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Cast Iron Speeds and Feeds

Guide · Updated October 28, 2026

Gray cast iron is one of the friendliest metals to machine: the graphite in it breaks chips into small flakes and lubricates the cut. The challenges are the hard, sandy skin on castings, abrasive dust, and ductile iron, which machines more like a mild steel.

Check the finished size

Iron parts are often precision bores and flats. Verify your mic or bore gage against gage blocks before the finish pass. The free Gage Block Calculator builds the stack.

Step by step

Identify the iron: Gray iron (class 20–40) breaks into dusty chips; ductile (nodular) iron makes stringier chips and needs lower speeds.
Pick a starting SFM: Gray iron: about 60–100 SFM with HSS and 250–450 SFM with carbide. Ductile iron: about 20–30% lower.
Get under the skin: Take the first pass deep enough to cut below the cast skin, which holds sand and hard spots that dull edges fast.
Choose a chip load: Iron tolerates healthy feeds; for end mills start near the steel chip load for the diameter.
Cut dry or with air: Gray iron is usually cut dry with air to clear chips; coolant turns the dust into an abrasive paste.
Control the dust: Vacuum chips and cover the ways; iron dust is abrasive to slides and screws.
Watch the edges: Exit edges can break out; chamfer first or slow the feed as the tool leaves the part.

Worked example

Squaring a gray iron block with a 1/2 in 4-flute carbide end mill at 350 SFM: RPM = 3.82 × 350 ÷ 0.500 = 2,674 RPM. At 0.002 in per tooth, feed = 2,674 × 4 × 0.002 = 21.4 IPM. The first pass goes 0.060 in deep to get below the skin; the finish pass takes 0.010 in. With HSS at 80 SFM the same cutter runs about 611 RPM and 4.9 IPM.

Gray vs ductile iron

Gray iron is weak in tension and its graphite flakes make it cut freely. Ductile iron has round graphite nodules, is much tougher, and gives longer chips, more heat and more tool wear. Treat ductile like a 1045 steel and slow down.

Common problems

Rapid edge wear on the first pass means you are riding in the skin. Chipped edges at exit mean the edge broke out the part. A black, gritty paste on the machine means coolant is mixing with dust; switch to air.

Tips

Compare: aluminum and stainless speeds and feeds.
Basics: feeds and speeds basics.
Face milling: how to face mill.

FAQ

Do you use coolant on cast iron?

Usually not; gray iron is cut dry with air or a vacuum.

What SFM for cast iron with carbide?

About 250–450 SFM for gray iron as a starting point.

Why is the first pass so hard on tools?

The cast skin holds sand and hard spots; cut below it in one pass.

Is ductile iron harder to machine?

Yes; run it 20–30% slower and expect longer chips.

Is cast iron dust harmful to machines?

It is abrasive; keep it off ways and screws.

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