Do engineering drawings imply solid and uniform parts?

[iusedtobehydrogen]

If I were to have a drawing of, let's say a cube, and the material specified was simply "ABS", and after sending the part to a vendor I recieved an average quality 3D print instead of a solid piece, could the part be said to be out of spec?
In my view, the discontinuities inherent in normal 3D printed parts would mean the part is out of spec. In other words, if really did want a solid piece for strength reasons or any other reason, I would not have to specify that it not be 3D printed. But a friend from work who is a drafter disagreed. What say you?

 

[SWComposites]

LOL. Nope. You have specified way too little info. Even specifying “ABS” is too generic. You will get exactly what you ask for and nothing more. Thats why good drawings have a long list of detailed requirements.

 

[mfgenggear]

OP
The drawing must be very specific for the call out of the raw material. And specification for the material. Foe different grades. As stated ABS is to vague. To insure no voids then non destructive testing is required. Which is appropriate for the type of material.

 

[GregLocock]

War story 1. A railway company sent out a spec for some beer glasses, intended to be 1/2 a pint. They specified the lower diameter, the upper diameter, and the distance between them. A cluey engineer realised that a frustum was not the minimum surface area, sent in a quote significantly less than everybody else, and was awarded the contract. And that's why the LNER (?) had so many elegantly curved water glasses, and had to buy another batch of 1/2 pint glasses as well.

War story 2. We wanted to build our cars out of thinner steel with a tighter tolerance. We pay for steel by the length*width*nominal thickness. The steel company said if we wanted tighter tolerances we'd have to pay more. So we measured what we were getting and discovered they were already delivering (say 0.70-0.72) to our original thicker spec of 0.70-0.79, and were charging us for 0.745.

 

[iusedtobehydrogen]

LOL. Nope. You have specified way too little info. Even specifying “ABS” is too generic. You will get exactly what you ask for and nothing more. Thats why good drawings have a long list of detailed requirements.

On ABS being too generic, perhaps some will balk, but I have seen it several times. Designed, drafted, purchased, without issue. Military parts even. In practice, what I describe is not an issue. If you send a drawing to a machine shop and the part can be milled from a bar, that is what will occur generally.
The question is, if the machine shop suprised the purchaser with a 3D print where 3D printing was not mentioned in the drawing, purchase order, or other correspondance, would that be correct. Most 3D prints are mostly air. If a weight wasn't specced, did they actually give what was asked for?
You could take it a step further even. Say it's an aluminum part. Say under material it just says "6061" (again, not super well defined, but common in the real world). It would be bizarre, because 3D printing metal is pretty expensive, but the part could theoretically come back as a 3D print with significant voids and weakness at layer lines. If the drawing never said anything about it, and the purchase order didn't either, is it acceptable based on the drawing?
This is just a hypothetical between coworkers, not an issue I am currently facing.

 

[Stick.]

For reference, when I order iron castings, instead of just specifying "ductile iron", I send along about 45 pages of material and quality specifications and about 30 of those are just to ensure the casting has an acceptable level of internal flaws. Sure I could try and argue with the vendor that a sand inclusion isn't iron and therefore doesn't meet the material requested, but that would be a losing battle. It's far easier to just tell the vendor up front what I'm expecting, in as much detail as is required so that we both know what's expected.

As for your 3D printed 6061 example, yes I would say that a 3D printed part would meet the spec if all you specified was "6061 Aluminum". If you want something more specific than that, you need to specify it. Particularly with plastic parts and how ubiquitous/cheap 3D printing has become, you need to anticipate it as a possible manufacturing method for your parts and adjust your material specifications as needed.

 

[mfgenggear]

Again the requirements by the contract will dictate of certified material is required.
In aerospace the answer is yes.
Some commercial products that don't have a safety issue may not require certified material and a 3d printed parts are acceptable.

 

[mfgenggear]

If a commercial part material is critical
Do to safety issues and has high tensile requirements then yes certified material is required.

 

[ctopher]

I agree. Simply putting ABS the part meets spec. But, the drawing should have called out some type of finish, which most likely a 3D printed will not meet.
To be more specific, call out the material spec on the dwg. You may also call out approx Mass/weight.

 

[3DDave]

Usually this is handled for metals by including the material specification reference, like SAE AMS QQ-A-250/11 which includes a lot of detail about how it got formed. No matter what, 3D printing cannot meet the requirements of that spec.

Thermoplastics are expected to be formed by some type of molding, whether as raw material or production of the finished piece. 3D printing makes a mess of that. I guess now one has to specify "SOLID STOCK".

The default condition for ABS is loose pellets and the primary consumption appears to be injection molded items. Turning that into filament and using a 3D glue gun to print a part isn't that far off.

