Feed and Speed Calculator for PCB / FR4: Chip Load Chart & RPM
FR4 is woven glass fibre set in epoxy. It cuts cleanly and it destroys cutters, because you are effectively machining glass. Treat tooling as a consumable and plan the job around that.
For pcb / fr4, the recommended chip load runs from 0.025–0.051 mm per tooth on a 1/8″ cutter to 0.076–0.127 mm on a 1/2″. Typical spindle speeds sit between 15,000 and 24,000 RPM.
Your chip load is 0.0765 mm per tooth, which is within the recommended 0.051–0.102 mm range for pcb / fr4 with a 6.35 mm upcut spiral.
PCB / FR4. FR4 is glass fibre in epoxy and it is brutally abrasive - treat cutters as consumables. The dust is a respiratory hazard; extract it at the source and do not dry-sweep it.
Upcut spiral. The baseline the published charts assume. Chips are lifted clear of the cut, which keeps heat out of the workpiece at the cost of tear-out on the top surface.
How to cut pcb / fr4 on a CNC router → - the full how-to: which bit, workholding, chip evacuation and reading the cut.
How pcb / fr4 fails when the chip load is wrong
Tool wear is the failure mode, and it is fast. A cutter that has isolated traces perfectly for an hour will start producing ragged edges and lifted copper with no warning. The abrasive fibre wears the cutting edge round rather than chipping it, so there is no dramatic moment of failure to notice.
Setting up a cut in pcb / fr4
Small chip loads and high spindle speeds, with the smallest cutter that will do the job. Fishtail or diamond-pattern router bits designed for composite are worth their price here - general-purpose end mills wear out several times faster.
The thing most people get wrong
The dust is the real hazard and it is worse than wood dust. Glass fibre particles are a respiratory irritant with no safe informal handling. Extract at the cutter, wear appropriate protection, and never dry-sweep it - this is the material on this list where the health control genuinely is not optional.
Chip-load reference for pcb / fr4
Recommended chip load per tooth by cutter diameter, before any adjustment for depth of cut or bit geometry. Values between the 1/8″, 1/4″ and 1/2″ anchors are interpolated.
| Cutter diameter | Chip load (mm/tooth) | Chip load (in/tooth) |
|---|---|---|
| 1/16" (1.59 mm) | 0.025 – 0.051 | 0.0010 – 0.0020 |
| 1/8" (3.18 mm) | 0.025 – 0.051 | 0.0010 – 0.0020 |
| 4 mm | 0.032 – 0.064 | 0.0013 – 0.0025 |
| 6 mm | 0.048 – 0.096 | 0.0019 – 0.0038 |
| 1/4" (6.35 mm) | 0.051 – 0.102 | 0.0020 – 0.0040 |
| 8 mm | 0.057 – 0.108 | 0.0023 – 0.0043 |
| 3/8" (9.53 mm) | 0.064 – 0.115 | 0.0025 – 0.0045 |
| 1/2" (12.70 mm) | 0.076 – 0.127 | 0.0030 – 0.0050 |
These are estimates derived from published tooling-manufacturer data, not test results. Always prove a new setting on scrap first. Sources are listed on the methodology page.
Other materials
Already running a feed rate and want to know what chip load it produces? Use the chip load calculator.