Milling know-how
Feed rate and spindle speed without the math: the material presets explained
A wrong feed rate is the most common cause of poor surfaces, broken cutters and melted edges. The good news: for hobby machines a robust rule of thumb is enough, and exactly that rule powers the material presets in step2gcode.
The basis is the cutter diameter: as a starting point, step2gcode uses feed = diameter × 300 mm/min and a step-down of one diameter per pass (1×D), both for softwood. Each material scales this basis down with its own factors.
Use the presets directly: to the CNC milling tool →
How to do it in step2gcode
- Choose the material preset (e.g. wood, hardwood, acrylic, aluminum).
- Enter the tool diameter. Feed rate, plunge rate, spindle speed and step-down are calculated from it.
- Override individual values manually if needed, the fields stay editable.
- Run the first cut conservatively, listen to the sound and watch the chips, then increase if appropriate.
The presets at a glance (6 mm endmill as an example)
This is how the factors translate into concrete values for a 6 mm endmill:
| Material | Spindle speed | Feed rate | Step-down |
|---|---|---|---|
| Wood (softwood/MDF) | 18,000 rpm | 1800 mm/min | 6 mm (1×D) |
| Hardwood | 16,000 rpm | 1260 mm/min | 4.5 mm (0.75×D) |
| Acrylic/plastic | 12,000 rpm | 1080 mm/min | 3 mm (0.5×D) |
| Aluminum | 10,000 rpm | 630 mm/min | 2.4 mm (0.4×D) |
Why materials react so differently
Acrylic melts when the cutting edge rubs the same spot too often: hence lower spindle speed but still a brisk feed, so every tooth takes a real chip instead of rubbing. Aluminum is tough and conducts heat into the cutter: small step-down, moderate feed, and the cutter rewards you with tool life. Wood forgives almost everything; burn marks mean the feed is too low, not the spindle speed too high.
Flute count: why the single-flute cutter often wins in acrylic and aluminum
Besides feed and spindle speed, a third detail decides between cutting and rubbing: the number of flutes. How much each tooth removes per revolution is the feed divided by spindle speed times flute count. At the same feed, a two-flute cutter therefore takes chips only half as thick as a single-flute.
For acrylic and aluminum on hobby machines, the single-flute cutter is usually the best choice: it has a large chip gullet, ejects every chip cleanly and takes a proper chip per tooth before heat builds up. Exactly that prevents the two classic failure modes: melted, re-welded edges in acrylic and aluminum-clogged cutting edges (built-up edge) that ruin a cutter in seconds.
For wood and plywood, two-flute cutters are the all-rounder with a good balance of removal rate and surface finish. Three- and four-flute cutters come from metalworking on large, rigid machines: they need high feed or low spindle speed to keep the chip load right. Hobby spindles rarely run slower than 8,000 rpm, so four-flute cutters in aluminum often rub more than they cut there.
For the step2gcode presets this means: the values fit the common single- and two-flute cutters. If you use a cutter with more flutes, raise the feed accordingly or lower the spindle speed so the chip load per flute stays the same.
When to intervene manually
Full-width slot instead of a contour: when the cutter is buried across its whole width, halve the step-down. Flexible machine or long, thin cutters: reduce as well. And always: an even cutting sound and real chips (instead of dust or melt beads) are the best feedback your machine can give you.