Wednesday, 31 May 2017

Crossbow Project

Now this project has only been going about 45 years, so it's still in the early stages.
I'm hoping to build a crossbow that is NFAS compliant so I can have a go at field shooting with it, dunno if I'll use it more than a few times, but I've always fancied a decent accurate crossbow.
Over the years the plan has evolved in keeping with my improved workshop, skills and the improvements in crossbow design.
I'm going to be making a laminated prod, bamboo backed with maybe a heat treated Yew belly. I've been experimenting with the amount of Reflex/Deflex, but like most things with bows there is a huge amount of compromise.
Adding lots of reflex gives good early draw weight and adding some deflex gives a good force draw curve the problem is that you get the early poundage at the expense of available draw length and which is the big draw back (chuckle) of crossbows.
To do some experimentation I got some medium tensile aluminium sheet off E-Bay and sawed out a pyramid profile prod with tiny horn nocks. I then did force draw curves with various degrees or deflex and reflex bent in. Being only a medium strength Ali' I couldn't draw it too far without it taking set, but it's probably roughly equivalent to wooden/laminate bow. I used a slightly scaled down bow for reasons of cost (750mm ~ 29.5") where the real bow (excuse if I mix the terms bow and prod) will probably be about 31-32".
The graph shows a straight prod vs heavily deflexed/reflexed. You can see I've drawn the straight one an extra 3" before the set kicked in ( I should really have repeated the test, but it gives you the idea). I'll probably end up with a bow somewhere between the two.

Modern crossbows (decent ones) tend to have individual limbs and are centre shot, rather than having the bow mounted below the track and set at an angle or the bow having it's tips kicked upwards, this removes the friction of the string along the top of the track (or stock or whatever you want to call it). The bow mounting will be one of the challenges, but a bit of thick plate steel and some welding should do the job, I'm not going to be trying to cast Aluminium... (well not just yet!)
The other big challenge is the trigger mechanism/safety catch. I've made 'em before but not with a safety catch, there are some good videos on Youtube, and I've basically copied this one so it won't be my own design, just my implementation of it. I've made a mock up in plywood initially so I can see it's eminently doable.

Meanwhile the Yew Primitive is waiting to be signed and waxed having had the last few tool marks taken out and some more coats of Danish Oil.


  1. Could you hurry up a bit? I figure I've only got about 10-15 years left and I would like to see this finished.