Wednesday, 26 July 2017

Crossbow Development

I think I'm homing in on the optimum prod length etc.
Having shortened the prod to about 34.5" after the nock shearing off the performance has stayed pretty much the same as it's taken more set. I've found what I consider a sensible brace and I think I spotted why I got a couple of misfires (putting aside the whole "fire" misnomer for now). I think what has happened is that I've pushed the bolt back, but it has stopped up against the fingers of the latch rather than going snugly between them and onto the string, this has allowed the string to impact the very top edge of the bolt, scrape off a very thin sliver and propel the bolt in a haphazard manner. (making a nasty noise and scaring the crap out of me in the process)
To prevent this happening again I've slapped myself around the head with a rolled up copy of the D G Quicks catalogue, I've also opened out the fingers of the latch slightly and tidied up the back of the bolts.

I think the optimum prod length for this bow is 36", but before rushing in and cutting down the mkII prod, I have first heat treated it.
I did that last night. The pic shows the set up, with the side cheeks clamped on to keep the heat off the glue line and the back. I put copious layers of masking tape over the back and sides fbefore clamping up. I've strung it this morning (with some difficulty). It has certainly raised the draw weight and I haven't quite mustered up the bottle to cock it yet!
I'm leaving it strung for a while and I'll give it some exercise before plucking up courage to test it through the chrono.

This project is beginning to pall a tad, but it's good to persevere and get to a conclusion. The rear sight I made looks really good. Dunno what I'm going to do for a front sight, let's see if the crossbow works decently first.

Update:- I had to re-make the string catchers as the heat treating had weakened the glue and burnt the horn.
It shoots fine with the speed about 219fps, which is reasonable being an increase on the un-heat treated version. I'll now take 1" off each tip and hopefully it will be about optimum.
I'm now using the 14" bolts, 5/16" cedar with 70 gn points as my "standard". The lighter bowpistol bolts don't gain much speed and may be too light for the bow.
The bolts slide home into the latch better now too.
Update 2:-
Bit of a downer, the MkII prod always seemed slower than the MkI. Shortenning it by an inch has done nothing and it's showing a slight chrysal where there is an "island" of growth rings on the belly. It shot 120fps, but has dropped back to 110, I may stick on a fresh belly lamination ('boo? Horn? more Yew?) or go back to the MkI or maybe shoot it, or maybe have a break and do a warbow. It seems like I've pretty much pushed the wood as far as a can, shame I'd have liked to get to 250 fps, but we can't always get what we want.
There's been a lot of work in there, mind the trigger mechanism is working well, so maybe it will get an outing some time.

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