Crossbow Workout

The crossbow got a good work out yesterday at the Celtic Harmony Samhain Festival.
We were running an archery have-a-go. The bow shot nicely, but at one point the string flew off, I added a few twists to effectively shorten it and increase the bracing height and re-fitted it.
A few of the club members had a go with it and found it easy to handle. One of our ladies in period costume had a go, I should really have taken a pic but didn't have my camera to hand.
Experimenting at home shooting through the chronometer shows it's not particularly fast and the actual kinetic energy imparted to the bolt is similar to the last Yew bow I made.
E.G 80# at 12.5" ends up about the same as 40# at 28".
For those who like figures the kinetic energy is calculated as 1/2 x mass x Velocity squared.
The figures need to be in kilograms, and metres per second to give an answer in Joules.
personally I dislike units like Joules as they mean nothing to me , can anyone tell me what a Joule feels like? An old fashioned imperial unit like a horse power at least give you some feel, as you know what a horse looks like.
The figures below are in 'grains' which is what arrows are usually weighed in and feet per second which is what arrow speeds are usually measured in. You can see it's a weird mix of units and if you haven't heard of 'grains' before, a grain is about the weight of a grain of barley.
The heavy bolt (11/32") in the picture is 440gn @ 125fps = 20.7 Joules of kinetic energy.
A light bolt (5/16") 150gn @ 170fps = 13.0 Joules of energy.
The heavy bolt goes pretty slow, but being 4 times heavier ends up having much more energy.
It would be interesting to plot loads of different weights and see which actually gives most energy.
Here's a couple of extra pics. The front view shows how the string sits level, it also shows how I was a bit sloppy making the prod as the left end curves up a bit more abruptly than the right. Still it was just a quick experimental bow to try out the Laburnum, the other pic shows the bolt clip which is Ash and looks a bit 16th century in style.

Crossbow Finished Pics

Here's a pic of it cocked and some showing the early stages of binding on the prod.

Crossbow Test Shot

Before binding on the prod I wanted to test the bow, but first I needed to make a groove for the bolt to run in. Some crossbows have a groove, others have a small piece of horn or somesuch with a notch for bolt at the end of the 'runway' 'track' 'barrel' 'channel' (or whatever you want to call it) with the back end of the bolt being held by the trigger mechanism.
The trigger mechanism I'm using doesn't actually hold the bolt, so I need a shallow groove, the advantage of a groove is that it lowers the bolt slightly so the string is then pushing the bolt on it's centre-line rather than it's bottom edge, which can cause the string to go under the back of the bolt or the bolt to bounce upwards.
I made a shallow square groove using a plough plane which has a very narrow blade. My Brother gave me the plane for my Birthday many many years ago specifically for that purpose. I rounded the groove using an arrow shaft with fine sandpaper wrapped around it. This wasn't working particularly well so I took a 7.5mm drill and ground the butt end of it at a slight angle on my small grinding wheel, this made an excellent scraper/chisel for working along the groove.
The prod was temporarily bound on with some rubber strip (cut from EPDM roofing sheet, but old inner tube is much the same) I cocked it and put the bolt on, making sure the back of it was just in front of the string, so that the string didn't pop up and knock the bolt off the runway.
THWACK... it smacked into the target at very close range, the trigger pull was smooth and easy, there was no real kick and the bolt was buried up to the flights in the target. Excellent!
I'm now working on the detail. I'm adding a sliver of Ivory (from an old piano key blank) at the front face of the trigger slot which retains the string when the bow is cocked. This will effectively block off the groove in which the bolt rests, stopping it from being pushed back too far. A clip of Ash will also reach over the top of the trigger slot and press lightly on the bolt to stop it falling off or bouncing about, I may incorporate a back sight into the retaining clip. The clip also stops the trigger peg coming out the top of the stock. I shall probably just glue it in place with hide glue. One advantage of hide glue is that it can be undone with heat and humidity, whereas epoxy is rather permanent.
Once I've done these few bits I can bind the prod in place and really test it.
Pics tomorrow.

