Week 03 - Computer-Controlled Cutting

Here are the assignments of week 03.

Group Assignment:

• Do you lab's safety training
• Characterize your lasercutter's focus, power, speed, kerf and joint clearance
• Try different joint types

Individual Assignment

• Cut something on the vinylcutter
• Design, lasercut and document a parametric construction kit, which can be assembled in multiple ways

Group Assignment

Please refer to the group page to read about how we performed our group assignment.

Individual Assignment

Vinyl cutting

In last week's assignment I scanned the mating surfaces for my free-wheel hubs to make new gaskets for them. Here is the improved cutting file, after running through one iteration in Fusion.

Now it was time for cutting on the machine in our Lab. It is a Roland CAMM-1 GX-24 Servo vinyl cutter and it can load up to 700 mm wide material, but the cutting area is 584 mm wide. The maximum cutting speed is up to 500 mm/s.

My instructor found some impregnated paper gasket in a local machine shop in 0,25mm thickness.

As this is not a standard vinyl sheet, we needed to adjust the knife cut depth and force.

The adjustment of the knife is done my making test cuts on a table. The protrusion is set by turning the knife holder slightly between each test cut and checking the results. A good depth is set, when the cut barely cut's through the whole thickness of the material and doesn't damage the surface below, but you can separate the cut part from the sheet.

Cutting force:  220 gf
Speed:          20 cm/s
Offset:         0,25 mm

After the sheet was loaded, we made some test cuts with the machine that appeared to be satisfying.

Then it was time for cutting the gaskets.

Peeled off from the sheet - here are three sets.

Fits nicely!

Parametric construction kit

I tried to come up with something that hasn't been done before in FabAcademy and decided to make a molecule construction kit. The idea is similar to 3D molecule contruction kits you can buy for educational purpose, but cut from acrylic sheets.

Because it's just a proof of concept, I start with the atoms for some simple molecules and the set could be extended further. It consists only of Hydrogen, Oxygen, Carbon and Sulphur for this assignment.

The design was made in Fusion using parametric table and configuration. By that I could adjust the kerf and clearance according to the results from the group assignment and select each atom or connector from a drop-down list. If you look carefully, you will notice arcs in the corners of the joints for stress relief.

Then I selected a face, made a new sketch and projected all geometry to that sketch. With a right-click on the sketch I could export that geometry to a *.dxf file.

Then I had to import the single cutting files into Inkscape and arrange them on a sheet that fit's into the empty space of an acrylic sheet of desired colour, that I found. I also tried to select sheets that were as close to 3 mm thickness as possible. In worst case a single sheet could vary from 2,6 to 3,1 mm from side to side.

When the line width was adjusted to 0,02 mm I saved the file as *.pdf. Below you can see a strip of Hydrogen atoms ready for cutting.

The machine in our lab is an Epilog Mini 24 laser with 40W power.

Power:        100%
Speed:          7%
Frequency: 5000 Hz

The cutting went without any issues. It took a while to peel off the protective film from the backside, but it's a good occupation while waiting for the next batch to finish. Obviously, I selected different colours for the different elements. Unfortunately, there wasn't any yellow acrylic in our lab, so I had to use this bright green for sulphur...

Here are some of my favorite molecules assembled. Can you identify them?

The design files can be found in my repository.

Challenges, solutions and findings

The vinyl cutting process was straight-forward and I didn't face any major challenges or problems. There are however two things I noticed.

• Independent of the shape of the sheet loaded, the vinyl cutter will recognize the largest rectangular box, that fit's into the outlines of the sheet and use that as available cutting area. This means if you have a long, trapezodial shaped sheet and want to cut several smaller objects, it might be clever to split the sheet up for beeing able to utilize the whole width.
• The origin of the sheet is located in the upper left corner, when standing in front of the machine and looking down on the sheet.

The cutting of the acrylic sheets was a bit more challenging, because of the poor quality of some sheet and the material properties. If we hadn't done the group assignment I would have produces some scrap pieces to begin width, before I would have figured out the reason. Also in acrylic sheets the clearance of the joints is more crucial than for MDF, because it is a quite brittle material and tends to crack, when deformed too much. One trick is to avoid sharp corners as I did. However, because of the white acrylic sheet being slightly thicker, than the other, I still experienced some cracks in the clear arms when assembling them with hydrogen atoms and beeing a bit uncautious.

If I would make an educational molecule construction kit for students, I would do some more research on where to get consistent quality acrylic sheets or maybe go for some other - less brittle - plastic. Also I would try to design a few more joint types and see if I can come up with a more robust one.

Key Learnings

• The vinyl cutter is a versatile machine that might deserve more attention and can cut more than vinyl.
• The acrylic sheets we have in our lab have quality issues. The thickness is uneven, which makes excact fits difficult.
• Always measure the thickness of a sheet in all four corners before cutting, if you are using joints.
• Colour mapping is a very handy feature, but unfortunately (and for no good reason) limited to only 16 colours.
• Acrylic is quite brittle and joints need to be designed carefully.
• MDF is a quite forgiving material to work with.
• Make a kerf and clearance test for each new material you try. A good point to start with is a kerf of 0,2 mm.
• Cutting with a 40 W laser through 3 or 4 mm thick and dense material is quite slow. Experiment with speeds around 10% or slower.