Making a Shoe Anvil requested a larger anvil for driving nails into shoe soles. I had some 6x6x1/4″ (presumably) A36 lying around so I gave it a go in multiple parts. To get the outline I took a lot of profile measurements by hand from an existing anvil and scaled it up.

For workholding, I bolted the stock plate onto a sacrificial plate, drilled the stock holes and tapped them, bolted down the indiviual parts to cut out on the stock plate, and then milled them out down to the stock plate. When I had all the pieces, I bolted the post hole in alignment with the sole using an adjustable parallels and a 1-2-3 block, bolted the toe piece on, and clamped it in the vise by the post hole so I could contour the bottom. To help stiffen the overhang, I used a machinist jack, but cutting on the overhang was still noticably louder.

Decorations and 3D Printing Thoughts Part 1

I get the impression that things are much less decorated than they used to be. This has been explained to me as because of the increased cost of labor, which has some issues but to me wins out over other explanations. Some other explanations I’ve heard are that people are overstimulated in life and don’t buy decorative stuff so they have a place to recover, that plainness and lack of decoration is the new class signalling, and that a different civilization is responsible for all the nice stuff we see historically but don’t see built any more. These may later explanations may be more correct but they are unverifyable, while the first points to a specific effect that is definitely true. An issue with the first explanation is that the ultimate cost of e.g. nice decals, impressions onto stamped metal, has also been made lower due to technology, but maybe less so than the thing itself. Regardless, I appreciate nice looking things and soulful things and want to live in a world with more of them.

Costs can be split into manufacturing costs and design costs. Something neat to me about additive manufacturing is that for a lot of methods, decorations are free, or atleast very low cost. The decorations can also be different for each object produced. For example, Desktop Metal has a binderjet printer that prints wood (sawdust+binder) and dyes the material to resemble different types of wood (Forust). Because it’s a binderjet printer, small model changes for e.g. fake engravings don’t affect print time or cost, and because the material is already volumetrially dyed having custom colorations and patterns and images should also come free. This is the best example, but all binderjet printers and resin LCD printers should have basically free 3D decorations, and many 3D printer methods that allow color have the same print time cost regardless of the color used. Of course, all these methods rely on 3D printing actually being a competitive way to design a mass produced product, but in my impression that’s already what a lot of 3D printing companies are betting on anyway. There are also many cases where literally the same object is sold from different places with several fold differences in cost so I think it’s very possible that most people would be willing to pay a small premium for custom items.

This leaves design costs. While I think there’s some human intervention required, there’s a lot of neat generative deep learning work that could be used to take the load off of human artists. For example, given a model of an object to print, a human probably needs to label visible surfaces vs. critical surfaces (surfaces which must remain unchanged for function). At the lowest end of engineering difficulty, aesthetic patterns or simple random textures/tilings can be applied. There are manufacturing methods which produce results like this already, where every customer gets a slightly different object in a nice way. At higher levels of engineering difficulty, generative deep learning models could produce stylized and themed decorations according to the desires of the consumer. I’m still thinking this side of everything through, so I’m collecting examples of technologies and art. While I have some experience making and training generator deep learning models, it’s not something where I’ve ever been happy with the result.

Silicone Stamps

I was a bit interested in making a personal stamp (like for sealing letters), so I threw together a design in Blender, printed it, and cast it in silicone. Just a note, I was incredibly impressed with Blender’s sculpt mode-it felt like I was playing with actual clay, and no matter what I did the result was always a manifold mesh. I have plenty of experience where programs can’t even maintain manifoldness after boolean operations on relatively simple shapes.

For the stamp itself, I think I used smooth-on’s ecoflex gel (don’t remember now). Silicones are nice because they are generally non-stick, so no extra mold-release is needed. When cast, they often can transfer details finer than the eye can see and this level of detail is used for some types of lithography.

As 405 nm floodlamps and SLA resin were more common where I did this than waxes, I used resin first. I was pretty happy with the results.

I also tried beeswax (not show), shellac and polycaprolactone, but wasn’t as happy with either of those.

