Friday, 30 June 2017

Prod Development

I've been tinkering with the design and experimenting with the MkI prod whilst building the MkII.
I was pleasantly surprised when the MkI survived being strung and even more surprised when I got it to within a couple of inches of full draw. I thought I'd add some binding over the cracked area to maybe stop it buckling and splitting open. I also added some little horn string bridges or string catchers to hopefully help stop the string popping over the top edge of the bow.
Maybe I'll have the courage to try cocking the bow, if that holds I'll try it through the chrono'

I've also adjusted the stock to get the prod mounting about 3mm higher. In the final design I may go even higher, but the problem with that is it needs more cut away to allow the bolt (arrow) to travel effectively through the prod. I've checked online where I bought that aluminium angle and they have it thicker and with longer sides to the angle, so I could have a much taller vertical section where the bow is mounted and extend it up above the bow to form an arch under which the bolt travels, this would serve several purposes. It would add strength and provide a sight bridge on which I could mount the fore sight. Going up to 3/8" thick section would maintain the rigidity.
None of these changes may be necessary, but one needs to have a game plan for when failure kicks you up the backside... with engineering the time to worry is when you've run out of back up plans and ideas!

Thursday, 29 June 2017

MkII Prod etc

I've been through some of my Yew and found that a load from last year was all but worthless, but I did manage to get two short clean lengths for the crossbow limbs from 4 staves. I'd paid good money for the Yew while it was in the tree, but some you win, some you loose, I've had other stuff for free, so I can't complain.
I'd also gone through my billets too trimming them down and eventually I ended up with 7 potential limbs, from which I chose the best two.
Doing all that has cleared some of my wood storage shelves, so that's a result, I can get some Yew off the garage floor and onto the shelves now.
The new Yew limbs are clear of any knots this time, although it took a bit of careful laying out to achieve that. I've done the Z splice, which is slightly tricky. As the limbs are angled back in deflex they have to be supported at that angle when the splice is cut, I did this by making up a block of plywood and sticking the limb onto it with double sided tape to support it at the right angle as I sawed the splice
Meanwhile I'd glued and clamped the cracked limb and I've actually braced it and pulled it almost to full draw! I may pluck up courage and pull it all the way, but it will just be for curiosity and test purposes, I'll never really trust it.
Hopefully the pics will show what I mean.

I think I can see why the string went up and over the limb. The tip of that limb is bent up very slightly and I think the reason for that is that the lower edge of the limb is thicker than the upper... the reason for that is the bamboo backing being thicker in the middle than the edges. The top edge of the bow corresponds to the edge of the bamboo (thin edge). The lower edge of the limb tapers upwards and as it narrows it goes from the thin edge of the bamboo slat to it's thicker center section.
So the top edge of the limb is say 5mm Yew plus 1mm Bamboo, where the lower edge is 5mmYew plus 3mm Bamboo. This difference causes the limb to twist up slightly towards the weaker top edge.
On the mk II I will try and keep the thickness more even.

Tuesday, 27 June 2017

One Shot Wonder

I got the bow mount all finished and tested the clamped up bow on the tiller, it was fine and drew to 16" inches with no problem, I briefly flashed it to 18 inches and felt confident that it would be fine,
I got the mount fitted to the stock and plucked up courage for a test shot, I used a bolt which is usually shot from my bow pistol, my heavier bolts are fletched with 2 flights, are larger diameter and wouldn't fit correctly.
The good news was that the trigger held fine and the safety catch worked. The shot seemed fast, but the bow string went over center and one limb smashed. I can't actually be certain how it failed, maybe the shock (or vibration) broke the limb which then allowed the string to go over the top, or maybe the string stretched enough to go up a tad and over centre. If I'd had string bridges or grooves on the belly to catch the string, or maybe buffers like most modern crossbows and compounds have it would have been ok.
The limb failed at the patch, which is predictable as the patch is ok in compression but has no strength in tension if the bow goes past it's unstrung state and flexes the wrong way.

It would be easy to think it's a major failure, but in fact much of the work hasn't been wasted. I still have the form for the bow and the bow mounting, I also now have good dimension for the prod and confidence that it will take the draw length.
Onwards and upwards... with hindsight I'd have fitted string buffers/catchers, but one has to take that test shot at some point and maybe I was just too impatient, although if I'd used a heavy bolt, I could have got lured into a false sense of security and simply had the failure later.
The big shame is that I didn't shoot it through the chrono'.
I may try and mend that limb just so I can do more tests.

