Monday, May 27, 2013

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.

Sunday, May 12, 2013

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.

Saturday, May 11, 2013

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. :)