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Backpressure Homemade

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#1 Buffdaddy

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Posted 30 May 2011 - 04:42 PM

After extensive testing (and failures) in the area of backpressure tanks, I finally have a viable design. This one is based off of the spudding train of thought (large piston) as opposed to Nerf (small piston, like in a Big Salvo).

So, without further ado, I present the B.U.F.F. (Credit to Sharvil for the name, haha)

Materials list:
1/2" PVC
1/2" CPVC
3/4" PVC
1 1/4" PVC
1 1/4" PVC tee
1 1/4" PVC coupler
1 1/4" to 3/4" PVC bushing (2)
3/4" to 1/2" PVC bushing (2)
1 1/4" PVC cap
3/4" PVC cap (3)
3/4" to 1/2" CPVC bushing
3/4" PVC cross
3/4" PVC plug
1/8" ID tubing to 1/4" NPT adapter (McMaster #5116k302)
Check valve (McMaster #2987K21)
OPV (McMaster #50265K23)
Bell Dual Action Bike Frame pump

Total: about $30, including the pump.

----------------------------------------------------
Now for construction!

NOTE: There is a new method for piston construction HERE that's much simpler. Would be easy to drill two holes and add the check valve to the rear. Plus, it seems to seal better.

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First, we make the piston. Take two 3/4" endcaps (the flat kind you find at smaller stores, like Ace, True Value, etc) and cut them roughly in half. Take one endcap, drill a small hole, and install a McMaster check valve, with airflow going towards the interior of the cap. You will then mount the pieces on a roughly 2.5" piece of 3/4" PVC, as shown. The rings must be reversed in position, so that the flat, uncut sides face the ends of the piston. Use you super glue or other favored epoxy to put everything in place. Cut/drill a tiny hole in the side opposite the check valve, to bleed air into the chamber. Also put a craft foam circle at that end. Finally, Take your o-rings and slip them on. The piston is complete.

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Now, the chamber: You need ~3 inches of 1 1/4" PVC. To compact my design, I sanded one end with my dremel until it was thin enough to stick past the "hump" of the PVC tee, so to speak. This was purely for compacting the design, and is not necessary, as the wall is thin enough that, without care, you can crack it, etc.
On the other end, put in a 1 1/4" to 3/4" bushing, with another bushing to reduce to 1/2". 1 1/4" to 1/2" Bushings DO EXIST, but don't seem to be very common at hardware stores, at least not in slip couplers (There were ones made for having threaded 1/2" PVC, I just decided not to use them). Hammer some 1/2" CPVC into 1/2" PVC, and place in the chamber. Place a 1/2" to 3/4" bushing at the end of that, to provide a flat sealing surface. Slide in your piston, and look at the marvelous seal!

Please note that you should use silicon grease on your o-rings and all that jazz. It helps.

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Now to cap the chamber: This is where you have to take some guesswork. You only want the piston to have a small motion (1/4" at most), so you'll have to adjust the fit accordingly.

Using another 3/4" endcap, place (but not glue) in another 1 1/4" bushing. (I recommend just stepping down to 3/4" for the bushing, as you'll see later). Line it up to be flush with your 1 1/4" pipe, and test the piston movement manually with a pencil or something. Shave off the open end of the cap until you get the desired fit, then epoxy into place. put a craft foam bumper on the end of that, to help make the thing last. Finally, drill/dremel a bleed hole in the middle so air can actually pass. Now you can throw on a 1 1/4" coupler and connect everything. The chamber is complete.

Edited by Buffdaddy, 20 August 2011 - 10:57 PM.

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#2 Buffdaddy

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Posted 30 May 2011 - 05:27 PM

Now for the rear of the blaster:

Posted Image

In the last version, I used a 3/4" PVC tee, with different branches for things. The rear has a Bell Dual-Action Bike Frame pump, that slides tightly into the coupler. The top is a plug, with a 1/4" NPT adapter attached. This is where the OPV goes. The bottom has a 3/4" to 1/2" bushing, a length of PVC on the end of 1/2" CPVC (use the hammer again), with a garden hose nozzle placed over it. Epoxy seals everything together.

