mini-ulrich questions

I am new to all this, and have spent the last week reading through the medieval/early crossbows forum, and  Iolo's primer (thanks Geezer, really enjoyed it).  Anyway, I do like the curvy Ulrich-type of crossbow and think I am going to cut my teeth with a mini version (1:4 scale), using a pin-lock like in the loose-lam crossbow by kenh.

I have a number of questions about the prod.

I am going to make a wooden prod; at small scale, a laminate prod is probably un-necessary but am unsure as to how to proceed. I know the prod must describe a long shallow v when viewed from the front elevation but carving the prod so that the limbs reduce in thickness when viewed from above (like on a long-bow) will cut through the grain - is this a no-no? Any advice on making solid wood prods gratefully accepted as I want to do this as a technical exercise, preparing for a larger bow at a later stage.

  A fly in this ointment is UK crossbow law - I want to involve my kids in the project but to do so the 'bow will have to qualify as a 'toy' as I believe the current legislation is that no-one under the age of 17 can have anything to do with a crossbow - even if supervised by an adult over the age of 21???

Thanks in advance
Stuck

I actually think that in a small crossbow the loose laminate style prod is easier even than in larger builds.

Prods do not have to have that "shallow V" profile from the front, they can be exactly like miniature English Long Bows, if you will.  Or even plain ole rectangular.  Steel prods often have that V shape, but wooden prods often look thick and bulky all the way to the nocks.

The strung prod is tipped at an angle (as seen from the side) such that the string in the braced position just touches the deck of the tiller... it will be a bit higher at the cocked position but not more than half the thickness of the bolts.

What's the UK definition of a "toy" crossbow?  Under X pounds draw weight?  What the bobbies don't know can't hurt anyone.  Americans would say what you do in the privacy of your workshop is your business. But that's us...

Any of the standard bow making woods will do.

Canada has those 'Nanny knows better' proclivities as well.  Fortunately, not where crossbows are concerned, yet.  I think of my x-bow as a toy - all 250 lbs of it.  Go for the full Monty, nanny be damned. 

1:4 bow can do 1:64th the damage of a full size one.
Sounds like a toy.

Hi Guys, I have tried to find a definition of 'toy' in the UK and failed, am assuming it is as much to do with sharp pointy bolts as anything, as long as the 'bow is not unreasonably powerful, but 1:64th of the power does sound like a toy.
Did look into sourcing some chain-link tension bar but couldn't get anything near-by but have lots of wood in garage, including some really nice ash and a near-by timber supplier with lemon-wood, yew, and some other bow-wood in the size I need.

Regarding yew, does it have to have the sapwood - is heartwood useless for prods or will it be OK? 

One-more-thing (I love Uncle from Jackie-Chan cartoons), the string prod tip, presumably this is a glued on tip of a harder material? or can you cut a notch into the prod? If you do cut a notch, is it cut straight like in a long-bow or angled to lift the string toward the top edge of the bow?

Man - I wish I lived near your timber supplier!

Pin nocks or side nocks -- either will work.  You don't need separate tips.  They don't have to be angled; keep the string down the center line.   The whole bow gets angled to bring the string up to the tiller deck.

Fair play, Paul at Isca Hardwoods (no relation, no interest) in Newport is good as gold, not a huge amount of any one timber but a wide variety and can usually get something in if you give him enough notice.

Stupid question, but does any body use lime (linden) for anything? it is the wood-carvers timber of choice - even grain distribution, medium density, takes outstanding levels of detail and  has almost no figuring. Don't know how springy it is or whether it takes a set - anyone?

Just found this on a seperate discussion on this forum, presumably I will be aiming to achieve an equivalent for the wooden prod.

Yep -- that's a nice looking pin nock.  Can be wider and thicker, of course... No reason not to use Lime/Linden/Basswood although it does not have as great of Modulus of Elasticity as some other common bow woods.  Ash and Red Elm both make great bows.

Ash 11.00 GPa
Basswood/Linden 10.07 GPa
Elm (Red) 10.28 GPa
Hickory 14.92 GPa
Lemonwood 15.75 GPa

Progress, kind, of...

I spent Sunday afternoon whittling down a prod from a piece if yew I found in the garage.  I now have something that is 10mm thick by 20 mm wide at the centre, tapering to 15mm wide by 5mm deep at the tips.  It was uniform in width and depth but I couldn't bend it by more then a half inch (sorry to mix measures), now I can deflect it by about 3 times that if I put it between some bricks and stand on it (seriously) - I am assuming that I will need to steam or boil it to apply some pre-bend so it can be spanned?

