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.
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Students will be able to
I can explain what kerf is and how it changes a cut.
I can draw a slot for a press fit.
Timing
Predict and Try 5 min · Worked example 8 min · Vary 4 min · Test comb (teacher-led) · Practise and Make 8 min
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
What you will see
A question to ask
The fix
A student adds the kerf to the drawn slot.
Does the laser make the slot bigger or smaller than drawn?
Bigger, so draw it smaller.
Answers to the checks
a 3 mm slot: A slot is drawn 3 mm wide for 3 mm plywood. The laser's kerf is 0.2 mm. The plywood… It slides in with play: the slot is cut 3.2 mm wide, 0.2 mm more than the board.
Try: make it snug: Draw the slot narrower until the plywood is a press fit (snug, no glue needed).
A slot drawn about 2.85–2.9 mm: cut 3.05–3.1 mm.
Back from the board: 3 mm board, 0.2 mm kerf, 0.05 mm clearance wanted. Draw the slot (mm)? 2.85 Cut slot = 3 + 0.05 = 3.05; drawn = 3.05 − 0.2 = 2.85 mm.
Find the mistake: Sam's joints all wobble. Click the line where Sam's reasoning first goes wrong. line 2 Line 2: the beam has a width. It burns away about the kerf, so a 3 mm slot is cut wider than 3 mm.
Vary: a finer beam: This cutter's kerf is 0.1 mm. Find a press fit for the same 3 mm board.
Drawn about 2.9–3.0 mm: less to take off.
A square: A 50 mm square is cut with a 0.2 mm kerf. About how wide is the cut square (mm)? 49.8 Half the kerf off each edge: 50 − 0.2 = 49.8 mm.
Finger joints: Finger joints come out loose. What do you change? Make the fingers wider and the gaps narrower by about the kerf Allow for the kerf in the drawing: wider fingers, narrower gaps.
thicker board: 6 mm MDF, kerf 0.25 mm. Find a press fit.
Drawn about 5.8–5.85 mm.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: a finer beam
Open task: any program that does what the step asks.
thicker board
Open task: any program that does what the step asks.
Stretch and support
Stretch: Make the slot width a variable in your parametric model: #board - #kerf + #fit.
Support: A card: drawn = board + clearance − kerf.
Exit ticket
Why does a laser-cut slot come out wider than drawn?
The beam burns away a strip of material (the kerf), so each edge of the slot moves out.
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.
Where it leads
Back to Capstone projects (an enclosure or a chassis), or Design and print.