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Pullback Kit Assembly

Getting a handle on the basics
homemade writeup spring 3D printing

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#1 Aeromech

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Posted 15 July 2015 - 01:14 PM

Always exercise proper caution when working with tools. Only you are responsible for your safety and your property.

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Preface
This blaster is designed to be an easy to assemble blaster that also teaches the fundamentals of homemade building. This blaster tutorial will include techniques such as:
-Tape and goop for seal creation
-Open slot cutting
-Countersinking and chamfering
-Mounting screws internally in pipes using opposite-wall entry holes
-Assembling a rainbow catch
-Cutting a unidirectional catch in Nylon rod

What You'll Need
Included in kit:
-All 3D printed parts
-All hardware
-All springs
-Skirt Seal
-Nylon rod
-PVC Bushing
-1/4 and 1/3 circumference spacer

Not Included:
-13" of 1" PVC pipe
-14" of 1-1/4" PVC pipe
-Plumbers' Goop
-Silicone Grease
-Packaging Tape
-Course sandpaper
-Fine sandpaper
-Tiny piece of blue tac

Tool required:
-Safety Goggles
-1/2" or greater countersink bit
-Long shank countersink bit, 1/4" diameter*
-Dust Mask**
-Safety Gloves
-Orange Duct tape
-Small rat tail file or square file <3/8"
-Wood Saw
-Hobby Knife
-Thin, long shank screwdriver (Radio Shack sells these)
-Power Drill
-1/8", 5/32", 5/16", 1/2" drill bits
*This costs 25 USD online. You If you choose to use a drill bit instead use EXTREME CAUTION, as you risk drilling through your holes and I do not recommend it.
**If using a rotary tool. Work outside, as PVC dust is nasty.


For this writeup, descriptions will be shown BELOW their respective photos.
Put on your safety goggles and let's get started.



Plunger Tube
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If you haven't done so already, cut the 1" PVC to 13", and the 1-1/4" PVC to 14" overall length. Gently sand off the ends to ensure they are square.

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Wrap the PVC busing in packaging tape. Wrap it about a dozen times in the tape, and if it doesn't fit in the 1-1/4" PVC, remove one layer at a time until it is a SNUG fit. This should be as tight as possible before any kind of adhesive is added. This is the trick to geting a good forward seal.

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Cut off the excess tape when you are satisfied with the fit. I am using my pocket knife here, but a hobby knife is MUCH safer. You'll want to go outsde for the next few steps, as we'll be working with plumber's Goop.

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Place in the bushing about halfway into the PVC. Orientation doesn't really matter, but if you want it to look a little cleaner, line up one of the faces with the writing side (bottom) of your pipe.

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Goop around the entire circumference of the bushing on the tape. Make sure the entire circumference is well covered.

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Push the thing all the way on and wipe off the excess that pops out the top. If it doesn't want to go all the way in, tap it in place with a hammer.

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I sealed the end of the bushing with duct tape to make it less stinky. This will be removed upon completion of the blaster.

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Measure 7/8" from the from of the tube and make a mark.

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Use the spacer provided to mark out 3 equidistant spots about the circumference at the 7/8" mark just made. This means that there should be 3 point marked about the circumference, each 120 degrees apart from each other.

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Use the 1/8" drill bit to drill through the pipe and the bushing

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Countersink these parts so that the flat-headed (I often call them called cone-headed)screw heads will fit flush.

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Sink three 6-32 x 3/8" flat head screws in the holes, as shown.



Preparing the Stock

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Measure 1.25" from the rear of the 1" PVC, and make a mark on the bottom side (writing side).

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Drill a 5/32" hole at this point...

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then drill this out to a 1/2" hole. Mine is messed up because I used an improper, stepped drill bit. If this happens to you, it isn't that big of a deal, as it will be covered by the stock later.

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Draw two parallel lines extending forward from this hole. Make sure they are spaced half an inch apart, this channel will eventually hold the priming bar, and will prevent the bar from rotating. It is better to cut out this channel to be half an inch wide, and then widen it later with sandpaper if needed, than to make it too big the first time and have to scrap the part.

DO YOU HAVE YOUR SAFETY GLOVES ON YET? YOU ARE GOING TO NEED THEM FOR THIS NEXT PART. IF YOU ARE A KID, GET AN ADULT FOR THIS PART.

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A long straight saw makes long straight cuts. Use your wood saw to cut down the length of the line drawn. I hold it from the back of the tube, and cut from the front, until I have enough of the slot cut out to hold it from the front, and get my hand out of the way of the saw. Use just the first couple of inches from the end of the saw to make shallow saw strokes, and BE PATIENT. This is the most difficult part of the build, and if you get past this, the rest is basically just a bunch of drilling.

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There we go. Whew. Glad that's over. This one was actually cut a tad too wide, and I needed to make a new stock tube. Again, it will save you a lot of aggravation to just cut the slot 1/2" wide and then spend 5 minutes of sanding than cutting these darn slots again.

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Wrap the open end of the stock in ten or so layers of tape just like the front bushing.


Preparing the Body Tube

All measurements will be referenced to the back face of the body tube in this section.