Now for the bottom of the chamber: this serves as a handle for the blaster, and can be made to any length you want, depending on how much power (and how much pumping) you'd like. I just used 2", enough to put the cap on.

You'll notice the snap-on tee I have down there. I mounted it by carving a hole in the cap, shoving 1/2" pvc through, and putting another cap on the inside, with epoxy sealing everything up tight (was already snug as is, just needed a little). The snap-on tee fit the pump in an earlier iteration,as shown:
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For purposes of this writeup, I have left that out in the materials list. Should you decide to do it, you know how.


Diagrams of the above instructions, for clarification:
Chamber and piston construction:
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Rear and pump holder construction:
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Edited by Buffdaddy, 30 May 2011 - 10:18 PM.

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#3 Buffdaddy

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Posted 30 May 2011 - 05:46 PM

Some tips to help you guys:

-Endcaps usually have small ridges on then, for gripping, etc. Be sure to sand those off.

-In case you can only find curved endcaps, all is not lost, you can still construct your piston. Here's an example, using a 1 1/4" endcap and 2" PVC (yes, you can scale my blaster up to that size if you want ridiculousness):

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Sand the end of your cap a little, so there's a flat surface and area to better place adhesive. Make the craft foam cushion the size you need, place a large glob of 2-part epoxy in the middle, and set your cap on it. Then, take the pipe, and place it OVER your cap, to keep everything straight. Let cure for many hours. For best results, wrap the cap in E-tape, to make it fit snugly and straight in the pipe.

-Finally: USE SOLVENT WELD UNLESS NOTED. I used 2-part epoxy on everything, because I need something to hold decent, but still be able t tear it apart if things went wrong. You should use solvent weld to make sure everything will hold.

FIRING VIDEO:
http://www.youtube.com/watch?v=N_vd4WLhDPA
Ranges: at video time, I was having trouble and getting only 60-70. But I've seen 80-90feet easily. My initial pistonseems to leak more air than it should around the o-rings, so hopefully I can fix that in another iteration and update the writeup.

Edited by Buffdaddy, 10 July 2011 - 07:06 PM.

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#4 Buffdaddy

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Posted 30 May 2011 - 06:06 PM

Finally, a pic of what I originally was trying to do:
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You notice the pump placement and handle, with homemade trigger/blast button. These things didn't work, and will be noting in my continuing build thread.

The second version?
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The te at the bottom would be capped off, and small diameter vinyl tubing would connect the interior points (pump, trigger, OPV, chamber) to reduce dead space. You could also add a fitting at the back of the bottom tee for, say, connecting to a hard tank or bladders of air.

Sadly, I seem to have bad luck making homemade blast triggers. But it's certainly possible.
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#5 blitz

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Posted 30 May 2011 - 07:05 PM

(drools like a dog on a bad day.)

That is really cool. Its about time that someone made a homemade Back-Pressure tank. It also uses simple parts that anyone can buy. I love it. Period.
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#6 SlightlySane813

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Posted 30 May 2011 - 07:29 PM

Absolutely Genius.

Mad props. This is going to be a great addition to the new concept of homemade air-tanks.
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#7 sharvil06

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Posted 30 May 2011 - 07:52 PM

I love the name haha! Good luck in the contest. This is just a great homemade backpressure tank!
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#8 Ozymandias

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Posted 30 May 2011 - 11:12 PM

Impressive.

Question: what is the advantage to a back pressure blaster?
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#9 taerKitty

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Posted 30 May 2011 - 11:51 PM

Question: what is the advantage to a back pressure blaster?

