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Telescoping Plunger Tube Blaster (TPB)

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concept writeup homemade telescoping spring

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#1 Aeromech

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Posted 16 June 2014 - 02:24 AM

Preface
Sketching/drafting is one of my favorite time killing activities, and my mind inevitably wanders to Nerf related things. I got to performing some calculations, and realized that using the 2" PETG as a plunger tube instead of 1 1/4" PVC yields a 61% increase in cross sectional area in the plunger tube. That's resulting from a diameter increase of only 27%! At this point I was hooked on the idea, and despite the known problems with this design, I decided to give it a shot anyway. Inspiration draws from a wide variety of sources, initially Zorn's FAL-1, MIG's Multiple Orgasm, with the final product (I suspect) more similar to Nine's ART.
Read results at the bottom before you decide to build this one.

In this writeup, photos are above the reference text.

Consumables
1-1/4 to 1/2" PVC bushing
1 to 1/2" PVC bushing
2" OD PETG*
1-1/4 Clear PVC*
8-32 x 2" Machine Screw
6-32 x 1/2" pan head screws
6-32 x 3/4" machine screws
1-1/4 PVC Tee
2" PVC
1/2" PVC
1/2" PVC endcap
1/4" thick Polycarbonate*
11" long [k26] spring or stronger*
1-5/8" Bath plug
1/2" PVC Insulation Foam
3-16" Light Duty Rope
Packaging tape
Plumber's Goop
Silicone Grease
Catch spring

Tools
Course File
Rat Tail File
Drill with Drill bits set
Permanent Marker
Ruler
Screwdriver
Hacksaw
Vice
6-32 Tap
8-32 Tap
1-1/2" hole saw
1-3/8" hole saw
1/2" Countersink
1/2" Nylon rod
Table Sander
Trusty Knife
Rotary Tool with cutting bit (optional)

Note: (*) indicates this item was purchased from McMaster-Carr.


Plunger Head

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I could have bought a skirt seal from McMaster Carr, and I probably should have. But for the sake of spontaneity, I picked this up instead from Ace Hardware. If you do this, I reccomend getting a skirt seal, it will probably save you a lot of weight in the plunger tube. I will likely swap out this part in the future.

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You know what to do.

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Your finished plunger head should resemble this guy. I put the bath plug to a belt sander to trim down the height a bit. The idea was to reduce the weight of the plug, as it is made of a very dense rubber type of material. It is very rigid and I am confident it will not break. I used polycarbonate cut using a 1-1/2" holesaw to make my "sandwich" here, but you can use washers if you like. This was my attempt to keep the already heavy plunger head to a minimum. Throw an 8-32 x 2" Screw in there to attach it to the plunger rod later.


The Catch
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Make one of these. Standard Rainbow Catch. I don't even use Templates anymore. Just use a holesaw. Make an extra "endplate," as shown next to the catch assembly.


Plunger Rod

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Tap that rod with the 8-32 Tap. This material I've found is notoriously difficult to self-tap, and even tapped, it's tough to screw in the plunger head.

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Make yourself a catch Notch. Make the notch longer than the one pictured. I was having catch issues due to this notch being too short lengthwise. The catch face should be about 7 inches from the rear of the plunger head assembly, as shown.

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Drill a 5/16 hole about an inch from the rear of the plunger rod. Tough to see here. We'll come back to this later.

Rear Bushing

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This part is made by connecting together a 1/2" PVC endcap, a 1/2 to 1" PVC bushing, and just enough 1/2" PVC to connect the two pieces flush.

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Round off the protruding edges using a file or belt sander. Also shown is a big hole in the back of the endcap. Make it happen.

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Wrap it in tape as if you are making a traditional Rainbow, so that the part is a relatively tight fit. Doesn't need to be airtight.

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When inserted into the 1-1/4" PVC, this part should not interfere with the interface of the PVC and the PVC Tee which will make up the stock.


The Body

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I like to blueprint my parts.

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The easy way to make slots. The rear bushing we made before will support both halves of the split tube. Forget all that drilling nonsense. Just keep the slots about 1/2" thick.

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Make 'em look pretty. File flat the slots.

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Here is everything assembled in the body tube. A lot of things happening at once... I'll give you the measurements in the morning, but basically, the front of the blaster is the right side. The spring rests on the free-standing plate on the right. The assembly on the left is obviously the catch. The little piece of gray/black material is some of insulation foam used to cushion the travel of the pump.

