Monday, 31 October 2011

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.

Saturday, 29 October 2011

Crossbow Finished Pics

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

Friday, 28 October 2011

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.

Monday, 24 October 2011

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.

Saturday, 22 October 2011

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.

Friday, 21 October 2011

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.

Thursday, 20 October 2011

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.

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.

Wednesday, 19 October 2011

Grain/gram Scales, Crossbows, Speed and Power

I've just bought myself some digital scales for weighing arrows and suchlike. As I've started making a crossbow prod from Laburnum my thoughts strayed to the comparative speed and power of the crossbow vs the longbow.
there is a lot of daft speculation on the subject and it's largely irrelevant as the two weapons are/were so different.
In medieval times crossbows with wooden and composite prods were out ranged by the longbow, the introduction of the steel bow gave the crossbow extra range and power but at the expense of weight and speed of shooting, thus it became a defensive weapon or effectively a snipers weapon.
Doubtless there will be plenty who will argue with this synopsis.
Anyhow I shall illustrate with some facts and figures using my 75# longbow and my 275# crossbow (both on the website)
The Longbow shoots an arrow weighing 28.93 grams at 166 feet per second.
The Crossbow shoots a bolt weighing 54.55 grams at 158.7 feet per second, (the slower speed may surprise you).
Because the crossbow prod has heavy steel limbs it is slightly slower and shooting a lighter bolt doesn't increase the speed much.
If I convert the figures to kilograms and metres/second to get a figure for kinetic energy in recognised units (Joules) I get 37 J for the arrow and 63 J for the bolt.
(Using the formula 1/2 x mass x velocity squared)
You see the bolt is going slightly slower, and will have less range* but has almost twice the energy when it hits home.
Of course the crossbow is only a representation of a 'light sporting bow' and my longbow isn't a full 'Warbow' but it illustrates that range and power aren't necessarily the same and that you have to match the weigh of the projectile to the bow to maximise the energy as well as the range, it also shows the limitation of the crossbow.
To get real long range from a crossbow they had to go to immense draw weights.
E.G. Sir Ralph Payne-Gallwey refurbished a siege crossbow, draw weight 1200 pounds (that's over half a ton!) It would shoot a bolt 460 yards.
The other factor of course is draw length, the crossbow having a much shorter draw. In the above example the string only came back 7".

The point of all this is the primitive crossbow I'm going to make is hopefully going to be about 70-80# draw weight, pulling back about 8-9" but it will doubtless be less fast and less powerful than my longbow.
The prod is made with the tips curving up slightly to help the string lift up over the stock, I've also used a bit of wood with a natural deflex, this will help to avoid over straining such a short bow, and allow me to get a bit more draw length.
This is all just done by guesswork/experience, so I fully expect it to explode the the tiller, especially as I haven't used Laburnum before.

Blimey I've trimmed about 3/16" off the width and put a loose string on it. It winched back to 70# on the tiller very quickly at just a few inches deflection, it looks scary as hell.
I think I'll clean it up and maybe glue some rawhide or leather onto the back for extra safety before doing too much more. I thnk it could gould go bang rather spectacularly.
It looks odd having such a short bow on the tiller. I'll post a pic in a day or so.

*As a point of interest a crossbow bolt is a bit more aerodynamic than an arrow. there is an optimum ratio of length to diameter and a short bolt is closer to this. There was a 'Scientific American'article on this many years ago. This shows the cover of the relevant issue (Jan 1985).

Sunday, 16 October 2011

Oak & Apple Explain More

Not quite sure which bit requires further explanation. The primitive crossbow will be explained further when I get round to building it!
It will have a wooden bow (prod) held onto the stock with a binding similar to the repro' mediaeval bow on my website. The trigger mecanism will be a groove across the stock which the string pulls back and drops into. A wooden peg will press up through the stock to push the string up out of the groove, the peg being pressed up by a long wooden trigger beneath the stock
The apple scratter was inspired by this link and there is tons of stuff on Youtube about building apple scratters (grinders) and presses.
To cover more about gathering apparently rotten Oak, I went back and took a couple of pics from where I'd cut it. You can see from the Ivy growth and rot that it has been down for a fair time. The fresh cut face has now started to split as it has had a week to dry out, but you can see the heart wood is sound.

