Specialty bits: roundover, chamfer, dovetail, keyhole, T-slot and core box
These bits cut a shape rather than a plain wall. Several of them, dovetail, keyhole and T-slot, cut wider below the surface than at it, which means they must enter from the side at full depth and can never be plunged in and out mid-cut.
Edge profile bits
| Bit | Cuts | Used for | Notes |
|---|---|---|---|
| Roundover | A quarter circle on an edge | Softening edges on signs, trays, boards | Sold by radius. Many have a bearing, which CNC work usually does not need |
| Chamfer | An angled flat on an edge | Crisp bevelled edges, breaking sharp corners | Sold by angle. A 45 degree chamfer bit has a 90 degree included angle, and works as a V-bit for wide shallow work if you enter it as 90 in CAM |
| Cove | A concave quarter circle | Decorative edges, finger pulls | The inverse of a roundover |
| Core box | A half round groove | Fluting, decorative channels, rounded bottom pockets | Essentially a large ball nose with a short cutting length |
On bearings. Bearing guided bits are made for handheld routing against a template. On a CNC the machine already knows the path, and the bearing is at best redundant and at worst an obstruction that limits how the bit can approach. Bits without bearings are usually the better choice for CNC.
Undercutting bits, and the rule they share
Dovetail, keyhole and T-slot bits all cut a shape that is wider below the surface than the opening at the top. That geometry has one absolute consequence:
Keyhole bits
A keyhole bit cuts the slot that lets a sign hang flat on a single screw. The routine is fixed:
- Plunge straight down at the point where the screw head will enter, to full depth.
- Travel horizontally the length of the slot, cutting an undercut channel.
- Travel back along the same line to the entry point.
- Retract straight up out of the entry hole.
The screw head goes in at the plunged hole and the sign slides down so the head is trapped in the narrow channel. Two practical points: put the slot high enough on the back of the sign that it does not break through a thin panel, and remember the sign hangs from the top of the slot: the entry hole sits below where the screw will end up, and the channel runs upward from it.
Dovetail bits
Angled sides producing the classic dovetail socket. Used for joinery and for sliding dovetail assemblies. They are fragile at the neck, they must be run at a single full depth, and they do not tolerate heavy feeds. Test fit on scrap of the same species, because a dovetail that is a thousandth too tight will split the workpiece when you assemble it.
T-slot bits
Cut a T shaped channel, most often for track and fixturing. Same undercut rule. Common on CNC tables themselves, for building fixture plates and hold-down systems.
Feeds for shaped bits
Starting point Profile bits usually have a large effective cutting diameter at their widest point, which means the edge is moving fast. Lower the RPM from what you would use for a small end mill, keep the feed reasonable, and take the full profile in one pass if the bit and machine allow it, because a stepped profile cut leaves a visible line where the passes meet.