Friday, 28 July 2017

Last Throw of the Dice

Sometimes I'm my own harshest taskmaster and I was beginning to feel I'd had enough of the crossbow project. It's actually performing ok having settled down to about 215fps with the MkI prod, but I'm not really happy with ok.

I realize there are no deadlines, there isn't a spec', there is no boss or customer to satisfy. On the other hand I don't want to plough in vast amounts of time money or effort, so what materials do I have to hand that would beef up the belly? It would be nice to hit 250fps reliably.

Ah, there's the old horsebow which I smashed by overdrawing, I could use the fibreglass laminations off that, that would give them their third outing! What about adhesive? Some of the specialised bow making epoxies are ludicrously expensive but I've got the epoxy resin left from when I made the glass fibre socket for the take down bow. As a rough guide to dimensions (mainly thickness) I have the glass fibre/rockmaple prod which exploded at the start of this project.
So a plan is coming together... Boo back Yew core glass fibre belly, what's not to like?
I've already split/peeled the glass lam's off the belly of the horse bow and started rasping down the belly of one limb of the prod.
Here's a pic of the chrysal and the various prods/bows/lams I'll be working with.
Just out of interest, I reckon that with the two Boo Yew prods being re-worked at different lengths I've gone through about 7 iterations of prod. A lot of work, but you never get anywhere if you are too keen to quit. Of course it helps if you enjoy what you are doing and you've only got yourself to please... mind I'm reliably informed that there is some decorating that needs doing (Yes Dear!)

I'm feeling a bit more cheerful about the performance I've been getting, it's better than the cheaper split limb (centre shot) 90# crossbows and only just slower than the more expensive ones. The bows that are up in the 240 - 300 fps range are much higher draw weights and have 36" prods and longer power strokes.

PS:- For anyone who can't spot the chrysal or doesn't know what they are looking for. It's a silvery hairline fracture travelling up and left from the lower edge of the limb, just above where you can see a pencil lying on the bench.

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.

Sunday, 23 July 2017

Crossbow Capers

Well it's good news and bad news. I took the crossbow up to the Cloth of Gold field archery club as a visitor of my mate Mick the blacksmith.
The test shots went fine and he took some video of a shot.
I then tried one shot at 10 yards to check the sighting and it kicked like a mule... the bolt was in the target about an inch low, the string was nowhwere to be seen and the right nock had sheared clean off.
My guess is that the brace height was a tad low and the string had gone over the bow taking the right nock with it. The string had then pivoted on the left nock and flown off to my left and slightly behind me, as that is where it was found.
Well it wasn't a disaster as I'd taken good old Twister with me as a back up.
Twister shot like a dream as if to say "you don't need to mess with crossbows!" I hit some great first arrow long shots, and I could tell that even with sights on a crossbow, it's all about distance estimation.
Mind I did manage to smash the points off 6 arrows! It is rather stony at Cloth of Gold. It was showering off and on and I was pretty tired, so after 30 targets I took my leave with Mick and Rob shooting on.

I shall take 1/2" inch or so off each tip and try the crossbow again, all these small changes will eventually be applied to the MkII prod.
From a structural standpoint I think I made a mistake by over doing the sanding on the back of the bow at the tip where the horn overlay is glued on. Sanding it flat makes it possible to glue on the horn but removes some of the strength of the bamboo. This time I'll flatten it much less  as the bamboo has more strength than the Yew. I'll also glue some horn to the belly and file the string grooves into that (rather than gouging into the Yew belly), as a sort of string bridge/string catcher. I'll also take care to mark the minimum brace height on the track of the crossbow so that I can easily check if the string needs a few extra twists.

I've also been making a better rear sight with a sliding sight block of horn with a bit of spring wire holding it in a steel frame. I can file sight marks into the frame so that the block will slide up and click into place.

Thursday, 20 July 2017

Good Improvement

I've taken 1" off each tip of the MkI prod and made a new string, I can feel some increase in draw weight, but I'm not keen to try and fiddle about weighing it on the tiller.
It is certainly faster:-
212 fps using the 230 grain 14.5" bolt  (100gn point).
244fps using the lighter 156 grain bow pistol bolt which is only 12" long with a 50gn point (NFAS rules state 14" minimum).

