Reference
Machinability guide
How fast each grade cuts, relative to AISI 1212 at 100. The number is useful — but only if you know what it does and does not measure.
Ranked
All ten grades, fastest first
- EN1A PB
Free Cutting
145ExcellentThe highest machinability index in our range — roughly 45–60% above EN1A.
- SAE 1018
Low Carbon
70GoodBetter finish than commercial MS thanks to controlled chemistry, but still a ductile steel that produces a long chip.
- MS
Low Carbon
65GoodMachines cleanly but tends to produce a continuous, stringy chip because there is no free-machining sulphur.
- EN8D
Medium Carbon
60ModerateMachines like EN8 — slightly lower sulphur means marginally longer chips.
- 16MnCr5H
Case Hardening
58ModerateIdentical to 16MnCr5 — machined soft at ≤ 207 HB before carburising.
- 20MnCr5
Case Hardening
52FairSlightly harder to machine than 16MnCr5 because of higher carbon and chromium.
Index relative to AISI 1212 free cutting steel = 100. Higher is faster to machine.
What drives the number
Three things, and only one of them is hardness
The first and largest factor is chip control. A clean, ductile steel produces a continuous ribbon that wraps around the tool and the workpiece. Sulphide inclusions — deliberately added in EN1A, EN1A PB and EN8M — fracture the chip as it forms, so it breaks into short curls that fall clear. On an unattended automatic machine that is the whole difference between a completed shift and a jam.
The second is friction at the tool face. Lead, in EN1A PB, is insoluble in steel and sits as fine particles through the bar. At the cutting zone the heat softens and smears them into a microscopic lubricating film exactly where the chip slides across the tool. That is why EN1A PB sits at 145 while plain EN1A sits at 95 — the mechanisms are different and they stack.
The third is hardness and carbon content, which is what most people assume is the whole story. Higher carbon means more tool wear, which is why EN8 sits at 60 and the case hardening alloys sit lower still. But note that EN8M has more carbon than EN8 and machines a third faster — because chip control outweighs hardness.
What the index is good for
What it is not
Do not buy machinability you do not need
EN1A PB costs more than EN1A. On a low-volume job with plenty of machine capacity, that premium buys nothing. On a lights-out automatic running to schedule, it pays several times over.
Do not lose properties chasing it
The sulphur that lifts the index reduces transverse ductility and impact toughness, and makes sound welding unreliable. A part that fails in service costs more than every second you saved making it.
Chip control matters more than raw speed
A grade that cuts 10% slower but never jams the machine is faster in practice than one that runs quickly and stops twice a shift.
Chemical composition and mechanical properties shown are as per the applicable standard and are indicative. Actual values for your consignment are certified in the Mill Test Certificate supplied with each dispatch. Available sizes and tolerances are confirmed at the time of enquiry.
Tell us what you need
Tell us the part and the volume and we will tell you honestly whether the faster grade is worth it.