Bit anatomy: diameter, shank, flutes and stickout
Cutting diameter is the part that cuts and it is what CAM needs. Shank diameter is what the collet grips, and it can be different. Cutting length limits how deep you can go. Stickout, how far the bit hangs out of the collet, controls deflection more than anything else you can adjust for free.
The dimensions
| Dimension | What it is | What it controls |
|---|---|---|
| Cutting diameter | The widest part of the cutting portion | The width of the cut, the minimum inside corner radius, how much material it can remove |
| Shank diameter | The plain part the collet grips | Which collet you need, and how stiff the exposed shank is: a 1/2 inch shank flexes far less than a 1/4 inch one |
| Cutting length | How much of the tool has cutting edges | The deepest the tool can ever reach over all passes, not the depth of one pass |
| Overall length | Tip to end of shank | How much room you need under the spindle |
| Stickout | How far it protrudes from the collet | Deflection, chatter, accuracy. The one you control. |
| Helix angle | How steeply the flutes spiral | How much the cut is a shear rather than a chop, and how hard chips are pushed along the flute |
Cutting diameter is not shank diameter
A bit with a 1/4 inch shank can have a 1/8 inch cutting diameter, or a 1/2 inch one. They are different measurements for different purposes, and getting them mixed up in CAM is a common and expensive error: the software calculates toolpaths using the cutting diameter, so telling it the shank size produces a pocket that is the wrong size or a profile that does not fit.
Stickout, the free accuracy upgrade
A router bit sticking out of a collet is a cantilever beam, and cantilever deflection grows roughly with the cube of the length. In plain terms: doubling the stickout makes the bit roughly eight times easier to bend.
So the single most valuable habit with bits is to insert them as deeply as the collet safely allows and only expose the length you actually need to cut. The effects are immediate:
- Walls come out square instead of tapered.
- Chatter goes away or gets much quieter.
- Dimensional accuracy improves without touching a single setting.
- Small bits stop breaking.
There is a limit. The bit must not bottom out inside the collet, and the flutes must not be gripped by it. See collets for the right way to do it.
Carbide, steel and coatings
Carbide's brittleness is why a bit breaks rather than bends when something goes wrong, and why a bit dropped on a concrete floor may have a chipped edge you cannot see. It is also why, in solid wood, a dull carbide bit usually got that way by overheating rather than by wearing out. Sheet goods and filled materials are the exception: the glue lines and mineral content in plywood, MDF and solid surface abrade an edge steadily, which the materials pages say in their own words.
Judging quality without a lab
- Look at the edge under magnification. A good edge is a clean line. A cheap one looks ragged even when new.
- Check for runout in your own machine. A bit that wobbles cuts oversize and loads one flute more than the other. See runout.
- Notice how long it stays sharp. This is the real difference between a budget bit and a good one, and you only find out over time.
- A cheap bit is not a waste. Budget bits are genuinely useful for learning, for cutting reclaimed material that might hide a nail, and for jobs where you expect to destroy something.
When a bit is done
Carbide does not announce it. The signs are indirect:
- The same settings that used to cut cleanly now burn.
- The cut sounds different, usually higher and harsher.
- Edges fuzz where they used to be crisp.
- The machine works harder for the same cut.
- The bit is noticeably hotter at the end of a pass.
Mark bits as they age. A used bit that looks identical to a new one in the drawer is how a ruined workpiece happens.