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Milling cast iron: end mill speeds and feeds

Published carbide end mill SFM and chip load for gray, ductile and malleable cast iron from Harvey Tool, Kyocera SGS, Fullerton and 6GTools, plus dry cutting, dust control and worked examples.

Hatched bars are published ranges for carbide end mills, orange ticks the catalog's nominal value, open dots 6GTools's single values. Fullerton's ranges are for uncoated tools. Sources [1]-[4].

Gray cast iron is one of the friendlier metals to mill: the graphite flakes break the chip into small pieces and the material doesn't gum up an edge. It is abrasive, though, and the published numbers split into two camps. Harvey Tool's general table gives carbide end mills 100-400 SFM in gray cast iron and only 80-140 SFM in soft ductile iron [1]. Kyocera SGS's general-purpose catalog runs cast irons up to 220 Bhn (gray, malleable and ductile) at 335 SFM (268-402) for profiling and 245 SFM (196-294) for slotting [2]. Fullerton Tool's chart, written for uncoated tools, is the most aggressive: 350-500 SFM for gray iron, 250-400 for ductile and 200-350 for malleable [3].

Coating moves the number a lot. 6GTools lists soft cast iron at 450 SFM uncoated, 540 with TiN and 600 with AlTiN, and medium cast iron at 300, 360 and 400 [4]. Fullerton says to add about 15% for TiN, 25% for TiCN and 40% for TiAlN to its uncoated figures [3]. Start at the catalog figure for the tool you actually have, not the top of somebody else's range.

Chip load for a 1/2" end mill

Published feed per tooth at 1/2" diameter. Grey bars are gray and malleable iron, gold bars ductile iron, and the SGS bar covers all cast irons up to 220 Bhn. Sources [1]-[4].

Harvey and Fullerton both give gray iron twice the chip load of ductile iron at 1/2": .004 vs. .002 in/tooth [1][3]. SGS uses .0020 for every cast iron in its group, with .0008 at 1/4" and .0015 at 3/8" [2]. 6GTools lists .0025 at 1/2", .0017 at 1/4" and .002 at 3/8" for all its cast iron rows [4]. For light radial cuts, add feed to make up for chip thinning, as in the chip thinning guide [8].

Which cast iron do you have?

Gray iron (Sandvik's group K2) wears tools by abrasive flank wear and thermal cracks, and its typical part problem is frittering, small chips breaking out of the edge where the cutter exits [5]. Sandvik's fix for frittering is to check flank wear, lower the feed per tooth, and use a more positive geometry [5].

Ductile, or nodular, iron (K3) is a different material at the cutter. Sandvik says ferritic and ferritic-pearlitic nodular iron machines much like low-alloy steel, so steel recommendations apply, while pearlitic nodular iron is more abrasive and needs cast iron grades [5]. Austempered ductile iron (ADI) is harder still: Sandvik puts its tool life at about 40% of regular nodular iron with about 40% higher cutting forces [5]. If you can't tell gray from ductile, run the ductile numbers until the chips tell you otherwise. Gray iron makes powder, ductile iron makes short curled chips.

Dry cutting, dust and the casting skin

For gray iron, Sandvik recommends running dry in roughing to keep thermal cracks down, and choosing a wet-milling grade only if fluid is needed to control dust. For nodular iron it recommends PVD-coated grades and wet machining [5]. Dry milling makes fine abrasive dust. SDF Tools recommends extraction when milling dry, keeping the holder, taper and fixture seats clean of dust, and not letting the cutter recut it [6]. See the runout guide for why that matters. MetalShopCalc adds that the outer skin of a casting is hard and sandy, so take the first cut under it, and wear a dust mask [7].

Worked example: 1/2" 4-flute profile in gray iron

SGS profile: 335 × 3.82 ÷ 0.5 = 2,559 RPM, and 2,559 × 4 × .0020 = 20.5 IPM, the same numbers printed in the catalog [2]. Radial width up to 0.125" (0.25 × D) with 3 or 4 flutes, 0.25" with 2 flutes, and axial depth up to 0.75" (1.5 × D) [2].

The same tool at 300 SFM with Harvey's gray iron chip load: 300 × 3.82 ÷ 0.5 = 2,292 RPM, and 2,292 × 4 × .004 = 36.7 IPM [1]. That's nearly twice the SGS feed, so prove the program at the lower figure first and step up while watching the exit edge for frittering.

Worked example: 1/2" 4-flute slot

SGS slot: 245 × 3.82 ÷ 0.5 = 1,872 RPM, 1,872 × 4 × .0020 = 15.0 IPM, no deeper than 0.4 × D = 0.2" per pass. With 2 flutes SGS allows 1 × D deep at 7.5 IPM, and with 3 flutes 0.5 × D at 11.2 IPM [2].

FAQ

What SFM should I run a carbide end mill at in cast iron?

Published starting points for solid carbide in gray cast iron run 100-400 SFM in Harvey Tool's general table, 335 SFM (268-402) for profiling and 245 SFM (196-294) for slotting cast irons up to 220 Bhn in Kyocera SGS's general-purpose catalog, 350-500 SFM for uncoated tools from Fullerton Tool, and 450 SFM uncoated or 600 SFM AlTiN-coated for soft cast iron from 6GTools. Ductile iron gets less: 80-140 SFM soft in Harvey's table and 250-400 SFM from Fullerton.

What chip load should I use for a 1/2" end mill in cast iron?

For a 1/2" carbide end mill the published chip loads are .004 in/tooth for gray iron and .002 for ductile iron (Harvey Tool and Fullerton), .0020 with SGS general-purpose end mills for all cast irons up to 220 Bhn, and .0025 from 6GTools.

Should I use coolant when milling cast iron?

Gray cast iron is usually milled dry. Sandvik Coromant recommends running dry in roughing to limit thermal cracks on the cutting edge and to use wet-milling grades only if fluid is needed to control dust. Ductile (nodular) iron behaves more like low-alloy steel, and Sandvik recommends PVD-coated grades and wet machining for it. Dry cutting makes fine abrasive dust, so use extraction and keep the holder and taper clean.

Steel guide Chip thinning math

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