CNC turning and milling operations: differences, axes and the wood router

The two core methods of machining are turning and milling. Both shape a part by removing chips; the difference is what rotates. CNC (computer numerical control) automates both in the same way: the path the tool follows is defined in a program and the machine repeats it with the same accuracy every time. This article compares the two methods, lists their typical operations and shows where the everyday tool of the furniture shop — the CNC router — fits into the picture.

Turning versus milling

On a lathe the workpiece rotates and a stationary cutting tool moves along or across its axis. On a mill the tool rotates, the workpiece is clamped to the table, and tool and table move relative to each other along the axes. That one difference largely decides which part is made on which machine:

Some machines bridge the two: a lathe with live tooling (C and Y axes) adds milling and drilling to a turned part, and a 4th-axis rotary on a mill turns a part much like a lathe while milling it. Still, the basic distinction is which method gives the part its main shape.

CNC turning operations

Lathe operations are named relative to the axis of rotation. The most common are:

• Facing: brings the end face square to the axis; usually the first operation in a program.
• Turning (straight/cylindrical): the tool travels along the axis to reduce the diameter, in roughing and finishing passes.
• Profile (contour) turning: tapers, radii and stepped outside shapes.
• Threading: external and internal threads, with the tool feed synchronised to spindle rotation.
• Grooving and parting off: circlip and relief grooves; cutting the finished part off the bar.
• Drilling and boring: a drill on the centreline, then a boring bar to bring the bore to size.

Controllers offer ready-made canned cycles for roughing, finishing and threading; how they are called depends on the maker. For the general structure of G-code and its dialects see CNC programming languages: G-code, M-codes and controller dialects.

CNC milling operations

Milling operations are distinguished by how the tool enters the material and which surface it produces:

• Face milling: brings a large area down to one level; surfacing a router's vacuum table (spoilboard) is the same operation.
• Contour (profile) milling: cuts the outer or inner boundary of a part; the basic operation for cutting parts out of a sheet.
• Pocketing: clears a closed area down to a given depth; a raised-panel recess or a hinge cup.
• Drilling: vertical holes; System 32 rows and connector holes in furniture.
• Slotting / grooving: back-panel grooves, glass rebates, V-grooves.
• 3D surface milling: freeform surfaces in roughing and finishing passes; moulds, reliefs, carving.

Most furniture work is 2.5D milling: the drawing is two-dimensional and each shape gets a depth. True 3D machining (freeform surfaces) shows up mostly in moulds, carving and relief work.

What do 3, 4 and 5 axes mean?

There are two ways of working in 5 axes: 3+2 (the rotary axes lock at an angle and the cut runs like 3-axis) and simultaneous 5-axis (all five move together). More axes mean a more complex machine, CAM software and post-processor; in a panel-based furniture shop, 3 axes plus aggregate heads cover most jobs.

The CNC router: milling for wood

The CNC router in a furniture shop is, by method, a milling machine: the tool rotates and the sheet is held on a vacuum table. What sets it apart from a metal mill comes from the material: wood and MDF are soft, so spindle speeds are very high (typically in the tens of thousands of rpm), the table is large (full sheet size), workholding is by vacuum, and chips are removed by dust extraction rather than coolant. Routers also cut plastics, composites and light aluminium, but they are not built for heavy metal cutting.

Typical router jobs — door contours, panel pockets, decorative V-grooves, hinge and shelf holes, back-panel grooves — are the milling operations above adapted to sheet material. For the steps from door drawing to toolpath see Cabinet door drawing in Alphacam: membrane, panel, American, framed, glazed and louvred doors; for a wider classification of manufacturing methods see Manufacturing technologies in furniture production: CNC, CAD/CAM and automation.

Choosing tools for wood and MDF

On a router, tool choice directly sets the finish quality. The commonly known options:

Spindle speed, feed rate and chip load

A good cut on a mill or router is a balance of three values: spindle speed (n, rpm), the number of flutes (z) and the chip thickness per tooth (fz, mm). The feed rate follows from them:

• Feed (mm/min) = spindle speed (rpm) × number of flutes × chip load per tooth (mm) — F = n × z × fz
• Example: 18,000 rpm, 2-flute tool, 0.1 mm chip load → 18,000 × 2 × 0.1 = 3,600 mm/min.
• Cutting speed: Vc = π × D × n / 1000 (m/min), with D the tool diameter in mm.

The logic: each flute must remove a chip of a certain thickness on every revolution. With high rpm and low feed the chip gets too thin; the tool rubs instead of cutting, heats up, leaves burn marks on wood and dulls early. With too much feed the load on the tool rises, the finish suffers and a small-diameter tool can snap. The right range depends on the material, the tool diameter and the maker's chart; the numbers above only illustrate the calculation. Alphacam stores these values in its tool library; for the terms see Alphacam and furniture CNC glossary.

Alphacam's Lathe, Mill and Router modules

Alphacam follows the same split with its modules: Lathe for turning, Mill for metal milling and Router for wood, MDF and plastic sheet processing (there are others such as Stone, Profiling and Wire; module and level names vary by release). The module a furniture shop uses is Router. Module details are in What is Alphacam? A CAD/CAM guide for furniture and wood CNC, and a general classification of CAM software in Types of CAM software: by process, axes, integration and industry.

GMacros does not make tools for turning or metal machining; its macros and add-in work only inside Alphacam Router, for wood and MDF panel parts. For tools that draw door, panel and cabinet parts from dimensions, see the door macros page.