Cutting Parameters for Mold Steel: A Practical, Shop-Floor Guide
Table of Contents
Know Your Material State First
Before touching parameters, confirm hardness and heat-treatment state.
Unhardened state example: Cr12MoV after normalizing is ~250 HB with tensile strength around 750 MPa.
Hardened state example: after quench & temper, hardness can reach ~60 HRC, tensile strength ~1,800 MPa—more than double the strength of the normalized condition.
As hardness and strength rise, contact stress and cutting force rise, friction increases, and thermal loads intensify. For milling hardened steels, cutting speed and chip load must be controlled more tightly than in pre-hardened or annealed stock.
Cutting Speed (vc): How Hardness & Tooling Shift the Window
What hardness does to speed
The rule of thumb is simple: as hardness goes up, the safe cutting speed goes down (for a given tool material and geometry). Finishing generally allows slightly higher cutting speeds than roughing due to smaller chip loads and improved stability.
Typical relationship: finishing vc ≈ roughing vc × (1.20–1.25).
When switching to higher-grade tool materials (coated carbide → ceramic → CBN), permissible vc can jump 2–5× compared with traditional YT-class carbides, provided the setup is stable and heat is managed.
Why higher speeds can sometimes help in hardened steel
At elevated cutting temperatures (approaching ~1,000 °C in the shear zone), the workpiece’s cutting hardness drops significantly. Tool materials may soften slightly, but their toughness increases, making the edge more resistant to chipping. With the right tool class (coated carbide/ceramic/CBN) and controlled engagement, a moderate increase in cutting speed can actually extend tool life when milling hardened steel.
Convert vc to rpm:
n [min−1]=1000⋅vc [m/min]π⋅D [mm]n\,[\mathrm{min^{-1}}] = \dfrac{1000 \cdot v_c\,[\mathrm{m/min}]}{\pi \cdot D\,[\mathrm{mm}]}n[min−1]=π⋅D[mm]1000⋅vc[m/min]
Feed Per Tooth (fz): Respect Minimum & Maximum Chip Thickness
Chip thickness has two hard constraints in hardened steel:
Minimum chip thickness: to avoid “rubbing,” a_min ≥ 0.03–0.035 mm. Below this, the edge skates on the work surface, friction spikes, and tool life collapses.
Maximum chip thickness: to avoid edge chipping in hard material, a_max ≤ 0.10 mm.
A practical, reliable band for hardened-steel milling is fz=0.05–0.10 mm/toothf_z = 0.05–0.10 \,\mathrm{mm/tooth}fz=0.05–0.10mm/tooth (adjust for tool diameter, edge prep, and engagement).
Convert fz to feed rate:
vf [mm/min]=fz [mm/z]×z×n [min−1]v_f\,[\mathrm{mm/min}] = f_z\,[\mathrm{mm/z}] \times z \times n\,[\mathrm{min^{-1}}]vf[mm/min]=fz[mm/z]×z×n[min−1]
Feed-per-Tooth Guardrails for Hardened Mold Steel
Parameter | Recommended Range / Limit | Rationale |
Minimum chip thickness amina_{min}amin | ≥ 0.03–0.035 mm | Prevents rubbing and thermal wear |
Practical fz band | 0.05–0.10 mm/tooth | Balances edge load and tool life |
Maximum chip thickness amaxa_{max}amax | ≤ 0.10 mm | Reduces risk of edge chipping in high-strength material |
Radial (ae) & Axial (ap) Engagement: Combine Small ae with Large ap
For hardened steels, keep radial engagement small to reduce average chip thickness and cutting force, and use a relatively large axial depth to distribute load along the edge. This combination improves surface finish, reduces vibration, and evens wear.
Face milling (indexable face mills)
Radial step (ae): about 0.6 × DC is a practical upper bound in many setups; in harder conditions, reduce ae to control force.
Axial depth (ap): 0.2–1.0 mm for hardened surfaces; use the lower end as hardness rises.
Tempered steels ≤ ~38 HRC: you can step up to ap = 1.5–2.5 mm if the setup is rigid.
End milling (solid carbide)
Radial step (ae): 0.1–0.3 × DC. Use the lower end when fz is high, tool overhang is long, or walls are thin.