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Take the 1-1/4" PVC Plunger Tube/Body tube that was being worked on earlier, and Draw a line down the bottom of the tube on the writing side.
Mark 3-1/8" from the rear face and drill this mark out with the 5/32" drill bit.

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Measure 1.5" from the rear face, 2", and 2.5" on the bottom line. Drill out these holes with the 1/8" drill bit.

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Use the spacer to mark a point opposite the bottom line. We want a top line to be directly opposite the bottom one, as we will be inserting screws from the top-down. Just follow along.

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Measure out 1/5", 2", and 2.5" from the rear face, and mark them on the top line. Drill them out with the 1/8" drill bit,

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Drill them out to 5/16". Additionally, mark a point 3-7/16" (between 3-3/8" and 3-1/2") from the rear face, but do not drill it.

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Line up the spacer as shown with the mark that was just made. The sides should be 45 degrees offset from the top line. Mark points on the 4-divisions marks, so that 4 points are spaced 90 degrees from each other, and each is 45 degrees offset from the top or bottom line.

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Drill all four of these holes to 1/8", and countersink them.

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Like this.

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Next, you'll need an intake hole. Mark a point 4.5 inches from the rear face on the bottom line, and drill it out to 1/2"

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This is the internal countersinking, where the long shank countersink bit is used. It is not impossible to use a drill bit, but it is very difficult, and you could very well destroy the work you've done so far if you use it. If you have to use a drill bit, my advice is to use a VERY low speed, hold your part and the drill very tight, and constantly back out the bit so as not to grab the material and drill all the way through.



Assembling the Rainbow Catch

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Insert one screw, 6-32 x 1" flat head, all the way into the rear catch plate.

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Screw on the forward catch plate by rotating it into place. The middle catch piece should fit loosely between the two plates and be able to move up and down freely.

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Sink in the other screw through both plates. The middle catch piece is shown here as well.

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This is actually a bit of a tight fit. You should be able to clearly see some daylight between the plates and the middle catch piece. Loose catches are good catches, yo.



Assembling the Plunger Head

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Gather your materials. The screw used is a 6-32 x 2" pan head. Not shown is the #6 washer that should also be used.

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Here is the assembled plunger head. Note the washer on the front and the orientation of the front plate. The wide side should be facing outward.



Preparing the Plunger Rod

Remember I said the slots were the hardest part? I lied, this is the hardest part.

A word on catch rod theory: Omnidirectional catches are a bit easier to make, and, as the name implies, catches no matter how the rod is twisted around. This makes them ideal for non-guided rods in some designs, but in this one we have the benefit of that guiding slot we made earlier in the stock tube. The reason I chose the unidirectional style of catch is because an omnidirectional catch almost always binds with the spring that is wrapped around it, and creates a "Sticky" sort of prime when the spring binds with the deep depression of the omnidirectional rod. In conclusion, there is no reason this could not be made as an omnidirectional design, I just want a smoother pull.

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Stick your rod in your clamping device of choice. I am using a cheapo pana-vise that has seen better days.

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Mark out two points from one of the ends of the rod, 3-3/4" and 3-3/8" This will henceforth be the "front" of the rod. Score it with your woodsaw so the cuts are about 40% of the way through the rod. Make sure the cuts are "flat". The idea is to create basically a flat surface at the termination of theses cuts. Just keep reading, you'll see what I mean.

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Use your hobby knife to CAREFULLY remove the large portions of material from the trench. DO NOT USE A GIANT POCKET KNIFE LIKE I DID.

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Here's what it looks like after rough material removal. Continue doing this until it is flat on the bottom of the trench. I used the tip of a rat tail file here, but a small knife should work.

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Now it's relatively flat

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Now, at the FRONT of the trench, AND ONLY THE FRONT, make a cut 45 degrees from the front wall of the trench. Smooth it out a little with sandpaper. Cool, catch notch is all done!

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Using the 1/8" bit, drill 1-1/4" depth on both sides of the rod. Make sure they are centered and straight!



Internals Assembly

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Test fit the pullback handle onto the REAR of the plunger rod. Use one or two wraps of tape on the plunger rod to make it fit snuggly.

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Use the 6-32 x 1-1/4" pan head screw in conjunction with the remaining #6 washer to adhere the pull handle onto the plunger rod.
Also difficult to see here is the orientation of the catch notch. Line up the catch notch with the top screw hole on the pull handle. This is necessary for proper function.

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When the notch and screw hole are aligned, drill a 1/8" hole into rod through the hole, until you just begin to scratch the 1-1/4" screw used to hole the pull handle in place. Sink a 3/8" flat head scre into this hole to lock the rotation orientation in place.

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Assemble the innards as shown, with the protrusion on the rainbow catch facing backwards and down, and the hole on the bottom of the middle catch piece also facing down. Add the spring, and then screw in the plunger head. Boom.



Installing the Handle

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Use three 6-32 x 3/4" flat head screws to install the handle. Hold it tightly against the body tube and line it up with the three 1/8" holes on the bottom spaced 1/2" apart. Insert the screws from the top in the 5/16" assembly holes and tighten them down ONLY HAND TIGHT. DO NOT OVER TIGHTEN THESE SCREWS. Just ensure your countersinks are deep enough to essentially "hide" the heads of the screws.

