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How-To: Build your own CNC machine (Part 2)

By Will O'Brien

posted Jul 4th 2006 9:37PM

How-To

In today's How-To, we're still pimping out our dremel tool with parts from old printers. In Part 1 we got started with the controller and covered all the basics. Today we'll get into the details and get busy with the power tools. And that, of course, is always the best part.

Once the board is finished, building the controller is pretty easy. We highly recommend using sockets for mounting the 5804 chips. The thin, flexible legs are much easier to fit into a hand-drilled board. (We were out of 16 pin sockets, so we used pairs of 8 pin sockets.) The rest of the board is standard fare.

The controller is designed to connect to the parallel port, and each connection is helpfully labeled with the pin of the Sub-D 25 connector. We prefer the solder type connectors. Assembly is quick and easy if you have a set of "helping hands" alligator clips.

Electrically, unipolar stepper motors have four coils inside. Every motor we've salvaged has had six wires, so we'll go over that type. To have six connections, each pair of coils has a common lead, while the opposite end has a dedicated lead.

Identify the wires by measuring the resistance between the leads with a multi-meter. If the wire are connected to separate sets of coils, the resistance will be very high. Resistance across two coils will be double the resistance of just one coil. On some motors, the common leads are connected.

Each axis of the stepper controller has six output connections. Each group of three wires connects to a pair of coils.

Linear slides are key to the design of a functional machine. These slides are a half successful experiment. We used 1/4-inch steel rod from the hardware store and some brass and steel bushings. The brass material slides easier, but ultimately we think the smaller size and unfinished rod is too prone to binding. Alignment is critical, but they can work well for very short travel.

Salvaging matching rods from old printers is more optimal. Imagewriter IIs have metal carriages with pressed in brass bearings. The cast material is on the brittle side, but some careful dremel work can really pay off.

Getting appropriate materials for the project can be a challenge. In this case, we're using two of these handy half inch thick cutting boards from Sam's Club. They're about $10 each. Higher quality plastics like delrin can be obtained from suppliers like McMaster-Carr.

The threaded rod needs to spin freely with the motor, but still needs to be anchored. We picked up a 1/4-inch inner diameter ball bearings off of ebay. We drilled a hole the same size as the bearing, then cut a slot in the piece with a miter saw. Finally, we drilled a hole for a machine screw.

The bearing is sandwiched between two nuts on the threaded rod. They are tightened with two wrenches. Then the bearing is inserted into the block and the machine screw is tightened down. It's a simple and effective design. We usually put one at each end of the threaded rod.

To build the mechanical base of the machine, it's important to put in some design time. Determine how much material you have, draw out your design and estimate how much material you'll need to achieve the size of machine you're going for.

Spend time laying out each axis. Then break it into its components so you can begin laying out your cut sheets. This was our original layout for the first axis of our machine.

We cut our cutting boards using a standard table saw and a circular miter saw. If the blade is sharp, you'll end up with some very nicely finished edges.

The first axis for this table is simple. The base acts as a large channel for the table. We've found that the plastic is soft enough that it doesn't have to be tapped for threads. Just drill the hole with the same bit you'd use if you were tapping threads (like a #21 for 3/16 threads) and bevel the outer edge a bit. Machine screws will thread right into the plastic, and the threads will hold surprisingly well. However, tapping the threads for extra precision isn't a bad idea.

Originally we wanted to use two 1/4-inch rods to maintain alignment, but thanks to the channel design, just one was sufficient. The second rod was a source of binding. We suggest incorporating a larger rod or two from a printer.

The holes for the rods and screw were drilled at one time on the drill press before assembly. The bearing block was added once the screw was aligned. The locknut isn't necessary. If you want to hand align the machine, This is a good place to add a knob or wheel to spin.

Next week we'll build the rest of the machine, mount the tooling and finish the job. See you then!