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Throw some speed holes on there. Seriously, they work.


Plunger Tube

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Take the 1-1/4 to 1/2 bushing and wrap it in packaging tape until a tight fit in the 2" PETG. Secure with Plumber's Goop inside the tube and allow it to dry overnight. Drill, tap, and countersink holes for the 1/2" screws.

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The plunger tube needs a slot on the bottom to allow the rainbow catch screw to clear. Make on about 3/8" and 8" long.

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Here it is on the body tube.

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Make two of these parts, about 2 inches wide and 1 inch tall. The purpose of these parts is to resist rotation. Take one of these parts and Size them so it just fits inside the 2" PETG Plunger Tube. When the PETG tube is screwed onto the wings of this part, the wings will fit right inside the slot on each side, and the 1/2" hole in the middle will allow the plunger tube to slide right through it. This prevents the Plunger Tube from rotating all over the place, and also provides a linear constraint on how far the plunger tube can be pushed forward.

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Screw in the winged part to the rear of the PETG tube from both sides. Tap it into the PVC, and countersink. Secure with Pan-head screws. You are NOT screwing into the plunger rod here, this part is meant to slide over the plunger rod.

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Here is the winged part in the blaster, underneath the screw.

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Here it is again. Remember this part is united with the plunger tube, and is meant to slide over the plunger rod.

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Take the SECOND winged part and screw it into the plunger rod. Make sure the rod is oriented with the catch notch facing up, so the catch works properly. Again, THE WINGS INTERFACE WITH THE SLOTS, PREVENTING ROTATION. I secured this part by drilling a 3/8" hole into the plunger tube, drilling a 7/64 screw into the part and rod, and then screwing in a 6-32 machine screw, as shown.


Low Profile Plunger-Stop

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Take your Ace Hardware Rope and string a 2-3 foot length through the hole in the rear of the plunger rod. Drill another hole on the underside of the body tube to get it in.

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You see where I'm going with this. That large hole drilled in the rear bushing is to assist in the positioning of the rope.

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Drill a hole in the top and bottom of the endcap. Thread each side of the rope through the hole and tie it off. To get the right length, you'll need to fire the thing a few times, and tie off both ends. Drill and countersink the rear bushing into the body tube using two of the pan headed screws, one on the top, and one on the bottom.


Lower Receiver

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If this doesn't make sense, just follow along, it's pretty easy.

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Make 2 short screws from the pan headed screws.

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Take a section of 2" PVC and make a slot about 4 inches long through the bottom. This slot allows the assembly to slide on and off of the Body Tube, and allows the catch screw to peek out.

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Take your 1-1/4" PVC Tee and wrap a few wraps of packaging tape around the middle arm. Stick it right into the 2" PVC. Push the Tee all the way Referencing the first photo, drill four holes through the 2" PVC through all layers, in an X-shape. Place two shortened screws on opposite sides of the tube, and Put the longer screws in the remaining two holes. The idea is that disassembly may be achieved by backing out the two long screws about 3/8". The "Lower" may then be removed by pulling the PVC Tee off the body tube. Hence the need for the low-profile plunger-stop.

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Lower assembly removed. A handle is secured onto the bottom of the 2" PVC, not the body tube. This allows for a modular system. You can swap lowers if you have different asthetics on a different lower, or if a blaster has a catastropic break, make a second "upper" assembly, and just replace that. Save yourself the trouble of making another handle. Just my two cents.

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Pumped back. Plunger tube doubles as the pump itself.

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Plunger caught back, Plunger tube/pump pushed forward. Ready to fire.

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Fully Assembled, just needs a grip.


Results
Every blaster is a learning experience. Some will have you bashing your head against a wall more than others. This was one of those. Troubleshooting probably accounted for 6-7 hours with this thing.

I had to reduce the draw length from my standard 6.25" to about 5.5-5.25". At the same time, the volumetric increase is still crazy compared to the standard 1-1/4" PVC plunger tube. Which is why I was baffled why it was actually shooting with less force than my rainbow pump, I'd wager 150-200 fps on a chrony; my Rainbowpump by comparison rocks 220-250.

The problems that plague larger diameter plunger tubes still apply, Namely:
-Increased friction resulting from a larger circumference plunger tube, Where C=(Pi)(Diameter)
-Increased Mass from non-traditional plunger head construction. The bath plug is super dense. As mentioned, I'll likely be replacing it.

