Computer-Controlled Cutting
Summary
Assignments
Group Assignments
- Security Training
- Characterization of our laser-cutter
individual Assignments
- Cut something with the vinyl-cutter
- Make a construction kit
Security Training
Patrik, our local instructor, gave us a tour of the laser-cutting room.
The fire extinguisher and the fire blanket are just at the right of the door, easily accessible for anyone in the room and someone just coming in (in case of emergency).
All laser-cutters have a "locking device" that has 2 purposes:
- To use the laser-cutter, you have to unlock through the fab lab's booking app. You only have access to this app if you followed the laser-cutting and safety training.
- Every 5 minutes, it beeps until you press a button. If you don't, it turns the laser-cutter off automatically.
Characterization of our laser-cutter
Description of our fab lab laser-cutters
Our fab lab possesses several laser-cutters of 3 different models:
- Epilog Fusion Pro 36
Laser : CO2
Laser power : 60W
Work area : 914 x 610 x 228mm - BRM PRO 1300
Laser : CO2
Laser power : 150W
Work area : 1300 x 900 x 200mm - Lasersaur (DIY)
Laser : CO2
Laser power : 100W
Work area : 1220 x 610mm
Our Lasersaur was recently updated.
Hence our instructor asks us to characterize it to update its documentation.
Focus distance measurement
My fellow students, Fabio Coelho and Jonas Grimaud, tested different focus distances.
The Lasersaur control panel has a Pulse button. They pulse the laser to bore a hole in a paper sheet, with different focus distances.
By visual inspection, we determine that the optimal focus distance is 15mm.
Kerf estimation
They also perform a kerf test by cutting 9 side-by-side rectangles in a MDF board.
The gap between the resulting frame and the rectangles is due to the 10 vertical cuts made to create the rectangles.
The test was made twice with a gap of 2.87mm and 2.46mm.
We take the mean value to obtain the Lasersaur kerf: 0.267mm.
Operating parameters choice
I made a power/speed test grid for 2mm cardboard on the Lasersaur
I decided to use the following parameters for this material
| Action | Power | Speed |
|---|---|---|
| Cut | 80% | 20mm/s |
| Engrave | TBD | TBD |
press-fit test
My design would use press-fit slots. I started by sketching a test comb:
To make it parametric, I defined 3 values:
| name | value | description |
|---|---|---|
| toothWidth | 4 mm | width of the comb's teeth |
| thickness | 2 mm | material thickness |
| increment | 0.05 mm | slots width increment |
The slots width is incremented by increment. The central slot width is equal to the material thickness.
Left-most slot is 1.75 mm wide and the right-most is 2.25 mm.
I laser-cut 2 of them. Each actual slots should be one kerf larger than in the sketch.
As the lasersaur kerf is 0.27 mm, I expected that the "perfect" fit should be between the 1.75 mm or 1.8 mm slots.
Both are tight fits, I the decided to use 0.25 mm as kerf parameter in my design.
construction kit
I decided to create a dice tower.
It's used by gamers to roll dice fairly. Dice are dropped into a hole in its top and bounce on multiple ramps inside the tower and going out at the bottom of the tower.
Design idea
I first design my tower without thinking about assembly, just to have an idea of the global design.
All pieces are flat rectangles except for the front wall that have a door.
I used the Part Design workbench in FreeCAD and defined all the tower dimensions in a spreadsheet to make my design parametric.
We can see the result on the 2 screenshots above. The left one shows the entire tower.
On the right one, the front and right walls are masked to show the inside pieces.
We can see, from top to bottom. If we follow a falling die:
- it first meet the top ramp (light green), that guide it to the rear
- the 2 middle ramps force it to the center of the tower, between the 2 inner walls (dark pink)
- Finally, the bottom ramp (blue) send it out of the tower on the base plate (dark green).
My first idea was to modify the sketches to create the slots to press-fit the pieces together (that's why I used the Part Design workbench).
But, I quickly realize that it'll a very time-consuming task.
Hence, I started to look for a laser-cutting workbench for FreeCAD.
I found Laser Cut Interlocking. I started by following the examples given on its github page.
This workbench has only 6 tools, available in its toolbar.
Box test
I tried the first two:
- Box: create a box
On the left panel, we can see the box part that contains 6 sub-parts already correctly placed to form a box. These parts are simple panels, without tabs or slots.
On the right panel, we see the dialog panel allowing us to set the box geometry. We can already note that the parameters in this dialog box can't be linked to a spreadsheet. After the box creation, it can't be modified.
- Slots: create new parts with slots, based on existing simple panels.
On the right panel, we can see the dialog box for this tool.
The first step is to select all the parts that has to be connected, as seen on the left panel where I selected the 6 panels. We add them by clicking on Add parts. Next step is to add the faces to be "slotted".
In the next figure, I hid the top panel to select the face and behind panels' top face. we need to select the "edge" faces.
There are 3 types of connections: tab creates a given number of tabs, screw, add a screw to the tab and continuous that creates tabs along all the selected edge.
As both selected faces are identical, we can use add same faces.
the bottom of the dialog box allows us to configure the connection. Here, I choose continuous and set the number of elements to 6.
After clicking on OK, we obtain this result:
We can see that a new part MultiJoin was created. It contains a copy of the original parts and new sub parts that are their tabbed version.
All these copies of the pieces seems to be an obstacle to create a parametric design.
I looked for more advanced tutorials to see if it's really a problem. I found one (french-speaking) video that explain how to create a more complex object.
The main drawback is that the workbench doesn't work with Part design objects.
Crosspiece test
I then tried to make a simple parametric press-fit assembly, to test the Crosspiece tool:
It's only two identical panels crossing each other perpendicularly.
I used a spreadsheet to add parameters:
| Name | Value | Description |
|---|---|---|
| length | 40 mm | pieces length (x) |
| width | 40 mm | pieces width (y) |
| thickness | 2 mm | material thickness |
| x_angle | 0 ° | rotation angle around the x-axis of panel2 (measured from the xy-plane) |
| y_angle | 0 ° | rotation angle around the y-axis of panel1 (measured from the xz-plane) |
| kerf | 0.25 mm | Laser-cutter kerf |
The crosspiece tool gives the expected result:
I then tried with another angle, by changing the parameter:
By updating the crosspiece, I had an unexpected result:
The slot in panel2 was angled and could not be laser-cut.
Bottom ramp design
I had to design the bottom ramp without lc workbench.
I made a sketch , extruded it. Next I had a chamfer on the outer edges of the slots.
I used Export tool to create a SVG file of the ramp:
Cutting and assembly
Cutting goes without problem.
The tower assembly goes well, except for the top ramp.
Its lateral slots don't fit in the side walls.
We can see on the photo that I damaged the "fingers" when trying to assemble it.
There are 2 other ramps in the tower that use the same assembly, with the same parameters.
I still need to investigate this problem.
I plan to make a test piece for this T-shaped assembly to try different parameters, beginning with the kerf.
Deepnest
Laser Cut Interlocking export tool is pretty bad for space optimization as we can see on the exported svg file:
Instead of reorganizing them by hand, I decided to test Deepnest.
It is pretty easy to use:
- Import all svg files containing the parts to cut
- Add a rectangle the size of the material sheet, with the Add button.
It represented the size of the board in which the parts will be cut. - Select this rectangle as the canvas, by checking its tick box.
- Click Start Nest
After a while, Deepnest will give you a much more organized board to cut:
You can also check
- uncheck common edges
- add a minimum distance between pieces
Vinyl-cutter
work in progress