What Is Chip Load in CNC Machining? (Feed per Tooth Explained)

If you ask seasoned CNC machinists what single parameter most heavily influences tool life and surface finish, the vast majority won’t say spindle RPM or table feed—they will say chip load.

Also known as feed per tooth ($f_z$ or IPT/mm per tooth), chip load is the theoretical thickness of the chip that each cutting flute shears away from the workpiece during one revolution.

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The Physics of a Cutting Chip

When a cutting edge enters the material:

1. The tool’s rake face exerts immense compressive stress on the workpiece.
2. The material yields along the shear plane at temperatures exceeding $600^\circ\text{C}$ to $900^\circ\text{C}$.
3. In ideal shearing, approx. 70% to 80% of the generated cutting heat is carried away inside the curled chip itself.

If your chip load is too thin, there is insufficient thermal mass in the chip to carry away the heat. Instead, thermal energy dissipates directly into the cutting edge and the workpiece, leading to rapid crater wear and premature tool death.

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Typical Starting Chip Loads by Tool Diameter

As a rule of thumb, cutting tool manufacturers design flute geometry such that chip load scales proportionally with tool diameter ($D$):

Tool Diameter (Metric)	Tool Diameter (Imperial)	Aluminum Chip Load ($f_z$)	Steel / Alloy Chip Load ($f_z$)	Stainless / Ti Chip Load ($f_z$)
3.0 mm	$1/8\text{"}$ ($0.125\text{"}$)	$0.02 - 0.04\text{ mm}$ ($0.0008 - 0.0015\text{"}$)	$0.015 - 0.025\text{ mm}$ ($0.0006 - 0.0010\text{"}$)	$0.010 - 0.020\text{ mm}$ ($0.0004 - 0.0008\text{"}$)
6.0 mm	$1/4\text{"}$ ($0.250\text{"}$)	$0.04 - 0.08\text{ mm}$ ($0.0015 - 0.0030\text{"}$)	$0.025 - 0.050\text{ mm}$ ($0.0010 - 0.0020\text{"}$)	$0.020 - 0.035\text{ mm}$ ($0.0008 - 0.0014\text{"}$)
10.0 mm	$3/8\text{"}$ ($0.375\text{"}$)	$0.06 - 0.12\text{ mm}$ ($0.0025 - 0.0050\text{"}$)	$0.040 - 0.075\text{ mm}$ ($0.0015 - 0.0030\text{"}$)	$0.030 - 0.050\text{ mm}$ ($0.0012 - 0.0020\text{"}$)
12.0 mm	$1/2\text{"}$ ($0.500\text{"}$)	$0.08 - 0.15\text{ mm}$ ($0.0030 - 0.0060\text{"}$)	$0.050 - 0.090\text{ mm}$ ($0.0020 - 0.0035\text{"}$)	$0.035 - 0.065\text{ mm}$ ($0.0014 - 0.0025\text{"}$)
20.0 mm	$3/4\text{"}$ ($0.750\text{"}$)	$0.12 - 0.22\text{ mm}$ ($0.0050 - 0.0090\text{"}$)	$0.075 - 0.140\text{ mm}$ ($0.0030 - 0.0055\text{"}$)	$0.050 - 0.090\text{ mm}$ ($0.0020 - 0.0035\text{"}$)

Note: These ranges serve as general starting planning values. Always verify specific grade recommendations in your tooling supplier’s technical catalog.

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Chip Load vs. Machine Rigidity

While cutting tools can often handle heavy chip loads, your machine’s spindle motor and frame rigidity set the true ceiling. If your CNC router or light VMC experiences:

• High-pitched squeal or vibration chatter
• Steps or ridges on the finished wall
• Spindle load meter exceeding 100%

You must reduce the chip load or depth of cut to match your machine’s mechanical limits.

Calculate your exact tool advance with our free Feed Rate Calculator or Chip Load Calculator.