O-flute and plastic cutting bits
An O-flute is a single flute bit with a deep polished round gullet. The polish stops plastic chips sticking, and the single flute makes chips thick enough to carry heat out of the cut. That combination is what prevents melted plastic welding back into the slot.
The problem O-flutes solve
Plastic does not burn like wood. It melts, and molten plastic is sticky. If it stays in the cut it welds itself back onto the wall of the slot, and now the bit is cutting through a solid rib of re-melted material it created itself. The cut gets worse with every pass, the edge is rough, and eventually the bit seizes.
Everything about plastic cutting geometry is aimed at one thing: get the chip out of the cut before it can melt and stick.
What makes a plastic bit a plastic bit
Settings that stop melting
Starting point The instinct with plastic is to slow down and be careful, and it is exactly wrong.
| Setting | Direction | Why |
|---|---|---|
| RPM | Lower than wood | Less friction heat per unit of material removed |
| Feed rate | Faster | Thicker chips carry heat out of the cut |
| Flutes | One | Thickest possible chip, biggest gullet |
| Depth per pass | Moderate, not tiny | Very shallow passes make thin chips that melt instantly |
| Air blast | Yes if you can | Cools the cut and clears the slot |
| Coolant | Sometimes, a light mist | Effective, messy, and not always compatible with your machine |
Where a wood bit is good enough
For a shallow engrave or a single light pass in thin acrylic, a sharp single flute wood bit often works fine. The problems start with deep cuts, through cuts, and anything where the bit spends real time in the slot. For those, the correct geometry matters and a proper plastic bit is worth the money.
A note on different plastics
They are not interchangeable. Cast acrylic machines beautifully; extruded acrylic melts more readily. HDPE is soft and gummy and wants a very sharp tool. Polycarbonate is tough and tends to leave a burr rather than a chip. See acrylic and HDPE for the specifics, and note that CHIP does not give settings for PVC because of the gas it releases when cut.