On the bright side, the air seal is fucking perfect.


Verdict
Basically, don't build this this if you don't plan to improve it. Because I'm this deep in the rabbit hole anyway, I'll post some kind of update when I get a proper skirt seal from McMaster.


Notes
The thing likes shorter barrels. It's sweet spot is around an 8-9" length of CPVC.

From rear of stock to front of bushing, this blaster is about 3 inches longer than my rainbowpump. When equipped with it's 9 inch barrel and hopper, compared to my rainbowpumps 12" barrel and hopper, both blasters are exactly the same in length. Go figure.

I could have used an omnidirectional, telescoping plunger rod, a la multiple orgasm, but I've had bad experiences with omni rods before. I don't want to talk about it.

The entire hopper and barrel assembly moves back and forth with the plunger tube every time the blaster is pumped. It's oddly satisfying. Could it rattle off in the middle of a war? Perhaps, but the interface between bushing and PVC is very tight. I'm not too worried about it.

The slightly longer length allows for a great grip on the pump/plunger tube, especially if you like to be supa tacticool with your arm extended all the way to the front of the pump.


Thanks for reading. Questions/ comments/ singing telegrams are always appreciated.


Edited by Aeromech, 23 November 2015 - 03:02 AM.

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#2 Drev

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Posted 16 June 2014 - 10:04 AM

I built something a lot like this except using 1 1/4" and 1" PVC. This type of blaster is really cool when you can move the plunger tube as the pump grip, but this type of design just isn't as efficient as the more basic pump action types of homemades. Even with a standard sized plunger tube, I had to make the draw between 5-5.5" just to get the mechanism working. This type of design, while fun, is not very practical. Anyways, good job making the writeup to document the process of building this. Even though it might not be the best blaster out there, someone someday could pick up this design and improve upon it, making it a practical blaster to use. With any new kind of homemade, it is better to document it than just doing nothing. This is a good writeup for what the blaster is. Not super detailed, but still shows how to put the thing together and make it work.
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#3 Birch

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Posted 16 June 2014 - 03:49 PM

What I have found with my expand a blast, which has the same problems as your homemade does with the power, is to not make the seal perfect. Doing this reduces friction, which I have found adds enough speed to the plunger rod to counteract the loss of volume and actually add extra power.
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#4 KaneTheMediocre

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Posted 16 June 2014 - 05:14 PM

While I've done some relevant work with large plunger diameters in the BullPAC series, I don't recall making a blaster called the FAL-1, so I think you've misattributed credit. Also, if possible it's nice to link to the blaster threads rather than just mention the names, although I often forgo this when finding the threads is difficult or if I'm feeling lazy at the time.

Regarding barrels, since the larger PT produces a lower pressure but greater air flow rate (given equal priming force) you may have better results with an all-loose barrel, or at least a partly loose barrel. This in part depends on your plunger speed, so your heavy plunger head may necessitate a tight barrel, but since you are working to reduce your plunger mass I thought this might be relevant.
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#5 Aeromech

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Posted 16 June 2014 - 06:38 PM

...this type of design just isn't as efficient as the more basic pump action types of homemades.


Tell me about it. My next design follows a RBP more closely, those are rock solid.


What I have found with my expand a blast, which has the same problems as your homemade does with the power, is to not make the seal perfect. Doing this reduces friction, which I have found adds enough speed to the plunger rod to counteract the loss of volume and actually add extra power.


Your words make sense, but that air seal is so damn effeminate... If I must I shall, but not without exploring my other options first.

While I've done some relevant work with large plunger diameters in the BullPAC series, I don't recall making a blaster called the FAL-1, so I think you've misattributed credit. Also, if possible it's nice to link to the blaster threads rather than just mention the names, although I often forgo this when finding the threads is difficult or if I'm feeling lazy at the time.

Regarding barrels, since the larger PT produces a lower pressure but greater air flow rate (given equal priming force) you may have better results with an all-loose barrel, or at least a partly loose barrel. This in part depends on your plunger speed, so your heavy plunger head may necessitate a tight barrel, but since you are working to reduce your plunger mass I thought this might be relevant.


Fixed the writeup to reflect this. It was Zorn's creation I meant to reference.

