So, for no other reason than simply because I can, I decided to make a medieval crossbow. Also called an arbalest in medieval times.
It's one of the more complicated projects I've attempted, simply because I've done bugger-all working with wood before. I have this unshakeable preconception of it as a nasty piece of recycled nature that lurks, waiting until you are attempting the most vital part of any shaping procedure, & then gleefully splits along the grain with wild abandon, scuttling all your plans out of simple human-hating spite. Metal was created by man, so it's much easier to work, but wood, wood is vengeful...
Anyway, seeing as I'd have to base the dimensions of the wooden stock on the draw length of the bow, or "prod", as it was called back in the day, I decided to make the bow first. First stop, Trademe, where I scored a set of leaf springs from an old Jaguar. Naturally, leaf springs are made from spring steel, which is perfect for making a strong, long-lasting bow. So I set out to disassemble the leaf sets & choose a suitable blank for the bow.
The first step was to clean the leaf up, which I did by hitting it repeatedly with a mallet, & then got rid of the rest of the rust with a sanding flap disk & my trusty angle grinder #2. Angle grinder #1 is bigger, & always has a cut-off disk mounted on it, so I used that to cut it to length, & rough-shape the ends into nocks, for holding the string on.
Now, the trick to working with spring steel is keeping the temperature low. If the metal gets too hot, it ruins the temper of the steel, softening it, sometimes to the point where it will simply bend & stay bent. Quite embarrassing if you then pull the trigger & the arrow just falls off the front of the bow. So, after every 20 seconds or so of working the metal, I had to wrap it in soaking rags to keep the temperature down. I kept it down to hot enough to touch, which is probably a bit retentive, as steel doesn't change it's structure until it reaches several hundred degrees, but better safe than sorry, I figured.
After cutting the nocks, I started on the long tapering cuts, in order to make the bow bend properly. It needs to be tapered for several reasons. The first reason is to spread the stress evenly along the bow. If the bow was the same width along it's length, the increased leverage from the tips would force most of the bend to the centre of the bow. By making it easier to bend as the leverage decreases towards the tips, the bow can be made to bend evenly along the entire length .The second reason is to remove weight from the ends. Seeing as the bow has to accelerate the tips in order to straighten & fire the arrow, the lighter the tips, the faster they accelerate, & the faster the arrow is shot. In order to go slowly to keep the heat down while making the long cuts, I had to take almost 2 hours for each cut, stopping & cooling it for 5 minutes every 20 seconds of cutting or so. I have now learnt patience.
The downside of using leaf springs, apart from the difficulty of working them, is the fact that they usually have holes through the centre, creating a weak point right at the place of maximum leverage. I decided to go OTT, ADD, & possibly even NATO in reinforcing the centre, creating a steel sandwich with a high-tensile steel bolt holding it all together. The front slice of the sandwich also gave me a place to attach the foot stirrup, which makes the bow a lot easier to cock. The stirrup is made of mild steel bent & hammered into shape, & then welded onto the front plate.
After completing & assembling the bow, I took it to an archery pro shop to get a string made up. I figure a properly made high strength string is going to keep me whole longer than anything I could come up with myself.
When fully strung, with a 6" brace height, I can draw it back another 12" unassisted, for a full foot of power stroke. Draw weight at 12" is 180 pounds, which should produce a pretty respectable speed with a heavy bolt.
Stay tuned for the next post in the series, in which I impose my will upon the wooden stock.
It's one of the more complicated projects I've attempted, simply because I've done bugger-all working with wood before. I have this unshakeable preconception of it as a nasty piece of recycled nature that lurks, waiting until you are attempting the most vital part of any shaping procedure, & then gleefully splits along the grain with wild abandon, scuttling all your plans out of simple human-hating spite. Metal was created by man, so it's much easier to work, but wood, wood is vengeful...
Anyway, seeing as I'd have to base the dimensions of the wooden stock on the draw length of the bow, or "prod", as it was called back in the day, I decided to make the bow first. First stop, Trademe, where I scored a set of leaf springs from an old Jaguar. Naturally, leaf springs are made from spring steel, which is perfect for making a strong, long-lasting bow. So I set out to disassemble the leaf sets & choose a suitable blank for the bow.
 |
| I choose you... spring. |
The first step was to clean the leaf up, which I did by hitting it repeatedly with a mallet, & then got rid of the rest of the rust with a sanding flap disk & my trusty angle grinder #2. Angle grinder #1 is bigger, & always has a cut-off disk mounted on it, so I used that to cut it to length, & rough-shape the ends into nocks, for holding the string on.
 |
| Not traditional, but effective... |
Now, the trick to working with spring steel is keeping the temperature low. If the metal gets too hot, it ruins the temper of the steel, softening it, sometimes to the point where it will simply bend & stay bent. Quite embarrassing if you then pull the trigger & the arrow just falls off the front of the bow. So, after every 20 seconds or so of working the metal, I had to wrap it in soaking rags to keep the temperature down. I kept it down to hot enough to touch, which is probably a bit retentive, as steel doesn't change it's structure until it reaches several hundred degrees, but better safe than sorry, I figured.
After cutting the nocks, I started on the long tapering cuts, in order to make the bow bend properly. It needs to be tapered for several reasons. The first reason is to spread the stress evenly along the bow. If the bow was the same width along it's length, the increased leverage from the tips would force most of the bend to the centre of the bow. By making it easier to bend as the leverage decreases towards the tips, the bow can be made to bend evenly along the entire length .The second reason is to remove weight from the ends. Seeing as the bow has to accelerate the tips in order to straighten & fire the arrow, the lighter the tips, the faster they accelerate, & the faster the arrow is shot. In order to go slowly to keep the heat down while making the long cuts, I had to take almost 2 hours for each cut, stopping & cooling it for 5 minutes every 20 seconds of cutting or so. I have now learnt patience.
 |
| Halfway through the long cuts. |
The downside of using leaf springs, apart from the difficulty of working them, is the fact that they usually have holes through the centre, creating a weak point right at the place of maximum leverage. I decided to go OTT, ADD, & possibly even NATO in reinforcing the centre, creating a steel sandwich with a high-tensile steel bolt holding it all together. The front slice of the sandwich also gave me a place to attach the foot stirrup, which makes the bow a lot easier to cock. The stirrup is made of mild steel bent & hammered into shape, & then welded onto the front plate.
 |
| The "slices of bread" for a steel sandwich. Not as tasty as you might think. |
After completing & assembling the bow, I took it to an archery pro shop to get a string made up. I figure a properly made high strength string is going to keep me whole longer than anything I could come up with myself.
When fully strung, with a 6" brace height, I can draw it back another 12" unassisted, for a full foot of power stroke. Draw weight at 12" is 180 pounds, which should produce a pretty respectable speed with a heavy bolt.
Stay tuned for the next post in the series, in which I impose my will upon the wooden stock.
Love it, Dan! Can't wait to hear/see how well the arrows fly! Please, just don't aim it at Aimee... we like her in one piece! And, nothing wrong with OTT & ADD or even NATO when needed!
ReplyDeleteLooking awesome :D Can't wait to see the next installment!
ReplyDelete