Axial depth (ap): depends on diameter, stickout, and hardness.
Roughing, > 45 HRC: ap ≤ D is a safe ceiling to protect the edge.
Finishing with very small ae (≤ 0.03 mm): you can increase ap to 1.5–2.0 × DC to spread wear vertically.
Example: with a Ø6 mm end mill in finishing and ae ≤ 0.03 mm, ap up to ~12 mm is feasible in rigid setups.
Stickout (L0): keep as short as possible. Shorter L0 boosts tool stiffness, suppresses runout and chatter, and improves finish.
Engagement Recipes for Hardened & Tempered Mold Steels
Operation | Material Condition | Radial Step aeaeae | Axial Depth apapap | Notes |
Face milling | Hardened (>45 HRC) | up to ~0.6 DC (reduce if unstable) | 0.2–1.0 mm | Use smaller ap as hardness rises |
Face milling | Tempered (≤38 HRC) | up to ~0.6 DC | 1.5–2.5 mm | More permissive ap if rigid |
End milling (rough) | Hardened (>45 HRC) | 0.1–0.3 DC | ≤ D | Control force and deflection |
End milling (finish) | Hardened (>45 HRC) & ae ≤ 0.03 mm | Very small | 1.5–2.0 D | Spreads wear, improves finish |
Tool Material & Coatings: Picking the Right Edge for the Job
Coated carbide (TiAlN/AlTiN, etc.): baseline for most mold work. With proper edge prep and coolant strategy (often MQL or dry for hardened finishing), coated carbide can achieve stable finishing and moderate roughing.
Ceramics: excel at high-speed finishing on certain hardened steels; need stable, continuous cuts and light ae to avoid thermal shock.
CBN: the premium choice for >55–60 HRC finishing and light roughing. Delivers long life at elevated vc, with strict runout control and rigid fixturing.
Relative speed uplift vs. traditional YT-class carbide: 2–5× for ceramic/CBN under suitable conditions.
Worked Example: Setting Feeds & Speeds from First Principles
Task: Finish mill a 58–60 HRC tool-steel cavity wall with a Ø6 mm, 4-flute coated-carbide end mill. Setup is rigid; we’ll use the “small ae, large ap” strategy.
Choose engagement
Let ae = 0.02 mm (≤ 0.03 mm target for finishing)
Let ap = 10 mm (≈ 1.7 × D, within the 1.5–2.0 D guide)
Pick fz
Stay within the hardened-steel band: fz = 0.06 mm/tooth (between 0.05 and 0.10, above a_min)
Pick vc
For finishing in hardened steel, step up slightly from roughing (≈ +20–25%) within your toolmaker’s safe range. (If your internal shop range says, for example, 80–100 m/min for this tool and material, start near the lower end for the first pass and climb in small increments while monitoring load and finish.)
Compute n and vf (illustrative math using vc = 90 m/min):
n=1000⋅90π⋅6≈4,774 min−1n = \dfrac{1000 \cdot 90}{\pi \cdot 6} \approx 4{,}774 \,\mathrm{min^{-1}}n=π⋅61000⋅90≈4,774min−1
vf=0.06×4×4,774≈1,146 mm/minv_f = 0.06 \times 4 \times 4{,}774 \approx 1{,}146 \,\mathrm{mm/min}vf=0.06×4×4,774≈1,146mm/min
Validate against chip-thickness limits
fz = 0.06 mm/tooth ≥ 0.03–0.035 (min) and ≤ 0.10 (max) → OK
Cut, inspect, and iterate
If you see rubbing or heat tint, increase fz slightly (toward 0.07–0.08) or raise vc modestly to leverage thermal softening of the workpiece.
If you see micro-chipping on entry/exit, reduce ae or reduce fz in 0.005–0.01 mm/tooth steps.