It would be nice to get upto 250 fps, I don't expect to reach the maximum allowed 300fps
I'm pleased with the 244 for now but will experiment further.
Hoping to try it out on some 3Ds at the weekend, I have some 70gn field points, so I'll make up some 14" bolts with those and some low profile fletchings that should arrive today. Got to make a trigger guard too so that it is NFAS compliant. (Update:- done it!)
It will be interesting to see how far I can push the boo/Yew, I may take a tad more off the mkII or try heat treating the belly.
The bow is still comfortable to cock simply putting the butt against my belly and heaving, no need for a stirrup, which I've often thought a recipe for a pulled back. I think the optimum would be a stirrup of a suitable length so you can use your legs rather than bending your back. Too easy to twinge your back.
It's still slightly nerve wracking have such a highly stressed bow up at face level, but I think the failure mode would be relatively a benign collapse or de-lamination rather than the explosion you get with self Yew.

Wednesday, 19 July 2017

New Prod, Old Prod...

Would you Adam and Eve it?
The MkII prod is actually a whisker slower than the MKI ! Am I despondent? Well just a little.
First problem is the draw weight is lower than I wanted, that's because once I got it braced (which was a nightmare) I could see one limb was stiffer, so I had to ease it off which lost some weight of course.
The good thing is that I now have two similar prods that I can experiment with.
I'm shortening the old one by an inch off each end, which will take it to the original intended length of 36" I'll see if that gains poundage and/or speed. I think I'm maybe near the limit of the materials as the prods have taken a little set (but that's to be expected). I can still heat treat the belly of the MkII if I am careful to keep the heat off the glue line.
I shot the MkI again to provide a reference and it's down to about 195fps, shooting one of my lighter bow pistol bolts takes it up to about 215fps, so that's another area for experimentation.
I'm hoping to maybe test it at a field shoot on Sunday, but we'll see how it progresses, hopefully I'll manage to avoid destroying two prods.
I glued the two 1" off cuts together as a joke and posted it on facebook as an infinite draw weight, zero draw length prod :-)

Meanwhile I bought a quick change toolpost for the lathe. It's only made of Aluminium Alloy and designed for those cheap 7"x 10" Chinese lathes. It's a tad big for my little lathe and looke oout of proportion so I jigged it up and turned 5mm off the top of it, this necessitated making a wooden plug as a steady and also filing the cam on the centre section to suit. The tool holder for a boring bar was also too high, but inverting it and drilling/tapping the holes solved that. It's got a lot of slop in it, but once locked up seems to locate the tools at a reproducible height which is the point of the whole thing.

Saturday, 15 July 2017

The Forgotten Crossbow

By Brother Ian who lives down on the South coast was up this way to buy a banjo (a Gibson Earl Scruggs signature model). He dropped in to visit to look at the crossbow I'm building and have a good talk about trigger mechanisms and stuff. He brought with him a crossbow that I'd made him as a Christmas present over 45 years ago, he wanted to give it back to me as he didn't really have room to store it nicely and he no longer used it. He was a bit worried that I'd be somehow insulted, but felt it would have a better home in my "collection".
To be honest I'd forgotten how good it was and I'd conflated that bow with some of my less successful early attempts. It was joy to see this one again as it has oodles of style with a sort of retro Art Nouveau target crossbow look.
It's only a small bow, basically a scaled up bow pistol with a stock.

My Brother filled me in with the story of its use... at that time he was working for the Natural History Museum in workshops out in Cricklewood. In the heat of summer they'd open the windows high up in workshop but feral pigeons would get in making a mess everywhere and with the very high ceilings there was no easy way to get them out, unless of course you had a light weight accurate crossbow! He says he must have shot more than a dozen easily, but the bolts didn't last long rattling around indoors, so he made blunt ones out of aluminium tube weighted at the and with round head nails (flat end outermost).

Feeling it in my hand again was a joy as the grip had been sculpted perfectly to fit my hand, the Aluminium Alloy prod is about 22" long and the whole bow only 24" long, draw weight is about 45-50# but it's perfect for close range target (or pigeon) work.
The biggest surprise was the trigger mechanism, I'd forgotten that I'd made one of that type and the advantage of it was soon apparent. The string slips down into a groove when cocked, but the groove is slightly angled such that the sting would just slip back out again if not held down by the catch closing above it. The advantage of this is that the upward force on the string as it tries to slip out is only a tiny fraction of the draw weight, so the trigger mechanism isn't subject to the great force that it would normally be and can thus be a more subtle and delicate affair with a nice light pull.
In case that has confused anyone, imagine the slot is sloped at 45 degrees and the pull on the string is 100#, that 45 degrees slope would direct equal amounts of force into the stock and upwards trying to slip the string up and out. If the slot is completely vertical, all the force in into the slot and the string won't pop out on it's own. So you see the angle of the slot controls how much force is on the mechanism. There is always a downside though, and in this case it is that the string isn't constantly touching the bolt like with the more conventional trigger mechanism.
The next day I made a new string and my Son and I had some fun shooting into the garage, although the aperture in the rear sight was a little small for shooting into the relative darkness of the garage. I opened it up by about 0.2mm and this helped, along with the additon of some extra illumination of the target. This opens up a whole area of investigation as there are many types of sights, V U, aperture etc all with pros and cons.
I've also shot it through the chrono and it gave a respectable 165 fps, a lighter bolt would gain some speed if needed, but it's only really meant for short range target work.

