How to build a Spud Gun

Alright, you want build your own potato launcher. These instructions will help you on your way to an effective yet simple spudgun that should provide hours of enjoyment for you, your friends, but probably not the old man down the street who gets mad when your car backfires!

Instructions

Difficulty: Moderately Easy

Things You’ll Need:

• 10" of 4" SCH40 PVC water pipe (chamber body) 36" of 1 1/2" SCH40 PVC water pipe (barrel stock) Or get yourself a piece of SGTC rifled barrel stock. 4" PVC coupler 4" x 1.5" PVC bushing (if not

1. To make sure everything goes together properly and smoothly, proper conditioning and sizing of the materials is necessary. Pay particular attention to the shaping of the pipe ends, as this is a major factor in proper solvent welding with the fittings. PVC PIPE SIZING Cut the two pipes to size, 10" and 36" with the saw unless you already did so in the store. Using the file, take off the sharp corners on the inside AND outside of the pipe. Radiusing the inside helps reduce the amount of potato scud that can build up in the launcher, and breaking the outside corner ensures proper solvent welding, if this is not done leak paths may result. Cut ends should be as square as possible. To one end of the 1 1/2" pipe it is suggested that the inside be chamfered about 0.020" and the outside filed or turned down to almost meet the inner chamfer, creating a blunt knife to help size the potato. It will work without out this step, but a lot messier! PVC FITTINGS Inspect the fitting for really bad weld lines or possible large dislocations that may result in premature failure. Remove all paper tags or labels by peeling, scraping, even using a bit of primer to clean off the last adhesive. File off any sharp sprues that may hamper your ability to grip the fittings firmly when assembling. IGNITION SYSTEM Cut the wire into two equal lengths. Strip and twist each end about 3/4" back. Taking the BBQ igniter, upon inspection you should find a plug on the tail end (one electrode site) and close inspection should reveal a very fine wire embedded in the side body of the igniter, close to the button lip.(if you have a 2 pole ignitor -- side burner type -- this step will be much easier) This fine wire is attached to the igniter nearest the button, as the wire is just pressed into this slot which might be about 3/4" long. Carefully extract the other end of the wire with the knife, removing all but the last 1/4" or so from the slot. Carefully twist this wire to one end of one electrode wire stripped previously. Immediately solder this connection together taking care to not break or extricate the fine wire from the plunger body. Cool with spit (really!), dry, and tape this joint back against the body with electrical tape, using enough to immobilize the wire with gentle tension. Take the wire supplied with the igniter, the one with the plug that fits on the base, and cut leaving about 2" from the plug end that will fit into the base of the igniter. Strip and twist t
2. Now you get to stick all the stuff together, using pipe glue and tape and more solder if you like! Just don't breathe too much pipe glue solvent...it'll make you nuts! I Chamber Assembly: Prime both ends of the 4" pipe, also priming the 4" bushing, both 4" coupler sockets, and 4" slip on the adapter. DO NOT get any primer on the adapter threads! Immediately apply pipe cement to the three primed 4" fittings, then LIBERALLY to both ends of the 4" pipe. Start both the coupler and adapter fittings on the pipe, start the bushing in the coupler, then right the assembly with the threads up, pressing it together with body weight while twisting about 1/2 turn. Hold this position for at least 30 seconds. The pipe ends should bottom each about 1 1/2" inside the fitting socket. If they both do not bottom, either apply more force IMMEDIATELY somehow to get it to, or you are screwed and have to throw it out--start over with that. If the fittings and pipe are properly conditioned this should not be a problem. Wipe up any spilled glue with the rag, but leave a good bead at the external pipe/fittings line. If any glue got on the threads get it out of there as fast as possible! Any glue on your hands should just be left to dry and then peel it off. Using solvent to remove it will just increase your exposure to it. II Electrode Installation: Wait about 10 minutes for the glue to set. Using the proper size drill bit, put two radial holes 90deg apart about 1/2" back from the front edge of the adapter fitting--the plastic here is about 1/2" thick as well. Drive in the two machine screws nearly all the way, leaving about 1/8" between the screw head base and the plastic. The ends may touch or be far apart--the electrode gap will be calibrated after the electrode wires are connected. III Barrel Introduction: Prime one end of the barrel and the 1.5" socket on the bushing. If one end of the barrel was chamfered to a knide, prime the square 'regular' end of the pipe. Apply glue, liberally to the pipe, and press together, again using a 1/4 turn motion to bottom. Hold for 30+ seconds. After releasing place the unit upright for several minutes, to let the glue set a little more. Solvent welding is more of a curing than a drying process. Solvent is lost, hence the term drying, but the action of the solvent effectively cures all those individual plastic parts into one continuous physically joined piece of plastic. Properly done, solvent welds are always stronger than
3. IT IS HIGHLY RECCOMMENDED THAT YOU WAIT AT LEAST 24 HOURS FOR THE GLUE TO FULLY CURE AND SOLVENT TO DISSIPATE. Many of the unfortunate potato launching accidents occur due to over anxious constructors/operators getting a little trigger happy a little too early. Keep in mind that the solvent fumes are extremely flamable. A few notable events include people chemically/thermally removing a majority of the hair on their head, or rapidly separating the supposedly single piece of PVC plastic back into its purchased components (or even smaller pieces!) Just wait...it'll be worth it. In the mean time this is a great opportunity to gather what you may need when you do start running this device. On the top of this list is certainly a bag (or two...or three...) of good fresh potatoes. Never use old sprouting, skunky potatoes, this is just bad form! Buy a bag for $2 and relish in their crispness! Also a fuel, probably starting with hairspray, the old mainstay. Traditionally the 'Aqua-Net' brand has yielded to best results, but it will be mighty hard to find those folks who grew up on that. Once you get more experience you might graduate to propane...but wait a bit. A good location is also required...don't use this device in the middle of your subdivision out of your garage or you are guaranteeing that old grandpa down the way will call the cops!
4. Again, find a suitable location to deploy your potatoes, as the device can get quite loud with a good fuel load. Out in the sticks is best, but just not downtown! 1. After waiting the necessary time for the solvent vapors to dissipate, now you can test the ignition device. Depressing the plunger briskly should produce a good spark to jump across the screw threads. If all connections were made correctly and the gap is about 1/4" it should spark every time (or at least 9 times out of 10). If it doesn't check your connections, maybe set the gap a little closer (never less than 1/8") but get it to work, or potatoes are going nowhere (unless you throw them--boring!) 2. When a reliable spark is verified, test fit the threaded plug into the adapter. It shold engage the threads at LEAST TWO TURNS. If not, check for crud in the threads, and if clear, obtain a 60deg triangle diamond file, and size down the plug threads a little. The threads are at a 60deg angle, so careful filing about the first few threads is ok. 3. With the cap off, load a potato! With the chamber on the ground, place a potato over the muzzle of the launcher, and press it down with your palm, shaving off the excess, creating a cylindrical potato plug. Make sure the potato contacts the wall firmly all around, or it will not fire or not that well. Loading the potato sideways is acceptable, as it creates a more stable projectile anyway. Ram the potato down to within 2" of the breech of the barrel, using a smaller diameter PVC pipe or broomstick. Don't push it too far or it will fall out into the chamber...and just be subject to a short baking cycle. 4. With the spud firmly seated in the breech, pick up the launcher and holding with one hand, dispense a few seconds of hairspray directly into the chamber. Don't use too much; it will just make a mess. Quickly close by screwing on the end cap hand tight, do not use a tool to tighten to two turns or you will likely never get it off again. 5. Call out "Fire in the hole!", point the launcher in a safe direction, and depress the BBQ plunger. That potato you rammed down there should exit at a quite rapid rate with a sizable report, and depending on where you aimed and at what angle you pointed, it went anywhere from 6 inches to ~200 yards from the muzzle. Pretty cool. If you clicked the ignitor a few times and it didn't work, you are not alone, check out my next section. 6. If it worked, great! Remove the end cap (this may require a tool), vent t

