How to use this calculator
- Start from the rated chip load. The feed per tooth the tool maker publishes for your material — that is a chip thickness, not a feed.
- Enter diameter and stepover. Thinning is pure geometry of ae/D; it only exists below half diameter.
- Program the compensated values. f_z′ and the table feed from this screen restore the rated chip at the light stepover.
- Check runout at light chips. Keep tool runout under ~10% of the programmed feed per tooth or it dominates what each tooth actually cuts.
How it works
A tooth stepping over less than the cutter radius sweeps a shallow chord — the chip starts thin, peaks below the programmed feed and leaves thin. The peak is set by the engagement geometry alone:
h_max = f_z·√(1 − (1 − 2a_e/D)²) · f_z′ = h ÷ that factor
Tool makers rate a chip thickness; the program asks for a feed. Below half-diameter those are different numbers, and the gap is free feed rate. Spindle speed comes from the SFM to RPM converter, the baseline feed arithmetic lives in the chip load calculator, and the uncut-path version of these speeds sits in the speeds & feeds calculator.
Worked example
Verified against the live calculator
A 1/2 in 4-flute at 8,000 RPM running a
0.050 in HSM stepover (10% of D), tool rated at
0.002 in/tooth:
factor = 2√(0.1 × 0.9) = 0.60 · f_z′ = 0.002 ÷ 0.60 = 0.0033 in · V_f ≈ 107 ipm
Programmed at the naive 0.002 in the table would run 64 ipm and the chip would measure 0.0012 in — 40% under rating, rubbing and hardening the wall it is supposed to cut. The compensation buys 67% more feed at the exact chip the maker rated, with the engagement arc only 36.9°.
Frequently asked questions
What is radial chip thinning?
Below half-diameter stepover, each tooth sweeps a chord that never reaches full feed thickness: the maximum chip is f_z × √(1 − (1 − 2ae/D)²). At a 10% stepover the chip measures only 60% of the programmed feed per tooth — feed must rise to compensate or the tool rubs instead of cutting.
How do you calculate the chip thinning factor?
Factor = 2 × √((ae/D) × (1 − ae/D)) for stepovers below D/2, and exactly 1 at or above half diameter. Divide the rated chip load by the factor to get the feed per tooth to program: at ae/D = 0.10 the factor is 0.6, so a 0.002 in target programs as 0.0033 in.
Why does chip thinning matter in HSM and trochoidal milling?
Those paths live at 5–15% stepover. Uncompensated, the tool runs chips far below its rated load — rubbing, work-hardening and recutting at full RPM. Compensated, the same half-inch 4-flute at 8,000 RPM legitimately feeds 107 ipm instead of 64 — 67% more material at the same chip the maker rated.
Does chip thinning apply to slotting?
No. At half-diameter engagement and beyond — including full slots — some point of the arc reaches full feed thickness, so the factor is 1 and programmed feed per tooth equals the chip load. Ballnose work adds a separate axial thinning effect from shallow depth of cut.
Method & assumptions
- Chord-geometry relation for side milling (Machinery's Handbook milling section; standard HSM practice across tooling vendors) — straight-flute approximation, sharp tool.
- Applies radially; ballnose and shallow-DOC face work add a separate axial chip thinning factor not computed here.
- The verdict flags factors below 0.3 — at very light chips, runout, edge prep and material spring dominate and the geometric model flatters the cut.
- Machine, holder and workpiece rigidity still cap the usable feed; the compensated number is what the chip wants, not what every setup can take.