I'm looking into the "Telescoping Barrel" idea, with a tight fit initially, and a loose one after a few inches. That being said, I'm going to see if I can get away with shotgunning this thing. Low pressure/high volume leads me to believe I may be able to throw on a 1 to 4 coupler on the end, and fire off 4 stefans from short barrels.
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#6 Nerf Gra

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Posted 16 June 2014 - 07:07 PM

It would seem to me that a more powerful spring could help alleviate some of your problems. It might be harder to prime but if it turns this thing into a monster it would me worth it IMO
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#7 Zorns Lemma

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Posted 16 June 2014 - 09:11 PM

You should begin the writeup with an overview of the blaster and its operation, followed by a part build of the major components, grouped in order of how important each component is (aka the plunger rod is what almost everyone puts first, but is in fact one of the things that matter least).

This makes these weird tube blasters much easier to understand as each piece now has a context. This was a mistake I made in almost all of my writeups, realizing only now as everyone else's is quite inscrutable.
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#8 makeitgo

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Posted 17 June 2014 - 11:07 AM

Nice concept.

I've found that with larger diameter plunger tubes, in order to compensate for the increased volume being forced out, a stronger spring is needed other than a [k26]. The reason it seems to output less 'power', is because the plunger rod stops accelerating forward sooner, due to the increased volume its trying to push out of a 1/2" diameter hole. The power comes from the acceleration. In essence, the plunger rod reaches a terminal velocity sooner in a larger plunger tube than it would in a smaller diameter one when using a [k26].

As for the mass of your plunger rod assembly. The 2 polycarb disks you used are not necessary. Or at least ones that big. The drain plug is pretty sturdy and really only needs support from behind. Using a decent sized washer behind and a smaller one in front is all you need.

If you wanted or expected the overall length to be decreased by this design, the key is in the length of the plunger rod. The only way to decrease the length of a blaster (while maintaining your expected draw length) is to decrease the length of the effective plunger rod. In my MO, I did this by creating a section in the 'G-spot' that sleeved what would be the tail end of the plunger rod. Effectively decreasing the length of the plunger rod by the total length of the compressed spring length. In your case, you could shorten the plunger rod (and overall blaster length) by 5.5".

Posted Image
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#9 Aeromech

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Posted 17 June 2014 - 12:41 PM

You should begin the writeup with an overview of the blaster and its operation, followed by a part build of the major components, grouped in order of how important each component is (aka the plunger rod is what almost everyone puts first, but is in fact one of the things that matter least).

This makes these weird tube blasters much easier to understand as each piece now has a context. This was a mistake I made in almost all of my writeups, realizing only now as everyone else's is quite inscrutable.


Zorn, when I have the time (probably not til Friday with these damned summer classes) I'll throw up some sketches. I know what you mean though it took me a while to figure out how the FAL series worked.

Nice concept.

I've found that with larger diameter plunger tubes, in order to compensate for the increased volume being forced out, a stronger spring is needed other than a [k26]. The reason it seems to output less 'power', is because the plunger rod stops accelerating forward sooner, due to the increased volume its trying to push out of a 1/2" diameter hole. The power comes from the acceleration. In essence, the plunger rod reaches a terminal velocity sooner in a larger plunger tube than it would in a smaller diameter one when using a [k26].

As for the mass of your plunger rod assembly. The 2 polycarb disks you used are not necessary. Or at least ones that big. The drain plug is pretty sturdy and really only needs support from behind. Using a decent sized washer behind and a smaller one in front is all you need.

If you wanted or expected the overall length to be decreased by this design, the key is in the length of the plunger rod. The only way to decrease the length of a blaster (while maintaining your expected draw length) is to decrease the length of the effective plunger rod. In my MO, I did this by creating a section in the 'G-spot' that sleeved what would be the tail end of the plunger rod. Effectively decreasing the length of the plunger rod by the total length of the compressed spring length. In your case, you could shorten the plunger rod (and overall blaster length) by 5.5".


The polycarbonate discs are artifacts of my experience with skirt seals. I will try this method you suggest before my new skirt seal arrives.

Posted Image

Here is my original sketch of the design. If you can see the telescoping plunger rod was stolen lovingly eloped with this design. However, my fear of omnidirectional rods got the better of me. Having taken a closer look at the catch problems I was having with those, I think I figured out what I was doing wrong. I am actually quite happy with the length it is now, I may change this in a future design.

Anyway, the blaster I have now works with my "A2 Lower" design of lower receivers for the blaster. In short it's a system I use that allows interchangeability between the Rainbow type blasters I produce.

Edited by Aeromech, 17 June 2014 - 12:42 PM.

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