CNC chip load calculator
Chip load = feed rate ÷ (RPM × flutes). This calculator runs that relationship in both directions. Use the first mode when you know the chip load you want and need the feed rate to get there. Use the second - the more useful one - when you are already running a feed rate and want to know what chip load it actually produces, and whether that is inside the published range for your material.
Reference range for hardwood at 6.35 mm: 0.229–0.330 mm
18,000 × 2 flutes × 0.1500 mm
0.1500 mm is below the recommended 0.229 mm minimum. Too small a chip means the edge rubs rather than cuts — that is what generates heat, burns the work and dulls the tool. Feed faster or reduce RPM.
Need depth-of-cut and bit-geometry adjustments too? Use the feed & speed calculator.
Why chip load is the number that matters
Feed rate on its own tells you almost nothing. A machine running 2,500 mm/min with a two-flute cutter at 18,000 RPM is taking a chip of 0.069 mm per tooth; the same feed rate with a four-flute cutter halves that to 0.035 mm. One of those may be a clean cut and the other a burnt one, at an identical number on the controller. That is why swapping a cutter for one with a different flute count, without changing the feed, so often ruins a job that was working perfectly.
Reading the in-range check
The band on the right compares your chip load against the published range for the material and cutter diameter you selected. Landing below the range is the more common error and the more damaging one: a chip too thin to shear means the edge rubs, and rubbing puts heat into the workpiece rather than into a chip that leaves the cut. Landing above the range loads the tool and the gantry, which on a desktop machine shows up as deflection and chatter well before it shows up as a broken cutter.
Diagnosing a cut that went wrong
Work backwards. Take the feed rate, RPM and flute count you actually ran, put them into the second mode, and compare the result against the range. A burnt edge with a chip load below the range confirms the diagnosis, and the fix is to feed faster or slow the spindle. Chatter or a snapped bit with a chip load above the range points the other way. If the chip load was in range and the cut still failed, the problem is elsewhere - work-holding, tool sharpness, machine rigidity or a spindle that is not reaching its commanded speed under load.
Depth of cut and bit geometry are not included here
This calculator deliberately works on the raw relationship, so the arithmetic is transparent. It compares your result against the unadjusted table range for the material. If you are taking a deep pass, or running a downcut, compression, V or ball-nose cutter, those call for adjustments to the target - the feed and speed calculator applies them and shows each factor separately.
Outputs are estimates derived from published tooling-manufacturer data and the stated formula. They are not test results. Prove any new setting on scrap.
A worked example: diagnosing a burnt edge
You ran 2,500 mm/min with a two-flute 1/4″ cutter at 18,000 RPM in hardwood, and the edge came out scorched. Work it backwards: 2,500 ÷ (18,000 × 2) gives a chip load of 0.069 mm per tooth. The published range for hardwood at that diameter is 0.229 – 0.330 mm, so you were running at roughly a quarter of the minimum.
That confirms the diagnosis rather than guessing at it. At 0.069 mm the edge is not shearing a chip at all - it is rubbing, and rubbing puts heat into the workpiece because there is no chip leaving the cut to carry it away. The fix is to feed faster or slow the spindle, and slowing the spindle is usually the practical one: at 6,000 RPM that same 2,500 mm/min feed produces 0.208 mm per tooth, which is nearly in range without asking the machine to move any quicker than it already was.
The same arithmetic explains why swapping a two-flute cutter for a four-flute one ruins a working setup: at an unchanged feed the chip load halves, and half of an in-range value usually lands below the minimum.