The crossbow is one of those inventions that changed history. Prior to its invention, an archer might train for years before developed the strength and proficiency to be an effective warrior. With the crossbow a modicum of training could turn even the meanest peasant into a soldier. Additionally, through the use of mechanical cocking mechanisms, brute strength was no longer a limiting factor.
All that aside, here's my attempt to design and build a crossbow.
Please note, this should not be attempted by anyone, it can kill you, your dog, your ... etc. etc.
Step 1: The prod, or bow.
The overall length is 50" with a width of 2 1/2" tapering to 1/2" at either end. The thickness is 7/16"s.
The nocks, are like the nocks I used previously, how ever instead of tying them on, I used hardwood pins.
Please note, one edge of the bow is kept straight.
I did back the bow, with denim. I simply layed down a coat of Titebond and then a piece of denim which I rolled into the glue with a dowel, similarly to using a rolling pin.
Step 2: The Stock
The Stock was whipped up in ProE, it's really just two rectangles, one 3x20 and one 5.5x14.5 offset and joined with a spline to make it "pretty".
So grab the dimensions off the picture and cut out two from3/4 inch ply (it was what was handy)
Once the two pieces are cut, they are screwed together with wood screws, since they will need to be disassembled several times.
Now's as good a time as any to use your jointer to even out the top edges of the two stock pieces, if you don't have a jointer (I don't). screw the stock to a board and just use your tablesaw. But those top edges must be flat and level.
Step 3: The lock
Disassemble the stock pieces
Working on the inside face of one piece.
Start by measuring back from the front of the stock 20"s and then down 3/16ths. Use a compass to draw a 1 1/2" DIA. circle.
Measure up from the bottom of the circle 1/4" and then drop that line to the bottom of the stock. Run an orthogonal line from the end of the line inside the circle towards the end of the stock.
Within this area is where your trigger will be. Can I be anymore vague? Yes. but this will take trial and error. you can see in the pic where I sketched in lines to define the opening.
The hole is where the pivot for the trigger will be.
Step 4: Continuing the lock
At this point I used a router to clean out the lock area to a depth of 1/4".
Afterward, I layed a piece of paper over the cut out and traced the outline, because we need to transfer the cut out to the other stock piece.
Also at this point it is critical to drill a hole at the center of the circle since all your lines will be on the inside when assembled.
Step 5: Stock assembly
Glue and screw the two sides of the stock together, try not to fill the lock cut out with squeeze out. At this point also attach a piece of 3/16" hardwood to the top of the stock, I used a scrap of maple. Once the glue is dried, use a 1 1/2" hole saw to drill out the hole that the lock position was predicated on.
Then use a belt sand and router with a round over bit to tidy everything up.
Step 6: The Nut
The nut holds the string when the crossbow is cocked. It needs to be strong and split resistant. To this end I assembled a plywood made from red oak and epoxied together. Frankly the five layers are not sufficient and this bears rethinking, perhaps interspacing each layer with glass cloth would be better.
Anyhow, once the block from which the nut is being made has cured, it should be turned until it is a slightly loose fit to the matching hole in the stock. About a 1/32nd under is sufficient.
The width of the nut, is the same as the stock minus a hair for tolerances.
The bottom back half of the nut is cut away to form the sear, the top back half to form the fingers that hold the string. And additional notch is cut to make it possible for the bolt to be in contact with the string.
The nut will be held in the stock with blocks on either side.
Step 7: The Trigger
You still have that piece of paper from tracing the lock cutout? Good because that defines the space that the trigger has to fit and move within.
The top edge of the trigger has to be flat, and the nose square to the top edge, after that, just make it strong enough and small enough to hold the weight of the bow and still rotate to allow the nut to revolve.
I used a piece of hardwood plywood, a poor choice as it's still too splintery. To help fix that I reinforced the end with a carpet nail I found in my shoe.
Once you have the trigger positioned like in the picture, finish drilling the pivot hole, make sure it rotates completely out of the nut hole.
Step 8: Attaching the prod
To simplify the attachment, I use a bolt through the prod into the stock and securing into a nut hidden in a cross pin.
Step 9: The String
Is pretty ghetto, it's an endless loop style string made from hemp, it's 48" long and 16 strand. And badly made, hey, it's my first.
Step 10: So what's left?
Well, it needs to be finished, since it's plywood, I'll probably paint it, since plywood looks terrible stained.
What still needs to be done.
There's no safety.
An index pin to make sure the prod is square.
Pull the screws, drill and plug the holes.
Make a keeper for the bolt, if you tilt the crossbow right now, the bolt slides off.
What needs fixing.
Well, I'd hoped to have this ready for deer season, but, that started last week. I measured the kinetic energy using a ballistic pendulum at 28 Joules, this is below the recommended minimum of 33 Joules for hunting, so I need to make a stronger prod.
I might just glass this one and see what happens.
Alrighty, I'm done.