A laser beam burns away a thin strip as it cuts: the kerf. So a cut slot comes out wider than drawn (by the kerf) and a cut-out part a little smaller. Draw slots narrower than the board for a snug press fit.
A laser cutter is accurate to a tenth of a millimetre, yet your flat-pack box falls apart without glue. Where did the fit go?
Kerf is the width of material the beam burns away, typically 0.1–0.3 mm on a school laser. A slot is cut on both edges, so it comes out kerf wider than drawn. The same fit classes as printing apply: under 0 won't go in, 0–0.15 mm press fit, 0.15–0.4 mm sliding, more is loose.
Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Try: Try it on the board and compare the result with your prediction.
Investigate: Work out why it happens, one step at a time.
Modify: Change the problem on purpose and see what happens.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
A 3 mm slot
TryTry it on the board and compare the result with your prediction.
Try: make it snug
Try it yourselfYour edits are saved in this browser.
Draw the slot narrower until the plywood is a press fit (snug, no glue needed).
Use the board. This ticks when you get there.
InvestigateWork out why it happens, one step at a time.
Worked example: allow for the kerf
Worked exampleBefore each step, say what you think comes next.
cut slot=drawn+kerfBoth edges of the slot lose half the kerf.
want: board+0.05=3.05A press fit.
drawn=3.05−0.2=2.85Take off the kerf.
measure your kerfCut a 10 mm square, measure it: it is about a kerf smaller.
3 mm board, 0.2 mm kerf, 0.05 mm clearance wanted. Draw the slot (mm)?
Sam's joints all wobble. Click the line where Sam's reasoning first goes wrong.
ModifyChange the problem on purpose and see what happens.
Vary: a finer beam
Try it yourselfYour edits are saved in this browser.
This cutter's kerf is 0.1 mm. Find a press fit for the same 3 mm board.
Use the board. This ticks when you get there.
Modify TypicalChange the problem on purpose and see what happens.
Cut a test comb
PractiseA few on your own, to check you have it.
Kerf
A 50 mm square is cut with a 0.2 mm kerf. About how wide is the cut square (mm)?
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
Thicker board
Try it yourselfYour edits are saved in this browser.
Need a hint?
Cut slot = drawn + 0.25; you want 6.0 to 6.15.
Another hint
Drawn 5.75 to 5.9.
6 mm MDF, kerf 0.25 mm. Find a press fit.
Use the board. This ticks when you get there.
For teachers
An optional lesson. Students see a 3 mm slot cut wider than 3 mm, draw slots narrower for a press fit, work the drawn size back from the board and the kerf, cut a test comb on the school laser, and find a fit for a different board and kerf. About 35 minutes.
I can explain what kerf is and how it changes a cut.
I can draw a slot for a press fit.
Before class
Laser cutter use is supervised, with approved materials only (no PVC or vinyl: chlorine gas). Kerf depends on the machine, focus, power and material: measure yours. The board treats kerf as making slots wider by the full kerf (each edge loses half).
Running the lesson
Predict: pass round a loose test joint.
Test comb: the teacher runs the cutter.
The hard part (on purpose)
The kerf is too small to see. Cut a 10 mm square, measure it with calipers, and pass it round.
When students get stuck
A student adds the kerf to the drawn slot. Ask: Does the laser make the slot bigger or smaller than drawn? Fix: Bigger, so draw it smaller.
Stretch and support
Stretch: Make the slot width a variable in your parametric model: #board - #kerf + #fit.
Support: A card: drawn = board + clearance − kerf.
Where it leads
Back to Capstone projects (an enclosure or a chassis), or Design and print.
What each step is for
a 3 mm slot: Committing to how a same-size slot fits tests the 'cut exactly on the line' picture.
Try: make it snug: Narrowing the drawn slot on the board settles the prediction and finds the press fit.
Worked example: allow for the kerf: Working the drawn slot back from the board, the kerf and the fit wanted models the move. Sam's 'exactly on the line' is planted after the method.
Vary: a finer beam: A different kerf changes the drawn size for the same fit: measure your own cutter.
Cut a test comb: A kerf test comb (slots stepping by 0.05 mm) gives the school cutter's real numbers.
kerf: Kerf on slots and on parts.
thicker board: A new board and kerf needs the whole calculation again.