Kerf, Accuracy and Designing Parts
Kerf and its compensation, taper, press fits and finger joints, living hinges, minimum features and heat effects.
What the kerf is
A laser (or a saw or a router bit) removes a strip of material, the kerf, centered on the toolpath. A CO₂ laser typically makes a kerf of 0.1 to 0.3 mm, a fiber laser on thin metal 0.1 to 0.2 mm; a router bit removes its full diameter. If you cut exactly on the drawn line, the outside of a part comes out smaller than drawn and a hole comes out bigger, each by half the kerf on every side.
Worked example: a tab and a slot
- A 3 mm tab and a 3 mm slot are drawn to fit. With a kerf of 0.15 mm and the beam on the drawn lines, the tab is 2.85 mm (smaller by the kerf, half on each side) and the slot 3.15 mm: a gap of 0.30 mm where the drawing showed a perfect fit.
- Offsetting each path by half the kerf (0.075 mm) outwards for the tab and inwards for the slot restores the 3.00 mm sizes, and then a clearance you choose is added on top.
- The error accumulates: a row of 8 parts cut edge to edge loses 8 × 0.15 = 1.20 mm from the sum of the kerfs.
Measuring your kerf
Kerf changes with the material, thickness, focus, power and speed, so measure it for each job type. The simplest test is a comb of slots:
Worked example: a ten-slot comb
- Draw ten slots of 3.00 mm side by side as lines (a comb in a bar), cut it on the line, and measure the width across all ten slots with a calliper: 31.5 mm against the drawn 30.0 mm.
- The difference is 1.5 mm over 10 slots: kerf = 0.15 mm per cut. Averaging over ten reduces the measurement error ten times.
- Alternatively, cut a 20 mm square on the line: each side loses half a kerf, so it measures 20 mm minus one kerf, and the kerf is 20 minus the measured size.
- Measure at several places on the bed if the machine is large: the beam, the focus and the speed can differ in the corners (see Machine Hardware: Optics, Motion and Cooling).
Compensating for it
- In the drawing or the nesting software: offset the outline outwards by half the kerf for the outside of a part and inwards for holes and slots (a “kerf offset” or “beam compensation” setting does this); do it once, for every part, and never by hand.
- Offset by the measured kerf, not a guess, and note it with the material and settings (see Power, Speed, Focus and Assist Gas).
- Cut inside features first and outside last so that the sheet stays held and the part does not shift.
- For a one-off, cut a test joint first: it is faster than calculating.
- Do not compensate the same dimension twice (the software and your drawing both): check the result against a test square.
Taper
A focused beam is narrowest at its focus, and a thick sheet is cut by a beam that is slightly wider at the top. The kerf is therefore tapered: wider at the top than at the bottom, and the cut face is not square. For a thin sheet it does not matter; for thick sheet and for tight joints it does.
Worked example: the angle of the cut face
- In 6 mm sheet the kerf is 0.30 mm at the top and 0.14 mm at the bottom: each face moves by (0.30 − 0.14) ÷ 2 = 0.08 mm over the thickness, an angle of arctan(0.08 ÷ 6) = 0.76°.
- A part cut from this sheet is larger at the bottom than at the top by 0.16 mm in each dimension (the top is smaller). A tab that is 3.00 mm at the top face is 3.16 mm at the bottom, so a press fit that is tight at the bottom may be loose at the top.
- Reduce taper by focusing at the middle or a third into the sheet, a longer lens (see How Laser Cutting Works and Laser Types), a slower and lower-power cut, and cutting thinner sheet. Where a part must be square, choose the face (top or bottom) to be the reference when you measure and when you design joints.
Press fits, tabs and slots
Fit is a matter of clearance: the slot minus the tab. Wood and plywood compress and swell with humidity; acrylic does not compress and cracks if forced. Use the clearance that fits the material and the use, and test a joint on scrap (see Fits and Tolerances for the general theory).
