Autobow

Blaster Design and Instructions by KaneTheMediocre

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These instructions are preserved as reference to those who might build something similar, but are not recommended as exact instructions.  In particular, the front 1/2" PVC cross is insufficient for the load produced by the bow arms, and will eventually break.




The Autobow is a bow powered, clip fed, autoloading blaster, made to fire streamlines from standard nerf clips and drums. Range is 30-40 feet ish, but I'm hoping to tweak the design to fire at least 40-50.


Firing video:



Parts: 

Bow
-2 3/4" CPVC Flex rods
-2 1/2" EMT Stubs (~2 in)
-2 1/2" Thinwall PVC sheaths (~3/4")
-1 PVC Cross
-2 10-32 x 1" Screws

Bow Assembly: 

1. Superglue the thinwall PVC sheaths to the EMT, nesting the EMT inside the thinwall, such that the ends are flush.  The glue is not critical or loadbearing, it's just to hold it in place during assembly.

2. Put the assemblage from (1) in the PVC cross, PVC side first.  Make sure that the PVC goes all the way in, bottoming out in the connector.  This can be tricky, because the EMT will want to slide relative to the thinwall, even if you superglue it.

3. Put 2 screws, of no particular size, through the cross on each side.  Center the holes in the coupling region, so that there is plenty of material on both sides of the hole.  This is why getting the PVC all the way in was so critical in (2).  

4. File / dremel / whatever any sharp edges from the screw, then wrap many layers of duct tape over the screws for safety and structure. 

5.  Cut a slit into one end of the CPVC.  My slit was only the width of my hacksaw, but somewhat wider will be better (You'll see why when you string up the bow).

6. The CPVC should slide with some resistance over the EMT stubs.  I never did anything to anchor mine to the EMT stub, and they seem to stay on just fine.

Breech/Guide
-1 Breech Machined sch80 PVC
-1 Guide Machined sch80 PVC
-1 straight PVC coupler
-1 1/2" x 1/2" Wooden stick.
-Mad hot glue.
-Duct tape

Make the breech and guide per schematics.  I have the luxury of a mill and a lathe, if you don't you'll need to get  creative about this.  I think it could be done by nesting Mcmaster 1658T49 (5/8" OD x .527 ID Al), EMT tubes (.719"ish OD x .625"ish ID steel), and thinwall PVC (.840"ish OD .718"ish ID) Both are made from sch80 1/2" PVC, which is where the .840 OD and .526 ID come from. 









To make the breech hold clips straight, I made a hot-glue mold (the same process is used in the raider-bbb).

1. Put paper under the clip, and fold it out of the way.  It's a good idea to have the straight coupler on for this process:

http://img412.imageshack.us/img412/3655/paperunderclip1.jpg




2.  Put the assemblage in a vise, or otherwise prop it up so that you can pour hotglue on it.  Make sure you have a clear workspace without clutter which may be damaged by the hotglue, or just get in your way.


3.  Make sure the clip is aligned with the breech


4.  Pour on the hot glue.  I usually put something on top of the hot-glue which is friendlier to attaching stuff, or otherwise helps to reinforce, since hot-glue isn't known for its structural strength, or it's effectiveness as a glue.  Ideally, all we should ever use it for is moulding to a shape.  For the AB, I used a 1/2" square stick.


Bowstring / plunger
-1 string
-1 3/16" OD, 1/8" ID steel Tube (~1.5 in) (89955K11 is 6 feet for 20$.  Might find something smaller/cheaper, but since I can imagine quite a few uses for this stuff, 6' was worth it for me)
-E-tape
-1 PEX tube


To assemble the whole thing, you need to plug the PVC into the connectors as shown in the photo.  I prefer to arrange them such that the clip points downwards-.  This is absolutely necessary if you want to use a raider drum, but if you're just using clips it looks cooler if the breech faces sideways.  And the lefty/righty distinction is moot, because you can turn the AB upside down.

Cut the PEX tube to size.  Size the PEX by finding the distance from the front of the slot, to the end of the sch80 PVC.  Add 3/4" to that size, and put a hole 3/4" from one end.  Chamfer the front tip (far from the hole) so it passes through the breech more smoothly.

To string the bow, put the machined PEX tube into the guide such that the rear hole is visible through the slot.  Put the steel tube through the PEX.

Put a knot in a long piece of string, then put the string through the slot such that the knot stops it from being pulled farther.  Then feed the string through the tube, and through the slot, knotting the string as before.  Wrap the interface between the steel tube and string in E-tape, with gusto.  To tighten the string (which gives the bow its shape, among other things), wrap the string around the bow a few times, putting the string through the slot on the final wrap.

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