15 - wildcard week

introduction

Goal of this week is to design and produce something with a digital process not covered in another assignment, documenting the requirements that your assignment meets, and including everything necessary to reproduce it. Possibilities include but are not limited to wildcard week examples.

For my final project I have to make a cover plate. I have two ideas I would like to try. First I would like to create a nice flat plate for the buttons and sockets (the modular-faceplate). Since I have to know where the buttons and sockets (and furby-face) need to go, I also have to design the final PCB. Next up, I would like to experiment with a furry cover that I could use to give the furby eye mechanism an even more furby-look.

faceplate design

material

I have seen some very nice and colorful pla-scrap plates lately, so I thought I might give it a go. Earlier experiments using a kitchen blender to grind PLA scraps resulted in a desasterous amount of noise, and a kitchen blender gradually blending its internal plastic guts by fast rochetting bits of PLA. Instead I chose to go for manual labour instead and used cutters and hammer to grind bits of left-over PLA and misprints for this purpose.

The T-shirt press’ temperature setting can go up to 300 deg- PLA extrusion temperature is typically around 195-220 deg., so part of the experiment will be checking the best temperature setting, melting time etc.

I did a total of three presses using different temperature settings. For every press I used the same pressure, a similar pressing time (1-2minutes) and the same type of baking paper. The following picture shows the results at 195, 230 and 275 degrees:

Overall the highest temperature results in the best ‘flow’, but at some point the paper starts interacting with the surface. Although the effect results in a nice structure, for smoother results the temperature needs to be lower. Overall the best results were seen in mid-range (so 230 degrees)

faceplate design

The schematic for my final project’s electronics is a combination of the previous weeks. I use the input stage as designed in week 11, the output circuits of week 09 (the I2S DAC, I use standard 3-wire sockets for RC servos, as well as a 6-pin socket for eventual use of the external servo controller). Still using the Pico as carrier for the RP2040. The schematic drawing of the whole system can be found here as pdf. In KiCAD pcb I try to keep the width of the PCB within the rack module unit sizes (in multiples of 5.08 mm) while at the same time allowing placement of the furby face module. The height of the PCB cannot exceed 100 mm in order to be millable (eventually) and allow for mounting space in a rack. The full design can be seen below:

The kicad project (final PCB) can be found here:

• furbysynth.kicad_pcb
• furbysynth.kicad_pro
• furbysynth.kicad_sch

In KiCAD pcb I add sizes in one of the user-drawing layers using the measure tool. Subsequently I use these sizes in the OpenSCAD design file to specify locations for potentiometer holes, the display, encoder, jacks (and eventually the furby eye mechanism as well).

I use the projection function to create a 2D SVG file for cutting and import this directly in Lightburn (first quikly in inkscape for checking, but eventually no processing is necessary)

below the OpenSCAD file containing all sizes from KiCAD PCB:

module plate_cut() {
  projection(cut = false) {
    difference() {
      modularplate(11, 3); // 11 units ~= 70mm, 3 mm thickness
      // furby eye mechanism
      translate([17.5 + 5.08 / 2, 100, -0.01])cylinder(d = 32, h = 10);
      translate([17.5 + 5.08 / 2 + pupillary, 100, -0.01])cylinder(d = 32, h = 10);
      translate([17.5 + 5.08 / 2 + pupillary / 2, 100 - 22, -0.01])cylinder(d = 30, h = 10);

      // jack sockets
      translate([10.52, 22.098, -0.01])cylinder(d = 8, h = 4);
      translate([10.52 + 16.29, 22.098, -0.01])cylinder(d = 8, h = 4);
      translate([10.52 + 16.29 + 16.256, 22.098, -0.01])cylinder(d = 8, h = 4);
      translate([10.52 + 16.29 + 16.256 + 16.256, 22.098, -0.01])cylinder(d = 8, h = 4);

      // potentiomters
      translate([10.3321, 38.6004, -0.01])cylinder(d = 7, h = 4);
      translate([10.3321 + 16.256, 38.6004, -0.01])cylinder(d = 7, h = 4);
      translate([10.3321 + 16.256 + 16.256, 38.6004, -0.01])cylinder(d = 7, h = 4);
      // encoder
      translate([10.414, 53.0981, -0.01])cylinder(d = 7, h = 4);
      // display
      translate([26.9240, 51.4604, -0.01])cube([ 21.4884, 5.689, 4]);
    }
  }
}

Before cutting the file on laser (using the nicely coloured PLA mix plates) I do a final check for component location and allignment by doing a simple paper print job:

• furby-final.scad - OpenSCAD script (full design file)
• furby-final.svg - svg drawing for cutting the faceplate
• furby.lbrn2 - the lightburn file