If one puts finish, they may just sand it to meet the requirement. Weight? they use solid infill, which doesn't fix internal bonding problems.

It does suck that the variety of methods to manufacture items greatly affect the finished product when the mantra of design is that processes do not belong on drawings.

 

[EdStainless]

Drawings don't imply anything.
If it isn't dimensioned or stated then it is free game.
You need to be aware that production methods will be different for different quantities.
This is why process engineering and QA need to be signing off on drawings.

 

[3DDave]

I had some wire parts made, stainless steel. Normally after passivation the stainless parts look, well, stainless. These came in gleaming like they were going to a car show. QA was rattled by this, thinking they'd been polished and chrome plated. I called up the supplier and found out they had "bright dipped" the parts as part of their processing. It had no effect on the form, the function, so I told QA it was OK as there was no way I was going to try to list a possibly unlimited number of ways to make a part or to exclude all the other ways it should not be made. I certainly didn't want to continuously update drawings because suppliers did something that made no difference. Like one tumble deburrs and leaves that finish and another uses a wire wheel and another ... even giving a roughness won't stop it as this case was smaller than typical. Oh, they did too good a job and I should reject it? Or put that as a requirement and limit potential suppliers?

One thing that I wanted, but never got, was to have the requirement for engineering to sign off on procurement and manufacturing processes. There have been times where something that seemed quite obvious to me had a different interpretation that would have been obvious in a production/QA review.

An infuriating one was when I had worked out with a supplier to install an IR window into an aluminum housing using their adhesive. Procurement and program management decided it was better to let the aluminum machine house, that never handled glue or optics, the full contract to save the cost of a second operation. Great job. The test group ended up dropping $7,000 in germanium and fracturing it because, it turns out, not knowing about glue or optics and not paying attention to where the "put glue here" arrow went, they put it where it was "easy" and the only place it would definitely do no good. Oh, yes, that drawing was signed off by our manufacturing and QA prior to shipping it.

 

[mfgenggear]

to my experience it's the bean counters who
try to save pennies then it cost dollars.
that had nothing to do with engineering, QC or production eng.

 

[mfgenggear]

my success with out side operations was to have a dialog with the supplier. I ask for concurrent engineering. 99.9 % per cent
success. I asked for fix process once it has been tool proof. and supplier locked. it may have cost a little more , but saved cost of mrb, handling , packaging, stores, and the cost of paper work. the time of 3 departments to review analyze., disposition and rework.

 

[Jboggs]

Simple answer - yes. If no specifications are given for an inner shape (ID, wall thickness, etc.) then solid is the only other option. Anything else should be considered as "not to print".

 

[Mech1595]

Our customers usually specify the exact polymer grade, color code (if applicable), and/or related material standards on the print.
In the rare instances where I receive a print that just specifies ABS, PP, etc. with no supporting information, I ask the customer to clarify what they want. There's just too many different formulations (high temp, UV resistant, etc) with wildly different properties, all within the same family, to guess. If they want just a generic ABS/PP,etc, I'll just propose something we have in stock, and get written approval from the customer.

On ABS being too generic, perhaps some will balk, but I have seen it several times. Designed, drafted, purchased, without issue. Military parts even. In practice, what I describe is not an issue.

Same experience, I've had quite a few ITAR components come through that have a very generic specification called (Green ABS, etc). I'll ask for any specific material standards, and they'll just request some general purpose material. I'll still send them the grade for approval, but it's just interesting to see the contrast between those parts vs. something like Automotive.

 

[mfgenggear]

Mech
yes exactly or precisely

 

[3DDave]

Simple answer - yes. If no specifications are given for an inner shape (ID, wall thickness, etc.) then solid is the only other option. Anything else should be considered as "not to print".

I like that answer.

 

[rb1957]

damn ! we've all had bitter experience with this. To throw my log on the fire ... we had a simple plate 3/8" thick, about 8in2. We don't generally define grain direction, and will analyze what we think is most conservative. later talking to someone about this we found out that they had "salami" sliced a thick plate (to create the 3/8" slice). "No" I said "surely this would be made from 3/8" plate, I mean that's why we made it 3/8" thick". Nope, salami sliced. Well, we didn't specify not to so, ok.

anyways, for this example if you'd wanted injection molded, you should've said so. I understand you think you did, but if the words aren't there then it's open to interpretation.

 

[3DDave]

Oh, you wanted the properties to be OK through the thickness? I've had the opposite. Part did not need such critical properties, but did need to be made real soon. So nothing happens from the Quick Response Shop. Similar issue, .375 thick required. None on hand, don't want to order a big piece for just a little one. I say, is there 0.500 on hand? Sure, plenty. Can you machine that down? Uh, yeah, I guess. Bang head, mine, because I need to work with these guys for a long time.