During this week, I have utilized the vinyl cutter to decorate a t-shirt for my son and I have used the laser cutter to create a press-fit construction kit.
My son is a big fan of pokemon, so I thought it would be a good idea to make a surprise for him to heat press a pokemon trainer's t-shirt. I utilized the vinyl cutter to cut the adequate shape in the head press vinyl. I utilized a heat press to imprint the shape in the fabric.
Before starting cutting anything I wanted to understand well the process and get acquaintance with the machine. Vinyl cutter operates similarly to a printer, but it has a blade which make cuts in the material instead of depositing ink on it. The material to be cut, must be provide to the machine firmware as vector graphics (I have used svg format). Vinyl cutters are widely used in the field of advertisement and in organization of big events. The vinyl cutter in our Fab Lab is a Roland GS24.
In collaboration with the other Fabacademy students we made some initial tests in order to learn how to use the machine and configure the settings. At the same time, we wanted to know how the width of the lines in the svg file affects the cut.
We created in >Inkscape three different stars with three different widths: 0.05 pt, 0.5 pt and 25 pt. All the stars where aligned at the bottom using Inkscape alignment tool (See Figure 2).
We follow the next steps to print the stars:
We learnt that when the width of the border lines are bigger than the blade, the vinyl cutter utilize a the border middle line to do the cutting.
I designed the image to be imprinted in the t-shirt in a paper. The idea was to have the silhouette of Ash (the main character of the Pokemon cartoon throwing a pokeball. At the bottom should read the text: "Entrenador Pokemon", that is, Pokemon trainer in Spanish.I first search for an image of Ash throwing a ball using Google Image search engine. I chose the one shown at Figure 4, left. It was a gif file, so I had to convert it to vector graphics. To do that I opened in Inkscape and used the Path>Trace bitmap option to extract the path. I played with the different settings until i got the correct one. I just wanted the outer silhouette, so I did not need too much detail. Figure 4 shows 4 different settings. Top-left: Brighness-cutoff=0.9; Top-right: Brightness-cutoff=0.1; Bottom-left: Edge detection=0.9; Bottom-right: Edge detection = 0.1; I finally selected the one with less details (top-right).
After that, I had to remove the rest of inner details in order to extract the silhouette. To that end, I used the Edit path nodes tool. Since, the image had too many nodes (Figure 5, left. I utilized the Path>Simplify option to reduce the number of points. The result is shown on Figure 5, right. Next, I removed manually the inner paths (selecting the nodes composing the path and pressing the Delete button. I activated the Show path outline option, from the Edit path tool-bars. When this option is activated, a red line shows the paths.
When I managed to remove all the inner paths, and got the silhouette, I decided to create the Pokeball myself. The process is shown in Figure 6. The process is at follows:
After a few small position adjustments I got the results shown in Figure 7
Before cutting the final version with the heat press vinyl I wanted to check that everything was correct. So, I utilized first normal black vinyl. I followed the same instruction as in the previous section. Removing the vinyl from its support material is not an easy task. I utilized a tweezer and transfer paper. I attached the transfer paper firmly against the vinyl. Push hard with my hand, and after a while I removed it slowly. In some cases, I had to use the tweezer to unstuck some parts that where not attached to the transfer paper. Once I got it out i sticked it in my laptop. The process is shown in Figure 8
Finally, everything was ready for the heat press. I first MIRRORED THE IMAGE so it looks correctly when it is imprinted. This is specially important for the text, otherwise it would look up side down. I did that using Inkscape Flip horizontal option. When I got the correct file, I load the heat press vinyl in the machine. It has two different sides, the matte side and the glossy side. The matte part should be the part to be cut (it is the part that sticks in the t-shirt). The glossy part is used as a protector while applying the heat.
The first cutting attempt was a failure. I did not check before hand and the blade did not have enough force to cut the material. Hence, I had to retry the printing, applying a little bit more force (using the analog slider on the right of the vinyl cutter). The second attempt was successful. I only had to weed the leftover material and the result is shown in Figure 9
The final step is to put the t-shirt on the heat press. The heat press vinyl on top of it (with the glossy part facing up), set the temperature of the machine to 160 degrees and a time of 20 seconds, and apply the heat. I checked that it was correctly attached to the fabric, and removed the glossy layer. Tachan!!! a nice and unique Pokemon t-shirt
After trying different configuration settings with my colleagues I decided to build a simple parametric press fit piece to be laser cut out of a 4 mm acrylic. I chose Freecad as a modeling tool. The final result are shown in Figure 11. They are quite similar but the one on the right has a chamfer in the slots borders.
The process I followed to create this pieces with Freecad is as follows:
We wanted a parametric piece. Parameters that I decided to can be modified are: Material thickness, Kerf, square size and slot height. The slot width would be the thickness of the material - kerf. I created a Spreadsheet to define the values of the parameters using the Spreadsheet workbench. The first column shows the name of the parameters while the second column contains the values. Next, in order to refer to these parameters in the constraints, I had to create aliases for the corresponding spreadsheet cells. (Figure 14).
