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The Snap Crossbow

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#1 Carbon

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Posted 30 July 2010 - 11:13 PM

Working on my version of Kane's AaBow got me thinking about bows as a power source in general...which of course, led me to the Crossbow. I mulled over some needlessly complex ideas, until I realized that a basic SNAP could be adapted into one. So, here's the result:

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The main issue was dealing with the barrel. On my bow, the barrel sticks through a 7/8" hole bored through the bow and 1.25" tee. Since I wanted to use 1/2" CPVC for the arms this time, though, that wasn't an option. Here's how the assembly worked out for holding the arms and routing the air:
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As it turns out, a 3/4" tee will fit into the front space in a 1.25" -> .5" reducing bushing, if the supporting ribs are bored out. I cut them out using a drill press, and seated the tee in place with a rubber mallet. Seating the tee forced the elbow back into the reducing bushing, creating a seal.

For placing the CPVC elbow, I just drilled a 5/8" hole far enough up to keep the bottom of the CPVC elbow flush with the PVC tee. The elbow popped in with a bit of force.

Dealing with the arms: 1/2" to 3/4" CPVC couplers seat very well in 3/4" PVC tees (at least, using a rubber mallet). I bored them out with a 1/2" forstner bit on the drill press, and the 1/2" CPVC was able to slide through with a bit of friction.

The bowstring is 1/16" steel cable, anchored with U-bolts. The arms can be unstrung, as one side is just resting in a notch.

The plunger is a bog-standard SNAP-1 mk2 plunger head, with two alterations. I added one cotter pin behind the catchface endcap, since there is no spring keeping it forward. I also added a second cotter pin in the middle of the plunger head, to better anchor the plunger head to the plunger rod. The trigger is a basic clothespin trigger.

The barrel is currently a basic RSCB...not any limitation, since it needs to be pointed down to prime it, anyway.

I'll get ranges this weekend, as I adjust it.

EDIT: After replacing the ridonkulously short barrel, I'm getting ~80-90 feet. Which tells me that I need a way longer barrel on my bow, because no way should the crossbow be getting similar ranges...

Edited by Carbon, 31 July 2010 - 10:50 AM.

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#2 Broderick

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Posted 30 July 2010 - 11:26 PM

Wow, I am starting to work on a version of the AaBow myself. I just need a 1 1/4" Slip tee before I can start, since my Lowes is out. ;)
You should call this "The Real SnapBow". When I got to posting a write-up, that's what I would have called it. That's just some steel cable? I was planning on using Mini-Bungees. Do the PVC bow arms seem like it will crack at all? That's what I've been worrying of.
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#3 Carbon

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Posted 31 July 2010 - 12:33 AM

That's just some steel cable? I was planning on using Mini-Bungees. Do the PVC bow arms seem like it will crack at all? That's what I've been worrying of.

For a real bow, you don't want string stretch, at all. All the energy is provided by the bow arms.

PVC won't crack...it'll just fold when/if it fails. Anyway, I'm going to see what kind of range and energy the 1/2" CPVC can give me. If it ends up not being enough, I'll upgrade to 1/2" PVC, or change the length. A shorter bow increases the draw weight.
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#4 CaliforniaPants

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Posted 31 July 2010 - 12:38 AM

This whole bow building business seems terrifying to me. I've had pvc shatter and break on me before when bent, and this seems like a lot of constant stress to be putting on something not meant to bend and bend back. Its a really cool idea and allows for some neat stuff, but it just seems quite scary.

Also, are the bow arms one solid piece of pipe, or two separate ones?
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#5 Carbon

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Posted 31 July 2010 - 12:43 AM

This whole bow building business seems terrifying to me. I've had pvc shatter and break on me before when bent, and this seems like a lot of constant stress to be putting on something not meant to bend and bend back. Its a really cool idea and allows for some neat stuff, but it just seems quite scary.

Also, are the bow arms one solid piece of pipe, or two separate ones?

In researching PVC bows, I read a few writeups that recommended wrapping the bows in a layer of fiberglas tape, but none of them warned against eventual shattering. All things considered, though, some external reinforcement is probably not a bad idea....as the pipe ages, it *will* get brittle.

The bow is one solid piece of pipe, threaded through the tee (that's why I bored out the couplers). Two separate arms not glued or screwed in *would* be terrifying.
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#6 CaliforniaPants

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Posted 31 July 2010 - 01:38 AM

This whole bow building business seems terrifying to me. I've had pvc shatter and break on me before when bent, and this seems like a lot of constant stress to be putting on something not meant to bend and bend back. Its a really cool idea and allows for some neat stuff, but it just seems quite scary.

Also, are the bow arms one solid piece of pipe, or two separate ones?

In researching PVC bows, I read a few writeups that recommended wrapping the bows in a layer of fiberglas tape, but none of them warned against eventual shattering. All things considered, though, some external reinforcement is probably not a bad idea....as the pipe ages, it *will* get brittle.

The bow is one solid piece of pipe, threaded through the tee (that's why I bored out the couplers). Two separate arms not glued or screwed in *would* be terrifying.

