Lost and Foundry - Crossbows

Hey everybody what are you building?
I'm Roger and welcome to the Lost and Foundry. Today I'll be learning how to not to make a crossbow. If you already know how not to make a crossbow, this probably won't be exciting reading but if you want to hear how it's done the let me take you on a journey.

Right off the bat, lets talk materials.
I've got three options for the prod - wood, metal, plastic. Let's break down those choices a little so you can see where I'm coming from.

Wood. This is going to look The Boss, but is the hardest material to work with. Wood is an uneven density, with uneven strength in compression and extension. This means I can't just "cut" a crossbow prod from wood and have it work. I'll have to start carving and then get it under minimum tension so I can see it deform and slowly work it into shape.

Metal. This is me. I'm not a wood guy. Steel is my bag right now. Spring Steel is incredibly uniform as a stock material, and I can only make it worse by working it. Thankfully, I can make a jig that applies even force to both sides and get a nice smooth curve, and send it off for heat treat and temper so that it comes out just right.
The downside with this is that I'm not really going to "make" anything here. I'll be buying flatbar spring steel, putting a primary curve and maybe recurve in it and sending it off for heat treat.

Plastics. These are lightweight, and fairly easy to shape, with high restitution, high availability, good uniformity, and very low materials cost. There are good reasons why this the material of choice for a lot of people starting out. You start with something that looks like a straight pipe, and fashion it into the business end of a crossbow so it's a fairly satisfying build.

So I'm "plumbing" for option three - thermoplastics. I'm using a synthetic thermoplastic pipe which is easy to shape and thankfully there is a plumbers suppliers just up the road. Need a tool? Make a tool! and the first tool for me is a flattening jig to form the overall shape.

Lesson 1 - Keep your recurves even. This prod failed because of a tiny kink at the start of the recurve that folded and buckled under about 15-20lbs of force.

Lesson 2 - Don't attempt to cold cut. The material isn't shock resistant and you can cause fractures. I was marking the shape near the nocks with a cold cut chisel, and decided I could tap it all the way through. Of course that doesn't reflect reality at all.

Lesson 3 - Don't overheat the centre. This second prod failed because the centre deformed a little out of my control when forming the primary curve. Since it's being formed by hand you want to work as cold as possible so that there is no sagging. The rule of thumb here, is that if it's too hot to hold your thumb against then its too hot.

Lesson 4 - Flatten the centre a little.
This third prod failed at about 70-80 lbs of force, which is it's full draw weight. It was actually cocked and ready when it buckled in the middle. I think if I'd flattened it a little that would have been OK. I can adjust a little to account for this.
Also, the Furniture wasn't super smooth and I'm worried it was applying pressure unevenly.
Need a tool? Make a tool! And this third prod had even, symmetrical recurves from a high tech recurve jig I put together.

That concludes the first session. I spent a while contemplating those and moved up to a 1" diameter material. Firstly I think that will give me more poundage and I think that will give me higher tolerances for error - not that I want to keep making mistakes! I've adjusted my flattening jig to just touch the centre on a 30" prod so its time to move on.

Lesson 5 - Become Familiar with your materials.
1" pipe needs more heat than 7/8" pipe. Its an obvious one but being only familiar with the feel of 7/8" stock I didn't apply enough heat to my first 1", and too much to my second, before finding the Goldilocks zone and the third one was just right.
If you've got an offcut, short end, scrap, or spare bit of material then it doesn't hurt to get a bit of free practice working it before you damage more valuable stock. In my case, my valuable stock costs about 50p and working with the real macoy is a cheap way to get hands-on relevant experience but there are times when this won't be the case.

Tune in next week for a Lost and Foundry post when I assemble the finished article, and for a photo tour of the new workshop.