Personal Fab

A CNC MILLING MACHINE FOR LESS THAN A GRAND

By Tom Owad

THE OBJECTS WE BUY TODAY ARE MADE in enormous factories, typically thousands of miles away. Like the mainframes that preceded personal computers, these factories are filled with expensive machinery, controlled and maintained by dozens of operators and technicians. Also like the mainframes, they’re about to be displaced by home machines.

Neil Gershenfeld of MIT’s Center for Bits and Atoms has written Fab, a book that describes the possibilities that home fabrication machines will create, and how people today are using fabrication labs that the Center created. A critical component of personal fabrication is CNC — computer numerical control. A CNC machine is controlled by a computer that issues commands for each movement the machine makes.

One of the machines most commonly adapted for CNC control is the milling machine, which resembles a drill press with a movable table. The table has x and y axes that are controlled by motors. Using a language called G-code, the programmer can direct these motors to move to a particular location, traveling on a defined path at a defined speed. The z-axis is the spindle (or the table) and can be raised or lowered to adjust the depth of the cut. Fourth and fifth axes can be added to rotate the work.

A commercial CNC milling machine will cost from $10,000 to well over $100,000 — but retrofitting a manual mill will give you a home CNC mill for a fraction of that cost.

The Sieg X2 is a very popular mini mill for CNC conversion. Made by Sieg and marketed by Grizzly, Harbor Freight, and others, it sells for about $500. The simplest CNC conversion you can do with it is to automate the x and y axes, leaving you to manually adjust the spindle to the desired level. In this configuration, the x and y axes are controlled by stepper motors that are mounted in place of the handles. You can make the motor mounts using your mill or you can buy a mounting kit, such as the one sold by littlemachineshop.com (part #2668). This provides all the brackets and couplings you need to mount your motors. It’s still up to you to provide the motors and the circuitry to control them.

There are many boards, called stepper drives, available to control stepper motors. My favorite is the Microstep by EAS (embeddedtronics.com). EAS sells the bare circuit boards and the PIC chips, and provides parts lists for ordering the rest from Digi-Key. They even provide the source for the PIC and layout for the printed circuit board, if you’d rather do everything yourself.

An easier and more popular option is the Gecko Drive (geckodrive.com). This comes fully assembled and supports more powerful motors than the EAS Microstep. Both options require an unregulated linear power supply. These are about as simple as a power supply gets, and are easy to build.

The stepper drives can be controlled by an ordinary onboard PC parallel port, but if you use a PCI parallel card, you reduce the risk of damaging your motherboard. EAS and many others sell parallel port breakout boards that make it easy to interface the stepper drivers. To control the machine, there’s Enhanced Machine Controller or EMC (linuxcnc.org), a very powerful open source controller that can convert G-code into motion.

You now have an entire CNC mill, if a simple one. The costs pencil out as follows:

So, about $825 and a lot of dedication will get you a basic home CNC milling machine that can cut metal, plastic, wood, and circuit boards. A fun first project might be to build your own woodenclock:pathcom.com/~u1068740.

Tom Owad is the owner of Schnitz Technology, a Macintosh consultancy in York, Pa. He spends his days tinkering and learning, and is the owner and webmaster of applefritter.com.