3-axis, fully-manual machine tools built much of modern manufacturing. American-made tools from world-famous manufacturers were built of rugged cast iron, and in the hands of a skilled machinist, they could be, and often were, used to create nearly anything.
Today, the choice is a lot more complicated. Should you rely on an old cast-iron classic? Or do you need a new 4-axis CNC machine? Or should you go straight to the top, and invest in top-of-the-line 5-axis machining centers capable of perfectly rendering incredibly advanced designs?
We’re here to break down the difference between 3-axis, 4-axis, and 5-axis machines. We’ll look at the differences between each, and see where and when you need to use an old workhouse or the latest CNC monster.
What are “axes”?
The most basic linear axes are the X-axis, Y-axis, and Z-axis. X refers to vertical movement, Y to horizontal movement, and Z to depth – i.e., movement from front to back. All CNC machines are built around those basic Cartesian axes. Today, the more advanced machines add in a 4th or 5th axis to expand the range of what’s possible without re-mounting a workpiece.
What is a 3-Axis Machine?
Three-axis CNC machines are the workhorses of modern manufacturing. Most manual lathes and mills are capable of 3-axis movement. That movement can come from the workpiece or the cutting head, or both. Horizontal mills often have Y-axis movement on the bed, with the cutting head moving horizontally above the workpiece along the Z-axis. Depending on the exact machine, either the bed, the cutting head, or both might have X-axis movement.
Vertical milling machines use the same concept in different ways. Most vertical mills keep the workpiece fixed in one place. All the movement comes from the rotating cutting head.
What is a 4-Axis Machine?
Four-axis machines add a rotating motion to one of the primary axes, typically the X-axis. Instead of X-Y-Z, 4-axis machines have X-Y-Z-A. The A-axis is the rotating motion around the X-axis. Rotating the workpiece along the new axis allows operators to undertake angled cuts or cuts on new faces of the workpiece without changing how the piece is mounted.
What is a 5-Axis Machine?
Five-axis machines take it one step farther, adding a second rotating axis, usually the B-axis. X-Y-Z-A-B, or X-Y-Z-A-C, will rotate around both the X and Y or X and Z axes. In practice, that looks something like this:
With 5-axis machines, interior cuts, angles, and unusual finishes are all possible. The workpiece can be moved in nearly any direction, allowing full range cuts to be made during a single operation. Add in a tool head with multiple cutters, and the design possibilities are nearly endless.
Adding those fourth and fifth axes vastly expands the range of designs you can build with a CNC machine. And that’s the key difference – the design.
Different design capabilities with 4- and 5-axis machines
When it comes to design, 3-axis machines can only machine all 3 axes on a single planar surface. Think of a block of metal fixed onto the bed of a mill. If the desired feature for each surface of the block is a single slot, that feature can only be cut into one face of the workpiece at a time. Start with side A, then turn the machine off and re-mount to access side B, etc.
Angled work, in particular, becomes challenging. To cut an angled feature with a 3-axis machine requires unusual mounting; it’s doable in the hands of a skilled machinist, but it isn’t something that is inherently capable with a 3-axis machine.
Adding a 4th axis (or 5th) vastly expands the range of motion. As you can see in the picture above, with a five-axis machine, almost any design can be rendered exactly. If you need to machine the surface of airfoils, vanes, or other non-linear shapes for the aerospace industry, you can do so on a 5-axis machine without remounting the part. There’s no need for a precarious mounting system with workpieces fixed at odd angles. Instead, the whole process can be programmed in advance. 5-axis CNC milling machines, for instance, will rotate both part and cutting tool as needed in the course of the operation.
When to use a 3-axis, 4-axis, or 5-axis CNC machine?
Simple, high-volume parts are best suited for a 3-axis machine. If you’re grinding out surface features, cutting key slots, and drilling out shallow holes, then a 3-axis CNC milling machine is all you’ll need. It’s worth noting that angled features can be cut on a 3-axis machine, they just require more planning and more frequent mounting/remounting.
Cylinders and features on multiple surfaces work best for 4-axis CNC machines. Adding a rotating axis to a CNC mill gives it many of the same capabilities as a CNC lathe. Both workpiece and cutting tool can rotate, meaning that a 4-axis milling machine can be used for surface work on cylindrical shapes. And to go back to our earlier example – if you need to cut an identical feature on every surface of a block a 4-axis machine saves you the trouble of remounting. Simply rotate the workpiece to the next surface, machine the feature, and move on.
Highly complex, one-off parts that require high-quality machining are ideal for 5-axis milling machines. Simultaneous 5-axis CNC machines can spin and rotate the workpiece in nearly any direction, allowing operators to machine features virtually anywhere on the workpiece. The entire process is continuous. 5-axis machines can render incredibly complex designs in a single operation, without requiring any re-mounting.
5-axis CNC machines are the biggest, baddest, most complex machine tools available today. They also carry a corresponding price tag. Most smaller machine shops and toolrooms don’t need the full capability of a 5-axis machine; an older 3-axis machine, along with some creative workpiece mounting and a bit more work, will do the trick just fine.
For more specialized applications, a 4-axis machine might be perfect, particularly when regularly dealing with more difficult geometries like cylinders.
Only the most advanced applications require the full range of a 5-axis CNC machine. In those situations, 5-axis CNC machining centers provide automation, high-speed and high-precision manufacturing solutions.