Monday, 26 June 2017

Prod Mount

I've been away over the weekend in Lincoln, which is a beautiful city with a nice medieval feel.
I'm finishing off some flight arrow for my mate JT, but while the fletchings are gluing I've been doing more to the crossbow. Last week I made a block which I glued onto the bamboo back to provide a flat surface to clamp it all together when mounted. The block is made of Ash and to match up to the bamboo back it is curved in two planes (you can see in the pic), thist took a lot of fiddling around sawing it rough on the bandsaw and then touching it up repeatedly on the nose of the belt sander, finally a curved scraper smoothed out the irregularities. I had to get a reasonable fit to allow a decent glue line and to allow it to be clamped hard in the vice while the glue cured without splitting the wood or damaging the bow. Hopefully this will give enough strength to allow me to cut away a portion in the centre where the bolt (arrow) will fly through, this will let me mount the prod fairly high to avoid the string bearing down too much on the track.
The pics show the arrangement, note I haven't cut the front plate to accurate size yet, as it is sensible to get the holes correct first, then true up the plate, rather than trying to drill two holes perfectly aligned.
I made it out of aluminium this time mainly because it's easier to work, in fact the bandsaw managed to saw through it using an appropriate blade.

Tuesday, 20 June 2017

Prod Glued Up

I took a lot of time drawing the prod shape onto some ply to make the former, no point getting it wrong. I didn't want too much reflex to over strain the bow, but conversely I wanted a decent draw weight. the compromise was to make the form a bit longer giving an extra inch of reflex to each tip. It's no problem sawing an inch off if I don't need it.
I was a bit worried about pulling the Yew down into the former and damaging it, so I though I'd put in a gentle heat bend and also chamfer the end a whisker to help it pull down without snagging on the rough sawn surface of the ply, (I used the other half of the former for the heat bend).
Note, I covered the former and the face of the bamboo with masking tape, this proved to be well worth while as it still took a bit of tugging to pull it off the form once cured. I clamped it up in the centre and mid limb to hold it in position whilst I strapped it up with rubber strapping, it was then put into the summer house which I'd left closed all day and was nice and hot, that should ensure a really good and quick glue cure, but I still resisted the temptation to unstrap it last night.The bow feels very stiff and looks good, I'll get it cleaned up, put some nocks on and try it on the tiller before worrying about the bow mounting... mind I've been mulling that problem over all week.
Final pic shows the rubber strapping as I've started to unwrap it.

Update:-
I've got nocks on it and with a string that's just taut, taken it back to just over 80#
It's damn scary!


Saturday, 17 June 2017

Can't Keep a Good Man Down

Started on a 'Boo backed Yew prod, I was going to heat treat the Yew but there were a couple of nasty knots that I had to chisel out and fill. I chiselled right through and put in simple rectangular patches, the boo backing will help hold it together and they should be fine with simple square ends as they are in compression. I've spliced the two limbs together with a very short Z splice at a deflexed angle, I'll add the reflex at the glue up stage, I built up the splice area with some Ash before cutting the splices to beef up that area.
I've got the bamboo planed up, might get it glued up tomorrow, but maybe not as it Father's day and we'll be picnicking.
The two limbs were cut from the same billet and one has a bit of sapwood showing, but it will have mostly gone by the time it's ready for the boo back.


:-)

Thursday, 15 June 2017

Crossbow Progress

Hopefully I'll have it ready for some testing later today.
I struggled to get the prod strung to even a low brace last night, but managed to draw it a good way. The bow mounting seems secure, the string sat a few mm above the track, but I'll be able to adjust the bow mount to suit.
I've been working on mounting the trigger mechanism. A big gap is sawn into the stock and the side plates fitted to form a pocket for the mechanism. The stock is slightly wider than the mechanism, but I've milled the side plates to protrude into the hole and take up the slack... the pics show what I mean.
Bear in mind this is only a semi rough try out, it was going to be a really quick dirty try out of plywood, but I found the Ash plank, so it's okish standard. The side cheeks would be of something better than ply in a final version.
Note the method of milling the ply is ... VERY dangerous unless you take small cuts, have a strong grip and a lot of confidence/stupidity. Every man is his own safety officer. You have to make sure that when (not "if") it snatches, it will push your fingers away from the mill and not pull then into it. Not something to do if you are tired or hungry. It would be easy enough to construct a holding tool to do it safely if one was doing more than just a quick try out or milling harder wood (I knew the ply would cut off easily... I would be reluctant to try that with Ash!)