Monday, 10 July 2017

MkII Prod

It's a bit scary trying to pull this prod, it feels like a bit of steel girder, but I have to take my own advice and if I want a 100# prod, I'd better pull it to 100#
So I got a string on it, just long enough to slip over the nocks and heaved, I took it back to 100# and it seemed ok, I took some video and gave

it plenty of exercise. I'm working on the principal that if I can get it drawing 100# initially that will allow it to settle down to a usable 95# or so.
I think the left limb looks a tad stiffer but it's hard to say as the lighting isn't square on and even.

Meanwhile I've spliced two of JT's broken flight arrows, one had split at the nock and the other had it's point snap off. A splice near the tip shouldn't be subject to much bending force, so it's a good way of salvaging a flight arrow which would take a lot of work to replace from scratch.

Sunday, 9 July 2017

Crossbow Sights

I've made some quick adustable sights for the crossbow and got it sighted in a 10 yards, made another couple of bolts too for some shooting this morning.
Very strange thing, the two new bolts kick high and left by about 20" at 20 yards whereas the two original ones go straight true and right next to each other. the odd two are made form the same batch of shafts, have the same points and fletchings from the same source although not from the same actual packet. My guess is that the fletchings are interfering with the bow mount, I might try an unfletched bolt.

So, I just used the 2 good bolts and managed to get some idea of trajectory out to 40 yards, and some long shots at 120yds.
My mate JT had a go with the crossbow too and he put 2 bolts withing about 1/4" of each other.
The trajectory was rather disappointing and I think the prod has lost weight. I've since re-weighed the prod on the tiller, it's down to 70# , I s'pose this isn't too surprising as it been exercised now and has had time to settle, the damage to one limb probably hasn't helped either.

I've got the bow mounted on the bench so I can try and work out what's happening with the rogue bolts. First step see if I can repeat the result...
Test 1:-  Shoot one good, one bad bolt. The "bad" bolt is 18" left and high compared with the good one!!!
Test 2:- Trim the fletchings of bad bolt to a low profile. It still flies high and left by about 18".
Test 3:- Remove fletchings of bad bolt. It flies true, striking the target just below the good bolt with a slight nose down attitude.
Conclusion:- it's the fletchings wot dun it guv' and no mistake.

I can't be certain exactly what is happening but my guess is the quill part of the fletching is catching on the 'shoot through' part of the bow mounting, which I shall adjust and then try the second bad bolt.
See results (left) the bad bolts had orange cock feathers, the good bolts had white.
I cleaned up the shoot through aperture, one trick being to reverse the front part and bolt the two together, that showed up any asymmetry and by filing the two together and reversing them back and forth I've enlarged and evened the aperture in the two plates The bow was them bolted back inbetween the plates and the wood rasped to match.

I've also got a few minor issues with the trigger mechanism, so I've ordered two 600 grit diamond needle files off the interweb. They do sets in Toolstation but I have it on good authority that they are 'crap' (prob too coarse for what I need).

Thursday, 6 July 2017

Crossbow Shoots Cleanly

I've got it shooting cleanly now, I twisted up the string to increase the brace height by about 1/4", but I think the stiffer prod mount is also a factor, it was probably moving a fair bit on each shot, that's to say if it put in 2 or 3mm of permanent bend, it must have been moving at least twice that dynamically and that may have been what was throwing off the string. My guess is that I could probably take off the inner string catchers.
I'll probably do more experimentation and development before finishing the MkII prod, after all I want to try to get all the mistakes out of the way on this one.
Now I can take more than one shot I've paid more attention to the feel of the bow and trigger pull, getting lined up for the shot and almost shutting my eyes to concentrate on the feel, the trigger pull is quite hard, but some of that is just feeling the raw edges of the steel, it's also not quite at the right angle to the finger. The bow feel to kick slightly, but again its all nasty corners and an unfinished grip.
It's a good time to strip down the trigger mechanism and examine it. I've been working on it this afternoon as it was rather stiff and is a long pull, I'm ok with the long pull, but some adjustment of the bent (that's the step on the tumbler that the sear engages in) was needed. I jigged it up in the vice so I could use a fine slip stone to just ease it a tad. Trigger mechanisms will wear, some can be adjusted but they will all need some maintenance at some point. A trigger mechanism than can slip could be lethal. A little while back I saw one on a crossbow which had the tumbler made of Aluminium alloy with no steel insert, as I rotated it I could see considerable wear, I mentioned it but was told it was fine and had been round a couple of NFAS courses (I was pretty horrified). We really do need to understand the equipment we use. There was a thread on one of the archery forums about release aids for compound bows. To be frank it was scary and IMO just wrong... here's a quote:-

