Author Topic: Del's Primitive Crossbow  (Read 18236 times)

0 Members and 1 Guest are viewing this topic.

Offline Del the cat

  • Member
  • Posts: 7,534
    • Derek Hutchison Native Wood Self Bows
Re: Del's Primitive Crossbow
« Reply #15 on: October 30, 2011, 05:07:47 pm »
Yes I saw that video "game over" :)

Must be difficult to hold a straight stock crossbow against you shoulder. Is it not sufficient to rest it against your cheek. (maybe grow a beard or something if it hurts to much)
Yeah, one reason for building these things is to see how they actually feel.
I shot the cross bow a fair bit today at the club and ir felt pertty good, either grouping tight or in a vertial line as the height estimation is tricky without sights. It's fairly smooth to shoot and there's a longish, but light squeeze on the trigger, one of the ladies had a go too and didn't have any problem with cheek rattling. There were a lot of visitors to the club as it was the Samhain festival and the bow drew a fair bit of interest.
Del
Health warning, these posts may contain traces of nut.

Offline Del the cat

  • Member
  • Posts: 7,534
    • Derek Hutchison Native Wood Self Bows
Dimensions and Stuff
« Reply #16 on: November 03, 2011, 06:44:28 am »
Most of it is worked out from the actual bow (prod) dimensions and just sketched out by eye.
I looked at images of primitive crossbows on the internet for the trigger and coupled that with the experience of the similar mechanism on my Chinese Repeating Crossbow. If you haven't seen that, then look at my website (google delsbows to find it). The trigger mechanism works surprisingly well, the slot is cut square down into the stock, just deep enough for the string to drop into, no need to go down too deep else you end up with a very long trigger pull. You need to round off the corners of the slot so that it doesn't wear the string too much, but if you round them off too much the string can jump out on it's own! If that happens a little re-shaping of the slot will solve the problem. Facing the groove with horn or somesuch is a good ide as it will polish nicely to avoid string wear.
Anyhow here are some dimensions (mixed imperial and metric for convenience)
Prod:-
31.5" long.
36mm x 16mm at the centre
17mm x 12mm at the tip
Bracing height 3.6" from back of bow, the string pops off if it is much less than this (happend twice at about 3", so I twisted up the sting a bit more)
Draw, (back of bow to trigger slot) 12.5"
Stock:- all dimensions from front of stock (prod end) unless otherwise stated
Overall length 28.5" (I like a shortish stock and that felt comfortable to me)
Prod mounting slot about 1" deep.
Distance to prod binding hole 4". Hole about 15mm diameter (round off the corners)
Distance to small hole which is the start of the tigger 9 3/4" (I drilled a hole to help avoid splitting)
Overall trigger lever length 11" (trigger is about 11mm thick and 23mm wide.. tapering here and there to look pretty :))
Depth of stock  about 2 3/4"
Width of stock 1 1/8"
Trigger slot depth 4.9 mm
Gap between top of trigger lever and underside of stock 4.9mm this gives just enough lift on the peg to push the string up and out of it's slot.

The Binding for the bow can look really nice, but it's tricky to get it tight, a piece of leather between bow and stock helps but it's one of those fiddly jobs that you will probably end up doing twice to get it right. The ideal material is sinew, maybe rawhide. I've used well waxed hemp twine before and it was fine. The first stage of binding gets it tightish, the zigzag binding the clenches the main binding even tighter.
If you don't mind what it looks like rubber strapping to bind it in is very effective as it hold it's tension.

Bolts, I just use 11/32" arrow shaft and some 100 gn points I had lying around (125 or 150 would prob be better). The trick is to match the bolt weight to the bow, heavier weight bolts will be a bit slower but will shoot smoother with less rattle on you cheek, the ywill hit home harder too. As for length it depends how cheap I'm feeling you can get 2 bolts from a standard arrow shaft, or 3 if you don't mind 'em a tad short. The one in the pics on the thread is a short one.
Del
Health warning, these posts may contain traces of nut.