New to designing things for fabrication and need help with tolerances for manufacturing

[Fr3nch]

So I am a research engineer specializing in thermo-fluid systems and have never designed a mechanical component with all the information needed to fabricate it from start to finish. Ive always had an assistant that was able to take my solidworks drawings, containing basic dimensions, and turn them into something that was machinable. I have taken a job with a startup company am now responsible for taking our basic designs and turning them into something machinable for one off prototypes.

I understand that general tolerancing, GD&T, etc are very complicated disciplines of engineering and I respect that, but we are only making small batches of prototypes and I dont need to know everything. I would really like to find a quick and dirty introduction to this stuff that will allow me to be functional very quickly. All I need to do is provide enough information for one prototype to fit together. Any suggestions?

 

[KENAT]

Ok, how important is it that your parts reliably fit together first time?

Are there aspects of function that are critically dependent on parts meeting relatively tight tolerances/closely confirming to ideal dimensions?

Are you willing to 'throw money' at the problem - i.e. find a shop that will routinely machine to relatively tight tolerances even though you may not always need them?

There are places like

http://www.protolabs.com/firstcut

that you can send your model and they'll machine just to the model. They'll have some stated 'process capability' or generally achieved tolerance that they claim to meet. So long as you design with that in mind they may work ok for prototypes. They offer limited materials & finishing etc. though.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MikeHalloran]

If you have an in-house fabricator, go out in the shop and ask questions.

If it's going outside, go there in person, with two prints and a red pencil. Mark up both prints in cooperation with the shop people. Bring one set back and update your drawings.



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

Start by understanding the difference between tolerance and process capability.

You do not "tolerance for manufacturing".

The root of tolerance is tolerate. How far off from your "perfect" design can you tolerate and still have things work correctly.

Deciding what tolerances are necessary is a pure design engineering function.

Next you find a fabricator, machine shop or whatever with the process capability to make your thing within those tolerances.

Good designers need to understand how what they designed will likely be manufactured, and what the manufacturing method is capable of achieving.

Good designers design things that can be made with sloppy tolerances by relatively imprecise (generally less expensive) means where ever possible. They use tight tolerances only where necessary to achieve functional requirements.

Good designers listen to machinists and toolmakers and have conversations with them that start with sentences like "This is what I need this part to do. I was thinking about this, and this and this when I designed it. But I've love to get your thoughts on it."

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[tbuelna]

If all you need are prototype quantities, and you are able to produce good quality solid models, then forget about making drawings and go to a Model Based Definition approach.

If you have a high-fidelity digital model of your prototype component, it can be produced and inspected entirely by using the digital model. With modern CNC equipment, you can machine a single part to extremely close tolerances. And it is not too difficult to machine a single set of mating parts to close tolerances. Just tell your machinist and programmer what you want to do with these prototype parts. They'll figure out how to make it work.

 

[dgallup]

Who is going to make your prototypes? If you are handing the models or drawings off to an experienced in house machinist who understands your product and how it functions then you can get away with the minimalist approach. Since you said this is a startup company, I doubt that is the situation. If your purchasing department is going to send drawings out for the lowest bidder half way around the world, you better nail down every tolerance, material spec, surface finish, edge condition, etc. if you want to get a decent part back.

Model based definition does not mean that you don't have to specify every tolerance and requirement, it just means it's all contained in the model. Since it's rather unlikely that your prospective 3rd world manufacture uses the same CAD system, you're probably going to need a drawing anyway. It's also a lot harder to interrogate the model and verify that everything is properly defined than to just look at a drawing and check it.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[3DDave]

The initial question is genuine, but it's not totally realistic to expect to do this quickly. Expertise comes with experience, and experience comes with time and effort. Particularly where the task is to communicate using an essentially new language. Best bet - make friends with the guys who will be making the prototype parts.

Here's an order (off hand, needs work) to performing this particular task:

0) Finish requirements analysis
1) Preliminary concept
2) Preliminary stress analysis
3) Preliminary material selection (based on stress, thermal, et al)
4) Preliminary cost analysis including manufacturing and inspection (this will be very wrong first time through)
5) Get feedback from manufacturers on (4) particularly for process variation. This will set manufacturing method based on qty and material and will set expected variation and cost
6) Perform tolerance allocation (based on 5)
7) Perform tolerance analysis (based on 6)
8) Adjust tolerance allocation and concept to meet performance requirements, restart at (5)
9) Finalize and fully document design
10) Finalize tolerance analysis, material selection, and stress analysis

Of course, at each step, verify that the item still meets the requirements. And do a good job of establishing quantifiable requirements.

The concept for this list is that the design will be such as to accommodate the variation expected from the manufacturing process that is selected. The trick is understanding that really cheap processes can result in really expensive designs to perform in spite of usually large variations. Often it is the case that a large increase in manufacturing cost can simplify the design and cut the overall system cost. For example, it is really cheap to dig a shape in sand and pour metal into the open mold, but it will often require expensive alignment processes later on, where an investment casting or permanent mold casting will be more expensive, but can be accurate enough to be self-aligning.