What are the axis referred to for CNC machining?
Standard nomenclature for CNC machining refers to the direction of motion that a workpiece can be machined from. X, Y, and Z are linear axes with the Z-axis aligned with the spindle of the machine which holds the workpiece. A, B and C are rotary axes around X, Y and Z respectively. U, V, and W may be used to refer to parallel linear axes along X, Y and Z.
A standard 2-axis CNC lathe has X and Z axes. Bar stock is either fed or inserted into the Z-axis through the collet and a tool cuts as the stock rotates. This is used for round parts.
3-Axis Lathes and CNC Turning Centers
When a third axis (Y) is added perpendicular to X and Z, curves can be machined. These are driven by ball screw actuators. The Y-axis slides on linear guides or box ways. Many manufacturers have added other ways to move the tools, evolving the 3-axis machine into “turning centers” to be more descriptive. The line between 3-axis and 4-axis turning can be blurred as manufacturers add features to a 3-axis that allow for more machining angles but may not provide a full range of motion in the 4th axis.
In a 3-axis CNC turning center, tools are arranged on a round turret with tooling slots. The bar stock is fed through a bar feeder and the turret is programmed to rotate and articulate on to meet the bar stock to cut the material. Certain CNC turning centers have more than one spindle. In a dual spindle CNC turning center, the part is fed from the originated spindle to the secondary spindle where the other side of the part can have additional machining performed. The turrets on dual-spindle CNC turning centers have tool slots on both sides of the turret and can make more complex parts than those with a single spindle. The tool (on the turret) is programmed to move to the bar.
To orient a workpiece accurately for live tooling (like a drill), a rotary C-axis can be employed, creating a 4-axis machine. Tiny motors in the tooling mounted on the turret convert the lathe into a conventional milling machine. These servomotors hold position to allow for contouring motion in 4-axis CNC lathes. In this way, the machine can make profiling cuts using simultaneous X, Y and Z axes motion with the C-axis.
The complexity of the parts that can be made on these 3-axis turning centers is driven by the live tooling capabilities as well as the number of tooling slots on the turret. Some manufacturers mount independent milling heads with tool change capabilities make this machining center even more efficient.
The fifth axis to be added is usually the A or B axis. The machine has either an XYZAC or XYZBC toolpath. It’s this B-axis capability that sets apart this kind of CNC lathe. This rotates around the Y axis making compound angle cuts possible. It’s possible to do all milling and turning operations in one setup because the machine supports the entire range of milling and turning operations. This is the most versatile of all the lathes.
CNC Swiss Lathes
CNC Swiss lathes typically make parts under 2 inches in outer diameter. They operate by moving a fixed tooling jig to the bar stock. These tools cut very close to the spindle makes tool changes very quick. The key difference between CNC Swiss and other CNC lathes is how the bar feeder and spindle work together to produce parts. The spindle on a Swiss CNC lathe controls the bar movement against a stationary tool on the tooling jig. The bar does the moving instead of the tool. All of the cutting is done right next to the collet. This differs from a CNC turning center where the cutting occurs as the tool moves to the bar. Swiss Lathes are great for high production work. When combined with a bar feeder, they are designed to run “lights-out” and produce parts unattended. With proper programming and operator input, they can make precise parts to specification with a high level of repeatability, often with cycle times under a minute.
Selecting a CNC Lathe to Purchase:
CNC lathes have many options to optimize them for the production of different kinds of parts. For example, you’ll need to consider:
Bar capacity (maximum OD that can be bar-fed into the machine), Max part length, Tooling capacity, Number of “Live” tool positions, Sub Spindle capabilities, Mist collectors, Attachments for long tooling, Collection trays , Tooling such as collets, guide bushings (ask what tooling comes with the machine), CNC control type (Fanuc is common), Tooling options, Feed rates, Bar feeders (This may come with a used CNC Swiss lathe or may need to be purchased separately. The length bar feeder determines the length bars you will buy, the space needed and the scrap rates you will have.)
Selecting the Best Size CNC Lathe
When selecting the best size CNC lathe, you’ll want to consider the part sizes O.D. that you will be making. Just as the axis movement, live tooling, and a number of tooling positions drive the complexity of parts that can be produced, the bar capacity outer diameter size determines the size parts. Bar feeders push the stock through the collets for production runs. The maximum collected and chucked sizes possible are very different. The best efficiency comes from machines that are closest to the size parts you need though you can turn something very small from a very large stock. Some CNC lathes may already come with barfeeders, Make sure to ask the seller if it includes a barfeeder at the current price.