CNC Routers for Woodworking: Kinds, Parts, Bits, Feeds and Safe Use

CNC stands for computer numerical control. On a CNC router, motors move a spinning router bit along three axes — left and right (X), front to back (Y) and up and down (Z) — following a list of coordinates the computer sends. Because the moves come from a file, the machine cuts the same shape every time: lettering, pockets, curved parts, joinery and repeated parts nested on a sheet. The cutting itself is ordinary routing; how a router and its bits cut is explained in the router guide and router bits.

The work-area figure is the most useful one: it is how far the bit can travel on each axis, and it limits the largest part the machine can cut in one setup. Z travel and the clearance under the gantry limit how thick the stock can be. A handheld machine has no fixed work area — Shaper's Origin reads its position from marker tape stuck to the work and retracts the bit if it is moved out of bounds — but it is guided by hand across the work.

How the axes are driven sets how stiff and repeatable a machine is. Carbide 3D sells both ends of the range: its Shapeoko 4 runs on V-wheels with belts, and its Shapeoko 5.1 Pro on ball screws and linear rails. Onefinity's machines use ball screws on every axis. Makers state their own accuracy figures in their own terms, and none of the figures reviewed here can be compared directly with another maker's.

Many gantry machines are sold without the motor that turns the bit. Carbide 3D's Shapeoko 5.1 Pro and Onefinity's Apprentice both take a 65 mm mount and name a compact trim router — Makita's RT0701C, the kind described in trim routers — or a spindle driven by a variable-frequency drive (VFD) as the options. Either way, the motor's speed range is in its own manual, and the bit's maximum speed in the bit maker's.

Most machines come with software for each step: Carbide 3D includes Carbide Create for drawing and toolpaths and Carbide Motion to run the machine, and says the Shapeoko accepts standard G-code from other programs; Onefinity's machines run on RealTime CNC control software with a computer attached.

• End mills (straight spiral bits) cut pockets, profiles and slots. An upcut spiral lifts chips out of the cut; a downcut spiral pushes them down and leaves a cleaner top edge; a compression bit combines the two.
• Ball-nose bits have a rounded end for carved 3D surfaces.
• V-bits cut a V-shaped groove for lettering and engraving.

A V-bit cuts a groove the shape of its tip, so the depth and the width are tied together by the bit's included angle. The bit's maker states that angle; the geometry does the rest.

Feed rate is how fast the bit travels through the wood. Bit makers set it from the chip load — the thickness of wood each cutting edge takes per revolution. Amana Tool publishes the relationship on its speed charts: feed rate in inches per minute = RPM × number of flutes × chip load. Its chart for 1/8 in two-flute spiral bits gives a chip load of 0.003–0.005 in in softwood, hardwood and plywood at 18,000 RPM — by the formula 108 to 180 in/min, which the chart prints as 110 to 180. Amana's values assume a depth of cut of one tool diameter; it reduces the feed by 25% at twice the diameter and by 50% at three times, and calls every value a recommendation.

No figure here is a setting for your machine: the right chip load depends on the bit, and the speed on the motor. The formula is how the makers' figures fit together.

The work must not move at all while the bit is in it. Clamps, screws into the spoilboard and double-sided tape all work, provided every clamp and screw sits outside every toolpath — a bit that hits a steel clamp breaks. Vacuum tables, which hold sheets by suction, are covered in the shop vacuum guide. Parts cut free from a sheet can be held by small tabs left in the toolpath and cut away afterwards.

• Never leave it running unattended. Carbide 3D's first rule.
• Eyes and ears. Impact-resistant glasses or goggles, and ear plugs or muffs: Carbide 3D warns that hearing damage is cumulative and irreversible.
• Dust. A mask or respirator suited to the dust of the material, and dust collection at the bit — see dust collectors.
• Hands and clothing. Never reach into the working area while it runs; keep clothing, hair and jewelry clear; handle end mills with care.

• Laser cutters and engravers — diode and CO2 machines, laser classes, eyes, fire and fumes.
• CNC router projects — a slot-together organizer and a pocketed tray, with dogboned corners.
• How to choose a CNC router — desktop, gantry and handheld, by specification.
• How to choose a tabletop CNC — small machines compared by their makers' figures.