What Is Kerf in a CNC Machine?

• 1
  How kerf in cnc machine work is measured
• 2
  Why kerf in a CNC machine is built into the tool path
• 3
  Where kerf in a CNC machine decides the part
• 4
  What pushes the real kerf off the nominal value
• 5
  How kerf differs across cutting processes
• 6
  Kerf by cutting process

Kerf is the width of the cut. When a Ø10 mm end mill travels through aluminium, it leaves a slot about 10 mm wide. That slot is the kerf. It is not a defect and not a setting you can switch off. Every cutting process removes material, and the removed band has a width.

On a milling machine the kerf in a CNC machine is close to the cutter diameter plus a small amount of tool runout. A 12 mm three-flute cutter with 0.02 mm runout cuts about 12.04 mm. On a laser or waterjet the kerf is much narrower, often 0.1–0.5 mm, but it still changes the final outline.

The workpiece does not push back. The cutter removes whatever sits inside its swept circle. That is why the nominal diameter on the tool holder label is the first number a programmer needs, and why a used cutter measures smaller than its marked size.

Measure kerf the same way you measure any feature: cut a test slot in the same material, then check it with a caliper or a pin gauge. Do that once per tool and per material, and you stop guessing.

• 1
  New carbide cutterKerf matches the marked diameter within a few micrometres.
• 2
  Worn cutterDiameter drops, so the slot narrows and the wall grows.
• 3
  Laser or waterjetKerf is 0.1–0.5 mm and varies with power, speed, and focus.

A CAM tool path is written for the centre of the cutter, not for the edge. If the control drove the centre along the part outline, every wall would sit one tool radius away from where the drawing says. The part would come out undersized by the full cutter diameter on both sides.

Cutter compensation fixes that. The programmer enters the kerf value, and the control shifts the path sideways so the cutting edge, not the centre, touches the finished surface. On a Fanuc-style control this is G41 for a left offset and G42 for a right offset, cancelled with G40.

The offset direction depends on travel direction and on whether you are cutting a pocket or a boss. Get it backwards and the cutter eats into the part instead of leaving stock. That mistake shows up on the first article, not on the drawing.

Modern CAM software applies the offset automatically from the tool library. The library entry is only as good as the diameter you typed in. If the library says 6.00 mm and the actual cutter measures 5.94 mm, the finished wall drifts by 0.03 mm.

• 1
  G41Cutter offset to the left of the travel direction.
• 2
  G42Cutter offset to the right of the travel direction.
• 3
  G40Cancel compensation before the next move.

Internal corners are the clearest case. If a pocket has a 5 mm corner radius and you rough it with a Ø12 mm cutter, the tool cannot reach the corner. You need a smaller cutter, or you accept a larger radius. The kerf and the corner radius are tied together.

Thin walls are the second case. A 0.8 mm aluminium wall cut with a Ø6 mm cutter carries a lot of radial force. The wall can deflect, and the finished thickness varies along the length. Rough with a larger cutter, then finish with a light radial pass.

Slot width is the third. A slot cut with a single pass is exactly one kerf wide. If the drawing calls for a 6.00 mm slot and your cutter cuts 6.04 mm, the slot is out of tolerance before you start. Two passes with a smaller cutter give you control.

For parts with a tolerance of ±0.005 mm, we measure the cutter, record the actual diameter, and let the CAM offset carry that number. The kerf value in the program is a measured value, not a catalogue value.

On sheet processes the same logic applies. Laser and waterjet kerf is narrow, but it still has to be added to the nest. If you ignore it, the cut part is smaller than the drawing and the holes sit in the wrong place.

• 1
  Internal cornerSmallest reachable radius equals the cutter radius.
• 2
  Thin wallUse light radial passes to limit deflection.
• 3
  Tight slotCut in two passes to control the final width.

Tool runout is the biggest single factor. If the cutter sits 0.02 mm off the spindle axis, one flute does most of the cutting and the effective diameter grows. On a finishing pass that runout goes straight into the wall position.

Deflection is next. A long, slender cutter bends under cutting force, so the bottom of the slot is narrower than the top. Depth of cut and feed rate decide how much. Keep the radial engagement low when the wall matters.

Thermal growth matters on long cuts. A spindle and a workpiece both expand as they warm up. A machine that cuts a good first article at 08:00 can drift by a few micrometres by 14:00 on a long run. Warm up the spindle before the first cut.

Tool wear closes the diameter over time. A cutter that measures 6.00 mm at the start of a batch may measure 5.97 mm at the end. For long runs, schedule a mid-batch check of the finished wall, not just the first part.

All of these factors are small on their own. Together they explain why the kerf value in the program should be a measured number from the actual setup.

• 1
  RunoutRaises the effective diameter and shifts the wall.
• 2
  DeflectionMakes the slot narrower at the bottom.
• 3
  WearShrinks the diameter across a long batch.

Milling kerf tracks the cutter diameter. A Ø8 mm cutter gives roughly an 8 mm kerf, and you can change it by changing the tool. That flexibility is why milling suits pockets, slots, and contoured walls.

Laser kerf is narrow and fixed by the beam, not by a tool you can swap. On thin sheet the kerf may be 0.15 mm; on 6 mm steel it can reach 0.4 mm. The kerf also changes with assist gas and focus position, so the nest offset must match the cutting recipe.

Waterjet kerf is wider than laser and tapers through the thickness. A typical value is 0.6–1.2 mm, wider at the bottom of the cut. For a part with a tight tolerance on the outline, that taper has to be planned into the offset.

Plasma kerf is the widest of the sheet processes and leaves a rougher edge. It suits structural plate where a 1–2 mm edge variation is acceptable, not a mating face.

The engineering point is simple. Kerf is not one number you learn once. It belongs to the process, the tool, and the material in front of you.

• 1
  MillingKerf equals cutter diameter; you choose it.
• 2
  LaserNarrow kerf, fixed by beam and recipe.
• 3
  WaterjetWider kerf with a taper through the thickness.