I've found the old bastard string clamps wich are needed to string my repro' medieval light sporting crossbow, I'll use them for stringing this one.

These pics are all with it just dry assembled and roughed out.

Update:- Afternoon, I got it braced using the bastard string and clamps, mighty scary, I feel I may have to take some draw weight off the prod.
The good news is that the trigger mechanism held the strain, mind that's not full draw power yet. I'll make up a suitable string (I'd used one off a very short bow which I'd wound through my string adjuster to give me the right length)
Further Update:-
Got it braced, not sure I've got the strength of bottle to cock it!

BUGGER! It smashed when I tried to cock it, but at least I have a try out stock and trigger mechanism for developing a Boo/Yew prod.
Prob didn't help that it was over 40 years old!
Hopefully it will allow me to design a better bow mounting as I can see where the delamination has propagated from the mounting hole.






Tuesday, 13 June 2017

Trigger Mechanism

Blimey what a lot of fiddling and fettling! I've been at it off and on for about a week.
I forged a small spring to hold the tumbler down after it has been shot to stop it bouncing back. It took a couple of tries and snapped once, all good fun though.
There are 3 springs in total, one to operate the safety catch peg which also acts as a trigger return spring, one small leaf spring (from a bit of clock spring) to operate the little detent pin which gives a nice click to the safety catch and the forged spring to stop the tumbler bouncing back. I've kept the mechanism compact by having the 3 springs sort of overlapping each other. The safety catch is in the centre with the ident spring one side of it and the non return spring running along the other side.
It was built with very little actual measurement, basically it all starts with the nut and trigger made of plywood first.
I'll let the pics speak for themselves.

The pic of the spring shows the one that broke, however I softened it and re-shaped it. This time I heated it red hot and let it cool slowly, it was springy enough, but could be bent, so I didn't bother hardening and tempering a slightly soft spring is going to be more reliable than one which might snap. It was made from a length of silver steel.



Monday, 12 June 2017

A Brief Post

Enjoyed a 3D shoot at Avalon today 4 of us in the group (Scott, James, Mark and myself), some old friends, 2 longbows, a recurve (bare bow) and my primitive, I was last in the group but still hit some good shots (scored 472 over 40 targets). The weather was fine, and it was pleasantly cool in the woods, there was plenty of walking up and down and a series of testing downhill shots from awkward stances I blanked a couple but got a 3 nice first arrow long shots. On one of them, we'd watched two people shooting compound take two shots each to hit the target. Then we shot and three of us got it first arrow and the other got it second!
The styles of shoothing were very varied, James had a nice lean into it stance, full draw medieval look (drawing 60#). Scott's draw was shorter with a more target style, Mark sort of came down on the shot with barely a pause. My style is of course perfect ;-) and my scoring being worst was purely due to random external factors (a list of excuse is available on application).
My fave shot was a long slightly down hill on a tiger, last shot before a much needed lunch break... we'd had to wait for the compounds to finish in front of us. James said, you don't need all that fiddling, just step up and shoot (there may have been a few expletives too), he missed, but got it second arrow. I said, in a mock swaggering West Indian accent... (that's in a spirit of admiration rather than racism... think Usain Bolt) "Dis is how d' man does it honey" and promptly smacked it in first arrow.

One unnerving occurrence was when an arrow exploded when I loosed, it broke in two main halves plus a big splinter, the bits all went about three or four yards, so it must have started to move and flex a fair bit before exploding. It must have had some unseen damage, of course they always say you should flex each arrow and inspect it before use, but how many of us actually do?
Thanks to all at Avalon for an excellent course, organisation and catering.

On the way home I stopped for petrol and bought a pint of milk which I drank straight down whilst standing watching a kestrel which was hovering in the breeze above some long grass.

