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Discussion Starter · #1 ·
Hello all,

I've been fiddling with a mini crossbow since I got quarantined inside. I'm still green at bowmaking, but it didn't stop me from blowing through 4 tiny staves. I'm hoping by posting my struggles y'all could help me to figure out a better way, and perhaps some of you will be entertained or get learned off it.

First off, here's a video of my rapidly failing most recent attempt to give you an idea of the whole project.

Now here's the first rounded-belly bow of osage that whose back broke under tension. I confidently reflexed it and microwaved it for 40 seconds three times because I was excited to try the crossbow out. It's a little over ten inches long.
Finger
Wood Branch Finger Wand

The second was of black locust with an almost equilateral triangle profile down the length. It also ultimately broke its back, though before that I could see a number of areas of compression on the belly. It's about 12 inches long.
Finger Wood Ruler Hand Tool
Wood Blade

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Discussion Starter · #2 ·
(thread cont)

The third one was more of a flatbow, but when it broke I was so frustrated I snapped it a few more times and now I can't find it.

The fourth is the one in the video and still in the bow. For this one I glued together with titebond II 8-10 pieces of (maple?) veneer I had lying around and backed it with a piece of silk. Early in tillering I noticed a belly area with some compression where the glue had failed between two pieces (where the most obvious bend is now). I attempted to hit it with glue and slapped a strip of bamboo on the belly in hopes of fixing that spot, and that worked for the day before it finally gave in, the first layer of veneer on the back breaking under the silk. It is 12 inches long.
Wood Metal Brass
Finger Paper Wood Thumb Art
Bow and arrow Bow Thread Arrow Joint
Text Font Hand Textile Finger


So I hope you found that all interesting. Are any of you seeing my patterns of failure? Something I could do better? I seem to break them all close to the same spot, and I'm unsure of what might be my best strategy for making such a small bow.
 

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Probably want to move this to the crossbow section. However they are almost all modern equipment people.

Better yet the arbalist guild.

Short answer probably over drawing it for its length. Draw length should be no more the length of one working limb.
 

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Discussion Starter · #4 ·
I figured the part I'm having a trouble with is not specific to crossbow, but I will take a look at the arbalist guild as soon as my registration is approved. Thanks for the recommendation!

The working limb length starting just outside the stock body to the string groove on the fourth bow is about 5 1/4", while the draw length from the inside of the belly to the trigger groove is 4 3/4"

What would you say the limb length ought to be?
 

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That looks like a mini traditional crossbow. My first guess, without close inspection of the wood, is that you had a hinge in the tiller of the limb so you are over stressing at the beginning of the bend. You want a consistent bend throughout the limb.

To check the tiller we need to see the bow before the limb blew up.

I used to make boomerangs. This looks like a very similar project. Lot's of fun and educational. You can work through the important factors until you land on something that works.
 

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Discussion Starter · #7 ·
If they keep breaking so close to the center, I would guess my tillering is consistently too thin there. Does that seem right?

Also, would you think there are any problems with such a small size? I end up using just a small portion of the stave, barely two growth rings, if that. That why my last try was composite, to try and get more "growth rings".
 

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If you have a hinge then it means you have a soft spot between two hard spots. You have to thin the hard spots, which may ultimately mean you have to thin all the way down the limb.

I call a spot that bends too much a soft spot and a spot that bends too little a hard spot.

You need to strive for a continuous bend down the entire length of the limb.

I wonder if this is something that can be practiced with Popsicle sticks since you are already going mini. See how much you can get a Popsicle stick to bend by thinning and tillering.

Mount one end in a vice and use sand paper to tiller. Test by seeing how much more you can make it bend over an unmodified Popsicle stick.

I may have to try it. It would be a fun contest where we could all learn a bit about tillering.
 

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Discussion Starter · #9 ·
It certainly is a quick way to practice, and I feel like the stakes are high, there's less margin for error.
 

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Just to get something working...why not try continuous thickness through the length of the bow/limb? Maybe try steel so you don't have to worry about homogeneity of the material/imperfections?

I have an old powermaster pistol crossbow that used an aluminum bow and was a consistent thickness through the entire length. It hasn't fatigued yet.
 

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just for comparison the powermaster dimensions are approximately:

bow length (unstrung): 22 inches
bow length (strung): 21.125 inches
'brace height': 2 inches
'stroke length': 5.5 inches
'limb thickness': 0.25 inches for entire length

please forgive my incorrect crossbow terminology
 

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If they keep breaking so close to the center, I would guess my tillering is consistently too thin there. Does that seem right?
Yes, assuming materials don't have any particular weak points, make it thicker where it breaks.

You can tiller it by drawing a circle of a given radius, larger radius to start with, and tiller it so that it bends evenly, removing material selectively so that it conforms to the circle, then move to a circle with a smaller radius.

Keep in mind that if you're making a miniature crossbow, and using just plain wood, you're asking a lot from a little limb. To make it work reliably without breaking, it will mean that the limb as a whole will have to be pretty thin, so that the curvature puts less stress on the fibers, which means not likely to be hunting much more than bugs with it.
 

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Are you rushing through the tillering phase to get to shooting? Building a tiny selfbow still requires patience.

The thick center puts more stress on the limb. Each working limb should be at least as long as the draw length for the sake of durability. Wide and thin are the way to go with wood, and making the limbs at least 1" wide in the center will help. You can make the limbs pyramid in profile which requires very little thickness taper. So 1 1/8" wide at the fades down to 3/8" wide at the tips with a very mildly taper thickness, or even a uniform thickness, should work. Tillering is the same as a full size bow: go slow, and don't bend it any further if a hinge starts to develop.

Hickory and white oak will be some of the strongest woods easily available in terms of tension. If you get a design that works, you can heat treat these woods for exceptional performance... but that's after you get something working. The same can e applied to Osage and locust, but those woods don't tolerate rapid drying as well.

If you're completely new to bow making I recommend Paul Comstock's "The Bent Stick". It's a short, thorough, to-the-point booklet on bow building and will cover all the important bits. Here's a breif intro to tillering a bow to avoid set: http://www.primitivearcher.com/smf/index.php?topic=58212.0
 
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