Build Log: MFIC

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
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Blitz
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Tue Aug 13, 2013 9:21 pm

In celebration of my first machined part, I was feeling ambitious and was in the mood to get a headache from PVC cement fumes. Yeah... so I cemented the chamber pipe and the breech/receiver. With this completed, I can now work on the carriage, cradle, recoil system, and electronics.

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I should have video recorded this process; I literally jumped on top of the receiver and saddled it like the bomb in Dr. Strangelove. :)
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Gun Freak
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Tue Aug 13, 2013 9:25 pm

Any way you could snap a pic of the entire thing so far next to a common object for scale?
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Tue Aug 13, 2013 9:35 pm

Sure! Stand by.
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Tue Aug 13, 2013 9:37 pm

Gun Freak wrote:Any way you could snap a pic of the entire thing so far next to a common object for scale?
I was about to suggest the same. Maybe use one of those soda cans they give you on aeroplanes :D
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Blitz
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Tue Aug 13, 2013 9:44 pm

Here, a 12oz beer bottle, and a typical folding chair.

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jackssmirkingrevenge
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Tue Aug 13, 2013 11:29 pm

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hectmarr wrote:You have to make many weapons, because this field is long and short life
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Blitz
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Wed Aug 14, 2013 12:01 am

I think that's a compliment. :)
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Wed Aug 14, 2013 12:14 am

Phonetically, yes it is :)
hectmarr wrote:You have to make many weapons, because this field is long and short life
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Wed Aug 14, 2013 10:17 am

Holy Shat, that thing is the size of a friggen telephone pole!!
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Wed Aug 14, 2013 11:06 am

It's the diameter of a typical utility pole, yeah. :) (Depends on what class, but that's a bit technical. Sorry, used to be a lineman)

Won't be as tall; as much as I wanted to use a barrel more than 10 feet, transportation becomes difficult at that point. :(
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Blitz
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Sun Apr 11, 2021 2:14 am

After an 8 year hiatus... I figured I'd give some closure to what happened to MFIC. Long story short, she had a short but eventuful career.

I eventually finished her, lost subsequent build photos along with MFIC herself in my divorce that happened 3 months after my last post here. I'll try to give as many details as I can though to the finished product.

I'll start with the biggest challenge that I had, and that was the breech. I was never satisfied with the cast aluminum door I bought and ended up creating a literal breech block and breech housing out of UHMW plastic that I married to the back of the chamber-connecting tee with a big flange. I made the breech block in two halves; a front and back one, that was held together by eight 1/2" threaded rods with washers. This made it much easier to carve a groove on each side for the block to sit in, which I was able to accomplish with a router table (and lots of patience). The operating handle actuated a rolling wheel/cam that very closely modeled how the breech actually worked on most artillery with a vertical sliding breech block. Because of the properties of UHMW, friction was definitely not an issue. The UHMW that made up the breech housing, along with the breech block, when assembled, weighed around 17lbs. It looked pretty goofy at the rear end until I got the entire carriage assembled, then it started to look more proportional.

Another challenge this presented was the gap between the breech and the big Tee that led to the chamber. This was relatively simple; because I made the entire interior flush with the 6" sch80 piping I just made a small "standoff" that left the hole in the tee. I inserted this after each projectile, which put the projectile just forward of the chamber.

I lopped off 2ft of the barrel so it fit in my truck more easily and shed some weight.

As for the carriage, I went with a design that very closely resembled the carriage of the French modèle 1917 Schneider gun with elements from the design used for the Russian's M1909. In fact I remember ttaking a silly amount of photos of the Schneider gun that was on display close to where I lived. So the trick was I had to great the entire gun, chamber and barrel and all, as a single entity, knowing it'd all have to recoil together. As such I wanted to ensure the support rails were as close to the height of the bottom of the barrel as possible while giving enouch support for the chamber to minimize stress on the chamber elbow and tee. This required a LOT of reinforcing by means of steel straps that I wrapped around the barrel and chamber with a wooden support in between, which sandwiched between the upper and bottom pieces, I had four 2"" thick strips of UHMW that protruded about 4 inches on each side; these strips were the contact points btween the guide rails and the gun itself. And of course, due to the properties of UHMW, this allowed for smooth recoil.

Speaking of, yes, I'm really damn glad I incorporated a recoil dampening system. I heavily borrowed the design from the Britsh BL 60-pounder with the use of recoil springs that both dampened recoil and recouperated; to dampen the recuperating, I ended up using a pneumatic cylinder with adjustable valves on each end to provide resistance to the recuperating springs. But it wouldn't get in the way of the recoiling as it'd move freely when extended (I was scared I'd blow it from the sheer force and the springs I used were strong enough anyway). That cylinder is one of the only remaining parts of my cannon; it's a Parker P1LN050DNT152.400/NN3S. The piston rod's threaded which made attaching to the breach very easy. Will definitely be using it in a future build!