Worked example: clearances for a 3 mm tab (after kerf compensation)
| Fit | Clearance (slot − tab) | Slot width to draw | Use |
|---|---|---|---|
| Slip (loose) fit | +0.10 mm | 3.10 mm | Easy assembly, moving parts, acrylic |
| Snug sliding fit | +0.03 mm | 3.03 mm | Snug, glued joints |
| Press (friction) fit | −0.05 mm | 2.95 mm | Plywood and softwood, no glue |
| Hard press, glue-free | −0.10 mm | 2.90 mm | Dense wood only: test first |
A press fit in plywood of −0.05 mm: the slot is drawn 2.95 mm for a 3.00 mm tab. Always cut a test strip first with several clearances on the same scrap: the best value depends on the sheet, the kerf and the humidity.
- Add a small relief or round the inside corners of slots: a laser cuts a sharp corner with a radius of half the spot, but a square tab will not fit into a square slot without clearance at the corner.
- Use T-slots and captive nuts for strong joints in acrylic and wood.
- Wood changes with humidity: a 100 mm wide plywood sheet may change by 0.1 to 0.3 mm with the season.
- Acrylic joints: avoid sharp internal corners that start cracks; use a small radius and solvent cement.
Finger joints and boxes
Worked example: a finger-jointed box edge on 6 mm sheet
- Choose fingers about twice the sheet thickness: 12 mm. An edge of 100 mm is divided into an odd number so that the ends match: 7 fingers and gaps × 12 mm = 84 mm, with the remaining 16 mm split across the two corners (a margin of 8 mm each).
- Each finger (after compensation) is 12 mm − the clearance; each gap 12 mm + the clearance, with the clearance from the table above (for a snug fit in plywood, -0.03 to 0.05 mm).
- Without compensation the 7 kerfs of 0.15 mm along an edge give a cumulative error of 1.05 mm: the box would be that much too small or too large, and the joints would be loose.
- Use a box generator (for example the free “MakerCase” or “BoxGenerator” tools) that takes the sheet thickness and the kerf and produces compensated files, then check one box on scrap.
- Corners: allow a small corner relief and round inner corners to the beam; chamfer or round the outer corners to avoid chipping.
Living hinges
A living hinge is a pattern of parallel cuts that lets a rigid sheet bend like a hinge, so a part folds without a separate hinge. It works in thin wood (about 3 mm), acrylic in a limited way, leather and felt. The bend radius is limited by the thickness and the material: too small and the web cracks.
Worked example: a hinge in 3 mm plywood
- As a rule of thumb a plywood living hinge tolerates a bend radius of about 12 to 25 times the thickness: 36 to 75 mm for 3 mm sheet. A 90° bend of radius 30 mm needs an arc length of π ÷ 2 × 30 = 47 mm of hinge pattern.
- Parallel cuts spaced 1.2 mm apart, 0.25 mm wide, leave bridges of 1.2 − 0.25 = 0.95 mm: over a 30 mm hinge there are about 25 cuts. Narrower bridges bend more but are weaker.
- The cuts are in a staggered pattern, each run broken into segments so that the sheet keeps its strength along the hinge. Align the cuts across the grain so that the bending is across the fibers in wood. Try a small test hinge; the best pattern depends on the material.
Minimum features and heat effects
| Feature | Rule of thumb | Example for 3 mm sheet |
|---|---|---|
| Smallest hole | About half the sheet thickness, or 0.5 mm, whichever is larger; the taper makes a small hole in thick sheet conical | 1.5 mm |
| Web between cuts | At least the sheet thickness, or the part burns through or breaks | 3.0 mm |
| Narrowest slot | At least 1.5 × the kerf | 0.50 mm |
| Inside corner | A radius of at least half the kerf (it cannot be sharper) | 0.08 mm |
| Distance from an edge | At least the sheet thickness | 3.0 mm |
| Tabs (micro-joints) holding a part in place | 0.2 to 0.5 mm wide, on long edges | Cut off with a knife |
| Material | Typical heat-affected zone (illustrative) |
|---|---|
| Acrylic | 0.10 mm |
| Plywood 3 mm | 0.30 mm |
| Leather | 0.40 mm |
| Steel, fiber N₂ | 0.05 mm |
| Steel, CO₂ with O₂ | 0.30 mm |
- Heat affects the edge: wood chars, acrylic flame-polishes (and can craze), thin metal discolors; keep delicate features away from edges and each other.