First I try some test squares using the Genmitsu L8 at different speeds. I start with a speed of 500, 100% power (a fairly standard setting for cutting through 3 mm wood, acrylic etc). The PLA melts very easily so the challenge is to find settings that do not put too much heat into the material but ‘burn’ it away instead. I end up doing quick multi-pass settings (speed 1500, 4 x multi-pass) - sadly I don’t find the option (yet) to do the entire file in one go (to let the cut cool down) so still a lot of heat is put into the material

In the end I chose to cut the file with the best settings mirrored, so I can use a fretsaw and drill-press for cutting away the last bits, leaving the edges at the front of the panel sharp (and the backside more molten)

I also use Lightburn’s kerf-offset to deal with the 0.5mm kerf caused by molten plastic. I divide the drawing into two groups so I can set an inside- and outside kerf for the inside shapes and the outer contour

The following settings have been used:

vacuum formed fur-casing

A normal 1998 style furby has an exoskeleton covered with a sewn fur ‘coat’. Since the shape of the traditional furby is nowhere near a rectangular modular unit, I decide to design a 3D printed mold and use the vacuum former. I tried this before, for instance during machine week to make a transparent pigeon half. We never manage to mount it properly, so eventually this idea was abandoned.

I also tried makeing PLA molds for the vaccum former before, but always ended up melting the mold as well. In previous attempts I did not change the shell thickness and infil densitiy, so they would be good parameters to change for this experiment.

For the mold I used the following OpenSCAD function:

hull(){
    cube([70,60,3]);
    translate([17.5+5.08/2+pupillary,40,-0.01])cylinder(d=30,h=10);
    translate([17.5+5.08/2,40,-0.01])cylinder(d=30,h=10);
    translate([17.5+5.08/2+pupillary/2,40-22,-0.01])cylinder(d=27,h=17);
}

leading to the following *.stl file for print:

• furby-final-vacuum.stl

Next up I slice the mold in Cura, using 50% infil and 2mm shell thickness

I use a small Mayku FormBox vacuum former with some stock white PVC foil. The settings for the foil are 5 for the heating (so allmost maximum) and a 2:20 min heating time.

First the mayku will heat up (takes more than 15 minutes) so it is best to only put in the foil after the heating is done, otherwise it will start warping already. When the heating is done (light is green) the foil can be put in the double frame, lifted to the top (close to the heater) and the timer can be started (timer button at bottom-left corner)

The timer will sound a metronome which ticks quicker towards the end of the heating period. You can also check that the foil is hanging like a parabola (20mm dip). As soon as you put in the shape you can pull the frame and foil down. A switch will automatically turn on the vacuum (and also turn it off after a few seconds)

The vacuum formed shape is cut out and tested for size:

After releasing the mold from the vacuum formed plastic (fortunately very easy thanks to the simple shape) I cutout the shape, cut holes for eyes and beak and use BISON TIX (flexible glue) to glue red faux fur fabric over the shape:

Note that the fur has not been cut yet around the eyes and beak, this will be done as soon as the 3D print of the face parts is ready, to make sure the fabric can either wrap around the edges or has to be cut away completely.

The mold held out pretty well, at the cost of being slightly warped and using more material and printing time than usual.

learning outcomes

• Demonstrate workflows used in the chosen process
• Select and apply suitable processes (and materials) to do your assignment.

evaluation checklist

• Documented the workflow(s) and process(es) you used
• Described problems encountered (if any) and how you fixed them
• Included original design files and source code
• Included ‘hero shot’ of the result

lessons learned, tips and tricks

(or, the most insightful mistakes I made)

• PLA is very difficult to lasercut. I have not found the optimal settings yet (but also have not done a great deal of reading up on the topic. There might be different sources or insights out there)
• Stronger molds hold out better in the vacuum former, however, shell-thickness also increases the risk of warping. Another trade-off to consider and tune with

left for todo

• More final project stuff: finalise the 3D printed face, cut the cover, assemble the PCB, write software…
• try cutting PLA on a different laser (not solid state)
• try the dremel-friction welding trick Leo showed (!)

reflection

I was surprised by how easy it was to make a nice (roughtly 3mm) thick PLA plate using the heat press. I did not need to experiment that much with settings (time, temp) - the initial guesses proved quite ok. Also the amount you need for a plate was purely based on an educated guess. It was also surprising that for molding a plate of PLA out of scraps, you actually don’t need so much granularity. Just breaking apart the bigger scraps and mixing with smaller bits, especially left-over filament, worked fine too.

In order to determine the sizes for all mounting holes for the PCB I needed the actual PCB design. In order to fit that time-wise I had to do a very quick KiCAD design job. I started with two versions, one with all components on top and one with all SMD components at the bottom (so I could make a single sided mix- board of thruhole and SMD components). In the end I did not have time enough to re-design the board to be manufacturable on the mill, so I went for a board house (AISLER) instead. This week I hope to get the PCBs and check that I didn’t make too much irrepairable mistakes….

For PLA molds in the vacuum former, I think the approach of using more shell thickness and infill paid off, at the expense of material use and warping. However, the mold does not seem to deform as much as earlier experiments did.

copyrights and references