CNC machines consist of one of more axis, on which a mechanical piece of hardware is allowed to move. Choosing the number of axis to have for your CNC router or mill is the first, and potentially most important design choice you will need to make.
1 axis machines
These are the simplest possible type of CNC machine, where there is only one set path for movement. Keep in mind that an axis doesn’t necessarily have to be straight. An axis could be curved, or it could be rotational, where instead of being able to move a specified distance, it could rotate by a specified number of degrees. So take this image as representative of a linear axis. 1-axis machines are uncommon because of their extremely limited use, but not completely useless. For instance, you could have a 1-axis CNC machine block water in a waterfall to make a neat pattern as the water falls.
2 axis machines
These machines are more common, and allow for two separate, independent types of movement. A common example of this would be a common household printer. The printing head would be mounted on one axis, and the paper would travel along another, perpendicular axis. Another example would be a plasma cutter, where the cutter head can move to any (x,y) coordinate. So if you’re making strictly 2D work, 2-axis machines may be a good choice for you.
3 axis machines
These machines are perhaps the most common among hobby CNC routers. Typically, the x and y plane are parallel with the floor, and the z axis is vertical. This configuration lets you create a wide variety of 3d art or 3d machine parts. Another popular 3-axis arrangement would be to have one axis be rotational, like a lathe, another axis be linear going down parallel to the rotating piece, and a third axis perpendicular to the piece. If you’re just getting started in the world of CNC and considering building a machine, 3-axis machines are probably the best type to build first because of their simplicity, and usefulness. This website will focus mostly on 3-axis machines.
4 and 5 axis machines
These are the most complicated CNC machines, and are therefore the most difficult and expensive to build. Keep in mind that while 3-axis machines can indeed mill out 3d pieces of work, they can not mill out any conceivable 3d shape. For example, you may need to have hold drilled into the top of your piece, and on through the side of your piece. For that, you’d either need to have a rotational axis which holds your piece, or have a rotational axis which changes the angle the end mill is facing. Either way, that would require a 4-axis machine. Adding a 5th or even 6th axis would further enhance your ability to create very complex shapes with your machine. Keep in mind that the cost of software to generate 4+ axis tool paths can be extremely expensive, starting at a few thousand dollars.