The electronics for a CNC machine are reasonably complicated, and consist of:
– 3 motors, one per axis, which drive the leadscrews that position the axes, driven by
– 3 “motor drivers”, one per motor; the boards convert TTL-level “step” and “direction” signals into high-power coil energizations in the motors. Each board connects to
– one high-voltage (24v, 36v, 48v, 60v) DC power supply that supplies the big current and voltages that the motors require, and
– one “breakout board”, which turns signal pulses from the PC into 3 clean TTL-level pulse trains feeding each of the 3 motor drivers. The breakout board connects to the PC via either DB25 parallel (most common) or USB (harder to find), and sometimes has additional features, like being able to turn on and off the “spindle” (cutting tool), and sometimes requires its own 5vDC power supply.
All of the above (except the motors, which are connected directly to the machine axes) can be put into a single enclosure “black box” that plugs into an AC outlet, and provides motor outputs and PC input (as well as an Emergency-Stop switch, home and limit switch connectors, spindle control connector, etc.)
The full constellation of PC software is out of the scope of this article, but the “machine end” of that software train is a piece of software that translates “G-Code”, a specialized CNC cutting language, into a series of “step” and “direction” pulses that get sent to the motors via the electronics “black box”. One such piece of software (and what seems to be the DIY “industry” standard) is called Mach3. More on this in the “software” section.
For a low-cost DIY CNC machine with a total budget of perhaps $800 (assuming that the PC and its software are not part of this cost), the choice of electronics is critical, encompassing about $500-$600 of that budget. The fasteners and wood are cheap, comparatively.
One can spend a lot of time building the black box out and coming at it from various directions, but at the end of the day, the main two choices are these:
– How big must the motors be in order to drive the machine-as-designed, and
– What is the upgrade path for the electronics
There is a tendency to want to size everything “just a little big” to keep a possibly larger future upgrade path in mind. I feel that avoiding this tendency is a good idea, especially considering I don’t really have a good feel for the capability gained by incremental capacity improvements in the electronics.
Having said that, my choice boils down to essentially two options:
Option 1 “Right-size it”:
For $390, buildyourCNC sells 3x 387oz/in steppers, 3x 40v@3A drivers, a 36v/8.8A power supply, and a DB25 BOB. He’s sized this set to drive the rig in his plans. I don’t have a feel for how much “extra” capacity there is, but I have a feeling that he’d have a worse reputation on the ‘net if he’d severely undersized anything. I do get the idea that adding a second motor and driver in X (for an additional $100) is often helpful — my understanding is that the machine is not lightweight. For an extra $200, I can get an enclosure, E-stop, and all the connectors wired in already ($429 + $150 in motors from kelinginc).
Option 2 “Build for the future”
For $520, kelinginc sells 3x 387 oz/in steppers, a Geckodrive G540 4-drivers-and-a-BOB, and a 36v/8.8A power supply. The G540 will handle motors up to 50v/3.5A, has a 4th axis in case I need to beef up X, and is the industry standard stepper driver. I don’t know that I get a whole lot of “extra expansion” out of the Gecko, but certainly I can count on it.
There are larger motors (NEMA 34 and 42), larger drivers (60v/5A), larger power supplies (you get the idea). I am fairly certain I don’t want to underbuild. But overbuilding doesn’t feel right, either. I don’t know what I’m doing yet.
About the enclosure. I like the idea that I have a relatively dust-free and “throw the switch” solution. OTOH, I wonder if building an enclosure will seem like such a big deal when I have a CNC machine sitting around…