Assignment 3 - week 3 - 11/02

    Design, make, and document a press fit construction kit

    Computer Controlled Cutting

    This week the first part of the assignment was to create a press fit construction kit to be realised using mdf, acrylic or cardboard and the laser cutter. The kit should allow to create different objects and be parametric. Furthermore the second part of the assignment was to design and produce stickers of different sizes to be cut on the vynil cutter.

    A. First Part: Press fit kit

    For this part of the assignment I researched several designs, for press-fit joints. I found out that many interesting objects have been built this way. On the technical side I found that many variations of the press-fit joint are available. An interesting fact is that some of them are also used for fitting industrial products and construction materials. For example press fit can be used for joining copper tubes, and most plastic enclosures use some to hold part together without using any glue or screws.

    A.1 The desing of motorcycle using press fit

    My concept was to create a motorcycle using press-fit kit, I decided to design the press-fit motorcycle kit using rhinoceros. I use an existing model of motorcycles and try to adapt the design to my dad's motorcycle, I change the shape of the front wheels and the rudder control to the assembly is easier using press fit.


    Downloading link for my archives:
    Archives

    A.2 Principle of laser cutting

    The laser can handle a variety of cutting tasks. They range from precisely cut micrometer joints in paper-thin semiconductor chips to high quality cuts in 1.25 inch thick steel. In laser drilling, the laser beam creates very small to large sized holes without contact in metals, plastics paper and stone. Where the focused laser beam strikes the workpiece, it heats the material so much that it melts or even vaporizes. Once it has completely penetrated the workpiece, the cutting process can start. The laser beam moves along the part contour, melting the material as it goes. Typically, a stream of gas blows the melted material downwards, out of the cut. The gap is barely wider than the focused laser beam itself. In laser drilling, a short laser pulse melts and vaporizes the material with a high power density. The resulting high pressure drives the molten material out of the hole. The laser cutter can cut by burning away a little bit material. The laser follows the center of your vector line (drawing), burning away 0.1 mm on each side of the vector.


    A.3 Epilog Legend 36EXT laser cutting

    How to laser cut
    Step 1
    First you need a vector graphics file of the shape you want to cut or engrave. You can make these files with programs such as Inkscape, Coreldraw or Adobe Illustrator. Make sure you save the file as pdf. Lines that you want cut should be 0.01 mm thick. Everything else will be rasterized.
    Step 2
    Turn on the machine.
    Step 3
    Choose your material and place it in the machine. It is always best to make a dummy version in cheap cardboard first, to see if you got your measurements and settings right. If your are going to cut the material, make sure the honeycomb bed is in the machine. On the machine, press the [X/Y OFF] button followed by the green [GO] button for confirmation. Now proceed to move the laser by hand to where you want it to start cutting. If you need help aiming press the [POINTER] button to get a red dot on the lasers position. You can use the arrow buttons on the machine [up] and [down] to lower the bed for placing thicker materials. Bear in mind the machine starts at the top left corner of your design file. If you are satisfied with the position you can press [SET HOME].
    Step 4
    Open the Document in Adobe PDF reader on the computer. Press the [CTRL]+[P] buttons to print. Click on the [PROPERTIES] tab to set the laser cutter's options. Here you will need to determine the settings that correspond with the material you have placed in the laser cutter.
    Step 5
    First select either [RASTER], [VECTOR] or [COMBINED] from the selection in the top center. Raster means engraving only, vector means cutting only, combined means both. The next step is to set the piece size. Here you fill in the size of the material you placed in the lasercutter. Make sure you have [AUTO FOCUS] and [VECTOR SORTING] selected. The last step is to set the raster and vector settings in the right side of the dialog window. Below you will see a table with the settings for the materials we use in the FabLab. When this is done you can press the [OK] button to get back to the main dialog print window. In this window make sure you have the right printer selected and not fill but actual print size selected. When ready hit the print button. The job will be waiting in the lasercutter. Switch on the ventilation and the air pressurizer. And then hit the green [GO] button on the machine.


    A.4 First tests

    Before I started fabricating, i chose two materials (acrylic 3mm and mdf 3mm) and made tests to find the right notch. the right notch is 2.7 mm wide for acrilic and 2.8 mm wide for mdf . I chananged the "material" thickness in the right slider, and continued fabricating. i created some different connectors, and selected the one i liked.



    The major issues I found are related to my choice of using press-fit joint with an horizontal design for acrylic

    A.5 Acrylic for the mototcycle kit

    Using this design with acrylic has proven to be a challenge. For the acrylic we have used the following parameters on the Epilog Laser Cut - Legend 36EX

    The material is very irregular each test needs to be repeated many times, especially for the configuration of cutting power. In my first tests with acrylic with a speed:14%, power:100% and frecuency:5000 Hz, the cutting of material was not complete, I changed the speed:9%, power:100% and frecuency:5000 Hz, this setting was better.


