Most router problems are one of three things: wrong shank, wrong spiral direction, or a feed rate that cooks the carbide. Everything else is detail.
Use 1/2" whenever your router accepts it. Four times the cross-sectional stiffness means less deflection, less chatter, and a cut that doesn't wander. A 1/4" shank in a deep mortise flexes, and you can see it in the wall of the cut. Keep 1/4" bits for small trim routers and palm work where that's all that fits.
| Up-cut spiral | Pulls chips upward, clears well, excellent in deep mortises. Tears out the top face of plywood veneer. |
|---|---|
| Down-cut spiral | Presses fibers down, leaves a clean top edge on plywood and melamine. Packs chips into the cut, so take shallow passes. |
| Compression | Both geometries in one bit, clean on both faces of sheet goods. Expensive, and only worth it if you cut plywood parts for a living. |
| Straight flute | Cheapest, fine for solid wood dados, noisier and rougher than a spiral. |
Micro-grain carbide has smaller tungsten particles, takes a keener ground edge, and survives more regrinds. That's most of the gap between a $25 bit and a $60 one in the same profile. The rest is grind accuracy and how well the brazing holds under heat.
Practical rule: solid wood forgives cheap carbide. Plywood, MDF, and laminate destroy it — the glue lines are abrasive. If you cut sheet goods, buy the better bit.
Burn marks are almost never a dull bit. They're too slow a feed, too small a depth of cut, or too high an RPM for the diameter. Move faster, cut deeper, slow the router down. A bit that's actually dull leaves fuzzy fibers, not scorch marks.
| Up to 1" | 18,000–22,000 RPM |
|---|---|
| 1" to 2" | 14,000–18,000 RPM |
| 2" to 2 1/2" | 12,000–14,000 RPM |
| Over 2 1/2" (panel raisers) | 8,000–12,000 RPM, table only |
Large-diameter bits at full speed have dangerous rim velocity and will burn the work even if nothing breaks. Panel raisers belong in a router table, never freehand.
No more than the bit's diameter in one pass, and considerably less in hardwood. For a 1/2" mortise, three passes at 3/16" is faster in real time than one heroic pass that stalls the router and leaves a scorched wall.