You're getting there!  How long is the prod-to-be?  Rather than stand on it between bricks, make a simple tillering tree.  Then using a long string, draw the prod to a bit more than brace height and slip on the short 'span height' string.  You can steam or boil the prod and put deflex in it, but you don't need to.

prod-to-be is 14 inches (35cm) long.  How much curve is reasonable on such a short length - 3 inches brace height would be nice but I suspect that is well-past the failure point.

Okay, we're looking at two issues: proportions of the original and practicality.  If you've settled on a 14 inch prod, we're looking at a half-size replica of the original... or thereabouts. The original Ulrich V is 28-29 inches long and the prod about the same length... maybe squinch longer.  Bows from mid 15th to mid 16th century vary in proportion from 2/5 X 3/5 to 1/3 X 2/3: in the latter case that means the lock is approx 1/3 of the distance from nose from tail.  In the earlier case, the lock is 2/5 of the distance and the tail 3/5.  So if you have a 30 inch stock, you'll want the lock 10 inches behind the nose on the earlier bow, or 11-12 inches back on the later style.  With 10 inches, you're going to have @ 6 inches of power-stroke and 4 inches of brace.  If you build your bow half-size, expect 3 inches of power-stroke and 2 of brace. Does that help?  Geezer.

Hi Geezer, the 2 inch brace and 3inch draw sounds good as the 14 inch prod is where I am looking for. Anyhow, I did make a mini tillering tree but the prod snapped at 2 inches draw. I clearly need to learn more about working yew. One thing I did learn is that any yew prod will need to be drawn mechanically with a wippe or cranequin - very scale-like but not necessarily fun for a 9 year old....ash may be a better material this time around.

With that short of prod, I'm guessing 8mm thick at the tiller tapering to 4mm at the tips would be the place to start.   

One way to determine final prod dimensions would be to make several laminations say 2-3mm thick x 20mm wide tapering out to 10mm at the tips.  Stack them together like a old automobile leaf spring.  Start with two laminations taped together at the center, the outer one 14" long, the inner one say 12" long.  Cut nocks in the outer lam and try bracing the stack.  If two lams work OK but don't have enough power, add a third lam that is 10" long and try again.  Using this technique you can get a 2-4 piece "loose laminate prod" or a much better idea of the needed dimension for a final one piece shaped prod.

Good advice that, I have a stack of 5mmx15mm ash laths in the garage that I cut to make the ribs of a canoe a while back so a loose-lam adjustable prod is a great place to start.  As far as the yew goes I have downloaded a copy of the traditional bow-makers bible to see if I can work out where I went wrong - may well be it was too thick but also I may have cut through some of the sap-wood rings in the centre when straightening out a bump and weakened it - my biggest concern is that if I need an 8mm deep prod then the sapwood and heartwood ratio will be tricky to judge - mebbe I should go thin and wide, say 30mm x 6mm  constant thickness- like a flat bow rather than a more rounded cross-section like a long-bow.

Stuck,

If you are trying to make a yew wood prod that is proportioned like a composite, it will have to be deflexed.  That is to say, the shape should be bent in the direction of a drawn bow. If you make it straight, or worse yet, reflexed, you will "run out" of the elastic range of the wood at about the point where you get it strung.  There will be nothing left for the draw, and it will blow up. 

Materials can only withstand certain percentages of elongation of compression before they fail.  Steel bows are always deflexed for the same reason.  The horn and sinew composite can be reflexed in spite of its great thickness because these materials are able to withstand rather surprising percentages of compression and elongation respectively.

You can steam the roughly-tillered wood prod into a deflexed shape, and finish tillering it after letting it dry for a few days.

The alternative is to make the prod wide (top to bottom) and thin (front to back).  This will work, but it will not look as nice, and will not exploit the qualities of the yew to their fullest.

Mac

Thanks Mac, that is good to know, I'll try and rough out another prod on the weekennd and see where I get with it, I have been doing a bit of reading around and it seems the sapwood needs to be somewhere in the region of 5mm thick (?) and must have the same grain ring exposed along its whole length to avoid 'run-out' fractures when the limb is in tension - I certainly had a break in the grain at the point of fracture and pictures on-line of long bows with similar fractures also had breaks in the grain at the fracture point through knots/repairs.

