Kerf and Tolerances

Overview#

Kerf is the width of material removed by the cutting process. A laser beam, waterjet stream, plasma arc or router bit does not cut along an infinitely thin line; it consumes a strip. If you cut a 50 mm square along the exact outline with a 0.2 mm kerf, the square comes out about 49.8 mm and the hole you cut with the same logic comes out 0.2 mm larger than designed. For a single part nobody notices. For parts that must fit together (finger joints, tab-and-slot, press-fit bearings, nested assemblies) it decides whether the assembly snaps together, slides loosely or refuses to go together.

There are two common ways to handle kerf: compensate in the CAM/toolpath so the cutter path is offset from the geometry, or compensate in the design by changing the geometry. Which one you use depends on where the cut is generated. If a laser's own software cuts directly along your DXF line, the cutter path equals your geometry, so you must adjust the design (or use an offset function in the laser software). If you create toolpaths in Fusion's Manufacture workspace, Fusion can offset the path for you.

Also note that kerf is only one of several error sources: sheet thickness variation, machine calibration, material springback and swelling, focus errors, and the taper of the cut wall. Thinking in terms of fit classes rather than exact numbers keeps you out of trouble.

Where to find it#

• Design-side kerf: User parameters (Modify, Change Parameters) holding a kerf value; sketch Offset (O) and dimensions referencing the parameter.
• Toolpath-side kerf: In the 2D Profile operation on the Passes tab (verify tab name), the Compensation type (for example in computer, in control, wear, off) and the tool's kerf or cutting width. In the tool library, laser, waterjet and plasma tools have a cutting-width style parameter instead of a physical diameter. Option names differ by release; verify in your version.
• Laser software offset: Most hobby laser programs have an Offset Shape or Kerf Offset function.

Step by step#

Measure your kerf#

1. Create a sketch with a row of ten 10 mm wide slots spaced 20 mm apart, or simpler, a single 20 mm square. Name the file "kerf test".
2. Cut it in the actual material at the actual settings you will use for production. Do not use a scrap with different thickness.
3. Measure the resulting square with calipers on at least two sides. If the square is 19.82 mm, then 20 - 19.82 = 0.18 mm kerf (the cutter removes half the kerf on each side, so each edge moved 0.09 mm).
4. For better accuracy cut a comb: ten 10 mm strips cut side by side as one cutout and measure the total width. Divide the deviation by the number of cuts.
5. Store kerf = 0.18 mm as a user parameter.

Compensate in the design for a press-fit joint#

1. Decide the fit. For a tab that should push into a slot in 3 mm plywood measured at 2.95 mm:
   • Tab width = material measurement.
   • Slot width = tab width minus the kerf for a tight fit, because the slot cut leaves the slot wider by one kerf and the tab narrower by one kerf.
2. In practice: parts get smaller by one kerf for outer edges, and holes and slots get larger by one kerf. To make a slot end up at exactly 2.95 mm, draw it at 2.95 - kerf. To make a tab end up exactly 2.95 mm, draw it at 2.95 + kerf.
3. Add a fit allowance on top of that: 0.00 for tight press fit, +0.05 to +0.10 mm for a snug hand-pressed fit, +0.15 or more for a sliding or glued fit.
4. Write these as equations in the Parameters dialog: slot_w = thickness - kerf + fit and tab_w = thickness + kerf. Use the variables when dimensioning sketches.

Compensate in the toolpath#

1. In Manufacture, create or select a laser, waterjet or plasma tool. Autodesk's tool reference lists feedrates and quality control but no kerf-width field, and the 2D Profile reference mentions a "Kerf diameter" without detail, so check where your tool dialog takes the measured kerf (router: tool diameter).
2. In the 2D Profile operation, choose a compensation type. Autodesk lists two Compensation Types: In computer (Fusion outputs the compensated head position) and In control (the chain coordinates are output with compensation codes so the operator sets the offset at the control; done on finishing passes only). Sideways Compensation sets Left, Right or Center. G41/G42 are the usual codes for control compensation (general knowledge).
3. Confirm offset direction visually: outside contours offset away from the part, inside contours (holes) offset into the waste, so the finished part lands on the nominal dimension. The simulation shows the path on the correct side.
4. Cut the test coupon again. If the result is off, change the kerf value in the tool by half the error and repeat.

Options reference#

Typical kerfs for orientation (measure your own): CO2 laser in 3 mm plywood 0.15 to 0.25 mm; CO2 laser in 3 mm acrylic 0.1 to 0.2 mm; fiber laser in thin steel about 0.1 to 0.2 mm; waterjet 0.7 to 1.2 mm; plasma 1.5 to 3 mm; router bit diameter (3.175 mm = 1/8 in is common).

Tolerance and fit classes#

Fit is really a stack of tolerances:

• Material thickness tolerance. Plywood +/- 0.2 mm, acrylic cast +/- 0.2 to 0.4 mm, extruded +/- 0.1, sheet steel per gauge tables. Always measure.
• Machine accuracy. Hobby laser: +/- 0.1 mm over small distances, worse over the bed. Industrial lasers: +/- 0.05 mm. Waterjet: +/- 0.1 to 0.25 mm. Plasma: +/- 0.25 to 0.5 mm. Router: +/- 0.1 mm plus bit deflection.
• Burn and taper. Wood laser cuts have a slight V shape: the top is wider than the bottom, so the joint feels tight at the bottom of the finger. Plywood glue lines vary.
• Environmental change. Wood swells and shrinks 0.5 percent or more across the grain with humidity.

Practical fit classes:

Tips and pitfalls#

Tip: Kerf compensation is a single place to be wrong. Keep one kerf parameter and reference it in every joint dimension. When you change material, update only that parameter.

Tip: Avoid double compensation. If your laser software offsets paths and your design is already compensated, you get a doubled correction. Decide on one place.

Tip: Cut test joints with tabs on every face orientation. A machine bed may produce slightly different kerf in X and Y due to belt tension or head speed.

Warning: Compensation does not fix inside corners with a router. A square pocket will have corners with the bit radius unless you add relief geometry.

Warning: Control compensation (G41/G42) can fault the machine if the lead-in is too short or if a feature is smaller than the tool/kerf. Run Fusion's simulation and check your machine's manual.

Warning: Small features: a slot narrower than the kerf cannot exist, and a bridge thinner than a few kerfs may burn away. Keep the smallest feature at least two to three times the kerf.

Practice exercise#

Build a parametric test coupon. Parameters: thickness = 3, kerf = 0.18, fit = 0.05. Draw a 30 x 30 mm square tab block with a 3 x 10 mm tab on one side and a 3 x 10 mm slot (width thickness - kerf + fit) in a second 30 x 30 mm block. Export both as DXF in a single sketch, cut them, and press the tab into the slot. Expected result: the tab enters the slot with hand pressure and does not fall out when the block is lifted. If it is loose, reduce fit by 0.05; if it will not enter, increase it by 0.05. Record the final values.

Related#

• Joints: finger joints, tabs and slots
• Cutting toolpaths with 2D Profile
• Materials and cutting settings
• User parameters and equations (section 04)