Trigger Mechanism Try Out

I've cut out the trigger mechanism, drilled a hole for peg (11/32") and used a bit of arrow shaft as the peg.
I've dry assembled it and cocked it just to check it holds. There is tons of work slimming down the stock, shaping it nicely and binding the prod into place.
(Excuse the camera strap flapping about in the first pic)



The trigger mechanism will doubtless need some fine tuning too.
There is a fair bend on the bow, I don't really want to risk more, if I did want to work it any harder I'd take 1/2" off each limb tip.
Once it's finished I will be able to use it to test other prods.
I hope the pics are self explanatory.

A prod in the right direction

I've been feeling a bit coldy today but I've had a couple of little dabbles with the bow. I made a string, braced to about 3" (90 strands of fine linen thread).
I pulled it back to 12" 70# on the tiller. I then put it on the stock where I've cut the mounting slot and heaved it back to 13" !
Dunno how far I dare push it. Pulling it on the stock gives me the position for the trigger groove.

Crossbow Progress

Here's the prod with the rawhide bound on tight with string, one limb has been unwrapped to show the before and after. It will need to dry out some more and be cleaned up before I continue tillering it, I shall leave some of the grooved finish showing as I think it looks good.
I decided against Oak for the stock as it isn't seasoned, I have a nice chunk of Ash I'm using instead. The last pic shows roughly how it will work. The area I'm not sure of is the trigger, do I make it as part of the stock allowing it to hinge at a selectively weakened point at the front, (see the pencilled in arrow beneath the front of the trigger) or do I make a separate trigger pushed into a slot as a pivot it and bound on with twine? (Click on the pic to enlarge it and you will see how the trigger will push the rod upwards to push the string out of the groove)
Hmmm I'll have to think about it. The stock as drawn on the block of Ash looks horrible, to manufactured and not 'primitive', so it won't end up looking like that.
In the second pic you can see the butt end of the stock bends a litte to the left, this will make it more comfortable to shoot, it will probably drop down about half an inch too. I'm torn between making it shoot well and look primitive. Most shotgun stocks have a little cast to make them point correctly, with a dead straight stock it's hard to get your eye in line with the barrel or bolt in case of a crossbow.
I shall experiment with some offcuts of the Ash to get a feel for the necessary trigger dimensions.

Crossbow Prod

The prod is still pretty stiff so I checked the thickness and took it down some more, going 1.5mm thinner for each 3" along the bow from the centre.
It still seemed rather stiff, expecially in the centre so I took another 3/16" off the lower edge. I then adjusted the thickness taper from the tips which seemed about right increasing thickness by just 1mm for every 3" this time.
The pics show it on the tiller, but bear in mind it's not braced, the string is just slipped on and the bit of curve is natural deflex. It certainly is bending near the middle now and I'll probably work the tips and mid limb a bit more next.
It's going to have to bend a fair bit more yet, so I've glued some rawhide on the back using hide glue, as in this previous post. http://bowyersdiary.blogspot.com/2010/04/glueing-up-ashcherry-bow.html

The rawhide is a huge bone shaped dog chew from a pet shop soaked in warm water to unknot it. It's pretty slippy stuff to handle and gluing it on and binding it with string until it sets is a messy job. The glue gels quickly and it feels like it's not going to stick, but a waft with a hot air gun when it's finished helps to re-liquify the glue. You can see from the chalk marks on the wall that it is bending about as much as my other bows, so I'm not sure it can take much more.

The last pic shows how the upward curve of the bow moves the string line so that it won't press down too hard on the stock and waste power. The down side of this is that the bow tries to twist on the tiller.
It also shows some of the dark streaks on the belly of the prod, these are shallow cracks which hopefully won't be too much of a problem as they are longitudinal. There was also a knot which I filled with sawdust/epoxy mix.
I believe the stock of a medieval crossbow was called the tiller, being long and straight, maybe it was reminiscent of a boats tiller, and maybe then got applied to a tiller stick used for pulling back the string of a long bow when checking the curve of the limbs.