    The complete motorcycle, not totally cut

    A.6 Results achieved and lessons learnt

    I am satisfied with the overall results of the tool. The following picture shows the final result.



    Motorcycle assembled

I felt my design would be better suited for other kind of lasercut material such as mdf.

I followed the same process I used for mdf. Having a bit more experience I immediately started with several tests for the laser power and speed, creating a small test board with parameters in it, till I found the right settings.

Finally I was able to assemble the motorcycle in mdf using the new pieces. The fit was much more solid than the equivalent made in acrylic.


Also I experienced whith raster process in acrylic, as shown in figure following:

A.7 Conclusion

I liked the simplicity and immediate results Offering laser cutter available in the Tecsup Fab Lab. The use of the machine is very simple, the key element is the setting should be done before the final cut It is highly recommended to start working with the test material before the preparation of the design, you can save time if work is done in this order . Differences observed Between work in acrylic and mdf Were:
1. mdf absorbs moisture because it is made ​​from wood, this material is hygroscopic, in press fit joints can be skipped when the material swells.
2. Both the acrylic as MDF show variability in the thickness of the plates and even within a single plate variability also exists.
3. The model turned me mdf more stable than the model in acrylic.
4. For constructions using MDF press fit could use a lower tolerance because this material is flexible and allows a better fits, however with acrylic this is not possible as this material is broken.

B. Second Part: Making vynil cut stancils

B.1 Desing and cut of the stancils

During the week l used the laser cutter, but luckily found some time also to explore the possibilities given by the vynil cutter. I managed to find a small logo of my favourite football team in Peru: Alianza Lima and created some stickers for my library.The stickers where created starting from JPG files, but they were converted into PNG files for processing, using PNG files with the fab modules.

First sticker with small typeface
First sticker with small typeface

I initially tried to add to the sticker some text in a quite small typeface. This made the whole process really difficult, because letters were too small to stick well and the fill of some letters required some patience to be separated by the transfer tape. I eventually managed to transfer it on a piece of paper , but had to relocate some smaller elements practically by hand.

As a second try and using bigger letters, I cut again a similar logo, this time the process was very smooth, and I easily managed to transfer the logo on a piece of paper with the transfer material, as shown in the picture.

First sticker with small typeface
First sticker with small typeface

Downloading link for my archives:
Archives

B.2 Conclusion

I really liked the simplicity and immediate results offered by the large vynil cutter available in the Tecsup Fab Lab, much more advanced than the small A4 cutter I had used previously. The ability to cut in different parts of the material and to exactly know the area available for cutting allowed me to reuse the same vynil for several tests. Also the possibility to use the transfer sheet for moving the stickers from one surface to another is a great improvement from my previous workflow.

C. Third Part: Additional Research

I researched about other technologies also available for cutting

I show a list of references about other technologies:

  1. Waterjet: A water jet cutter, also known as a water jet or waterjet, is an industrial tool capable of cutting a wide variety of materials using a very high-pressure jet of water, or a mixture of water and an abrasive substance. The term abrasivejet refers specifically to the use of a mixture of water and abrasive to cut hard materials such as metal or granite, while the terms pure waterjet and water-only cutting refer to waterjet cutting without the use of added abrasives, often used for softer materials such as wood or rubber.Waterjet cutting is often used during fabrication of machine parts. It is the preferred method when the materials being cut are sensitive to the high temperatures generated by other methods. Waterjet cutting is used in various industries, including mining and aerospace, for cutting, shaping, and reaming.

  2. Wire EDM: Electric discharge machining (EDM), sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking, wire burning or wire erosion, is a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks). Material is removed from the workpiece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool-electrode, or simply the "tool" or "electrode", while the other is called the workpiece-electrode, or "workpiece".When the distance between the two electrodes is reduced, the intensity of the electric field in the volume between the electrodes becomes greater than the strength of the dielectric (at least in some point(s)), which breaks, allowing current to flow between the two electrodes. This phenomenon is the same as the breakdown of a capacitor (condenser) (see also breakdown voltage). As a result, material is removed from both electrodes. Once the current stops (or is stopped, depending on the type of generator), new liquid dielectric is usually conveyed into the inter-electrode volume, enabling the solid particles (debris) to be carried away and the insulating properties of the dielectric to be restored. Adding new liquid dielectric in the inter-electrode volume is commonly referred to as "flushing". Also, after a current flow, the difference of potential between the electrodes is restored to what it was before the breakdown, so that a new liquid dielectric breakdown can occur.

  3. Hot Wire: A hot-wire foam cutter is a tool used to cut polystyrene foam and similar materials. The device consists of a thin, taut metal wire, often made of nichrome or stainless steel, or a thicker wire preformed into a desired shape, which is heated via electrical resistance to approximately 200°C. As the wire is passed through the material to be cut, the heat from the wire vaporises the material just in advance of contact.The depth of the cut is limited only by the wire length. Width of cut is limited by throat, if any.