BP Valves let all the air out almost instantly. Pin valves are prone to 'bleeding', where the user doesn't pull the trigger quickly enough. This means some of the pressure is let out, either going around the dart, or pushing it down the barrel a bit.
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#10 makeitgo

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Posted 30 May 2011 - 11:53 PM

** edit - still trying to figure out the format/scheme.

@Ozimandias The advantage is possibly creating a semi-auto / fully auto air blaster. Power and extremely fast ROF.

Originally posted by SGNerf:

"This was 3DBBQ's V6 design (imho, the dude has amazing blaster style and colour schemes):

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You can see the video if it operating here:

http://www.youtube.com/watch?v=j1brsertLAg"

Edited by makeitgo, 31 May 2011 - 12:00 AM.

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#11 taerKitty

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Posted 31 May 2011 - 12:10 AM

Let's bring it back to the BUFF writeup, shall we?

We have a generic Adventures in Airguns thread already. I hope to be posting there soon.

Buffy, I am a kitty with very little brain, and can't see how the BP valve here is working Can you help me out?
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#12 Buffdaddy

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Posted 31 May 2011 - 09:27 AM

Haha, sure.

As setup in the "final" picture (I need to actually show it with the OPV on and such, I'll do that after work), you pump up the blaster with the dual-action bike pump. The pump already has its own check valve of sorts, so you have the input pressure. The piston slides forward and air bleeds out the piston's check valve to fill the chamber. Pulling the trigger on the garden nozzle lets the air out from behind the piston. Once the pressure on the piston is greater on the chamber side than on the rear (or pilot) side, the piston flies back and sends the chamber air out the barrel.

With the o-rings and the check valve in the piston, the whole thing is virtually airtight, so all the air in the chamber SHOULD be used purely for firing the dart.

I'm very good at seeing everything in my head. Explaining it....is another matter. The writeups will be clearer eventually :lol:

And yes, I'll get it to working status. I pressurized it last night enough to blow some of the weaker epoxy connections, and redid it with actual PVC solvent welding. Should be cured by now. But alas, you gotta work if you want the paychecks....

Edited by Buffdaddy, 31 May 2011 - 09:31 AM.

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#13 ricochet

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Posted 31 May 2011 - 11:05 AM

After looking at this for 15 minutes, I believe I actually understand the function of your design. Once you have a solid tank built that you're pleased with, a video would be my personal request to help with understaning. It looks very promising, I'm excited to see the final product!
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#14 Buffdaddy

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Posted 31 May 2011 - 11:13 AM

After looking at this for 15 minutes, I believe I actually understand the function of your design. Once you have a solid tank built that you're pleased with, a video would be my personal request to help with understanding. It looks very promising, I'm excited to see the final product!


Haha, yes, that has been the general consensus. I'll get the video done, don't fret!
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#15 Draconis

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Posted 31 May 2011 - 11:16 AM

Would you mind measuring the volume of chamber? I am interested in this design for use with larger ammo, partially because oh-my-gosh, it looks big. Like, scary big for micro darts.
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#16 taerKitty

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Posted 31 May 2011 - 02:05 PM

Drac, admit it. You want to shoot balls with it.

Er... that came out wrong. :)
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#17 Buffdaddy

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Posted 31 May 2011 - 03:30 PM

Would you mind measuring the volume of chamber? I am interested in this design for use with larger ammo, partially because oh-my-gosh, it looks big. Like, scary big for micro darts.



Drac, admit it. You want to shoot balls with it.

Er... that came out wrong. :)


LOL. Now remember guys, Nerf says "Do not modify balls" and "Do not aim at face"!

Anyhow, If you straight up made it, by my estimates you have about 80 cc. Since I installed my (now unused)pump bracket, the cap on the inside of the tank cuts the effective volume to 60 cc.

Another note: If you scale everything up to 2" PVC and 1 1/4" PVC, instead of 1 1/4" and 3/4", respectively, you have about 250 cc of space.

Obviously, you aren't gonna be singling the large version. But that would be plenty decent for a shotgun, balls, arrows, etc.
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#18 Draconis

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Posted 01 June 2011 - 01:58 AM

Drac, admit it. You want to shoot balls with it.

