Margin of Safety 4 upgrades

In the short period of time between Dragoncon and Sparkfun AVC 2017 I had a few upgrades in mind for Margin of Safety. First was a horizontal drum attachment with feeder wedges.

What is this nonsense.

The idea was to have the same wedge defeating power as a vertical with narrow wedges, except with a horizontal weapon behind it. Hitting the face of the wedge would impart a strong sideways force, which is unusual.

The hub motor is a new design, able to be used for both the horizontal drum and a new vertical drum, fixing weaknesses with the cantilevered motor on the previous design.

The new vertical drum. Wider and less vulnerable.

The new design requires CNC turning, which is outside of the tools available to me. So I sent it out for fabrication by professionals.

Cross section of the drum.

The drum is two main pieces, the hub motor, made of magnets and aluminum, and the hardened drum itself. It uses a taper fit to allow the drum to span the full width of the motor. With this new design the drum is fully supported on two bearings, the motor is fully protected and not cantilevered, and the wider drum allows for less accurate driving.

The drum without retaining screws.

Horizontal drum with wedges attached.

Horizontal drum without wedges.

I also experimented with a new color scheme for this robot: meat colored. I bought a stack of adhesive printer paper and cured meats and went to town at the scanner.

Plastic wrap on the scanner bed for easy cleanup.

Prosciutto and pancetta.

What have I done.

Sparkfun AVC showed the importance of keeping the 3D prints under compression. At every place where I broke that rule, there was cracking. First was the vertical attachment.

Cracks everywhere.

It split starting from every hole right along the layers. Second were the wheelguards. The rear end of the guards were kept under compression; those sides did fine. The front end was anchored to the front interface with plastite screws. Those came apart frequently, and I ran out of spares.

Boo cracks.

That failure also ripped one of the wheelguard screws out of the front interface.

Womp womp.

Luckily the robot held itself together to win Sparkfun AVC 2017 with a record of 6-1, and was the last robot standing in the rumble. I lost one match due to experimenting with horizontal drum. Due to it's length and narrow stance it requires exceptional accuracy to wedge under an opponent. The sides of the wedges are easy to get under and vulnerable to impact, both from the opponent and from flying across the arena. It is likely still useful against midcutters, though I have not tested that.

All in all, I feel this is an excellent design once the wheelguard issue is fixed, while the flexibility of different weapon modules allows focusing on the fun parts of design while maintaining a tested reliable drivetrain.

Part explosion.

Painted 3D Prints

Some nonsense I've 3D printed and painted. All prints are done at 0.1 mm in ABS on a Wanhao Duplicator 6, primed with spray primer, sanded and filled with wood filler, and primed again.

Rabbit with Arms

This was a birthday gift for my mom. Painted with acrylics and matte coated.

Final.

From behind.

Progress shot 1.

Progress shot 2.

Progress shot 3.

Progress shot 4.

Rise up With Fists!!

Miniotaurus

I made this for a friend. I don't know why. Painted with acrylics and a few black dots with permanent marker.

Blending overalls with horse is weird.

oh god why

lifeless

I guess you could ride it like a horse.

What future is it looking towards.

Beefy Venus de Milo

I like the idea that Venus originally had really swole arms. After final priming this was painted with a mixture of epoxy and bronze powder. The idea was to polish and age it. Polishing didn't work so well, it tended to wear through instead. But it still looks alright.

Metal looking.

Fresh from the printer.

Oddish

It's one of my favorite Pokemon.

Next to an oddish cross stitch.

The larger model on the right was used for painting.

Detail on body.

Detail on leaves.

A Guide to Better Dithering on Laser Engravers

Introduction

When laser engraving images that are not binary black and white, you usually get a couple options. The first is to use relief engraving, which uses the grayscale version of the image to control the power of the laser. This creates nifty 3D contours of your image, but often the difference in "colors" is small. The image only becomes visible when looking at it at an angle, if at all. The second option is to use the built in dithering. By using a pattern of dark or light dots, this ensures gives better control of how dark colors appear. However, the built in dithering is, in my opinion, pretty dumb. With the power of GIMP and a bunch of test engraves, we can do better.

From source image to engraving! Source image by jtobijah.
The dots are slightly different because I lost the original processed image, and forgot the settings.

Improving on vanilla dithering

The major improvements I worked out are:

• Using the actual colors of the engraved material when calculating dithering.
• Using a low DPI image, so the laser makes multiple passes per image pixel.