- Thermal expansion: during a long cut, a 300 mm acrylic part that warms by 30 K grows by 0.000070 × 30 × 300 = 0.63 mm. Most of that returns on cooling, but it is why a hot part is oversize.
- Stress and cracking: extruded acrylic cracks when engraved or when cemented under stress; anneal parts or choose cast sheet.
- Edges of acrylic can be flame-polished or left as cut; sanding and a clear coat hides the grain on wood.
Accuracy and where it is lost
Beyond the kerf, the accuracy of a laser part depends on the machine’s motion (steps per mm, belt stretch, square gantry), on the material (thickness, flatness, humidity), and on the cut itself (focus, taper, heat). The calculation of steps per mm is the first check:
Worked example: steps per millimeter and a tolerance budget
| Item | Value |
|---|---|
| Steps per mm | 80 steps/mm (200 × 16 ÷ 40 mm) |
| Resolution | 12.500 µm per microstep |
| A 0.2 % scale error over 600 mm | 1.2 mm |
A stepper motor of 200 steps per revolution driven at 16 microsteps turns 3 200 microsteps per revolution; a 20-tooth pulley on a 2 mm pitch belt moves 40 mm per revolution: 80 microsteps per mm, 12.5 µm each. A wrong value (a different pulley or belt pitch) scales every dimension: a 0.2 % error makes a 600 mm part 1.2 mm wrong, which is why you cut a test square and check it with a calliper.
| Source of error | ± mm |
|---|---|
| Machine scale and steps | 0.10 |
| Kerf compensation | 0.05 |
| Material thickness variation | 0.10 |
| Focus and taper | 0.05 |
| Thermal and humidity change | 0.10 |
| Combined (root-sum-square) | 0.19 |
The combined error of about ±0.19 mm is typical of a good machine: a press-fit clearance of ±0.05 mm is finer than that, so the sheet, the kerf and the machine must be tested together on scrap: the test cut is essential. Hold the tolerance you design for to what the machine can repeat.
- Check squareness: cut a large square and measure both diagonals; a difference means the gantry is out of square.
- Check scale in both axes separately.
- Measure sheet thickness with a calliper at several places; plywood and acrylic vary by 0.1 to 0.3 mm.
- Note the humidity for wood.
Related
- Laser Cutting: the overview of this guide.
- Power, Speed, Focus and Assist Gas: how power, speed and focus set the kerf and taper.
- Files, Software and Toolpaths: kerf offset and nesting in the software.
- Fits and Tolerances: clearances and press fits in general.
- CNC Machining: kerf and tool compensation on routers and mills.
FAQ
What is the kerf, and why does it matter?
It is the width of material the beam removes. If you cut on the drawn line, parts come out smaller and holes larger by half the kerf each side, so joints are loose and parts the wrong size. Measure it and offset every path by half.
How do I measure the kerf?
Cut a comb of ten slots of known width on the line, measure the width across them and divide the difference by ten; or cut a square and measure it.
Why is my press fit tight at the bottom and loose at the top?
The kerf is tapered: wider at the top than at the bottom. The slot is wider at the top face and the tab narrower. Choose a reference face, use a longer lens and a lower power and speed, and test the joint.
How accurate can a laser cutter be?
A good machine repeats within about ±0.05 to ±0.1 mm; part accuracy is limited by the material, the kerf and the taper too. Design the tolerance to what the machine can hold and test on scrap.
References
- Steen, W. M. and Mazumder, J., Laser Material Processing, Springer (kerf and cut quality).
- ISO 9013, Thermal cutting: classification of thermal cuts, geometrical product specification and quality tolerances.
- Box and joint generator documentation (MakerCase, BoxGenerator); laser makers’ guides to living hinges and press fits.
Last reviewed: October 2026. Kerfs, clearances and heat zones are typical and illustrative: measure your own on scrap.