Week 3: Laser cutter: computer-controlled cutting (CNC)

Introducing the laser cutter and 2D-3D assembly.

Prep for this week’s class

Watch the corresponding Fab Academy lecture. We’ll be covering part of this content.

Fab Academy lecture notes: http://academy.cba.mit.edu/classes/computer_cutting/index.html
Lecture from 2018 (Fab-20180207D_Lesson03, the second video on the page): Fab Academy 20180207 Recordings

Baseline

Who has:

Used 2D design software like Illustrator
Learned the basics of a 3D tool like Fusion 360
Cut something on a laser cutter

Computer Aided Design (CAD) software

We mostly use Fusion360, Rhino, Solidworks for 3D; Illustrator or Inkscape for 2D

Parametric design

Cardboard comes in different thicknesses
Lasers cut with different kerfs
Human bodies come in different shapes and sizes

Inkscape is not parametric, Rhino and Fusion 360 are.

Example in Fusion 360: minimal-parametric-laser-cutter-demo

Computer Aided Manufacturing (CAM)

CAD is design, CAM translates a design file into a format the machine understands and communicates with the machine.

Typically through printer driver (accessible in Print dialog on your 2D CAD software)
But we can also talk to the machine at a lower level using Fab Modules or Mods

The laser cutter

Applications

Cutting or marking/engraving by varying power
Raster or vector mode
Screen printing, by making a halftone (See holes, path)
Press-fit construction (example: GIK, gik.cad, gik.png)

Joints

Examples

Easy to design <–> Strongest properties

Slot - simplest, hard to align
Chamfer - rounded corners help align parts, and can compress materials into slots (eg corrugated cardboard). But still relies on sliding friction
Bistable (bump and slot), behaves very differetly for different materials
Flexure - design a bendable part with controllable flex
Pinned - secure joint with an orthogonal constraint (see also wedged mortise and tenon)

Tolerances

Slop vs security (range is about 0.1mm)
Brittle material vs compressible material
Parametric design is your friend

3D shapes

Kerfing (incomplete cut)
Flexures, living hinges (enables twists and bends). See also Inkscape living hinge extension
Extreme example, a moving platform without moving parts (gears, bearings)

Lasers (Light Amplification by Stimulated Emission of Radiation)

See Neil’s lecture (@ 45 mins) for technical details
CO₂ laser: good for wood, card, acrylic, etc. Need a fiber laser to cut metal.

Cutting mechanisms

burning
melting
evaporation
ablation

The material has to go somewhere, …

Airflow

Exhaust - draws combustible material out. (Machine: Extractor)
Assist - injects air at the cutting point. (Machine: Compressor)

If it’s not strong enough, that material stays around - Bad news
You shouldn’t see smoke hanging around in laser cutting chamber
Exhaust fumes are very bad news
Plastic will outgas for a minute after cutting. Leave the lid closed for a minute

Kerf

Some material is removed with the cut.
Some drivers (e.g. mods) allow for this, with offsetting.
Otherwise, you should allow for this in your design (parametrically!)

Safety

All laser cutters want to catch on fire
Card, MDF, plywood and acrylic are all really close to combusting when cutting.
Don’t step away from the machine. Always supervise.
Initial combustion: open the lid. Otherwise, smother or use the fire extinguisher.
Laser optics need to be kept clean, otherwise, heat can build up (so avoid smoke)

WIP: Check safety procedures

Materials

Cardboard - the good stuff bows, not kinks. Use old boxes. Very forgiving for joints
Plywood - Real thickness != real thickness - use the callipers.
PMMA/acrylic/plexiglass/perspex/lucite glue bend
POM/delrin/acetal - more elastic than acrylic

No-gos

No PVC - releases chlorine
Never put a material into the cutter unless you know where it came from - no random plastic.
Never put anything shiny (e.g. metal) into a CO₂ laser

Preparing artwork (for vector cutting)

2D vector file (EPS or DXF)
Make the format as simple as possible
All shapes no fill, nd 0.001mm stroke
Laser origin is top left

How to use the laser cutter

See Fab Academy docs here: https://docs.google.com/document/d/1kDd0A60eJdmJgIRt-zmAJwxszw0Gd61VXqHKnizv6hg/edit

Place material

Consider airflow
Alignment against the rulers
Support to distance the material from the bed
Flat surface - consistent z-height

Set origins and focus

Use laser pointer
Set x and y origins
Use focus tool to set z-origin

Settings

Power - too much melts, too little doesn’t cut. Use multiple passes.
Speed - too slow can cause combustion, too slow doesn’t cut
Pulse rate/Frequency - too close can melt, too far apart can leave gaps
Coordinate system, origin is top left
Vector mode for cutting, raster mode for engraving (but interesting grey areas for experimentation)

Extraction

Turn on the BOFA extractor (speed approx 330m3h)
Turn on the air compressor

Whilst it’s cutting

Always observe the cutting closely

After the cut

Leave the lid closed with the extractor on for 30 seconds
If you mess up the bed, clean it up - take it to the sink and carefully clean it without bending the sheet

Assignment

Design, lasercut, and document a (parametric) press-fit construction kit, accounting for the lasercutter kerf, which can be assembled in multiple ways.

Use cardboard: forgiving, cheap, recyclable. But beware fire risk.
Make test cuts, vary power, speed and frequency – Document your results.
Make kerf tests to find good settings. Use these results for your own (parametric) design.

What do I need to do to pass?

Design, cut and assemble a 3D card model using the laser cutter.
Demonstrate an ability to work safely.
Demonstrate a process of experimentation and iteration to improve your results.

Document all your work on your student blog, with photos and videos to show what you did, what went wrong, and how you fixed it. Cite external sources where you have used someone else’s work.

Update 2019-02-20:

Specifically, document how you established a good fit for the parts:
e.g. If they’re designed to stick together, how did you make them loose enough that you can assemble the pieces, but tight enough that the structure is stable.
Suggestion: make some test pieces that calibrates your laser kerf, and how much space you need to leave between mating parts

Extra credit

Use a parametric design tool to account for cardboard thickness, kerf testing and joint stability.
Make a model that uses more advanced joint techniques.
Make a model that doesn’t just consist of flat panels assembled together