Next, it is necessary to link the constraints with the parameters values. I went back to the sketch view, and in the model window>Data>Sketch constraints are solved. I selected each constraint, pressed the fx symbol and wrote there the alias of the parameter:
Spreadsheet.alias (Figure 15 ). Now the geometry is fully parametrized.
During the following step, I exported the geometry to svg format. I followed two different methods:
My colleagues at the lab should have all related information. We have been working with different laser cutter configuration settings and checking the kerf. For instance, Nataliya Shevchuk's description of week 3 includes a detailed description of our group work. It is difficult to specify who was doing what since Jari Pakarinen, Natalia Shevchuk and me worked together to accomplish the task.
Basically, the goal of the group work was to calculate the kerf of our laser cutter (a Epilog Engraver WinX64 Fusion). Roughly speaking the kerf is the groove or slit created when the laser cuts any material. When the laser cuts some part of the material desintegrates somehow (e.g. transforming it into powder). This makes the pieces are a little bit smaller than the original design. In the group work we had to define the kerf of the machine and how different settings affect the material. The kerf is important when working with press fit solutions. If you do not consider the kerf the tabs are not big enough, so they do not adjust correctly and the piece do not fit correctly.
In order to calculate the kerf, we created in Inkscape a 10 mm square. After that we cut in a 3mm green acrylic stock utilizing the laser cutter. We modified the cutting parameters to understand how different parameters affect the cutting. Finally, we measured the size of the cut square. The difference between the original size (10mm) and the cut piece would be the kerf.
NOTE on 10.06.2017: After reviewing the documentation I realized that we did one thing wrong. For measuring the kerf we should have measured the size of the cut squarers and the size of the hole left in the material. The kerf is then calculated as follows:
(Size of the square - size of the hole)/2
Fortunately, we have still the original pieces that we used for cutting, so I will update the documentation at the end.
The process to measure the kerf was as follows:
There are three values that can be modified:
As a base for the settings we utilized the one recommended by the laser cutter manufacturer:
We modified the speed from 5% till 15% (recommended value was 9%). We did not modify the power or the frequency. The following table presents he measure of the squares (and after the documentation update also the measures of the holes)
|Speed||Power||Frequency||Square Measures (mm)||Hole measurements(mm)||kerf|
|5||100||100||9.81 x 9.79||10.13 x 10.13||0.16 - 0.17|
|7||100||100||9.80 x 9.82||10.07 x 10.11||0.13 - 0.15|
|9||100||100||9.80 x 9.82||10.08 x 10.07||0.13 - 0.14|
|11||100||100||9.89 x 9.84||10.03 x 10.08||0.07 - 0.12|
|12||100||100||9.90 x 9.84||10.04 x 10.05||0.7 - 0.10|
|13||100||100||Not cut||Not cut||Not cut|
|15||100||100||Not cut||Not cut||Not cut|
From these settings we know that the maximum speed in order to cut the acrylyc would be 12%. Upper values wont cut the material. In addition based on the initial calculations we got that the kerf was between 0.15mm and 0.10 for this material. Note that we used a digital caliper to measure the size of the square, so errors might rise.
Mikko Toivonen worked in another method to calculate the kerf. Basically, he built several comb like models, in order to fit one with another and check which is the combination that fits better. The advantage of this method is that we avoid the errors added by the measures themselves.
Based on his model, we got that the kerf for this material and this laser was 0.18 mm
NOTE on 10.06.2017: I added the size of the holes in the previous table and make a new calculation of the kerf. We made a mistake while measuring the kerf and we did not considered the size of the holes. With this new method the kerf was between 0.16 and 0.07 mm. We noticed also that as we increase the speed, the kerf was reduced. So, KERF depends on speed. Anyhow, when I created the press-fit I had to use a kerf of 0.21, so it is important to always measure the kerf with the current settings and material that you are using.
During this week I learnt how to use the vynil cutter and the laser cutter. I learnt also how to make press fit pieces that fits without need of glue.
For the press fit is crutial to calculate correctly the kerf, otherwise the pieces won't fit. It took me a few cuttings attempt to make the pieces fit correctly together.
Inkscape and our vynil cutter did not understand really well each other. If you did not use the correct settings, the vynil cutter made strange things, for instance, rotating the models 90 degrees. I had to repeat a few times the cutting until I got the right settings.
Laser cutter is by far the most utilized machine in our Fab Lab. Some times we had problem to find slot available to make our tasks. Hence, it is important to reserve the machines well beforehand if you wannt be sure that you can finish your work on time.
Test the vinyl and laser cutter settings using some simple small models (e.g a small square) before cutting, is really important in order not to waste material.