Er... that came out wrong. :)


I thought that at this point, it was just a given.
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#19 shardbearer

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Posted 01 June 2011 - 06:22 AM

Are those volumes including the pilot volume and air inside the piston? Cause those seem pretty small, 3.7 and 4.9 ci.

Edited by shardbearer, 01 June 2011 - 06:23 AM.

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#20 Buffdaddy

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Posted 01 June 2011 - 06:10 PM

Are those volumes including the pilot volume and air inside the piston? Cause those seem pretty small, 3.7 and 4.9 ci.


Those are excluding the pilot volume, since that doesn't blast out the dart. Yes, those are small. HOWEVER, you're overlooking the major advantage of this design; since it's built inside a PVC tee, you can make the chamber AS LARGE/LONG AS YOU DESIRE. I'm just calculating for if you only use enough PVC to slap a cap on the bottom.

Also, I'm pretty sure I didn't calculate the air inside the piston. But I'm not going to worry about that, since the air would have to bleed out that tiny hole. In the grand scheme of things, it's probably insignificant next to the initial blast.

EDIT: Finally got around to some range tests this evening! Playing around with CPVC barrels of different lengths, I was getting anywhere from 80' to 100'. Hoppered was more disappointing, but by that time, the stefans were falling apart (my slugs are admittedly not the best), no I'll have to try with fresh ones. Not bad for 60 cc's and ~35psi.

Edited by Buffdaddy, 01 June 2011 - 08:30 PM.

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#21 polycarb

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Posted 02 June 2011 - 05:38 AM

Sorry if I missed it, but what are the dimensions on the O-rings?
Otherwise, the design is brilliant, and I'll look forward to making one.
EDIT: It appears that the craft foam is sealing the piston to the outside. Will this seal last for long enough?

Edited by polycarb, 02 June 2011 - 05:50 AM.

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#22 shardbearer

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Posted 02 June 2011 - 03:06 PM

Gluing a sheet of neoprene is the normal spudfiles aproach, and I think much more durable than craft foam.
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#23 Buffdaddy

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Posted 09 June 2011 - 11:40 AM

The craft foam is holding up much better than I had hoped; I've been firing repeatedly each day to test longevity, and it hasn't failed yet. I'll have to keep playing with it throughout the summer.

On another note: We have a new piston design, that's easier to construct!
Posted Image

Thanks to a suggestion fron Bob, I switched to using a 3/4" PVC coupler, and bushings/plugs at either end. Ready made o-ring slots, for one. I've aimed to minimize the pressurized space in the piston, and it's down to about 4 cc.

The one pictured still uses the McMaster check valve, but I have another one that's identical, except it uses the Homemade check valve Ice? posted. You know, a spring and 3/16" ball bearing in 1/4" ID tubing, with 1/4" OD tubing stuffed in either side. Using that both makes the piston that much cheaper and minimizes even more (wrap the 1/4" OD tubing at either end in e-tape to fit snugly in CPVC, and epoxy into place).

Also, with the exception of the bleed hole (still requires a drill), the only tools used have been a hammer, tubing cutters, and a file. Well, and adhesives, of course. But as far as cheap/easy to make? We're getting there.

If you use the homemade check valve, and use a Nerf pump with a built-in OPV, then you can actually build this without ordering McMaster (the #216 O-rings came from an assorted box, available at Harbor Freight as well as McMaster. Not sure if it's anywhere else).

Thus ends the update.

Edited by Buffdaddy, 10 June 2011 - 11:27 AM.

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#24 taerKitty

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Posted 10 June 2011 - 10:32 AM

404'ed. Perhaps it's ACLed to your network addr?
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#25 Buffdaddy

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Posted 10 June 2011 - 11:28 AM

Post image fixed; one letter was off. That's why you proofread, people.

Progress to (hopefully) be made this weekend on all my homemades.
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