Friday, 9 June 2017

Crossbow Metalwork

I've got the trigger mechanism almost finished, the safety catch works well but I haven't done the indents to make it click into position yet. The mechanism will hold my body weight but needs building into a temporary stock for testing and adjustment.
I've been making the prod mount out of angle iron with M8 threaded holes to clamp the prod... I was sold a dummy by the internet telling me that the tapping hole for M8 was 6.7mm, I managed to thread the first hole but it was rather tight and I wondered if I should go up a drill size? I told myself , no, use the drill it says. On the next hole the tap just seized up solid in the second, it would neither turn nor back off. I tried heating the angle iron but in the end just had to try brute force, which predictably snapped the tap.
I phoned my mate Mick the blacksmith who said I could pop over and use his good quality taps. He checked the drill size in his Zeus Engineering tables book and it said 6.8mm.. Grrrrr, stitched up by the interweb. His tap just went through it using finger pressure where I'd had to wrestle with my cheapo tap.
Anyhow, job done so I could make progress. Here are some pics, the prod will have leather between it and the metal work, I may even glue on an extra layer of glass on the belly from some old laminations I have lying around from the broken horsebow.
I'll post a pic of the trigger mechanism dismantled when it's all done... I did have to do a bit of forging on the trigger to stretch out and bend out a spur that projects back from the trigger to work the safety.

BTW. I ordered the Zeus engineering data book of the web.

Tuesday, 6 June 2017

Trigger Mechanism

I went over to my mate Mick the blacksmith, which is always good for a natter, he dug out some bits of steel, some EN9
Id filed the sear and bent to be about right but I wondered how square they were so I jigged 'em up in the lathe toolpost and put an end mill in the chuck and had a go at some milling, obviously taking very fine cuts as it's a small lathe not a milling machine. The finish is pretty good, but I was pleased to see that my original file work was V close anyway. I can't really jig up the nut to machine the working face, but I'll leave that soft initially until it has been adjusted and bedded in, then I'll harden it.
It's an interesting change to be working with steel and it shows what my little lathe can do, I might try and find a vertical slide attachment so I can do little bits of milling.
All good fun and it gives me something to do out of the rain.

Emily cat has been given free rein now with the cat flap permanently open now. She hasn't got me trained yet and I was a bit anxious when there was no sign of her this morning and it was pouring down. No need to worry she came clattering through the cat flap after a couple of hours and wolfed down her breakfast. It'll take a day or two to overcome my cat anxiety 'cos I'm an old softy, despite my carefully cultivated air of grumpiness ;-)

for the trigger and some angle iron for the prod mount. I spent a good deal of the afternoon sawing out the trigger, good exercise, little and often.

Sunday, 4 June 2017

Yew Primitive Collected

My secret buyer and his wife drove down from an undisclosed secret location... (he wants to surprise his club mates with the new bow). It was a lovely sunny day so we had a great time chatting and trying out the various bows and crossbows. He was certainly pleased with the bow and his wife who has moved to compound tried the bark on Hazel and shot some whistling arrows from Monkey bow. She found the Chinese Repeater a tad scary but liked the primitive crossbow. I shot a few arrows too.
In the afternoon I was tidying up and thought I'd have a go with good old twister, I strung it and picked up 4 arrows... first arrow thud, plumb centre of my little white cut out foam piglet... that'll do nicely! I didn't shoot the remaining 3 arrows.
I've started work on the trigger mechanism for the crossbow project and planning the bow mount. I might do a first try out to mount the old fibreglass and rock maple prod that I made many years ago. It was made on the same form as the bow that is on the Chinese repeater, but with a thicker core and is about 100# @ 15"
I might even do a quick plywood stock to test the various parts before touching a plank of decent hardwood.
I'll pop in on my mate Mick the blacksmith tomorrow to scrounge some carbon steel for the trigger, I had enough for the nut but don't have any more.
No pics today... you can draw on the screen with felt tip pens if you like ;-)
Gotta keep our spirits up and retain our sense of humour despite the murderous nutters who rampaged in London last night. It makes no sense, and if they don't want to be part of the world, I have no objection to them checking out early, but there is no need to take others with them.
It's comforting to see the concert in Manchester on TV, it shows that can't win and that must give a tiny crumb of comfort to those in pain.
It's not that hard to live peacefully if everyone just avoided killing anyone we'd all be better off.
Blimey, we archers all wander round with bows and still mange to avoid killing each other... even when the tea urn goes gold.
Stay safe...