When machining hardened materials (HRC40–55) such as H13, 4140, Cr12, S136 and 718H, the 6-flute end mill is usually the better choice for finishing and semi-finishing.
In processing hardened materials (HRC40–55) such as H13, 4140, Cr12, S136 and 718H, the 6-flute end mill is usually the better choice for finishing and semi-finishing. It achieves higher overall feed rates at lower feed per tooth, delivers more stable surface roughness, and significantly reduces vibration and tool deflection. 4-flute end mills still have advantages in roughing or soft steel with high chip evacuation requirements, but on hardened materials they easily produce surface waviness, rapid wear, and breakage risk.
6 Flute (PM Series) gives longer life, smoother surface and higher feed rates on HRC40–55 hardened steel
4 Flute is still good for heavy roughing or soft materials, but leaves waviness and wears faster on hard steel
Shop test on H13 HRC50: 6-flute life jumped from 42 min to 67 min per edge, Ra dropped from 1.15μm to 0.68μm
8-question checklist tells you exactly which tool to pick
4 Flute vs 6 Flute Core Parameter Comparison Table
Why hardened materials favor 6-flute cutters
Real shop case: H13 mold steel switching from 4-flute to 6-flute
8-question selection checklist
Amony PM Series 6 Flute recommendations
Frequently Asked Questions
| Factor | 4 Flute End Mill | 6 Flute End Mill (PM Series) | Winner on HRC40–55 Hardened Materials |
|---|---|---|---|
| Feed per Tooth | Higher (0.05–0.12 mm/z) | Lower (0.03–0.08 mm/z) | 6 Flute (lower load, longer edge life) |
| Overall Feed Rate | Moderate | 20–35% higher | 6 Flute |
| Surface Roughness (Ra) | 1.0–1.8 μm (visible tool marks) | 0.6–0.9 μm | 6 Flute |
| Radial Cutting Force | Lower | Slightly higher but more balanced | Tie (depends on machine rigidity) |
| Chip Evacuation | Excellent (larger flute space) | Good with high-pressure coolant or MQL | 4 Flute (in roughing) |
| Tool Life (H13 HRC50) | Benchmark | +40–60% | 6 Flute |
| Typical Application | Roughing, soft steel, aluminum | Hardened steel finishing/semi-finishing, molds, high-speed machining | 6 Flute (this scenario) |
Data from BT40 machine dry cut + compressed air assist – real shop records, not catalog values.
Hardened steels have low thermal conductivity and strong work hardening. A 4-flute cutter takes a bigger bite per tooth, so edge temperature easily exceeds 800 °C, causing rapid diffusion wear. The 6-flute spreads the load across six edges, keeping each tooth cooler and sharper much longer.
Hard steel is less rigid. The wider flutes of a 4-flute cutter easily excite resonance and leave fish-scale marks. 6-flute cutters have tighter tooth spacing and, combined with the variable helix in our PM Series, keep cutting forces smoother and vibration noticeably lower.
Although feed per tooth is smaller, the overall feed rate is actually higher. In our tests we dropped fz from 0.08 mm/z to 0.045 mm/z yet increased table feed by 28 % while the surface came out far cleaner.
H13 mold (HRC48–51), 30 mm deep cavity sidewalls and floor. Original tool: imported 4-flute, Vc=110 m/min, fz=0.075 mm/z, ap=0.3 mm, ae=0.6×D. Edge life 42 minutes, Ra 1.15 μm, light waviness on walls, two tool changes per shift.
D10 × 26 mm flute length, AlCrN coating. Vc raised to 135 m/min, fz=0.042 mm/z, ap=0.25 mm, ae=0.65×D. Edge life 67 minutes (+59 %), Ra 0.68 μm, walls clean with no waviness, only one tool change per shift, cycle time cut by 22 %.
Shop foreman’s words: “We always thought 6-flute would clog chips. Once we used proper high-pressure coolant we realized hard steel doesn’t need big flutes – life and finish are what really matter.”
Answer these against your job and machine. Majority “yes” means go with 6-flute.
Is the main operation finishing or semi-finishing? → 6 Flute
Material hardness HRC ≥ 42? → 6 Flute priority
Spindle can run 8000 rpm or higher? → 6 Flute advantage bigger
Surface Ra required ≤ 0.8 μm? → Must use 6 Flute
Large roughing volume and deep cuts needed? → 4 Flute for rough, then 6 Flute finish
Coolant pressure below 15 bar or dry cutting only? → 4 Flute safer
Thin walls or parts prone to deflection? → 4 Flute (lower force)
Want to lower cost per part? → 6 Flute (higher life spreads cost faster)
We printed this checklist and stuck it next to every CNC. After three months our hardened-steel tool waste dropped 31 %.
PM Series Flat 6 Flute Carbide End Mills are built specifically for HRC ≤ 55 hardened steels. Variable helix + high core rigidity makes them ideal for mold steels, Cr12, S136, 718H and similar grades.
AlCrN coating, First choice for HRC ≤ 55 steel finishing – stable Ra 0.6–0.9 μm.
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Get Free Comparison ReportOn HRC40–55 hardened materials, a 6-flute end mill is not an option – it is the right tool. It simultaneously improves surface quality, machining efficiency and tool life. Stop letting 4-flute cutters hold you back. Put the PM Series 6 Flute in the spindle and you will find hard-steel machining is far easier than you thought.
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