That is certainly reassuring to hear about the bow arms and the failure characteristics of PVC, hopefully mine was a fairly old piece of something. Did you pick up the wire from home depot or some other hardware store?
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#7 Draconis

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Posted 31 July 2010 - 02:09 AM

Carbon, I love it! Also, if you become insane or something, and want more power... You should be able to replace the CPVC arms with nylon rod very easily. Both McMaster and Fastenal carry 5/8" rod which, naturally, would fit right in to your couplers. Otherwise you could use the 1/2" rod and just drive on a piece of CPVC as an adapter. Of course, with all the Bow action lately, my backburned bow project won't seem as novel when I finally finish it. Oh well.
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#8 TantumBull

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Posted 31 July 2010 - 05:13 AM

This is sweet. I've been turned on to bows ever since seeing Hilt's bow in action at Effeminate IV. Maybe not as war practical as simply using a spring, but the exponentially increased fun factor makes it well worth it. Great job with the air routing too, that's pretty damn slick.
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#9 Carbon

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Posted 31 July 2010 - 09:08 AM

Pants: I bought the cable at True Value, but most any hardware store will carry it, usually by the rope and chain. It was 10 a foot. The cable anchors are the pricey part, since the instructions recommend using two per end....but they're still only 80 each.

Draconis: Thanks for the suggestion about the nylon rods. I may end up wanting to swap out the arms, since I have no idea as to how durable CPVC will be.

Tatum: Yup, that's what made me want to build one as well, the sheer fun factor. Not the most practical blaster type, but oh, so pleasing to use. The other half of the equation is using one with a hopper clip. I gave passing thought to a bow before, but loading one always seemed like a major pain. A hopper clip makes up for a bow's relative lack of accuracy by allowing for some really fun fast shooting.
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#10 nate the great

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Posted 31 July 2010 - 09:33 AM

I would highly recomend NOT using nylon rod. In my experience, I found that cpvc will bend when it fails but nylon will shatter.
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#11 altair9x9

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Posted 02 August 2010 - 08:12 AM

that is insane
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#12 KaneTheMediocre

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Posted 02 August 2010 - 01:48 PM

Nice work on the crossbow. I'm curious to know how long of an RSCB clip it can use at what performance loss, because I like that it's not as awkward to include in the frame as a hopper, and as you say it doesn't affect refire in this context.

Also, if you use a string, and cut V notches on the ends parallel to the ground plane, you can make the tension adjustable. Just tie a knot on each end of the string, and the V notch will pass the string but not the knot. Then, wrap the string around the bow arm outside of your starting knot. Do this on both sides, and your tension can be adjusted by changing the number of wraps around the tube. That said, you need do something to make sure whatever the string passes through on the plunger will not cut the string.

This whole bow building business seems terrifying to me. I've had pvc shatter and break on me before when bent, and this seems like a lot of constant stress to be putting on something not meant to bend and bend back. Its a really cool idea and allows for some neat stuff, but it just seems quite scary.

Also, are the bow arms one solid piece of pipe, or two separate ones?

In researching PVC bows, I read a few writeups that recommended wrapping the bows in a layer of fiberglas tape, but none of them warned against eventual shattering. All things considered, though, some external reinforcement is probably not a bad idea....as the pipe ages, it *will* get brittle.

The bow is one solid piece of pipe, threaded through the tee (that's why I bored out the couplers). Two separate arms not glued or screwed in *would* be terrifying.



The bow arms don't seem to fail that way. PVC connectors, on the other hand, REALLY don't like to be bent. Don't use a simple PVC cross, like the Autobow. The cross has broken and been replaced twice now. The Aabow uses a 1 1/4" T with short 1 1/4" stubs of pvc or a bushing stuffed in the sides, and 1" PVC all the way through the T. That seems pretty much invincible so far, even though it has a hole in it. I'm working on a smaller version with internal steel reinforcement to a normal sized PVC cross.

Edited by KaneTheMediocre, 02 August 2010 - 01:55 PM.

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#13 Carbon

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Posted 03 August 2010 - 11:40 PM

Kane: I added a longer clip tonight. Let it be said that Choppers rock. I routed the air from the bottom to keep it from getting too tall:

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I can get nine darts in it now. No apparent loss of force. I'll check actual range tomorrow to see if there's a loss. Amusing side note: if I route the air from the top, it looks like a pterodactyl.

Posted Image

The string I'm using now is also adjustable (I used bolt clamps, not the crimp style)...loosening and ightening bolts is a lot slower than your method, though. However, I have the advantage of being able to easily untension the arms, since the ends are a simple loop in a notch, and not wound.

Also, I can vouch for the strength of your bow method. I've put dozens of darts through mine at this point, and can see no signs of weakening.

Bob: Thanks for the suggestion on the arms, I'll see what I can find. And this is definitely screaming for a paint job. Decisions, decisions.
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#14 Zorns Lemma

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Posted 04 August 2010 - 12:55 AM

Rather than using a hopperrette, a BRYSC might actually give you the smoothest LOS, with the profile of the blaster. You have to point it down anyways so there aren't really any ROF differences.
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#15 Carbon

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Posted 06 August 2010 - 11:21 AM

Rather than using a hopperrette, a BRYSC might actually give you the smoothest LOS, with the profile of the blaster. You have to point it down anyways so there aren't really any ROF differences.

I tried out a BRISC last night, and it seems to be about the same. Biggest problem is the air routing: I think that since air leaving the plunger has to already practically bend back upon itself, I'm probably negating any benefit of the less turbulent airflow. That, and since I want the dart hopper on top, it adds dead space.

Anyway, range testing, with some amusing results. With the ten shot chopper, I was getting in the sixties. Then I discovered that I hadn't vented behind the plunger. Added a few holes. Bam, back up to 80s again. Which may mean going back to a four shot RSCB might break 100. Probably just going to try a smaller hopper, though, as priming really doesn't have to be done pointing down. I think I wasn't doing that at first out of fear of the arms snapping back directly into my face. It primes very nicely with one hand on the "foregrip" area, and the trigger hand pulling the plunger. (The balance of the blaster seems to require that.)

Edited by Carbon, 06 August 2010 - 11:24 AM.

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