I pilfered a worm gear setup out of an old circular saw I found at a junkyard; this became the elevation mechanism. I made a trunion on the guide rails that let me put the undersized brass gear on. Really wish I used a larger gear because of the weight stress on it, but it worked fine because I put a locking mechanism to hold the carriage in place after I adjusted the elevation. I did not use a traversing gear but I wish I did.

Let's see... Fueling and ignition. For ignition, I used two cheapo stun guns from eBay that I disassembled and combined the circuits to provide three spark gaps inside the chamber (glad I went with that removable plug, it was easy to work with although maybe that's because the chamber was 8" pipe lol). Venting was provided by the 80mm Delta fan that put out 80CFM.

Fueling was a challenge too, perhaps even moreso than the breech block. Because of the sheer volume there were higher tolerances, but I wanted to get it right. The fuel source was easy; I had a 20lb propane cylinder dedicated to the cause. I put a pressure gauge between the cylinder itself and the regulator was fixed as 30psi, so I ended up trying to calculate how long I'd have to have a specific valve open to transfer the amount of propane I needed. Then I just used a cheap ass solenoid valve that I got off amazon that ran off 12v, that was triggered by this wonderful device I recently learned about called the Raspberry Pi. I can open and close it precisely as long as I needed to for that 30psi to do its thing. But, I ran into my first mistake. The Pi's logic voltage wasn't 12v. oops... I learned I had to use a relay. Anyway, a crash course in python and low-voltage electronics (and like two months lol) and I had a system capable of deliving the perfect amount of gas. or at least as close as I could get it. It was enough though. I intentionally kept the firing circuit separate from the Pi and the fueling just because I didn't feel safe. In hindsight, I'd integrate it all, but I learned a lot more than I knew back then.

Trying to think what other details I missed out in the construction. I used tires from a boat trailer to move the thing around, it had to weigh around 400lbs overall (a last minute change was me making a small platform out of a milk crate to hold the propane tank off so I can move the entire setup) and moving it with my lawn tractor was a breeze. And cute. The looks I got when I went down the street about half a block!

The moment of truth, so what'd I fire? The only thing I actually shot were ice slugs that I cast from the 6" pipe I lopped off the barrel before. Because the modeling I did was with 5lb projectiles I ended up using 6" long sections which gave me right around 79 ounces if I topped them off with water. Good enough.

I was way too scared to touch this thing off where I lived, so I went to a coworker's property about 2 hours away. I had to use ramps to my pickup bed and a bunch of ratcheting straps to get MFIC up on the bed, took me a good 15 minutes... less than 5 minutes to unload though! Nice to have help. It was about 50'F out, which was a bit cooler than I'd have preferred but I didn't care. I figured the temperatures wouldn't be too much of a factor even though all of my flow calculations were at 70 degrees... But I'll never forget this all

After I set her up and get ready to fire I remember having a severe amount of anxiety and a rush of adrenaline. I think this is why I never thought about pictures for this and I hate myself for that, but oh well. I run a 12ga cable to the gun which is a good 20 feet away. We were behind a dumpster, he figured that'd be more than adequate protection. I poke my head around the corner and push the button, and I swear the earth stopped rotating for a second. MFIC roared to life and yeeted that miniature ice rink. My ears are ringing from the report and I loved it, I ran out from behind the dumpster to see where that ice chunk was and saw it shred upon landing just under 150 yads away. I told him to stand back while I saw how quickly I can reload, hearing that damn 80CFM fan whirling like mad I only hope I gave enough time for the gas to clear out while I reload. ! 8 seconds later I'm pressing that button and the MFIC belched out again, me in all my excitement forgetting how loud the first shot was, that one landing just 110 yards. I slow down reloading the third time, but I couldn't ignite. I wondered what went wrong. Long story short, the ignition system shorted and fried itself.

Well, I felt that was all worth celebrating but MFIC was never to shoot again. My goal was to build a large diameter cannon that was capable of delivering consistent shots rapidly with a muzzle energy comparable to that of a .50BMG; I think I succeeded. IRC HGDT predicted somewhere around 18000 joules. However, that was with a 10ft barrel and I know I lost some energy going to the 8ft barrel. Sure, my projectile was much heavier, but... power is power. I really want to do this again. And take pictures all the way through this time.

I hope this inspires others who are still around to build on a larger scale and help answer some questions to some of the complications larger builds bring up.

EDIT: I forgot to mention the overall travel of recoil was 8 inches and it took about 2 seconds for the gun to fully recuperate into battery again.
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Sun Apr 11, 2021 4:15 am

Welcome back. That's a hell of a story. I hope to see a build from you soon. I fell away from this hobby for a long time too, but then the bug bit me and I got back into it, although my last project wasn't as YUGE as this thing you made, lol. That was a hilariously huge canon!
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Blitz
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Sun Apr 11, 2021 5:58 pm

I actually have a legit need to make a small launcher for putting radio antenna wires in trees, so that's my next project for now.
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Mon Apr 12, 2021 4:46 am

40mm launcher ?
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Blitz
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Mon Apr 12, 2021 7:38 pm

40mm is a pretty odd size here in the US but it'd be somewhat similar. Definitely a bit smaller.
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