CNC Router Bits Explained: Which Bit for Which Job

The bits sold for a handheld router and the bits used in a CNC overlap, but CNC work leans on a different set. A CNC moves the bit sideways through the work for long periods, so how the bit clears chips matters as much as its shape.

For the handheld-router profiles (roundovers, ogees, rabbets), see 9 common router bits explained.

The cutters

Flat end mill (upcut spiral). The workhorse. The spiral pulls chips up and out of the cut, which keeps it cool in deep pockets and profiles. The cost: it lifts fibres at the top surface, leaving a fuzzy or chipped edge on ply. A two-flute end mill is the most versatile first bit.

Downcut spiral. The spiral pushes chips down. That leaves a clean top edge and holds thin stock against the spoilboard. The cost: chips pack into deep slots, so the bit runs hotter and can burn wood. Use it for shallow work where the top face shows.

Compression bit. An upcut section at the tip and a downcut section above. On sheet goods, both faces come out clean. It works only when the cut is deep enough that both sections engage. The bit for cutting parts from veneered ply.

V-bit. A pointed bit for lettering and V-carving. The machine varies the depth to change line width, giving crisp signs with sharp inside corners. Common angles are 60° and 90°; a narrower angle gives finer detail.

Ball nose. A rounded tip for 3D relief carving and smooth contours. The machine steps across in small passes, so 3D jobs take a long time.

Single-flute O-flute. One large flute for clearing big chips, for plastics like acrylic and for soft aluminium. More flutes in those materials leads to re-cutting chips, which melts plastic and welds aluminium onto the bit.

A starter set

• 1/4 in two-flute upcut end mill: pockets and profiles
• 1/8 in two-flute upcut end mill: finer detail
• 1/4 in downcut: clean shallow work and thin stock
• 60° or 90° V-bit: signs and lettering
• 1/8 in ball nose: 3D carving

Buy carbide from a known maker. The bits bundled with budget machines are usually the weakest part of the kit.

Feeds, speeds and chipload

Two settings control every cut: spindle speed (RPM) and feed rate (how fast the machine moves). Together they set the chipload, the thickness of the chip each cutting edge takes.

chipload = feed rate ÷ (RPM × number of flutes)

• Chips too thin (feed too slow or RPM too high): the bit rubs instead of cutting, overheats and burns the wood. Burnt edges and dust instead of chips are the sign.
• Chips too thick (feed too fast): the bit deflects, chatters, or snaps.

Start from the bit maker’s recommended chipload for your material, calculate the feed rate, and adjust by what you see and hear. Chips should be chips, not powder.

Holding the work

A bit is only as accurate as the clamping. Use clamps that sit low and clear of the toolpath, or double-sided tape on flat stock. Always check the toolpath preview for clamp collisions before you press start.