Chip Load, Chip Thinning and Engagement
Chip thickness, why a light radial cut needs a higher feed, arc of engagement, high-efficiency milling and how to choose a stepover.
What chip thickness does
The chip load fz is the feed per tooth, and it is the number you protect. The thickness of the chip each tooth actually removes depends on fz and on how deeply the tool is engaged. Too thin a chip and the edge rubs instead of cutting: it heats, work-hardens the surface (stainless, titanium) and wears fast. Too thick and the edge chips or the tool breaks, because the force rises in proportion.
| Chip too thin | Chip about right | Chip too thick |
|---|---|---|
| Rubbing, squealing, dust instead of chips | Crisp curled chips that leave the cut warm, not hot | Heavy chips, a loud thump, chipped edge or snapped tool |
| Heat goes into the tool and the work | Heat goes into the chip (see Coolant, Lubrication and Chip Evacuation) | Force and deflection rise; poor finish |
| Short tool life, work-hardened surface | Long tool life, good finish | Possible breakage and scrap |
Axial and radial engagement
- Axial depth ap is how much of the tool’s length is in the cut. It sets the force along the tool (deflection) and the removal rate, but does not change the chip thickness.
- Radial depth ae (stepover) is how much of the tool’s diameter is engaged. It sets the arc of engagement, the chip thickness and the heat.
- Full slotting (ae = D) has the tool buried on both sides, which packs chips and limits the speed. A light stepover leaves room for chips and cuts cooler.
Chip thinning
When the radial depth is less than half the diameter, each tooth enters and leaves the cut at a shallow angle, and the chip is a thin wedge whose thickest point is less than the nominal chip load fz. The average chip is thinner still. To get the chip you intended you must raise the feed per tooth by the chip thinning factor.
fz(adjusted) = fz × D ÷ (2 √(D ae − ae²)) for ae < D ÷ 2
Worked example: a 12 mm tool with a 1.2 mm stepover
- The radial depth is 1.2 mm, 10 % of the diameter. The tool is engaged over an angle of arccos(1 − 2 × 1.2 ÷ 12) = 36.9°.
- Thinning factor: 12 ÷ (2 √(12 × 1.2 − 1.2²)) = 1.67.
- A nominal chip load of 0.08 mm would really cut a chip with an average thickness of only 0.025 mm. Raising the feed per tooth to 0.08 × 1.67 = 0.133 mm gives an average of 0.041 mm.
- At 7 958 rpm with 3 teeth the feed rate rises from 1 910 to 3 183 mm/min.
- Without the correction the tool is working at only 60 % of its intended chip thickness and wears quickly while the cut looks slow.
Thinning multipliers
| Stepover | ae (12 mm tool) | Feed multiplier | Arc of engagement |
|---|---|---|---|
| 5 % of D | 0.6 mm | 2.29 | 25.8° |
| 10 % of D | 1.2 mm | 1.67 | 36.9° |
| 15 % of D | 1.8 mm | 1.40 | 45.6° |
| 20 % of D | 2.4 mm | 1.25 | 53.1° |
| 25 % of D | 3.0 mm | 1.15 | 60.0° |
| 30 % of D | 3.6 mm | 1.09 | 66.4° |
| 40 % of D | 4.8 mm | 1.02 | 78.5° |
| 50 % of D | 6.0 mm | 1.00 | 90.0° |
- Over half the diameter, no change. Under 10 % the multiplier is 1.67 or more.
- Cap the multiplier in practice (many programs cap it at 2 to 3) so that the feed rate stays within what the machine and the tool can take.
- Finishing passes with a 0.2 mm stepover would need a multiplier of 3.9, which is why a finishing pass uses a fixed small feed per tooth instead.
High-efficiency milling
High-efficiency (high-speed, dynamic or adaptive) milling uses a small radial depth with a large axial depth, so that the whole flute length cuts a little at a time. The chip is thin (corrected by the thinning factor), the arc of engagement is short so the heat is carried off by the chip, and wear is spread over the full length of the flute rather than the first millimeter.
