How hard can it be to copy a historic crossbow lath? Not as simple as you may think and I am currently on attempt number 3.
Copying a working bow is an odd experience. You have no excuses and no hiding place, you are literally learning from a master craftsman, and make no mistake, the original craftsman really knew what he was doing. This bow design is very highly stressed, really tricky to tiller, and I desperately wanted to say it couldn’t be done, but you’ve got no excuses when there is a well documented original sitting in a museum display case. Anyway:
Attempt number 1 was a kind of proof of concept, primarily I wanted to see if my measurements made any sense. The timber was not a good choice, with a “smile” profile – a strong left-right curve, but I did not want to waste a prime piece of timber on a proof of concept. The bow actually held together for a good while, the 1mm per inch taper worked well, producing a very strong bow and I was able to get it strung and drawn a couple of inches past brace height but eventually the left-right curve twisted the bow apart. There is absolutely no doubt that my poor tillering technique was the major factor in it failing, I left the centre far too strong for far too long causing too much stress, and set, in the outer thirds of the bow.
Attempt number 2 was going very well until suddenly it wasn’t. Learning from number 1, I got the centre moving early on, the taper was spot-on and made a very strong lath and I took a lot of time to carefully tiller out to brace height. The wood selected had a large central knot much like the original bow. I had often wondered why the original maker had placed the knot off-centre but I chose to put the knot dead centre as I thought the tiller would add strength to this potential weak point. This was a BAD decision. The tiller acted as a pressure-riser in the same way as breaking a stick across your knee. The bow exploded into 6 pieces without warning as maybe 300lb of draw weight suddenly let go just as I was putting the string on.
Attempt number 3 is currently underway. Unlike the original, and first attempts, number 3 is not high-crowned, the back is fairly flat, not rounded. Nonetheless, the bow was laid out with the same dimensions, a 30mm thick centre tapering at 1mm per inch to 16inch tips. As in number 2, the centre was worked on first, but the middle of the limbs was already bending and to get the centre working at the same rate, the middle had to be thinned a lot, in fact, it lost 5mm. The thickness taper is very different from the original but it will be interesting to see how it all works out, maybe even weighing the 3 bows to see if there is any difference in mass may be an interesting exercise.
I am now at brace height, which is where 1 and 2 broke, wish me luck!
Avenue Woodcarving 