Two things determine cut quality on a CNC router: the right tool and the right balance of speed and feed. However good the machine, the wrong bit or settings lead to burn marks, fuzzy edges and tools that dull early.
Types of router bits
Upcut
Pulls chips upward, giving good chip evacuation in grooves and pockets. However, it can cause edge chipping on the top surface, especially on laminated panels, and tends to lift the part.
Downcut
Pushes chips downward, leaving a clean edge on the top surface and pressing the part onto the table. Chips can pack in the kerf, so watch for heat in deep cuts.
Compression
The lower part cuts upward and the upper part downward, giving clean edges on both faces. It is often preferred for melamine-faced particleboard and laminated panels. To work properly, the depth of cut must pass the point where the flute direction changes.
Straight and profile bits
Used for general grooving, door profiles and decorative edges.
Chip load: the logic behind speed settings
Chip load is the thickness of material each cutting edge removes per revolution:
Chip load = Feed rate ÷ (RPM × Number of flutes)
If this value is too low, the tool rubs instead of cutting; it heats up, leaves burn marks and dulls quickly. If it's too high, the tool is overloaded, vibrates and can break.
A practical approach:
- Start from the tool manufacturer's chip load chart for the material.
- Reduce chip load as the depth of cut exceeds the tool diameter.
- Make a test cut and look at the waste: you want distinct chips, not dust.
Common problems and likely causes
| Problem | Likely cause |
|---|---|
| Burn marks | Chip load too low (RPM high / feed slow), dull tool |
| Fuzzy edges | Wrong bit type, dull tool, grain direction in solid wood |
| Chatter marks | Excessive depth, long tool stick-out, collet runout |
| Broken tool | Excessive feed, chip packing, collet slip |
| Chipped laminate | Upcut bit cutting the top surface |
Extending tool life
- Clean tools regularly when cutting resinous or glued materials.
- Insert the shank far enough into the collet and keep collets clean.
- Don't keep working with a dull tool; quality drops and the spindle is strained.
Note: The values here are general principles. The best result for each material, tool and machine combination is found through test cuts.
If you have cut quality problems on your machine, we can help determine whether the cause is the tool or the machine.