CNC Thoughts

Drill Press, Laney College Machine Shop

Drill Press, Laney College Machine Shop

Building a small CNC mill has been one of those projects I have thought about dozens of times over the years but have never initiated. It’s come up again now, and maybe this is the right time.

I would really like to be able to cut parts out of acrylic and aluminum sheet, as well as engrave text and mill out shapes. I can imagine many applications for milled acrylic as an enclosure for PCB based projects- potentially an asthetically pleasing, compact, cheap way of packaging a device.

So I’ve started shopping the eBay for linear motion parts, I would like to build the machine out of aluminum and use recirculating ball linear bearings and hardened rods, with Acme leadscrews. Great deals on new old stock parts can be found on eBay, but NOS bearings and rods are still pretty expensive, and are generally kind of a compromise between what you were actually looking for and what happens to be for sale. There are some parts on eBay now that I think would make a very nice set of axes, I have to decide whether they’re worth the (bit more than I’d like) price.

I am envisioning a gantry-type mill, using 24″ long rods, for a travel length of about 18″. An “Instant Z Axis” from CNC on a Budget is pretty tempting, as is their 3 speed spindle. I think his prices are very fair for the amount of material and work that goes into his products. So, a couple hundred dollars here, a couple hundred dollars on aluminum plate and linear bearings and you still haven’t got a CNC machine!

The most practical solution to simply getting a machine up and running would be to purchase a proven stepper motor based motion kit, like those from Hobby CNC. I would really like to go a different route though, I think stepper motors are a bit less than optimal- slow, noisy, generally not that elegant. Optimally, I would use any old brushed PMDC motors to drive the leadscrews, then attach a linear scale to each axis and run it closed loop servo-style. Capacative linear encoders are cheap and plentiful on eBay- search for “digital quill”. The data output on these is pretty well documented here and here, that’s not to say it wouldn’t be challenging to implement a system to read them and use them in a closed loop system with the motors. I bet it would work, though, possibly work well. I think it would be fun to build a custom control board with motor drivers, inputs for those scales and an onboard G-Code interpreter, then make a complementary app for the host computer that would work as a DRO and to feed the controller lines of G-Code.

Ok, not a small project. I should wait until I have a bit more machine shop experience then concentrate on building the machine and then pick up some steppers to get it running- the custom servo setup would be fun, though.

And, of course, I’ve got to finish the battery regulator system and the electric car first. But I’ve got a little cash burning a hole in my PayPal account, maybe it’s time to pick up some linear motion parts. Or maybe, as with so many ideas, it would be wiser to resist the impulse purchase and not regret it later- just like there are deals on eBay now, there will be deals later. Not an unusual quandary for me.

Posted on March 5, 2009 at 2:08 am by Henry · Permalink

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