CNC

A common Milling Machine has three plains of movement, each plain is known as an "Axis". More complex machines will have many other plains and even incorperate a rotory plain to cut more detailed objects, for example a "worm gear" or "crank shaft".

One plain will move the spindle up and down, another plain moves a bed horizontal to the Spindle a third plain may move the bed perpendicular to the spindle and a fourth plain may rotate the part below the spindle.

Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. The Spindle is the part of the Mill that holds the tooling, for example a spindle maybe a "Dremel" or Router.

Both types range in size from small, bench-mounted devices to room-sized machines.

Unlike a Drill Press, which holds the work piece stationary as one axis feeds into the work piece, Milling Machines move the work piece in a vertical motion against the rotating milling cutter, which cuts on its sides as well as its tip.

Work piece and cutter movement are precisely controlled to less than 0.001 in (0.025 mm), thanks to the Integrated Circuits on Printed Circuit Boards, lead screws, and stepper motors.

Many people will build machines from scratch or with the help of a kit, the more common do it yourself project found online is the Rep-Rap 3D printer, it's a three axis milling machine with an extruder nozzle attached in place of a Dremel or Router.

A great deal of software is available for this project and most of it's open source, complete with source code just in case you want to tweek things a little or use the software on other projects.

So how do you program a CNC mill?

Actually programming anything CNC is pretty simple, it just all depends on the application, and hardware. For example if you plan to interface your first project and computer using a USB port .vs a COM port like the old DB25 printer port the software will be a little different for each one.

In the case of the DB25 port, the open source "Machine Software" is "EMC", the hardware works by receiving electrical pulses from individual pins connected to the computer.

These pulses are sent thanks to the "machine software", the "Machine Software" in this case "EMC" converts "G-code" and "M-code" into high and low signals, or "0" and "1", "0" may turn the electricity off, and "1" may turn it on. The "Pin" on the DB25 that receives that "0" or "1" (High or low) is defined when EMC is configured for your machine, pin number 25 maybe the pin that controlles the motor that moves the "X- Axis", and maybe pin number 6 is the pin that controlles the "Y-axis". It really just depends on the "Printed Circuit Board" that connects to the computer and how it was designed.

In the case of the USB interface, additional hardware is needed on the "Printed Circuit Board", like a "TFID" "Integrated Circuit" to make the USB style "Printed Circuit Board" a "COM" port, the TFID receives Packets sent from the computer opposed to pulses and converts the packets into pulses on the PCB. 

The USB style interface also needs some smarts, usually in the case of the do it yourself project the Integrated ciruit used for this is called an "AVR", the "AVR" needs some timing help from a "Crystal" to work properly. This is called a PLC (Programmable Logic Controller).

With the TFID, AVR, and Crystal, the USB receives "Packets" containing a string of 0 and 1's from the Machine software, the packets are then processed thanks to that AVR IC, and sent to the appropriate driver.

Really the DB25 style interface is the most basic, strate forward way to "break-out", but the USB style "Break-out board" is more compact, allows for more connections, and is the new way to interface with the computer, the DB25 style port is rare on new machines.

So what about the "G-code" if anyone can program a CNC how does a person generate "G-code"?

The open source answer is... with the help of Skeinforge a Python Slicing Software, Skeinforge works with "Pronterface" and "Replicator-G" to name two programs, these programs can be used as CAM programs. Often people will use editing software like "Adobe Illustrator" or "CorelDraw" to render an Image and export the image to a .DXF or .DWG format.

The .DXF or .DWG format can then be opened using a CAM software and converted to G-code.

Some analogous technology is available and can work in connection of the stepper motors, see open loop and closed loop systems. Milling machines may be manually operated, mechanically automated, or digitally automated via computer numerical control (CNC).

Milling machines can perform a vast number of operations, from simple to complex. Some people make machines of their own and often use material found around the home or hardware store, often using Dremel tools or Router tools as spindles.