CNC Router Bit Basics: Flute Count, Shank Size and Feed & Speed

Flute count, shank size and feed & speed decide whether a cut is clean or a disaster. Here are the basics that matter on a CNC router.

Flute count vs material

Shank size

Shank must match the collet. Small shanks like the 3.175 mm bit suit detail work; 8-12 mm shanks like the 12 mm straight bit and 8-12 mm spiral bit suit deeper, heavier cuts with less deflection.

Feed & speed in practice

Rule of thumb: soft material = higher feed, fewer flutes; hard material = controlled feed, more flutes, coating. If the bit burns or chatters, raise feed or drop RPM. Keep spares of high-use profiles to avoid downtime.

Tie it together

Pick shank by collet, flutes by material, then dial feed & speed on scrap. A router bit set lets you keep the common profiles on hand so you can match the bit to the job every time.

Frequently Asked Questions

Q: What does flute count do?

A: Flutes are the cutting edges. 1-2 flutes clear chips fast for soft materials (acrylic, wood); 3-4 flutes give a finer finish in panels and metal. More flutes = smoother cut, slower feed.

Q: How do I pick shank size?

A: Match the router or spindle collet: 3.175 mm, 6 mm, 8 mm, 12 mm and 1/4″ to 1/2″. Larger shanks deflect less and suit heavier cuts; smaller shanks suit detail and engraving like the 3.175 mm engraving bit.

Q: What is a good starting feed & speed?

A: Start conservative: lower RPM with higher feed for soft wood, higher RPM with controlled feed for acrylic and metal. Test on scrap, then raise feed until finish degrades, then back off 10%.

Q: Why does my bit burn the wood?

A: Burning means too little chip clearance or feed too slow — the bit rubs instead of cutting. Raise feed, use fewer flutes, or switch to an up-cut long straight router bit.