CNC Milling, CAD, materials, and costs

In a previous thread I had asked about CAD programs, thanks to all who contributed.

Inspired by this guy’s CNC-machined PC case mod (check pictures down page, W O W.), I have decided to design some pieces with CAD which I plan to have CNC machined. I have some questions, however.

Is there an exportable file format that is recognized industry-wide by CNC machine services?

If so, Is is it such that these files can simply be loaded into the CNC machine computer, load a metal billet, press a button, and there you go, there’s your milled piece?

Or is there a specific treatment that must be applied to the file regarding the specific machine that will do the milling, or…? What is a CNC machine operator specifically looking for in a file that will allow him/her to use the file? Or is it that, by virtue of the file format, it is automatically workable? What has a machinist saying, “These are good files.” or, “These files are crap. The work of an amateur.”

Can any object that can be created in, say, autoCAD, be milled? What are the limitations on what can be milled in a CNC machine?

How expensive is CNC milling? I realize that costs are highly dependent on materials, size, complexity, batch size, etc… but I’d like to get a rough idea of the scope of the cost. For example, what would it cost to mill, say, a 2x size replica of a pencil, out of round stock aluminum? $2? $20? $200? What are we looking at here? How about stainless steel? Titanium?

Thanks all!

Wrong link, THIS is the one.

Paging Tuckerfan to GQ

Sorry, but I have never worked with a CNC machine — my machinist days are long gone. I’m pretty confident you will spend uber $$$ if you are setting up your own home shop. Make sure that one mod isn’t all you are doing.

Anyway, the limitations of a CNC milling machine will be similar to the limitations of a standard milling machine. There are certain things you cannot do easily with a standard Bridgeport mill, and good designers are aware of these limitations when they make the drawings.

For example, imagine you have a large block of aluminum that is part of a machine and you want to hollow it out, like an open box, to lighten it. Nobody would ever expect you to have square corners on the inside, since a milling cutter is round, like a router bit, and it is virtually impossible to cut a square inside corner with a milling machine. The draftsperson would likely have drawn the part with a radius on the inside curves, for this very reason.

Of course, there are many different cutters available, but most of them are going to do a poor job of concave cuts. The only fancy cutters I ever really messed with were round end ones that made a perfectly round-bottomed groove instead of the common flat-bottomed grooves normal cutters make.

The other kind of work is done using things other than milling machines, such as EDM machines.

A good designer will understand the expense/challenge of certain kinds of cuts, and then perhaps make the part in two pieces that are easier to machine that are then bolted together (typically with attractive recessed screws — we like a clean appearance in our work).

My (admittedly limited) understanding is that the standard low-level CNC “language” is G-code. The commands it embraces are rather simple (up, down, left, right, etc.). Practical machining typically involves a CAD program which has the ability to generate G-codes that allow a CNC mill to actually make a part.

There exist “virtual” milling programs that accept G-codes and depict what a CNC mill would do with them. They are useful for checking that a set of G-codes makes sense before you actually start turning metal (or plastic, etc.) into chips.

Ha! Ha! I say. :wink:

But seriously, there’s a lot more that goes into designing a manufacturable piece than just making some CAD solid model, and that comes from years of experience in what is feasible but expensive, what is practical to do, and what is strictly impossible. minor7flat5 gives a good basic coverage on what the issues are, but there’s a vast amount of knowledge and judgement that goes into making a machinable part. You need to have a real understanding of exactly how the part is going to be manufactured and assembled or you’re likely going to create headaches if you just generate a design and throw it over the wall to a machine shop, even with something seemingly simple.

In the past, every manufacturer has had their own scheme for implementing G-code instructions, and this often changes significantly from model to model. More recent high-end integrated CAD/CAM/PLM suites like Pro/ENGINEER and Unigraphics/NX/whatever they’re calling it this month have a more or less universal system for converting CAD designs to CAM instructions, but this is neither seamless nor without human intervention.

You’d be better off coming up with a conceptual design, then approaching a design/machinging house to develop it into a manufacturable final detail design, otherwise you’ll be costing yourself more in the long run and/or end up with a suboptimal final product. If you’re planning a production run of whatever it is you’re inventing, many design house that have manufacturing capability will do work for a nominal cost or even for free on top of the manufacturing costs. If this is a one-off thing, however, they’ll probably ding you for design costs as well as the cost of prototype manufacture and tooling.


To answer your questions in no particular order: The costs really depend upon a number of factors, with the hourly rate in machine shops around here going for about $60/hr (if only I got paid half that much). Some shops won’t touch “one-off” jobs, other shops can handle them with no problems. There are a number of different types of milling machines, some which use their own specialized language and others of which follow what’s generally industry standards, and certain machines can handle more complex jobs than others.

There are programs out there which will allow you to load an AutoCAD produced drawing in them and will then output the necessary machine code (generally as a .txt file) after you’ve input such things as machine type, material, and tooling. I do not know of any free Windows based programs which do this, however, and unless you’re willing to spend a couple grand on software, the Windows based stuff that’s relatively inexpensive is pretty kludgy to work with, from what I understand.

For what you’re wanting to do, here’s what I recommend: Check to see if there’s any trade schools in your area (or high schools/community colleges with a machine shop program) and give them a call. Odds are, they probably have a system for allowing the general public to get things machined for them at a relatively low-cost (where I went to school the fee was $15). You’ll probably have to supply the material, and don’t expect the job to be done quickly, but you’ll be able to talk with the instructor/student and find out what kind of equipment they’ve got and what the limitations of it are, without the meter running the whole time.

If you want to try your hand at generating the machine code and aren’t afraid of Linux, then check out the software offered here. If you’re feeling brave, you can check here for instructions on how to build your own CNC machine. Or, if you’d rather buy a homebased CNC machine, you can check out the magazines sold by these guys as various companies that build home sized CNC machines advertise in their pages and they also sell books on how to convert a standard mill to CNC. If none of those options appeal to you, you can post a request here, and see if anyone at that board (Yes, I do post there. Guess what my username is! ;)) can help you. If you’re not in a hurry, I can see if I can do it for you, or, if you can send me the CAD drawings in a format I can view them in (I don’t, at present, have the necessary software installed on my machines to view AutoCAD documents in their native format because it screws up my DVD-Burner for some reason.), I can take a look at them and give you some pointers.

Oh, and skip the exotic materials like titanium. That requires specialized tooling, which will cost you more than the titanium, and a lot of shops won’t touch it because it’s so hard to work with. If you’re looking for some kind of PC case components (google “WMD PC” for a truly jaw dropping case), I recommend going with aluminum. It’s cheap and easy to machine. Depending upon the alloy, stainless steel can either be no real problem to machine, or an absolute bitch to machine. I won’t even get into discussing machining things like monel or iconel as the applications for those things are pretty rare (not to mention the stuff ain’t cheap).

Feel free to drop me an email with any other questions you might have.

Well, that’s the way it was in my shop :). This certainly made life much easier, but I imagine it may have been an anomaly.

That’s like calling WWII “a bit of a rough patch.” In the shop I’m in, we’re making parts for a throttle cable that are little more than a cylinder with a slot down the center. The print is festooned with dimensions, angles, cutaway views, dimensions, tolerances, finish requirements, various other print symbols, revisions, sign offs from everyone from the janitor’s pets to the company president, and then you get to the Japanese characters which I have no idea what they mean or why they’re there. Trying to pick out the information I need on that print is a challenge.