Saturday, 15 October 2011

Longbow Fever

The other night I couldn't get off to sleep, longbows were going round in my head.
One of the guys I made a 60# bow for last year wants one of 90#. This could be really nice for me as 90# is probably about the most I'm going to manage to draw.
The Yew which was cut last November (From Audley End House by a tree surgeon) has been reduced to staves. I've picked out the longest 7'1" which gives me a bit of elbow room to find a decent bow in there, I've reduced it's dimensions a bit more and a couple of knots on the edge have been cut away now, so it's looking good.
I get a few people asking how long to season staves, so these pics will help show how work down a stave as the seasoning progresses.
Originally the log was quartered, a month or so ago it was debarked (that is somewhere in the blog). Now I'm reducing it further, including chopping away some of the excess sapwood, the pictures will give some idea of the size at this stage. The chippings on the floor give you an idea of how much I've removed, a bit at a time, with plenty of thinking in between is the way to work on bows. I'm sometimes aghast by beginners diving in and working too fast at the critical part of the process (the tillering)
I'm not sure when I'll start serious work on it but it almost certainly won't get any real flexing until the new year. This stave has a bit of natural deflex which I may possibly take out and heat treat the belly at the same time, although I might just leave it as is.
The guy who wants the bow isn't very tall and a 90# bow needs a reasonable length, I'll read up on the dimensions of the Mary Rose bows and make it about the length of the shortest of those. I don't like bows looking too tall for the archer, but conversely I don't want to over stress the bow and end up with a lot of set (permanent bend) in the bow, that's why I'm toying with the idea of the heat treatment. You can see I've still left plenty of sapwood which will stay on there if I do any heat treatment, as it will protect the actual layer of sapwood which will eventually become the back of the bow. the Yew is a good colour with clearly defined heart/sap wood, the rings aren't particularly tight, but it's similar to the Yew I was using at the start of this year which yielded some lovely bows.
I also need to be thinking about cutting a Yew limb for 2013, I have permission to cut it, but the paperwork hasn't come through yet.

Friday, 14 October 2011

Primitive Arrow, man make fire!

Ages ago one of the guys from Archery Interchange sent me some bits of Cow horn, antler and some Greylag Goose feathers. I finally got round to using some of the feathers on some of the Hazel shafts I'd cut ages ago. I've only done one for now.
I scraped off the bark, straightened it over a gas ring (they become quite pliable with heat) and stuck the flights on.
I cut the narrow end at the right diameter to fit a regular 5/16" point. The nock is cut with a saw and filed to a sort of keyhole shape so the string is a firm fit to push on, but sits loosely on the string once it is on there e.g the nock has a narrow entrance and is opened out at the bottom. This probably helps to avoid splitting, but that won't be an issue on a 40# bow.
Despite not being dead straight it flies straight enough.
there was a discussion recently on Archery Interchange amongst target archers about whether a bare shaft spins on not... what a load of overthink! Good old natural feathers induce spin because they are curved and one face is much smoother than the other. This is useful in helping to stabilise imperfect arrows. The modern target archer has virtually perfect shafts and puts symmetrical plastic vanes on which he then has to deliberately put on at an angle to generate spin, or to buy fancy 'spin wings' or some such, I even saw a plastic nock with the arrow slot cut on a spiral to induce spin and tiny stub vanes on it too. It just goes to show that sometimes simple and natural is sometimes best. (Of course, these guys doubtless shoot more consistently than I do and I'm just representing my opinion here, so apologies to any target archers reading this!)
My wife didn't like the smell of scorched Hazel in the kitchen, so I got an old saucepan from the garage, and bent some tabs out from the bottom. This making air holes and legs at the same time. I burned some offcuts of assorted bow wood in it. A very cheery conflagration which lasted long enough for me to straighten the other shafts.
The fire in a saucepan amused me rather so here's a pic.

Sunday, 9 October 2011

Oak & Apples

For a while I've been toying with the idea of a primitive crossbow, the short wooden bow would allow me to experiment with off cuts of timber and suchlike.
A crossbow stock wants to be heavy and stable, sounds like Oak to me. I went down to the woods and found a nice big fallen Oak limb, it looked much too big and much too rotten with the undergrowth established over it. The bark just crumbled off all black and manky, the sap wood was a white spongy mass, but I could see where the limb had split conveniently along the length the heart wood was solid as ... a chunk of Oak. I mean we used to build ships, barns and all sorts out of the stuff.
I sawed a 3' length, and took it home on my shoulder (it was getting heavier with every pace).
The bandsaw soon shaped it into two nice blanks for use at some future date.
Working the Oak also reminded me of project I'd been mulling over for a while, a machine for chopping up apples to make apple juice and cider. Another piece of that Oak soon became the roller for the machine, Oak works nicely when wet (and makes less dust). You can see from the pic that it's a nice chunk of Oak, cut from a quarter of the log so that the centre isn't in the block so hopefully it won't split as it dries. The roller has loads of stainless steel screws protruding from it to mash up the apples as it spins round, powered from an old 12V Dc motor I have lying around.
Anyhow here are a few pics rather than writing a load.

Monday, 3 October 2011


Here's a short video of the bow in action, it's a bit bleached out, I might to get a better sequence.