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Homemade Piston

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#1 Vassili



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Posted 29 May 2004 - 01:48 AM

Alright guys, for my next project, which I don't know will happen or not, I'm going to be building a homemade plunger and plunger tube. Problem is, I can't decide on how to make the piston. I'm thinking about just testing O-rings at the hardware store and see what fits nicely in 1" PVC, but it's going to be hard connecting the O-ring securely to the piston without being able to mold plastic. It looks as though wrapping some 1/2" PVC in e-tape worked nicely for Boltsniper's P99, but if you guys have any other ideas for making a nice airtight piston they would be greatly appreciated.
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#2 cxwq



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Posted 29 May 2004 - 02:02 AM

Actually I've done considerable work with this during prototyping for my xbow mod. Get two sizes of PVC (or CPVC, or whatever) where the larger size is your desired plunger tube diameter and the smaller has a loose sliding fit in the larger. Cut a 1" piece of the smaller tubing. Use the dremel bit (DRE192) that looks like a ~3/16 sphere with sharp things around it. Cut a pair of grooves to fit o-rings into, about 3/4" apart. The double o-ring adds stability to the plunger head and improves the seal. It does take some playing with various types of tubing and o-rings at the store to find a good match. You want the o-rings that you have a tough time stretching over the smaller tubing. That way the grooves will hold them better.

I'm using a threaded steel rod for my plunger shaft so I'm attaching the plunger head with nuts and some washers that are just smaller (OD) than the OD of the PVC I'm using. The down side to this is the weight of all that metal and the inertia that comes with it so I'm testing the strength of various epoxies in large voids like that.
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#3 texmustache



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Posted 29 May 2004 - 08:58 AM

If you choose not to go with cwxq's idea, try this.
Use 1" CPVC (For the plunger tube), 1/2" CPVC (For the plunger), 5/8" Rubber Gasket (Theres two sizes, get both and try them) and you'll need a 1/2" CPVC Endcap and coupler. You'll also need a bolt that fits in the gasket, and some washers and hex nuts. First you need to drill a hole in the center of the endcap the size of your bolt. Next, put these on your bolt in the following order: washer, gasket, washer, hex nut, then put the washer through the endcap hole, then add another washer and hex nut. It'll look somthing like this:


I - gasket
: - washer
= - hexnut
( - encap

then, pvc glue the endcap to the end of your 1/2" cpvc. Now all you need to do is add a spring, and close off the plunger tube ends.
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#4 Zero Talent

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Posted 29 May 2004 - 12:29 PM

Yeah, I'm doing something like these with a rubber washer, which is decidedly not as efficient as using a rubber gasket. I think I may have to visit my local hardware store today.

What are your opinions on just avoiding the plunger rod entirely, and instead priming the gun via a mobile barrel?
I'm trying to operate off this kind of system with my current Crossbow incarnation ([37/32" ID] 1" PVC shaft to 1/2" PVC coupler with 5/8"ID tubing on inside end, allows mobile 5/8"OD barrel or locked 1/2" PVC barrel), using a round, lathed (Power drill! Whee!) 7/16" Dowel to lock with the trigger mech, and a two-spring system, combined with an Arrow water bottle cap (cheep) to keep the thing straightened. It works, certainly, but I'm just not a big fan of barrel-mounted breeches with that much resistance, so any system like this, implemented on anything I use, is going to require a distant priming handle, using either a rod or a cord to carry the force. The hard part, of course, is getting this thing to look pretty. Not only that, I'm missing the aesthetics of the original plunger handle's position. The only real advantage is freeing up the stock to hold, say, a high capacity 1.5" PVC pressurized air tank for an air [Semi-automatic/regulated?] valve.
Yes, I'll be keeping a look out for that gasket. Not to poopoo O-rings, I just don't know if I'll find an 38/32">O-ring>36/32" at my local hardware stores. They're really horrible for stocking any O-ring greater than 1". If all else fails, there's always... Tape? :(

Excellent ideas, guys.
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#5 Vassili



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Posted 29 May 2004 - 08:01 PM

Wow, thanks guys. I'm leaning towards O-rings over a gasket, just because my local hardware store has half an aisle of O-rings. I don't know if I can get a hold of one of those spherical dremel bits, but I'm going to try Cxwq's idea for securing the O-rings.

Cx, is your xbow all back together and working?

Zero, why not relocate the charging handle to the stock, like a stock xbow? You could also put the charging handle in the foregrip like a real-steel gun.
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#6 Doom


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Posted 30 May 2004 - 09:29 AM

Well if you want something simpler than that, you could try something like in these pictures. I made this as part of a water pump with 1/2" ID PVC (schedule 40, it doesn't matter), but it's easily modable to make it work with larger diameters and as a plunger.


The last picture is the pump fitting into 1/2" ID PVC. To make this into a plunger type thing you could use a larger ID PVC and a larger rod. Then you should use a washer or something on the longer end of the rod while it has the electrical tape on to put the spring on. Then you get the spring, the endcap and glue it all together. :P
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#7 Jangadance



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Posted 30 May 2004 - 04:11 PM

If all else fails, there's always... Tape? :P

Told you, Zero! In your FACE!
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