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If the handle is slightly blocking one of the rainbow catch holes, simply use your countersink bit to SLOWLY AND CAREFULLY remove some material.

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Like that. It doesn't look pretty now, but It should be mostly covered by the screw in a few minutes.



Installing the Internals

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Take your silicone grease and SMOTHER the rubber seal in grease. If you think you have enough, add another couple finger-fulls. Seriously, a well greased seal is the make or break of these.

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Insert your plunger rod shish-kebab into the rear of the body tube. Here you can see the correct orientation of the rainbow catch.

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Here you can (sort of) see the taped end of the stock tube. Cut the tape flush with the slot.

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Use the stock tube as an assembly tool to push the rainbow catch into place, and also to turn it. The little nub on the rainbow catch interfacing with the slot allows for this.

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Line up the holes in the rainbow catch with the four holes you drilled to match it, you'll have to push against the main spring, but that's OK, this is what we want. Get one screw in and even if the others are slightly misaligned, things will work out OK. Sink them all in.

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Told ya. It's not that bad. You can touch it up with a TINY amount of Acetone if it really irks you.

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Use the spacer to measure out three points 1/2" from the rear.

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Insert the stock tube as far as it will go. Align the slot with the bottom of the body tube. Then drill and countersink your points, and sink in three 3/8" flat head screws. I ended up swapping this out for a blue one.

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Line up the stock with the handle...

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Just like that. Drill through the two screw holes and sink in the 3/8" pan head screws.



Finishing the Handle

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Insert the zip ties into the two topmost holes on the handle.

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Pull them tight, and make sure the clips are oriented like this. Chop off the ends and sand them down.

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Insert the 3/8" pan head screw about 1/4" into the trigger, as shown. This prevents it from swinging all the way out of place.

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Insert the tiny spring over the 3/4" flat head screw, and use a tiny piece of blu tac to lock it onto the screwdriver.

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Insert it into the bottom hole in the handle, all the way through.

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Insert it into the bottom hole in the body tube, until it grips into the bottom hole in the catch we installed earlier. Wiggle it around until it grips, then screw it down three or four turns. If it scrapes against the nylon rod, turn it out a turn, if it's too loose, screw it in another turn or so. You get the idea.

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Use a 1-1/4" pan head screw to go through the bottom hole on the side of the handle. Hang a #6 washer on there.

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Add the trigger in there and screw until the other side of the screw is flush with the side of the handle. DO NOT OVERTIGHTEN.



The Finished Product

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Damn son. Those be some sick lines right there. Make sure you go over the slots with some fine sandpaper for a smooth prime that wont eat your fingers. It fits all four of my small fingers and get a three finger grip for people with normal sized hands, though the best way to prime it is to use you palm and the web around your thumb with the stock against your shoulder, as if you were going to open-palm slap your shoulder. The priming handle does not hit your hand, and I honestly like it a little closer to the handle, but people were concerned about wrist-slap, so I adjusted for it. You can easily hack off an inch from the plunger rod if you want.

Additional Data for Nerds:
Ideal barrel length is 12 to 14 inches, Overall length is 42 inches with hopper assembly and a 14 inch barrel.
Length of pull (Distance from front of trigger to back of stock): 15-3/8"
Assembled weight with barrel and hopper assembly is 2 pounds even.
All 3D printed parts are 6 walls thick with a 40 percent interior density. handle layer height is 0.25mm, with all other parts at 0.15mm layer height.
Not designed to be dry fired.

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In bed, yo. Damn effeminate at dat angle.

Kits on sale soon.

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Edited by Aeromech, 23 November 2015 - 01:03 AM.

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#2 blazenb

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Posted 15 July 2015 - 02:31 PM

Props to you Aeromech very innovative where you put your priming handle.
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#3 shmmee

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Posted 15 July 2015 - 04:07 PM

Beautiful write up. The impact 3d printing has had on this hobby is amazing.

If you have access to a drill press you can cut your long channels easier by setting it to high, borrowing a spiral cutting bit from your dremel and fencing/bracing the pvc in place so it can't move anywhere but back and forth. It's a little more set up and prep but once done all you need to do is turn the press on, drop the bit into the pvc and pull the pvc backwards without rotating it. The spiral cutting bit in a drill press will be a whole lot quicker than a hand saw for cutting that long channel.
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#4 Aeromech

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Posted 15 July 2015 - 04:59 PM

Props to you Aeromech very innovative where you put your priming handle.


Posted Image
Borrowed the idea from a Kel Tec Sub 2000. The priming handle is on the bottom underneath the stock. Granted it's semiauto, so you don't need to charge it every time, but I draw a lot of inspiration from firearms.


Beautiful write up. The impact 3d printing has had on this hobby is amazing.

If you have access to a drill press...


Unfortunately I don't, as I just dropped a fuckton of money on the previously mentioned 3D printer. But yeah it's pretty nice. This blaster, including barrel and hopper assembly, takes under three hours to assemble once the parts are printed. A similar blaster would take 2 days using purely conventional methods.
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