock, but they have "aluminum flat bar 3/8" thick by 2.5" wide", they can't use it because it would be out of print (although they could use it and no one would know).
Indicating the raw mtl size ties down the shops to it and prices can go up.

Chris
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[Heckler]

Chris - exactly the point I tried to make.

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[ctopher]

I think we were thinking and typing the same thing at the same time.


Chris
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[drawoh]

DReimer and Heckler,

Wow!

What happens if you design a plate 3" wide by 8"? Do you tell them to order 3" bar or 8" bar?

What happens if the shop has to make a lot of these things and he wants to cut them all out of a large plate in some sort of rectangular pattern? Do you...

a. ...tell him to use the 3" bar like you told him (dammit!),

b. ...tell him that a lot of the stuff you specify is dumb (as per company policy of course), and it is safe for him to ignore it?

This is a slippery slope.

One of my pet peeves is people who design one-off tiny brackets, and specify the aluminium extrusion. Of course the fabricator is going to order a twenty foot length of the stuff, and of course it will not be warped or twisted.

JHG

 

[Heckler]

JHG - I know it's a slipery slope but sometimes it takes more than one person to rock the boat. And it seems that the powers that be are happy with the status quo. We had a project come through that should have made them change their minds....not only were material specs called out on the drawing but the revisions also. Most materials are made to the current specifications.

Best Regards,

Heckler
Sr. Mechanical Engineer
SW2005 SP 5.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

Never argue with an idiot. They'll bring you down to their level and beat you with experience every time.

 

[ctopher]

I think if the design is for plate and the shop makes it from bar, there is a problem with the shop. Call out mtl specs will help.

Chris
Systems Analyst, I.S.
SolidWorks Pro 06/PDMWorks 06
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ctopher's home site (updated 06-21-05)
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[DReimer]

drawoh:

Good question. Because we are using MRP, we will usually search the most logical materials to use, then find which we normally use the most. In your example, if we use 60' of 3" wide flat bar every year, and only 5' of 8" wide, we'll probably call for the 3". I'm not saying I like it!

As for your hypothetical scenario: that is why I want to change the system. Currently what would happen is the machinist would go for a walk to the front office, and interrupt the engineer to see if we can change the material to make it out of plate. Then he goes to the planner and asks the same thing: if the engineer says ok and the planner agrees he can change the material on the work order so at least the inventory transactions are correct. Then we go through it all over again the next time.

Actually, since I am the engineering manager I can change this practice if I like. The resistance isn't from me or the company, but rather from the machine shop. I'm trying to build a case for the change so that I don't have to put up with any more b*tching and moaning. [You wouldn't believe the griping when we changed our title block in such a way that you now had to flip the work order to see the drawing in order to find the material callout.]

 

[MadMango]

We keep our material call outs on the drawing title block generic:
HRPO
CRS
SST
AL ALLOY

If we need a spefic material we put "SEE NOTES" in the material block, and add the specification to the general notes, like HRPO 1018 per ASTM-A569. The shop default for material is HRPO 1008.

Our shop knows to follow the work orders generated from our MRP system. They get the BoM and Routings. The BoM defines the raw material to use and the quantity. When the shop doesn't have the specified material they come into the office and ask if they can substitute. If the engineer allows a substitution, a redlined temporary waiver is created and is signed by the engineer.

What kills me, we had a part that was CRS 1020, 3/4"x1/2"X7" long... engineering created a new part number for new bar stock 3/4"x1/2". Purchasing never ordered it, so the shop machined down 1"x3/4", talk about wasted time, engery and money.

[green]"Art without engineering is dreaming; Engineering without art is calculating."[/green]
Steven K. Roberts, Technomad
Have you read faq731-376 to make the best use of Eng-Tips Forums?

 

[ctopher]

Also, some shops will order mtl a percentage, maybe 25%, bigger than the finished part, to remove cracks or purosity or etc ... depending on mtl and shape of raw stock. Best thing overall, do not call out mtl size, saves time and $$ for everyone.

