Mill Gray cast iron (class 30) with a HSS end mill at about 70 SFM: 535 RPM for a 1/2 inch cutter, 4 flutes, about 0.0025 inch per tooth.
What RPM and feed for Gray cast iron (class 30)?
| Cutter | Ideal RPM | Chip load (in/tooth) | Flutes | Feed at ideal RPM (IPM) | Feed if capped at 2500 RPM |
|---|---|---|---|---|---|
| 0.125" | 2,139 | 0.0006 | 4 | 5.3 | 5.3 |
| 0.1875" | 1,426 | 0.0009 | 4 | 5.3 | 5.3 |
| 0.25" | 1,070 | 0.0013 | 4 | 5.3 | 5.3 |
| 0.375" | 713 | 0.0019 | 4 | 5.3 | 5.3 |
| 0.5" | 535 | 0.0025 | 4 | 5.3 | 5.3 |
| 0.75" | 357 | 0.0037 | 4 | 5.3 | 5.3 |
Tools for cutting Gray cast iron (class 30)
How does Gray cast iron (class 30) machine?
Gray cast iron is machined dry. It produces fine abrasive dust rather than chips, so keep it off the lathe ways and wear a dust mask. The outer skin of a casting is hard and sandy: take the first cut under it.
Why a HSS end mill?
HSS end mills are the forgiving choice on a light hobby mill: they tolerate chatter and interrupted cuts that chip carbide, they are cheap to replace, and they cut well at the modest spindle speeds benchtop mills offer.
Adjust speeds with the RPM calculator and milling feeds with the feed rate calculator. The cutting speed chart compares every material.
Frequently asked questions
What SFM for Gray cast iron (class 30)?
A starting surface speed of 70 SFM with HSS and 250 SFM with carbide, and 60 SFM for HSS drills. These are conservative for hobby machines; raise them in steps if the chips come off clean and the tool stays cool. Gray cast iron (class 30) rates good for machinability.
What happens if my machine cannot reach the RPM?
Run at your top speed. Running below the ideal surface speed only makes the job slower; it does not damage the tool. For milling and drilling, keep the chip load per tooth or feed per revolution the same so the edge keeps cutting instead of rubbing, which lowers the feed in inches per minute in proportion.
Other tools in Gray cast iron (class 30)
Other materials with a HSS end mill
- 6061-T6 aluminum
- 7075-T6 aluminum
- 360 free-machining brass
- 932 bearing bronze
- 110 copper
- 12L14 leaded steel
- 1018 cold rolled steel
- 1045 medium carbon steel
- All