LaserCanvas
Kerf and tolerance: making parts fit
Measure kerf by cutting a square of known size and subtracting what you actually get — typically 0.1 to 0.3 mm. Compensate only where parts meet: outlines move out by half a kerf, holes move in by half. For a press fit, make the slot the material thickness minus about 0.1 mm and let the kerf do the rest.
Measuring your kerf takes two minutes
- Draw a square of a known size — 20 mm is convenient — on the material you will actually use.
- Cut it with the settings you will actually use.
- Measure the piece that fell out with calipers, across both axes.
- Kerf = drawn size − measured size. A 20 mm square measuring 19.78 mm gives a kerf of 0.22 mm.
Kerf is not one number for your machine. It changes with material, thickness, focus and power — thicker stock usually gives a wider, more tapered kerf. If you do precision work in two materials, measure both.
When to compensate, and when it is a waste of time
| Work | Compensate? | Why |
|---|---|---|
| Decorative shapes, signs, ornaments | No | 0.2 mm is invisible and nothing has to mate |
| Boxes with finger joints | Yes | Every joint compounds the error |
| Gears, mechanisms, inlays | Yes | Fit is the whole point |
| Holes for bolts or dowels | Yes | A hole cuts oversize; an M3 hole drawn at 3 mm comes out loose |
| Stencils and masks | Usually | Thin webs lose a kerf from both sides and can vanish |
The last row is worth dwelling on. A 0.5 mm web between two cut-outs loses half a kerf from each side. At 0.22 mm kerf that leaves 0.28 mm — fragile. At 0.5 mm kerf it leaves nothing at all and the stencil falls apart.
Sizing a finger joint
The classic mistake is drawing a 3 mm slot for 3 mm plywood. The slot cuts about a kerf wider than drawn, so it ends up around 3.2 mm and the joint rattles. Two ways to fix it:
- Let the software compensate. Draw everything nominal — 3 mm slot for 3 mm stock — and turn kerf compensation on. The toolpath moves inward on the slot and outward on the tab.
- Compensate by hand. Draw the slot at thickness minus one kerf. For 3 mm ply and 0.22 mm kerf, draw 2.78 mm. Do not do both, or you will compensate twice.
Choosing a fit
| Fit | Slot relative to measured thickness | Use for |
|---|---|---|
| Press fit | Thickness − 0.1 mm | Boxes that hold without glue; needs a mallet tap |
| Snug fit | Thickness exactly | Glued joints — leaves room for glue |
| Clearance fit | Thickness + 0.1 mm | Parts that must slide or be disassembled |
Those are after kerf compensation. Test on one joint before cutting thirty — a 40 mm square test piece with one tab and one slot costs a minute and saves a sheet.
The taper nobody mentions
The beam narrows to its waist and widens again, so a cut through thick material is slightly V-shaped — wider at the top than the bottom, or wider at both ends than the middle if focused centrally. In 3 mm ply it is negligible. In 10 mm acrylic it is visible and affects how flush a joint sits.
Focusing about a third of the way into thick stock, rather than at the surface, spreads the taper more evenly and gives a squarer edge.
Common questions
What is a typical laser kerf?
Between 0.1 and 0.3 mm for hobby machines, with 0.15 to 0.25 mm the common range on 3 mm plywood. Diode lasers with a small spot can be narrower; thick material and high power give a wider kerf. Measure rather than assume — it is a two-minute test.
Should I add kerf compensation to my drawing or let the software do it?
Let the software do it. Drawing compensated geometry means your file no longer matches the real dimensions, which makes it useless on a different machine or material. Keep the drawing nominal and set the kerf per job.
Why are my box joints loose even with kerf compensation?
Usually because the sheet is not the thickness you assumed. "3 mm" plywood often measures 2.7 mm, and a slot cut for 3 mm stock will always rattle on it. Measure the actual sheet with calipers and size the joints from that number.
Does kerf change between the top and bottom of the cut?
Yes. The beam is narrowest at its focal waist and wider above and below, so the cut is slightly tapered. In thin material it is not worth thinking about; in material over about 6 mm it is measurable, and focusing partway into the thickness rather than at the surface evens it out.