--- title: "Laser cutting and kerf" subtitle: "Parametric design · Linked dimensions · Lesson 2" --- # Laser cutting and kerf **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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? :::teacher-note **In one sentence:** 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. **Students should be able to say:** I can explain what kerf is and how it changes a cut. I can draw a slot for a press fit. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **A slot is drawn 3 mm wide for 3 mm plywood. The laser's kerf is 0.2 mm. The plywood…** ☐ Slides in with a little play ☐ Fits perfectly tight ☐ Won't go in ☐ Falls straight through My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It slides in with play: the slot is cut 3.2 mm wide, 0.2 mm more than the board. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: allow for the kerf :::keep-together ### Back from the board 1. $\text{cut slot} = \text{drawn} + \text{kerf}$ — Both edges of the slot lose half the kerf. 2. $\text{want: board} + 0.05 = 3.05$ — A press fit. 3. $\text{drawn} = 3.05 - 0.2 = 2.85$ — Take off the kerf. 4. $\text{measure your kerf}$ — Cut 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)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 2.85 Cut slot = 3 + 0.05 = 3.05; drawn = 3.05 − 0.2 = 2.85 mm. ::: :::keep-together ### Find the mistake **Sam's joints all wobble. Click the line where Sam's reasoning first goes wrong.** ``` 1 The plywood is 3 mm thick. 2 The laser cuts exactly along the lines I draw. 3 So a 3 mm slot fits 3 mm plywood perfectly. 4 Sam's box wobbles and needs glue at every joint. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** 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. ::: :::typical ## Vary: a finer beam Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: :::typical ## Cut a test comb Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Kerf :::keep-together ### A square **A 50 mm square is cut with a 0.2 mm kerf. About how wide is the cut square (mm)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 49.8 Half the kerf off each edge: 50 − 0.2 = 49.8 mm. ::: :::keep-together ### Finger joints **Finger joints come out loose. What do you change?** ☐ Make the fingers wider and the gaps narrower by about the kerf ☐ Glue them ☐ Cut slower ☐ Use thicker sheet ::: :::teacher-note **Answer:** Make the fingers wider and the gaps narrower by about the kerf Allow for the kerf in the drawing: wider fingers, narrower gaps. ::: ## Make: thicker board Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Why does a laser-cut slot come out wider than drawn?** {{lines-3}} ::: :::teacher-note **Answer:** The beam burns away a strip of material (the kerf), so each edge of the slot moves out. :::