Chris
Systems Analyst, I.S.
SolidWorks Pro 06/PDMWorks 06
AutoCAD 06
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716

 

[MintJulep]

Makes no sense to me. Just because 1 part needs a 2" x 4" piece of stock does not necessarilly mean that 2 of the same parts needs a 4" x 4" starting stock.

Consider for example a simple bell crank:

L

If you need a lot of them do you layout this

LLLLL

or this

<<<<<

or maybe

<
>
<
>
<

or something that can't be done easily in ASCII CAD

The shop planer needs the ability to make the most efficient use of material. In fact, that's his job.

 

[ewh]

We put the material spec in the BOM, as well as a REFERENCE of the material size (plus 1/16") needed. Everyone in the loop understands the the spec is what is important, and the size is JUST a reference for the convenience of planning. As for assigning a p/n to stock on the BOM, I feel it only adds room for confusion and unnecessary additional cost (as is noted by several posts here). If planning feels that they need a p/n for stock, let them add it to their documentation and let them control those numbers, but leave it off of the drawing. As far as engineering is concerned, whatever the size of material used doesn't matter, as long as the finished part meets the drawing.

 

[DReimer]

From ewh: "As far as engineering is concerned, whatever the size of material used doesn't matter, as long as the finished part meets the drawing. "

That's my point exactly. But I get resistance on this, even from some of my engineering staff who have worked with our existing system for so long they can't seem to wrap their heads around the concept. You'd think a part designer would be all for NOT having to determine what size of material to use.

 

[ctopher]

ewh,
As usual, we agree.
Nothing wrong with indicating size as (ref).
Sadly, I have worked with engineers, inspectors and machinists that don't understand that () means ref.
I prefer to leave mtl size off completely. There would never be any question.

Chris
Systems Analyst, I.S.
SolidWorks Pro 06/PDMWorks 06
AutoCAD 06
ctopher's home site (updated 06-21-05)
FAQ559-1100
FAQ559-716

 

[thundair]

This will probably become a moot point with 'Model Base Definition'.
Also with more flat pattern capabilities. I.E. Laser, Water Knife and Plasma.
I am now working in Tool Engineering and we have a note that the material must be physically and functionally equivalent.
This makes it easier for the shop to upgrade without approval.

 

[ctopher]

Equiv to what? To work with the machine or the design? I hope you have a Materials Engineer. That note is very vague. A lot of shops do not know the equiv mtls. When I see that note, a lot of questions come up. I'm use to aerospace and military designs. Notes like that can't exist.

Chris
Systems Analyst, I.S.
SolidWorks Pro 06/PDMWorks 06
AutoCAD 06
ctopher's home site (updated 06-21-05)
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[thundair]

"SUBSTITUTION OF PARTS AND MATERIAL TO BE FUNCTIONALLY AND PHYSICALLY EQUIVALENT"

This is a copy and paste from our Tool engineeruing manual
I have seen this note at the last four companys I have contracted into.

Cheers

 

[Kwan]

Material size can be very important especially for structure designed to carry specific loads. The material strength depends of size and form of material. The strength of .050 thick 2024-T3 sheet is different from .50 thick plate. There are also big differences in form. Extrusion has different properties as compared to plate. There are also times when specifying grain direction is very important. The material spec (ie: qq-a-250/4) is also important to ensure the material has the required properties.

Just my 2 cents.

 

[drawoh]

thundair,

A fabricator I deal with recently could not find the beryllium copper I specified for a large Belleville spring, so he substituted plain copper. I am sure somebody thought "Copper, beryllium copper, what's the difference?"

Aside from its springy qualities, copper is about five times as conductive as beryllium copper. A power supply and voltmeter revealed the truth about why my parts would not work.

You and I might pick the same material for quite different reasons. The term "equivalent" is ambiguous. It is also likely that the supplier has limited knowledge of materials and engineering.

JHG