The problem I had was drawing and getting early misfires. The arrows fly high and long!!!! very scary. Some archers have sorted their draws to the point where they do not expect misfires. I gave up as I could not take the risk... when was the next one going to happen??

To give the guy his due he gave up on that type of release aid and decided not to risk any more misfires, but I find it worrying that there may be others out there who are using triggers and not fully aware of the risks they may be taking. Risks that could be greatly reduced with a better understanding of the need to maintain the equipment and the risks of not doing so.
Over the afternoon I've done more testing and even clamped the crossbow on the bench and measured the trigger pull, it is indeed too heavy at 12 lb !!!! I'll rework it a bit more.

I had a visit last night from a guy with big Yew stave from Salzburg which he'd got from Gunther one of the Austrian contingent at the ILAA Mary Rose shoot the other week. It was a very scruffy bit of wood with some serious damage at one end, one ring that seemed to have turned to black dust for a good few feet and numerous checks and cracks. On the plus side it was big! After much thought, mumbling and head scratching we ran it through the bandsaw to produce a couple of staves. Not quite sure what poundage of bow I'll get from them but hopefully in the 75-110# range. The staves are a "think about it" sort of project, I'll take 'em down further by degrees as the whim takes me.

Wednesday, 5 July 2017

Prod Mount Development

I'm using the MKI damaged prod to experiment and improve the mounting to allow more of a shoot through, with the prod mounted a little higher.
I don't want to risk making a mount that is fine at 80# but explodes when it's tried at 100# especially with the cut out extended down to allow for fletchings.
While making this mount I used the aluminium angle (4x2") up the other way with the 4" as the vertical, this meant the bottom, horizontal leg was too short... I thought I'd try some of that aluminium brazing rod which melts at 300 C (ordered on Ebay)
I could have bought larger angle, or I could have asked my mate Mick the blacksmith to weld it for me, but it's much more fun to try new stuff and where possible do it myself. After all I could buy a whole crossbow for a hundred quid!
. It has worked pretty well, no flux no messing, just a propane torch. the initial try out wasn't all that strong as I'd prepared the joint as you would for a weld. I then read the instructions which said for a butt weld, tin both surfaces and then sweat the parts together, like soldering rather than welding. That worked well, although with hind sight I'd have gone for the slightly harder 400C rods. We'll see how it hold up in use.
Please note :- Any resemblance to an early radio set is purely coincidental !

Update:- It performs better but there are still teething troubles. With the vertical section longer, there is more leverage and the mount has bent forwards slightly, it hasn't helped that I've narrowed the horizontal bit that runs underneath, it looks better but is weaker.
There are plenty of solutions, I could go up to 3/8" thick material or I might add side pieces which can be made from some of the scraps I have.
I did phone Mick the Blacksmith to see
if he could weld in a couple of bracing fillets, but he doesn't have the kit to weld Ali' , no matter, I'm not stuck yet!
Further update:-
See additional brackets in pics:-

Sunday, 2 July 2017

Good Results

I've taken the repaired MkI crossbow prod to full draw and got a Force/Draw curve for it. I've shot it again with the string catchers and a temporary string buffer (see pic) the shot was clean and fast but the string still went over the prod some how, but without damaging the prod! Close inspection of the tips show how the string has ridden up. Presumably the slightly heavier bolt (14" long with 100gn point), the string catchers and buffer all absorbed a good deal of the energy and saved the prod from breakage.
I can now experiment with  string catchers nearer the nocks and moving the string buffer slightly closer to the brace position.
Not much string groove in the belly on that left tip, so maybe that would help.

Meanwhile I've got the Mk II prod glued up and sanded to an even thickness taper ready for nocks.
All in all a very productive time. I'll also be able to try a shot through the chrono'.
Note the MkI has lost a llittle weight due to reshaping the lower edge of the tips and rounding corners. The Mk II is over 1 mm thicker and I'm aiming for about 90-100#

Just shot it through the chrono' 202.6fps, not bad from a 76# bow !
I'd moved the string buffer forward a tad but the string still jumped over the left tip. More development needed, before I get the MkII going.