The new place

After 2 years of bullshit from various 'authorities' and departments, each taking their thousands of dollars in fees, we've begun digging at the new site. 1400 sqft of work space, here I come!

Brush knife

The GF and I have recently been doing some off-trail hiking. Though we've found we can get up or down slopes of 70deg and more, we get seriously fouled and stopped by thorny plans. Now, it's not my goal to ever be hacking things down in the forest, but it's just as stupid to get lost/hurt/dead because you've lead yourself into a head-high thicket of 0.5-1" thorns you just can't seem to escape. So I figured it was time to own a real knife.

In terms of requirements, I wanted it to be thick and durable enough to also double as an ice pick for climbing steep snow. I also wanted it to be stainless steel since I don't take good care of things. I wanted a hand guard, since I seem to always get thorns in my knuckles the other times I've used even a 2' machete. Finally, I want it to be small enough that I can draw it easily. (If you've ever owned a 2'+ sword and tried to draw one of those things from a back scabbard or tried to hike with one on a hip scabbard you'll see what I mean.)

Building the knife itself was pretty straightforward. I took a 1" x 3/8" flat 316 steel stock and cut it to have a 13" blade with enough room for a handle. I then took it to the bench grinder for maybe an hour to get a blade onto it. When I put my hand onto the handle and marked where my hand would be. Unlike a regular knife or sword, this one is only for me and thus it can fit my hand *perfectly*. I welded little chunks of stainless steel supporting my hand from below, above and on specifically for my thumb to hold onto. I then finished the blade with a metal file till it was sharp-ish. I'll eventually fire and quench it, but I'll have to find someone with a furnace first.

Building the scabbard was more challenging. In the end, I went with a series of 3D-printed sections all bolted onto a steel band I'd cut holes into. I'll eventually replace the bolts with rivets when my friend returns my rivet gun.

3D printing for me is still more of process than a push-button affair. At this point, makerbot stopped selling the 3mm wire mine uses so I'm buying shittier 3rd party stuff now. The machine itself also needs to be manually warmed in the control panel before printing. But it still does the job. In this case I made the sections 40mm tall with two mounting holes. Each section has knobs on the inside that hold the blade on either side and don't let the edge touch the scabbard.

Spring gun prototype is done!

The spring gun is fully built. But it functions at ~12% efficiency, which is lower than I hoped for. I suspect this is because of losses in friction between the spring and the buffer tube that it's around. I tried oiling it but this produced no better results.

The pictures here are it shooting through my cronometer. As part of this, I learned that the crono requires it's backdrop/shield to be back-lit and expects this to be from the sun. Indoor it doesn't work with neon lights because the crono can see the 60Hz of those things. It turns out finding incandescent lights needed for the crono was harder than expected since they've become banned. So I just bought then from the manufacturer for a million dollars. Not sure where they get them.

In terms of performance we're expecting, we have to find the energy the spring will release: I measured the displacement of the spring with a 10lb (44N) weight and found it had about 2" (0.0508 m), which results in a K (spring constant) of 44/0.0508 = 886. The current setting is for it to release when it gets to about 3.75" (0.09525 m), which should yield 0.5 * 886 * 0.09525^2 = 3.929J

When fired, it launched a 0.5" ball bearing at ~35ft/s (10.67m/s). The ball weighed in at 8.9g (0.0084kg). It then must have had 0.5 * 0.0084 * 10.67^2 = 0.4782J. This means the machine is operating at 12% efficiency.

The next step is to check the efficiency at different release levels. If efficiency goes down as power increases that would be a bad sign. After that, it's time to try other springs.

Spring Gun progress so far

I'm making progress, probably about 2/3 done with the basic prototype. The goal of this project is to make a prototype that I can experiment with, not something I expect to carry around. That means weight is a second priority to reconfigurability.

Because the device I'm making is small and complex, precision of parts is more important than any other metal-working project I've done before. I considered actually 3d printing many parts but concluded that in many cases they would not be strong enough to handle the forces involved.

In practice, my digital calliper and the milling-machine's measures were invaluable. I calculated the size and position of each part/cut/hole at the level of 1/16" and in practice I was able to measure that each was placed within 1/32" with the exception of welding. It's amazing how significantly a piece of metal will warp during welding. I have not yet figured out how to solve this.

As noted, this involved a lot of milling. Among other things, I found that ~300-400 RPM on the mill was a good speed for steel. At lower speeds I think my mill takes too much torque and sometimes vibrates significantly. At higher speeds it needs more cutting oil and seems to produce more metal slivers instead of metal ribbons.

The act of welding it required in many cases placing parts on top of other stand off pieces. In this sense I found it very helpful to have a variety of shims: 1/8" flats, 1/4", 1/2", 1" square bars.

Spring Gun CAD

Springs have gotten strong. There are springs out there that can store 500J of energy and deliver that energy very quickly and efficiently. In fact, there are guns that already use springs and shoot with 22lr speed. Also, airsoft rifles often use springs; while their power is low they can be fully automatic because they are not considered firearms. If you've ever fire a full-auto airsoft, it's extremely fun.

After doing some simulation of springs and air pistons using pypy and multi-processing I found that actually the most power transmission happens when the air piston and the projectile have essentially the same diameter. This implied that probably the best design would be a projectile directly driven from the spring.

As with any simulation result we should ask 'does this make sense?' Well, if the spring pushed the projectile itself and was massless and had no internal heating then all the energy would be put into the projectile. Of course there will be internal heating, but that's also a problem with an air piston. And the spring is not massless, but it seems pretty light compared to the projectiles. So it's not a crazy conclusion to think the air part is not necessary. Assuming I could get the weight of the spring and any associated hardware to a minimum I should get good efficiency. If that turns out not to be the case in real-world testing I'll try testing air pistons with a different design.

