4th Axis machining is an interesting and important sub-part of the CNC milling world. Haas actually got it’s start building a 4th axis before it ever built entire CNC machines (pictured on the right). This is an article series to help beginners understand how and why a 4th Axis is used on CNC Mills. In the first installment, we’ll look at what a 4th axis is used for. In the second, we’ll look at how they work.

If you’re new to CNC, your first impression may be that the 4th axis is used in the same way a rotary table is used for manual machining. Indeed, there are many projects out there where someone converted a manual rotab to become a 4th axis. While there is a grain of truth to this idea, largely it’s incorrect. Most manual machining 4th axis work is about cutting features along an arc, something that’s hard to do with a manual machine. With CNC, cutting along an arc is easy–that’s what the G02 and G03 g-codes are for. There are cases where we do continuous machining with a 4th axis–in other words, we want the cutter to be machining as the 4th axis turns. But there are also many other applications. Let’s go over three major categories.