Quick-Pick Setup Matrix
One-Page Starting Matrix for Mold Steel Milling
Scenario | Tooling | Material State | Strategy | vc (relative) | fz (mm/tooth) | ae | ap |
Roughing pockets | Coated carbide | 40–50 HRC | Moderate ae, ap ≤ D | Base | 0.05–0.08 | 0.2–0.3 D | ≤ D |
Semi-finish walls | Coated carbide | 50–58 HRC | Small ae, larger ap | Base to +20% | 0.05–0.08 | ≤ 0.1 D | 1.0–1.5 D |
Finish walls | Coated carbide | 55–60 HRC | ae ≤ 0.03 mm, ap ↑ | +20–25% vs rough | 0.05–0.08 | ≤ 0.03 mm | 1.5–2.0 D |
Finish planar | Ceramic/CBN | 58–62 HRC | Small ae, light chip | 2–5× vs YT carbide | 0.04–0.07 | small | 0.5–1.0 D |
Tuning Tips That Protect Tool Life
Edge prep matters: A slight hone on the cutting edge stabilizes the micro-geometry and resists chipping in interrupted cuts.
Runout control: Keep ≤ 5–10 µm TIR for solid end mills in hardened finishing; more than that concentrates load on one tooth.
Coolant strategy: For hardened finishing, many shops prefer dry or MQL to avoid thermal shock; steady heat can be an ally. If flood is required (chip evacuation/deburring), ensure it’s consistent and not intermittent.
Tool stickout: Shorten L0—every extra millimeter costs stiffness.
Entry/exit paths: Use ramped entries and arc exits when possible to avoid edge shock.
Toolpath style: Prefer constant-engagement toolpaths (e.g., adaptive) in roughing to cap peak forces. For finishing, long uninterrupted passes with minimal ae yield the best surfaces.
Surface integrity: If burnishing or heat tint appears, increase fz slightly or reduce ae; verify that a_min is satisfied.
Fixture rigidity: Lock down overhangs and thin webs; even the best parameters fail on springy setups.
Troubleshooting Cheat-Sheet
Edge chipping at corners or on exit
→ Reduce ae; ensure fz ≤ 0.10 mm/tooth; consider a slightly higher vc to soften the cut; improve entry/exit moves.
Rubbing / glazed surface / blueing
→ Increase fz above 0.03–0.035 mm; verify ae is not too tiny without sufficient fz; consider a modest vc increase.
Chatter / poor finish
→ Shorten stickout, reduce ae, raise ap (within limits) to distribute load; slow vc slightly and match spindle to a stable “sweet spot.”
Rapid flank wear
→ Check coolant strategy; confirm hardness (is it higher than expected?); reduce vc a notch; verify chip evacuation.
Safety & Quality Reminders
Verify hardness with a reliable method (portable tester or coupon) after heat treatment.
Re-touch zero and probe walls after semi-finishing to remove stock uniformly before finishing.
Record actual spindle load, surface finish, and tool wear to close the loop on parameter optimization. Small, logged adjustments beat big guesses.
Quick Reference
Hardness & strength effect: Cr12MoV ~250 HB / 750 MPa (normalized) → up to ~60 HRC / 1,800 MPa after Q&T; higher hardness ⇒ lower safe vc for the same tool class.
Finishing vs roughing: finishing vc ~20–25% higher than roughing.
Advanced tools: coated/ceramic/CBN can run 2–5× vc vs traditional YT-class carbides.
Thermal effect near ~1,000 °C: workpiece cutting hardness falls; tool toughness rises slightly; slightly higher vc can extend tool life in hardened milling (with controlled engagement).
Chip-thickness limits: a_min ≥ 0.03–0.035 mm, a_max ≤ 0.10 mm; practical fz = 0.05–0.10 mm/tooth.
Engagement strategy: small ae + large ap improves precision and tool life; e.g., ae ≤ 0.03 mm with ap = 1.5–2.0 D in finishing.
Examples: Face milling hardened steel ap = 0.2–1.0 mm; tempered ≤38 HRC ap = 1.5–2.5 mm. End milling roughing >45 HRC: ap ≤ D; finishing with Ø6 mm, ap ≈ 12 mm feasible at very small ae.
Final Takeaways
Confirm hardness and choose the tool class accordingly.
Set fz first to exceed the minimum chip thickness without breaching the chipping limit.
Favor small ae and larger ap in hardened steel to flatten force spikes and even wear.
Use moderate vc increases in finishing (and with advanced tool materials) to leverage thermal softening of the cut.
Iterate with data—log load, finish, and wear to converge on a stable “recipe” for each mold steel family.