Traditionally, sealing was was made from shellac and beeswax with pigments. Modern sealing wax is literally hot glue. As I didn’t get anything melted to work well I wonder if it was because this used of a silicone stamp instead of a metal one.

I was thinking this would be neat to turn into a small self-contained device, with stamp material (resin or wax), stamp and activator (405nm light or heater). Maybe wedding planners would be interested in it.

Fauna and Flora and Fungi

Inductively Coupled Plasmas

After a bit of fuddling I’ve gotten some inductively coupled plasmas started. These pictures are each around 1E-1 torr. The two coils are 0.5uF and 5uF respectively. They are driven with a signal generator (@13.56 or 27.12MHz)->100W shortwave amplifier->MFJ 941B tuner->BNC aligator clips. The plasmas were started with ~500V 20kHz AC on different feedthroughs.

Tuning was difficult but was made easier by monitoring radiated power with a SDR. It helped to add a 220pF capacitor in series to move the impedance closer to something that a tuner would normally be used for. A 275pF capacitor would have yielded a resonant circuit at 13.56MHz and 5uF but the real part of the impedance would still have been mismatched. Using the balun tuner output failed, presumably because the 1:9 turns ratio moved the impedance further out of range. I also tried using a homewrapped ~10:1 inductor around a ferrite core (because the real impedance here is tiny compared to 50 ohms) and got some less impressive plasmas.

At higher pressures (not shown) the plasma is much more concentrated around the coil and dimmer.
Antenna analyzer fed through the tuner when tuned to achieve a glow.

Quick Schlieren Photography

I’m basically using the setup from here, with a used 700mm focal length telescope mirror. It’s a very clumsy setup and I’m very happy I got any results. Seeing it with your own eyes is kind of magical.

In this setup, light from a point source (LED covered in tin foil with a pinhole) is brought to a image half on a razor blade and the images are taken from behind the razor blade. Something that causes changes in air index of refraction (through heat from candles here) is in the light path. If there were no index of refraction changes in the air, one would see a uniform field of half intensity, but because the thing to be imaged changes the air’s index of refraction, you can see its effects.

Setup with LED, razor blade and mirror.
Phone camera, this is close to the appearance just looking with your eyes
DLSR camera

Biofeedback Smartwatch Update

I’ve been busy recently, with finishing my doctorate and a month long trip. However, I now have a fully wireless biofeedback smartwatch prototype.

The picture is not fully assembled, but everything works other than the lack of a power switch, usb plug, and IC for charging the Li+ battery on the back. I’ve switched from using a naked nRF52810 IC to a TAIYO YUDEN module, it is more expensive, but I was having a lot of trouble getting antennas to work. The wires to the right are for programming with SWD, and the pair of magnet wires soldered to the board itself are for power so I don’t rundown the battery in testing. To the right of the module are two holes for debugging with UART; it’s been really helpful to use that and the logic analyzer to test things during development.

On the finger strap are two electrodes for the GSR voltage bridge, and the MAX30102 SPO2 sensor, which is soldered onto a PCB out of view.

I’m using the following program to activate the ADC and SPO2 sensor and broadcast data over bluetooth. I’m using the nordic UART profile, which contains two characteristics, a read and a write one. I’ve had to learn a lot more about bluetooth than I wanted to for this project.

You can see in the above picture I can stream to my laptop, which live plots the data with matplotlib. I’ve had issues with plot update speeds… it sounds like pyqtgraph is a lot faster but not what I want. I’m probably going to try gnuplot if this is still an issue.

On the plotting side, it’s the same as usual, except with the source of data being:

subprocess.Popen("sudo gatttool -i hci0 -t random -b (nrf mac address) --char-write-req -a 0x10 -n 0100 --listen",shell=True,stdout=subprocess.PIPE)

Which writes 1 (0100 reverse endian) to the handle of the characteristic to enable notifications (0x10). Again, there’s a better way to do this with the actual gatt libraries.