Anyhow, I also think the prod I broke was nearing its limit and I should have done a lot more tillering to the outer ends of the prod as their curve was more of a  flat than an arc - so much to learn!

Yep -- making and tillering selfbows is a whole class of study unto itself... made more difficult when you want a micro-short powerful final product.  

You may find that a multiple-lam wooden prod, perhaps wrapped in suede or leather or some such for 'the look' will be an easier solution.  Not everything scales don (or up) well, and wood, IMHO doesn't scale down well in the sense of scaled down power like a bow.

IIRC, Lightly made her recreation with a steel prod "in the high 90s".  So let's call it 100# draw weight.  You 1/4 scale then needs a 25# prod 14" long.  No matter how you slice it, that's gonna be tough to make.

Update,

I have split off a couple more staves from my (only) lump of yew - it is a 20-inch round that has been sitting under my bench for the last ten years. The lump is pretty gnarled but I managed to get 3 half-decent staves out of it, the first one had to be shortened to avoid a knot - that's the one I broke last weekend, the others are about 17 inches long. 

Anyhow, I spent this weekend cutting the sapwood back from the 1/2 inch thickness that it was to a more usable 1/4 inch (3 or 4 mm) and tillered it so that the same growth ring was exposed along the whole length.  This evening I boiled it for a while and clamped it in a form for a 2 1/2 inch deflex I'll leave it set for a few days (thanks Mac) and tiller it properly next weekend.

Bravo!  Onward and upward!

Hi All.

My new yew prod is working nicely, would guess its about 40lbs, but hey, what do I know?  Anyway, I can cock it using 2 hands and a really firm pull, any more and I would need a mechanical assist.  I have also roughed out a yew stock, although it is a little too slender  - no room for a nut but fine for a pin-release  - the sap-wood is nice for replicating the bone inlay on top. Right update over, queston time, am I thinking clearly about the following? Oh, I cant work out how to post photos, sorry.

1. The prod has stretched the nylon string I used as a temporary measure after only half a dozen shots, so I was going to use some monofilament nylon kite string (12lbs) and make up a continuous loop string with say 10 turns? I know I should use dacron or some such, but this is a learning project and I have the kite string in a drawer in the kitchen.

2. On release, the string tends to travel under the bolt (OK pencil), when it catches it, then the bolt travels a long way very quickly - blink and miss it - I was thinking of pinching the bolt in a pair of studs, to mimic the appearance of a nut, instead of using a spring/finger on top - sorry I do not know the correct terminology. I am planning to not use a bolt groove but rather a flat plate and button. The studs will have a sloped surface for the string to travel up and slap the bolt more centrally.

3. I have no idea about open sights - if anyone could enlighten me I would be grateful...

Stuck

1.  Monofilament will stretch at least as much as nylon...  But what the heck.  Give it a try. 

2.  This is why we usually have a groove/track cut into the top deck of the prod.  That way the string does not tend to go under the bolt end.  And the bolt tends to be at the V of he string rather than floating around somewhere on the deck.  The groove doesn't need to be terribly deep - say 1/3 the bolt diameter.  If the deck is flat, the string has to be raised above the deck and it's much more finicky to get things to work properly and bolts to fly straight.  The bolt retainer can be made from metal or even springy wood.

like Kenh says, Monofilament will stretch like nylon.  It is nylon.

Here's the stuff you want.   It's not very expensive for 1/4 lb, and it goes a long way. http://www.3riversarchery.com/Brownell+B-50+Dacron+Waxed+Bow+String+Material_i4144X_baseitem.html

The "g" color looks most like linen, I think. 

Three Rivers is very reputable, and they probably have other stuff you need as well.

Mac

I am not sure what you're looking for Stuck,do you want a description of an open sight system,or do you want a description of one you can make yourself?Open sights can be defined as a targeting system that relies on the unaided human eye,consisting of a front sight,usually fixed,and a rear sight adjustable for windage(side to side)and elevation(up and down)there are 2 types,notch sights,where the foresight blade is centered in a notch,or aperture or peep sights,where the foresight blade in centered in a hole.Aperture sights are considered to be more accurate,and are simpler to make for a home builder,but some people just can't get on with them.Hope this helps,and feel free to ask further questions.
                                               Hermit.