Worked example: slotting against high-efficiency milling
| Slotting | High-efficiency | |
|---|---|---|
| Axial depth ap | 6 mm | 30 mm (2.5 × D) |
| Radial depth ae | 12 mm (full width) | 1.2 mm (10 %) |
| Chip load fz | 0.06 mm | 0.133 mm (corrected) |
| Feed rate | 1 432 mm/min | 3 183 mm/min |
| Removal rate Q | 103 cm³/min | 115 cm³/min |
| Arc of engagement | 180° | 37° |
- The high-efficiency cut removes 1.1 times as much metal per minute as the slot (115 against 103 cm³/min) with a far shorter arc of engagement and a much lighter, steadier load on the tool, because the tool is engaged for 20 % of each revolution and cools in between.
- The wear is spread over 5 times the flute length. The price: it needs a CAM system that keeps the engagement constant in corners (adaptive toolpaths), a machine with a fast controller and a rigid tool holder (see CAM and Toolpaths).
- It is also gentler on the machine in light hobby setups: the force is low and constant.
Climb and conventional milling
- Climb milling (the tooth moves with the feed) starts each chip thick and ends thin, so the tooth enters with a full bite and leaves the cut cleanly; it gives a better finish and longer tool life, and pulls the work toward the cutter, which needs a rigid machine with little backlash.
- Conventional milling (against the feed) starts each chip at zero thickness, so the edge rubs before it bites (work-hardening materials suffer), and it lifts the work. It is useful on machines with backlash and for scale or a hard skin.
- Use climb milling on CNC machines with ball screws; the first tooth of a climb cut meets a thickness of fz sin φ = 0.048 mm at the entry in the example above, and none at the exit.
Choosing a stepover and depth
| Cut | Radial depth ae | Axial depth ap | Why |
|---|---|---|---|
| Roughing, hobby machine | 25 to 50 % of D | 0.5 to 1 × D | Moderate force and good chip room; no thinning correction needed above 50 % |
| High-efficiency roughing | 5 to 15 % of D | 1.5 to 3 × D | Light, steady engagement; correct the feed for thinning |
| Slotting | 100 % of D | 0.25 to 0.5 × D | Chips pack on both sides; reduce depth and speed, use air or coolant |
| Finishing the wall | 0.1 to 0.3 mm | full wall height | Low deflection; fixed small chip load |
| Finishing a surface with a ball nose | set by the scallop height | small | See the scallop calculation in CNC Machining |
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| Tool is worn at the first millimeter only | Shallow axial depth uses only the tip | Use more of the flute length (higher ap, smaller ae) |
| Light cuts rub and squeal | Feed per tooth too low (no thinning correction) | Apply the thinning factor; raise the feed rate |
| Chips re-cut in a slot | Full-width cut, no chip room | Use a trochoidal or high-efficiency path; add air blast |
| Corners overload the tool | Engagement jumps in an inside corner | Slow the feed in corners or use an adaptive toolpath with constant engagement |
| Work lifts or pulls into the cutter | Climb milling with backlash or poor clamping | Clamp better, reduce the depth, or use conventional milling for the pass |
| Edge chips on entry | Tooth meets full thickness at once (climb entry into a wall) | Ramp or arc into the cut |
Related
- Speeds and Feeds: the overview of this guide.
- Cutting Speed, Spindle Speed and Feed Rate: the formulas used here.
- Cutting Force, Power and Torque: what the thinner chip does to the force.
- CAM and Toolpaths, Milling: Operations and Tooling and the CNC Feeds and Speeds Calculator.
FAQ
What is chip thinning?
When the radial depth is less than half the tool diameter, the chip each tooth cuts is thinner than the feed per tooth. Raise the feed per tooth by D ÷ (2 √(D a_e − a_e²)) to cut the chip you intended.
Do I need chip thinning on a hobby machine?
Yes, whenever the stepover is under half the diameter, but cap the multiplier (about 2) so that the feed rate stays within what a light machine can take.
Why is high-efficiency milling faster?
The small radial depth keeps the engagement short and the chips thin, so the tool runs cooler and wear is spread over the whole flute; this allows a much larger depth and feed, and a higher removal rate.
Climb or conventional?
Climb on rigid CNC machines with ball screws; conventional only on machines with backlash or for a scale or a hard skin.
References
- Sandvik Coromant, Metal Cutting Technology (chip thickness, entering angle and the effect of engagement).
- Oberg, E., et al., Machinery’s Handbook (milling cutters, feed per tooth).
- Altintas, Y., Manufacturing Automation, Cambridge University Press (average chip thickness and the angle of engagement).
Last reviewed: October 2026. Multipliers and chip loads are typical: use the tool maker’s data.