Each additional color added into the dithering algorithm vastly improves the range of shades represented by dithering and the quality of each shade. Since my method allows different "colors" to be etched at different speeds and powers, more colors can be squeezed out the material.

The Tutorial

Step 1

First, collect the tools. I used GIMP for all my image processing. I'm sure you can find the equivalent commands in your program of choice.

Step 2

Next is to figure out what colors you can etch onto your material. The more distinct colors you can make, the better, giving the dithering more options. A good way I find is to etch a bunch of these gradients at various speeds and work from there.

Test gradient!

For wood, etching makes things darker, but for acrylic, etching makes things lighter. Make a bunch of test cuts at different speeds and powers to see what colors you can get. I find slow cuts at low power create good darks on wood without etching too deep.

There is a problem, when you etch very dark colors the surrounding areas are scorched. While sanding off the scorches works, there are a couple exceptions. When you have standalone unetched pixels surrounded by light pixels, sanding tends to break them off. Or, if you're working with the cheapest craft plywood in existence you found in the scrap bin, any kind of sanding will quickly eat through the very thin veneer and reveal the glue below. What I did was use an low power at the highest speed to blast off a extremely thin layer off the top without any significant darkening.

Make sure you write down what laser settings you used for each power. As learned from experience, don't trust yourself to remember it.

Once the colors have been determined fire up GIMP to enter the colors into a custom palette. To do this first open up the palette window by clicking Windows -> Dockable Dialogs -> Palettes.

Palette window on the left, new palette on the right, with dark etch, light etch, and unetched material color.

Do not forget the color of the unetched or "laser sanded" material as well. Try to match the colors as close as possible.

Step 3

The classic test image.

Open up your image of choice. First we lower the source image's dots/pixels per inch (hereby referred to as DPI) to match that of the laser. I find 100 DPI is the highest resolution I can go before single pixels start failing to form. Make sure your image still fits on your material after changing the DPI. In GIMP you can do this through Image -> Scale Image.

It also helps to change image size to inches/mm to get a good idea of its real size at this low DPI.

Then comes the magic. Change the color mode from RGB or whatever it started as to indexed. Do this by Image -> Mode -> Indexed. The dialog below will come up. Choose the palette you created in Step 2, and pick a good dithering mode. The Floyd-Steinberg flavors creates dithering without (intentional) patterns while Positioned does. This converts your image to use only the colors in your palette, which in turn are all the colors your laser can recreate. If the output is too much of one shade, try tweaking the brightness and contrast before converting your image to indexed mode.

Converting to indexed. If you get big blotches of color, you either forgot to turn on dithering or actually have big blotches of color in your source image.

Since the material colors are input into GIMP, this is what the engraved material will actually look like.

When the image looks good, the next step is to convert it into an image with colors the laser software can use to define different layers (like solid red, green, blue, etc). To do this first convert the image back to RGB by Image -> Mode -> RGB. Then use the select by color tool to select a color, making sure the threshold is zero and no feathering or antialiasing nonsense is turned on, and set it to the laser software friendly color by using the fill tool set to fill the entire selection. It will look a litte funny.

Don't worry, the laser will know what's going on.

Finally increase the resolution to some multiple of what it was (here it was 100 DPI), so the laser makes multiple passes per pixel. I used 400. So, through the same image scaling dialog as before, first change the pixels per inch to 400, then change the image size to the same physical dimensions as before. This means multiplying its width and height by 4. Make sure you set interpolation to none or all your colors are going to be messed up.

Step 4

Unfortunately, I have no pictures of the laser setup because I don't have a laser anymore. I just have to believe in you, the reader, to get it right, eventually.

Set your laser cutting speeds. Based on the colors you used in your source image, set the laser speeds and powers corresponding to that color. So if you set the dark engrave to red in your image, set red to make the dark engrave in your laser program. Import the image into your laser cutter software and do whatever tweaks you need (adding a cut line, etc). Print your image, making sure the DPI setting on the laser is the same as your source image (here I used 400). Enjoy the wait.

Gallery

It even works on pictures. Source image by CopyX.
The dots are slightly different because I lost the original processed image, and forgot the settings. 

Paper Mario sprite.

Another Paper Mario sprite.

Marceline from Adventure Time. The chipping at the bottom can happen when you try to sand cheap craft plywood.

Somewhat irrelevant, but you can also laser engrave mashed potatoes.