Difference between revisions of "CNC machines: build or buy?"

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Latest revision as of 22:56, 20 May 2012

People frequently ask me if they should build their own CNC machine like the one I’ve built or just buy one. I don’t necessarily want to discourage anyone, but it really comes down to how much time and energy you want to spend building the machine versus how much time you want to spend making use of the machine. Presumably, you want a CNC machine to help you perform some interesting task, after all!

Building your own CNC machine is fun, and you learn a lot doing it. But you should go into the endeavor with your eyes open regarding what you’re getting into.

First, I’m going to assume that you want to build a large table CNC router that is on par with the reasonable low-end of CNC routers on the market today, like the base model Shopbot. There are lots of other options, though, like desktop machines with smaller cutting areas, or much lower precision machines built out of MDF and plastic that are great for many projects and a good way to learn about CNC at a low cost. I have no experience with either of these alternatives, so I’ll limit myself to precision CNC routers.

My goal was to build a machine that was accurate with repeatable positioning to within +/-0.005”, and which will stand up to years of use with reasonable maintenance. It should be rigid enough to cut half inch MDF or poplar at a reasonable rate, say 100 IPM, hold a 2-3hp router, and spin bits up to 1.5” in diameter. You should first establish similar goals for yourself: determine what precision you need, what materials you want to cut, and how fast you want to cut them. Use specifications on commercial CNC machines as a guide.

It’s not necessarily that much cheaper to build a good CNC machine than it is to buy a good one from outfits like Shopbot. It cost me just under $6,000 to build my machine in 2003, and at the time I could have bought a machine from Shopbot that was just as accurate, and a bit faster, for a little less than $8,000. Frankly, the difference in price was justified: the Shopbot had a more rigid frame, larger motors, etc. I also saved a reasonable amount of money buying some of the parts off of eBay. The components that go into it are not cheap, particularly the stepper motors, linear slides, and the framing material. These are three areas that you should not skimp on at all if you decide to build one yourself. However, these are the three that I frequently see people try to save a buck on, which only makes them sad later.

Building an accurate, rigid CNC machine takes a lot of time and precision metal work. At some point you will require the help of someone with at least a manual knee mill to flatten various mounting plates, and drill very accurate holes in key areas. The more precise you attempt to make the machine, the more precise you must be in machining the parts. Many materials are not actually flat when you get them, even though to the naked eye they might appear flat. Quality linear slides must be mounted to very flat surfaces or they will warp when the bolts are tightened down. This will cause the bearings will bind and nothing will move. Some background in precision machining is essential to constructing a good CNC router.

You’ll also need some background in basic electronics. These machines easily draw 30A at 220v between all the motors, drivers, and the controller. You should be comfortable working with both high voltage AC and low voltage DC.

If you do decide to build your own machine, I highly recommend two things:

  1. Look for a precision machining course at a local community or technical college. I took two courses in precision machining in the evenings at Lake Washington Technical College, which were excellent. Without them, I surely would have failed to build my first machine.
  2. CNC Zone. This is a fantastic community of DIY CNC builders, with resources for materials and components, advice on design and construction, etc.