In this design, we're using a grabber (red) and pulling back a plunger (blue) in the barrel (green). There will be a spring inside the barrel that pushes back. Once the puller gets to a certain point it releases. In this way, we can turn osculating linear movement to continuously load and fire the gun.

Franken Scooter

I crashed the electric scooter last winter. In the process, the fragile lithium-ion battery flew out and tumbled in the street. I put it back in. Since then it has run for only a few minutes at a time before some part of the scooter turns itself off automatically. Turning off and back on again doesn't seem to fix this. I have only been able to get it back on by pulling and re-adding the fuse that protects the battery from overloading.

I didn't want to spend the $500 for a similar li-ion so I scavenged some sealed lead-acid batteries instead. I needed to run all four of them in series to get the 48V the engine was expecting but I didn't want to charge them in series. Instead I installed a switch network that allowed me to charge them in parallel with a single 12V charger. Since the switches are in the circuit during drive and thus pulling a lot of amps, I doubled them up for each connection.

I used holes already cut in the sides of the scooter to run the wiring and a basic home-depot switch carrier. Of course, not all the switches I managed to find were the same or even fit in the switch carrier. A lot of hot glue keeps them together.

In practice, it doesn't have the acceleration of the lithium and it almost always shows 'battery low' on the console. But it was able to run up a 20deg hill for many blocks on end and still hits at least 20mph.

All in all, I consider it a win. :)

Robot Targets

Fail Train Automation Robotic Target Stand Set (RTS) - V1.0

Summary: The RTS is designed to teach reactive shooting, vigilance in searching for targets, and rapid target assessment for anyone involved in shooting sports or professions. The shoot-though design of the target stands makes the compatible with anything from airsoft to AP 50 BMG.

Detailed Description: Each of the four RTS targets can rotate to show either an enemy side, a friendly side, or the edge of the target (indicating no-shoot). The targets are shifted to the right so it is obvious whether the shot went to the enemy or the friendly. All four targets are driven from a single control box, connected by ethernet cables. The control box has four algorithms for the targets which are selected by a knob on the top of the box. A remote control is then used to turn the targets off or on. The control box has a self-contained battery which can last for several days of continuous use.

• 1 vs. 1 Algorithm (click for video) : The 1:1 algorithm is the most basic, showing a single friendly and a single enemy. After several rounds, the targets face you so you can score your work.
• 3 vs. 1 Algorithm (click for video) : The 3:1 algorithm is the more difficult, showing a single friendly and a three enemy. This is a challenging ratio and timing on purpose; it will help you find and improve the limit of your speed and duration.
• Chaos Algorithm (click for video) : The chaos algorithm shows an unknown number of friendly and enemy with an unknown amount of time between. The lack of predictability of this algorithm will help you break any patterns you might be learning from the 1:1 or 3:1 algorithms.
• Vigilance Algorithm : This algorithm waits an unspecified but long amount of time between events and launches attacks of unspecified strength. Only shooters who are most committed and patient will feel confident in their shooting against this algorithm.

FAQ:

• Q: How do I add/modify algorithms?
• A: The control box is powered by an Arduino micro-controller. Opening the cover and removing the black side panels will expose the Arduino's USB port. You can download and modify the existing programming by going to topchicksdigengineeringguys.blogger.com/codeforthetargetstands

• Q: The remote control seem to only go about 50 feet, how can I shoot at longer ranges?
• A: RTS can be connected to an external switch in addition to the internal remote control. Your RTS came with an additional cable connector that plugs into the front of the control box, right next to the batter charger. Closing the circuit on the two wires of that cable connector will turn the machine on in exactly the same way as the remote control does.

• Q: Workmanship on the RTS seems kind of poor, why is that?
• A: I've designed every piece possible of the RTS to not need exact tolerances. For example, the welds might look bad, but the targets are mostly made out of 1/8" thick, 1/2" wide mild steel and even a messy weld on this is very strong. In the end, this design means I can make them on the deck of my apartment on weekends instead of having to spin up and manage some factory overseas like everyone else does.

• Q: What if I shoot the RTS motors or controller box? They don't seem to be armored.
• A: If shot, they will be destroyed. Don't shoot them. I recommend practicing such that you never let bullets go significantly off-target. For example, shoot only within a small area on a given target and treat shots outside that area as 'misses' even if they hit the paper somewhere else. If you find yourself hitting outside of that area regularly, get closer. If you're already at point-blank range and still at risk of hitting the robotics perhaps it's time to practice on static targets for a while instead.

• Q: Can I plug the targets or the controller box into my computer? They seem to use ethernet cables.
• A: Do not ever plug the ethernet/network-cables from the RTS into a computer; they will likely destroy the computer. RTS does not actually follow the ethernet standards or protocols. Instead, ethernet cables were chosen here because they were cheap and readily available in many lengths. This means that owners like you can buy your own longer or shorter cables depending on your needs.

• Q: What's up with the see-through pipe section that sticks out the side of each target stand?
• A: This is the position sensor. Notice that when the target turns it moves the slider. That slider tells the controller where the target is. Though less durable than a shaft-encoder, this system does not require calibration or limit switches and saved you about $300 off the price of the RTS.

• Q: Why can't I select the target algorithm by remote control?
• A: Multi-select remote controls are more expensive. You can hack this by removing the cover and re-wiring the selector switch yourself.

• Q: What kind of deal is this; you keep talking about things you did to make it cheaper but it's not exactly pocket change?
• A: You should have seen what it would have cost with all the extras that 90% of people wouldn't use. Robots are just expensive that way. Unless they're made by children in China, which this one wasn't.