The next step towards building your router or milling machine is to decide what fundamental type of machine you want to build. This website will primarily focus on 3 axis milling machines which are the most common types of machines for hobbyists. There are two fundamental types of machines; mobile bed and mobile gantry designs.
Mobile bed design
A CNC with a mobile bed means that the actual piece of work you want to mill is mobile, while the gantry, or part of the machine holding the router, is stationary. To the side is an example of what a mobile bed CNC machine might look like. Notice that the bed is mobile in the X and Y axis, and the router is mobile in the Z axis. This is a common CNC design for small routing machines. The advantage to mobile bed designs is that they are simple to design and build. The disadvantage is that it can be hard to support the router because it needs to be extended over your work. A CNC router is only good if the router is held nice and stable. This is why mobile bed designs are usually small, typically with X and Y axis lengths of 12 inches or less. Sure, they can be much larger, but mobile bed designs aren’t typical for large CNC machines.
Mobile gantry design
A Mobile gantry design means that the work is held in the same location, and the gantry holding the router is the mobile part. An example of a typical mobile gantry design is shown to the side. Mobile gantry designs are good in a variety of situations, especially if you wish to work on large pieces of material. The design shown in this example has a long X axis, a shorter Y axis, and an even shorter Z axis. This is typical. The advantage of mobile gantry designs is that they are simple, and can be arbitrarily large without greatly added technical complexity. The main disadvantage is that it can be a little more difficult to move a large, heavy gantry.
Axis may be either fully supported, or partially supported. Taking a look a at the mobile gantry design above, this is partially supported X axis, fully supported Y axis. The X axis is only partially supported because it is supported only at the ends, allowing the machine to depress a little if a large amount of weight is put in the middle. The amount of depression is a function of material used, thickness, and length. It is possible to create a machine with fully supported X and Y axis, but at the cost of added design complexity. For example, if 2 motors, one on each side, were used to move the gantry, the X axis could be fully supported without interfering with the movement of the gantry.