Modifying our cheap CNC machine

As mentioned in a previous post, our CNC 3020 milling machine came with a little room for improvement. In this post we discuss some modifications we made to make our machine more useful.

Adding limit switches

Limit switches allow the CNC mill to home itself to a repeatable zero position. They also prevent the machine from going outside its permitted range of motion and crashing into 'hard' limits.

For most CNC machines, it's common to provide two limit switches for each of the X and Y axes (for maximum and minimum limits) and a single limit switch on the Z axis for the upper limit only. The 'safe' lower Z limit depends on the kinds of materials you have clamped onto the platform and the kind of cutting bit you happen to be using at the time. It's not the sort of thing you can simply guard with a limit switch.

Limit switches are cheap to obtain online. They are simple microswitches with a lever arm and roller attached to help actuate the switch with a reasonable amount of precision. We just superglued them onto the aluminium frame of the machine in positions where they would be tripped just before the machine would hit a hard limit. With a bit of creativity, you can find positions to locate the switches that sacrifice very little of the machine's range of motion.

The X limit switches will trip if the carriage moves too far to the left or right. The Z-maximum limit switch is mounted on the carriage and will trip if the spindle rises too far up. Small dowel pieces help give the superglue more surface area to bond to.

This Y-minimum limit switch will trip if the gantry moves closer to the front of the platform.

We wired both X axis and both Y axis switches in series, using the 'Normally Closed' connectors on the switches. This is the recommended option as faults in the limit switch connections show up immediately. We made the connections using light-duty stranded-core bell wire. We'll have to wait and see how durable this wire is in the long run, but it's very flexible and thin enough to fit into the existing cable trunking fairly easily.

For the other end of the connections, we were lucky. Although the CNC 3020 controller box doesn't provide any inputs for limit switches, there are holes on the circuit board inside where you can solder on a pin-header for X, Y and Z limits. What's more, after we soldered on a 6-pin header, we found these pins to be fully functional. They simply map onto three otherwise-unused parallel port inputs pins.

The PCB from the controller box after soldering a strip of 6 header pins into a bank of 6 vacant holes labelled 'LIMIT'. Presumably, a more expensive model would have come with those header pins already there to begin with.

We drilled holes in the back of the controller box and ran the limit pins to female banana connectors. We chose banana connectors because they're versatile: you can use them as binding posts for bare wire, or you can terminate the wires properly by adding banana plugs. (We also added a grounding post on the back for possible equipotential bonding to the CNC platform, which might be overkill...)

Banana socket binding posts added to the back of the controller box.

Ribbon cable linking banana sockets to LIMIT pins. (GND post not yet connected.)

Keeping dust out of the controller box

The controller box has a fan inside for circulating air around the heatsink, with an unfortunate side-effect. Milling dust is sucked into the unit from the nearby milling platform, where it collects on the circuit boards inside. This isn't a huge problem when milling wood as sawdust isn't conductive. It becomes a problem if we start milling aluminium or copper. Moving the controller box further away from the milling platform isn't an option; the cables are too short.

Our solution was a combination of filters and ducting. For the front and side vents, we cut filters out of kitchen scouring pads (the type that come in flat sheets). For the price, these make excellent dust filters. We hooked them over the air intake grilles with office staples.

For the bottom air intake (the most critical, as it pulls air across a large heatsink), there wasn't enough bottom clearance between the controller box and the bench to add a filter without severely limiting airflow, with the risk of overheating. Instead, we added sponge strips around the bottom of the controller box on three sides so that air could only enter from the left hand side – away from the milling platform. The sponges had the added bonus of lifting the controller box slightly, improving airflow to the bottom vent.

The rear vent does not need filtering as it's the exhaust vent for the fan, and should repel dust when the unit is turned on.