In the end I’m pretty happy with everyting. The GSR data is a lot better since switching to a TI adc (ads1246), and the setup is not that encumbering overall. I plan on working on more data analysis for biofeedback (e.g. getting actual SPO2 values, heart rate counters,…) before doing anything with a phone app.

Biofeedback Smartwatch

<Informal because I have no energy, also partially for my own reference later. I’ve been busy when not blogging.>

The PCBs I designed for the Bluetooth biofeedback thing arrived a few days ago. From Oshpark, these were ~10usd total, with free shipping and took ~2 weeks. I’ve never done any QFN soldering (the leads are entirely under the IC) and I didn’t get a solderpaste stencil. Big mistake. Spent a lot of painstaking time with an Exacto knife, sewing needle and tweezers. Also tried using a hot air station for the first time, but a pan on the stove ended up working better.

Small one is meant to be worn on a strap on the finger, the large one is meant to be worn on a strap on the wrist.
SWD pins connected, and it can flash!!!!!!!

Anyway, I got 2 assembled and one of them works with my st-link and openocd. Unfortunately I lost all of my rf inductors during assembly (also the only parts I lost), so I can’t test the bluetooth. I’m using the nRF52810, have downloaded the SDK kit from, and can get the examples to compile with the compiler from I’m mentioning where I got the compiler because I spent a few hours trying to get the gentoo package crossdev working before just downloading binaries.

openocd -f ~/openocd/tcl/interface/stlink.cfg -f ~/openocd/tcl/target/nrf52.cfg -c “program nrf52810_xxaa.bin verify reset exit”

Because it will take time anyway for new inductors to arrive, I did a redesign and ordered more boards. I hope to actually be able to wear the next generation around. I’ll put in some filler posts on computing before then.

For those who stuck with it

Sorting LaTeX references

I don’t use .bib files and just populate bibliographies like:


  F.~Romanelli, Phys. Fluids B \textbf{1}, 1018 (1989).

%.... (other citations)

One issue is that this can be out of order compared to the order of /cite{} in the text. The following python script goes through the citations in the text and prints the appropriately ordered bibliography. Replacing the bibliography must be done by hand, but it’s safer that way.



for line in f.readlines():
        for e in B:
          if e not in cites:

for line in f.readlines():

for i in cites:
  for j in refs:
      print j[0][:-1]
      print j[1]


Very much in the spirit of the 90-90 rule, this took much longer than I assumed.

So my goal is to put the galvinic skin response, hand temperature and SPO2 sensors onto a ring connected to a bluetooth ‘watch’, and have some phone app which can stream the data. To play around with a microcontroller which has bluetooth, I got the STEVAL-IDB008V2 dev board, which has a BlueNRG2 chip. Because I’ve only played with the Nucleos so far, I was overoptimistic about linux compatibility. On the plus side, my setup for development has gotten much more comfortable-I’m back to using emacs and makefiles, and I’ve learned how to use st-util to flash and debug and I’ve also gotten my first experience using logic analyzers with sigrok and pulseview.

The issue which held me up was actually getting the programs onto the chip. While the dev board has a JTAG header, the default application disables those pins so it cannot be programmed in that way. The alternative is to flash it with the UART bootloader, which requires the chip be booted into a specific mode and a different pair of pins.This board has a USB, but unlike the nucleos, it is not connected to a ST-Link. All of the software for flashing the chip was windows-only, and I had a lot of trouble getting that working.

Anyway, after much toil I managed to work out a way to reliably flash the chip using the windows software on a VM, but at this point I had already mostly written a bootloader using a nucleo as an intermediary. A warning on the very slim odds a reader will be stuck on the bootloader like I was. As far as I can tell, the bootloader will respond with a positive acknowledge even to invalid addresses, only to fail when receiving the data to flash. To work the last bugs out of the bootloader I was working on, I compared the output of the logic analyzer for each of them flashing the chip with the same data. This revealed that the address bytes I was using were off allowing me to fix the program.

~10usd logic analyzer that’s already worth it
Address bytes and acknowledge in Pulseview
Full writing in pulseview. First is the erase command, followed by blocks of 256 bytes.