Three axis CNC machines generally fall into two separate categories. Some CNC machines are routers while others are mills. These two types of CNC machines are for very different purposes, and as a result of that, are designed and built differently. For this article, the term ‘router’ is referring to the type of CNC machine, not the device used to rotate the cutting tool at high speeds.
Generally, CNC routers are used to cut soft materials such as wood, plastic and foam. Some CNC routers are even capable of cutting aluminum since it is a soft metal. Because routers are used to cut soft materials, they are generally built fairly lightly. Don’t get me wrong, large CNC routers can easily weigh hundreds of pounds, but that is still fairly light.
Because materials like wood often come in large sheets, many routers have a very large cutting surface such as 4×8 feet, and a fairly small Z axis, typically 2 to 12 inches. Because there is such a large travel area, routers typically have high cut and jog rates. For example, some Shopbot routers can jog at 12 inches per second.
Because the cutting size and travel speed of CNC routers is usually high, there is one tradeoff. CNC routers are typically less accurate than mills because soft materials usually don’t need to be machined precisely to the thousands of an inch.
A router may be equipped with either a router or a spindle to power your cutting tool.
A mill is an entirely different beast. Mills are used to mill hard materials, ranging from aluminum to hardened steel or even titanium. Mills are often used to mill machine parts, and because of this requirement, mills are very accurate. Generally, a mill is accurate to a thousandth of an inch or better. To achieve this accuracy, mills are often made with die cast iron or other extremely hard material which does not flex even under load. This makes mills several times heavier than CNC routers.
Because hard materials are cut much more slowly than soft materials, CNC mills have a much slower jog speed. Spindles are used instead of routers, since spindles are good at low RPM required for cutting metal. Because mills are often used with a rotary indexer, many CNC mills have a large Z-axis.
The cost of a mill is certainly higher than that of a router of equal size. In fact, mills typically have a very small cutting area. To get anything past 2×2 feet could cost you tens of thousands of dollars.