Material Specs on Mechanical Drawings

[hmxsgt]

I am a Quality Manager at an AS9100 Registered Company. We currently have a fairly new V.P. of Engineering. Of course we are already clashing. Go figure. Anyway our manual basically states that engineering will provide a material specification on drawings, basically to provide purchasing with detailed information when sourcing out material for manufacturing pieces. The engineer is using performance values of the material he wants used as an example:

Material: Steel with minimum material characteristics of 16% Elongation, 95ksi Yield Strength, and 115ksi Ultimate Tensile Strength.

His reasoning: In this case, we could have said to use 4340, but that is really overkill. We could not say to use 4150 because there are different heat treats for it, some of which have the necessary properties and some don’t. By giving the material properties, Bill can use available 4150 if he has acceptable certs for it.

Could someone please tell me if this would be an acceptable practice?

 

[HANDCALC]

Here's the typical engineering answer, it depends...

In the aerospace industry, the Bill of Material (BOM) normally specifies product, form, and temper. Additional engineering or quality requirements would be added as notes; calling up commercial or military standards as required:

Example (Aluminum):

1.0 by 1.0 x 0.125 thick square tubing,
6061-T6 Aluminum per QQ-A-200/8 (or QQ-A-200/16).
Weld per Specification XXX
Radiographic inspection per XXX
Heat-treat per XXX (if required)
Chemical Treat (Alodine) per MIL-C-5541E Class 1A
Apply Epoxy Primer XXX per MIL-PRF-23377 Type 1, Class C2
etc...

Quality conformance is measured against the engineering drawings, process standards and other documents that define the product being manufactured.

Most aerospace companies have Material Substitution documents for Purchasing to consult when looking for acceptable substitutes. Hopefully your organization has such a document.

If the product is flight-safety critical, the drawings could also state: "No Material Substitutions without Engineering Approval", or "No Material Review Board (MRB) Activity Permitted". In other words, no concessions, waivers, or deviations are permitted on this part.

Years ago, as an MRB Liaison engineer, I was asked by Purchasing to find an MS bolt substitute, which I did by consulting the standards, and following the supercession list. Two weeks later I dispositioned discrepant rotor-head bolts that were being manufactured from Spec-controlled drawings that clearly stated NO FASTENER SUBSTITUTIONS. Thanks to a meticulous quality inspector, I scrapped the parts, and learned a valuable lesson about what happens when you give people what they ask for, instead of what they need!

Enter a dialogue with your Engineering VP, share your experience, and make informed decisions. The short answer... it depends on the part requirements...

Good Luck

 

[RPstress]

It's normally the other way around! Purchasing (ok, ok, not quality) would like a bunch of options so they can buy the cheapest at any given time, and engineering want to control the material and its condition right down to the molecular level to minimise the risk of an aeroplane falling out of the sky.

For something safety critical you generally need close control over material condition and supply and traceability right back through the mill (for metal). A material spec is a good way to achieve this, although additional constraints may need to be added. (An interesting exception is some low alloy steel aircraft bolts; the specs often give several alternative steels which will do the job.)

For something less critical less control and traceability may be appropriate.

In the example you give fatigue and fracture characteristics are not controlled, nor are stress corrosion characteristics and many others. From a manufacturing point of view machinability is left open. Some ultra-cheap rubbish from China (sorry to generalise, but see the metallurgy forums) full of sulphur could be bought one week and something with better performance the next. Would that matter? The Chinese stuff would probably machine more easily...but it would be much more sensitive to cracks.

 

[rb1957]

how big is your place ?

in a small company it is usually easy enough for purchasing to ask the engineer if something else is ok since engineering (although they are all-knowing) can't anticipate all the options that purchasing can come up with. if it's ok (for the multitude of possible problems that engineering can conceive) then purchasing goes ahead and orders the new material and somewhere down the line the drawing will get changed (usually when inspction snags the material change !)

oh, wait ! you said you were AS9000 ... ah well, then purchasing need to write a query note to engineering, engineering need to document the study of the substituted material, change the drawing, update the stress calcs, etc, then approve the change (by which time purchasing will have come up with another alternative (usually because the previous one disappeared); sigh, and all in the name of progress!

putting the material requirements on the drawing is an even dumber approach, IMHO !