Tips & Warnings

• Debugging the Launcher There are a lot of reasons why your launcher may not be working. First and foremost NEVER take off the cap and start clicking away at the igniter. This again has led to human fires and that chemical thermal hair removal thing again.... Hairspray in cold weather just does not go well. Too much of the burnable stuff in the spray condenses on the chamber walls, making it very difficult to burn. You might try warming the chamber by the fire if you are camping or graduate to a lighter, hotter fuel (propane....) To 'defuse' a misfire you need to take off the cap, while pointing it in a safe direction. Allow the chamber to air out for several minutes, perhaps longer, to get the fuel load out. After you are certain the fuel has been vented, again verify the spark, just by turning the chamber enough so you can see where the spark should be. Never point the chamber bore directly at your face, or body, or anybody else. If it sparks then it is a fuel mixture problem. One of the most common fuel problems leading to misfire is too much fuel! Not usually a problem with hairspray, other richer fuels can be overloaded above their UEL value, and they wont work. Always try less fuel rather than more fuel. If you still can't get it to work I invite you to peruse the many other web sites that talk about how to troubleshoot your combustion launcher. Again I have made a partial list, and sadly just don't have the time to address every specific person to person launcher complication. I hope you do in fact figure it out and experience many hours of safe spudding
• These instructions/procedures are provided as-is. No guarantee is made that this device will function as well or better as I have experienced, intend, or describe it to operate. In other words, your results may vary, and they may even be very undesirable results, possibly resulting in personal injury or property damage. 2. The wording and statements/nomenclature pertaining to the construction and operation of this potato launcher are intended as such to help ensure one attempting to do this has a basic understanding of the equipment and materials involved. If some of the statements seem 'Greek' to you or you do not recognize some symbols or phrases, that is wholly my intent, and you should obtain assistance from a trusted source or not proceed at all. 3. These instructions are intended for a beginer or someone that has never built a spudgun before. The procedure outlined below is not necisarily the method myself or another experienced spudgun builder uses. These instructions will help guide you in building a "sound" piece of launching hardware. As you gain experience, you may find that different things may work better for you....but start simple. Thanks, and happy spudding! 4. The finished product should look something like the picture in the beginning. Keep in mind that PVC pipe/fittings are not approved by the manufacturer to be used for the purpose of constructing spudguns.