 

[Heckler]

Here is a thread that has some good information thread1103-171948

IMO it's okay to call out a material specification (if the design dictates) but please leave of the revision. If you need material certs then that should be stated on the purchase order.

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 4.0 & Pro/E 2001
o
_`\(,_
(_)/ (_)

This post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.

 

[hmxsgt]

O.K. so here is the outcome so far. I have been less then successful in trying to get engineering and the president to put specs on the drawing. Our purchaser purchased 4340 / AMS 6415 per engineerings recommendations. The material certs do not show elongation, yield strength, or tensile strength. It also is not hard enough (although the drawing does not call out for a hardness property) and now needs to go out for heat treat (says engineering). When this comes back my plan is to place a noncoforming material tag on it. The certs do not give me evidence of what the drawing calls out. Wouldn't it have simply been easier to state the spec. that was needed. Am I way off base here?

 

[HANDCALC]

Hmxsgt,

I'm hoping your Engineering group clearly understands the difference between controlling "Material Specifications" and "Minimum Material Strength Values". Make informed decisions, but you'll have to choose the approach that makes sense for your organization.

RPstress correctly identified why "Material Specifications" provides tighter Engineering and QC Controls. Generally Material Specifications are mill-marked onto the actual raw-stock and specified on both the Purchase Order and C of C. Minimum strength values are then found in the Material Spec, or an approved references like MMPDS-01.

"RB1957" must have worked for AS9000/AS9100/D1-9000 organizations to appreciate how bureaucratic these issues can be. I'm hoping I read his post correctly, that putting material STRENGTH requirements on a drawings is not a good idea?

 

[rb1957]

yeah ... everywhere i've uses QQ-A or MIL-SPECs to define the material, and yeah, trying to describe the critical strength requirements on the drawing just isn't (IMHO) ever going to work.

back to hmx's post, i don't think you could call the material "non-conforming" as engineering didn't specify what it had to conform to (or did they?) .... i guess it's a case that purchasing bought something they thought engineering wanted ... reading the post again, I think you got "screwed" because engineering didn't specify the heat treat they wanted (something they should have known, particularly if it's important enough to get the material heat treated after you've bought it (tho' possibly it works out cheaper that way, it's different if you intended to do it that way, or were backed into doing it that way !!) ... i suspect purchasing bought annealled 4340 (i suspect that the certs say pretty much only that this is 4340 Annealled, and you need the specs to figure out the properties ... typical i'd've thought) but engineering wanted 160-180 ksi

 

[hmxsgt]

I've come up with a tensile strength of somewhere around 152ksi per the HT that will be performed. The tensile requirement on the drawing is 115ksi. Couldn't this be to brittle now? This is what I'm talking about. Somewhere here says well it meets it then. Well to me not necessarily. just because it's better then the 115ksi. What if it was 350ksi. This just doesn't seem right to me.

 

[rb1957]

this, i think, is exactly the problem about putting a strength requirement on the drawing ... there are so many other issues that going along with strength (like toughness) and there are so many other issues in designing something (like corrosion).

btw, if the drawing requirement is 115ksi, why not H/T to 125ksi (that's pretty standard), or maybe 140-160 ksi.

if engineering could specify some alternate materials 4340 or 4130 H/T to 140 ksi ... maybe this would pacify your VP?

on 2nd thoughts, nah ...
YC (your cross)

 

[mechengdude]

My two cents:
I'm an ME and have worked for a large Aerospace company and at companies that supplied the "big players" (Boeing, Airbus, etc.).
I've always seen the material called out with a specification referenced. Example:
Aluminum 6061-T6 per AMS XXXX
Also in the material note we might put something like alternate material: Aluiminum 6061-T651 per AMS XXXX.
In special cases where the specification was not definitive enough additional properties would be referenced. Typically when a part is designed, the engineer has a material in mind. Not sure why anyone would be fighting putting it on the drawing. To me it seems like a bigger problem to put only a few requirements like YS or other mechanical properties. Going that route you'll end up with purchasing meeting your drawing but you'll end up with a design that tries to weld aluinum to steel but still meets your material requirements!!