My crossbow project

from what little i know, i am led to beleive that metal prods were introduced to allow for greater draw weights, which by that very nature meant mechanical means of drawing the crossbow were also required so pretty much as ''GNOME '' mentioned why a heavy draw prod, if you wish to draw by hand ? HOWEVER i also know sometimes its nice to have something that looks how you want it,

i know very little of crossbows, ( i'm primary a longbow man) but i have seen a 90lb draw longbow fail and when i say fail i mean ''expolde'', i think for safety reasons you should always look to the weakest point, and on the leaf spring it seems to me to be the hole, i have no idea how much spings cost, or ready made store brought metal prods, but i wouldnt have thought one made by a smith to any measurements you wanted would cost an awfull lot more,

i have made a couple of longbows, both of which may not be up to a professional bowers standard, but the pleasure of having something i have made from scratch is fantastic, in time i hope to also make a crossbow, so i find this and other threads really really interesting good luck with the project, look forward to seeing it as it progresses

Since the leaf spring is already tempered into a curve, you will not easily be able to see how the prod naturally wants to bend. When you make a thin flexible model, you will be able to see what shape makes a fair, even curve. When you then transfer that pattern to the spring, it should be almost perfect. Making the model will be kind of like making a pyramid style flat hand bow. I hope I am describing this properly.

Zigi,

Since I don't know what your current state of knowledge is, I will start with the basics. If any or all of this is "old hat" to you, please accept my apologies.

The process making a bow stave bend in the desired way is called "tillering". The bowyer puts the bow stave on a "tiller" or "tiller board" and draws the string. The string can be drawn by hand or with a pulley system, according to how powerful the bow is.

Most tillering is a subtractive process. The bowyer flexes the stave as much as he dares, and then judges how much material to remove, and where it is best to remove it. His goal is to make the stave bend evenly from one limb to the other. He must also keep the strains on the stave within acceptable limits at every point.

If he starts with a stave that is straight, and of an even thickness, he will have a relatively easy time telling when it is bending correctly. In this case, the bow stave will fall on a circle when it is flexed.

Unless it is very thin and light, a steel or wood bow stave for a crossbow can not start out straight. It must have some "deflex" Otherwise, the act of drawing it fully will cause unacceptably great strains in the limbs. That is to say, it will either break outright, or will take a permanent deformation. In the case of steel, there is another possibility; and that is that it will be so close to its elastic limits that it will fail in use. (This last failure mode is one that we need to be especially cautious of. )

If the bowyer starts with a stave that is deflexed, it will be difficult for him to tell when the limbs are flexing properly. He needs to look, not at the flexed shape, but at the difference between the resting shape and flexed shape. An experienced bowyer does this all the time, but it is an acquired skill that builds on an innate ability to "see" the shape.

The inexperienced bowyer might find it easier to employ a "tool" here. That tool is the model of which I spoke. While a powerful (wood or steel) crossbow stave must have deflex, a light and wimpy one does not. If the stave is modeled in a thin springy material, there will be no need for deflex. The model can be tillered to a nice graceful curve, and the resulting shape or "plan" can be copied directly in the thicker stave.

The model removes the need to visualize the difference between the final curve and the initial deflex. Instead you only have to see the difference between the final curve and straight.

The model also lets you do the tillering on something that only requires a few pounds of force to flex. This is much easier than having to flex the leaf spring you are starting with. Also, if you overdo it and destroy the model, there is little harm done and not much investment lost.

It is a difficult thing to write clearly about a process that happens in such a "wordless" place in the brain, but I have done what I can. If I can clarify anything, I hope that you will ask.

Mac

Yeah. What ^^ he said.

Now that is what we call usefull info! Wow, having a hard time really understanding it all.

ZigiMan wrote: can this calculation tells how to tiller the spring to get a prod with certain strength.

It is fairly easy to determine the energy storing capacity of a spring. Especially a solid metal one.

In your case the spring material can take a safe maximum load of 1,5 J / cm^3. And the bow limbs are approximately triangle shaped.

So you have average 2cm wide limb. 0.6cm thick and 87cm long. This means ~100 cm^3 working spring. Maximum load it can take is about 150J . The bow should be stressed to at least 100J to be efficient. ( Means maybe 200# weight , depends about length of string and initial curve)
String the bow and test its strength for a couple inch draw. Then it is easy to determine safe power stroke.

If you want lighter bow make the limbs proportionally narrower.
Make sure to leave the area around the central hole widest in actual material, to make sure it is the stiffest point of your bow.

When making your steel crossbow limbs, remember it's possible to make them too skinny out near the ends... if the stock isn't about twice as wide as it is thick, the tips can torque when you draw them. About three successive shots will be sufficient to throw your string off one end of the prod, if not break it outright. Geezer.

Geezer,

So for example, if the thickness of the prod is 6mm then the width at the tips should be at least 12mm?

ZigiMan

p.s. For those of you who have read my previous post, I didn't meant to teach you... If it's (as Mac said) "old-hat" I apologies, I just arranged the things for me by writing them (and to check that I'm right) and also for those who are newbie like me who also need to understand it...

happy to learn zigiman!

Nerd Flintstone wrote:
Make sure to leave the area around the central hole widest in actual material, to make sure it is the stiffest point of your bow.

A note on the centre of the prod; Alchem shrink-wraps the the centre 6' or so of their prods as they state this is the likeliest point of failure. I don't know how much that shrink-wrap would do in the event of failure I recall hearing about a prod failing with tragic consequence at a medieval fair or some-such.

This guy I know had a friend who was demonstrating a crossbow at a party and the prod broke and cut off his head and it fell in a punch bowl. So I heard. I would stitch on a leather cover.

Zigi,

You started off OK, but then you got side tracked on the curve thing.

What you want to have ideally, is a bow where every part of it is flexing elastically, and no part flexing so much that it is in danger. In general, this means that the tips of the limbs have to be weaker than the center. You can make them weaker either by making them narrower or by making them thinner, or by a bit of both. If you restrict your self to making them narrower, and keeping a constant thickness, the resulting shape of the fully flexed bow will be segment of a circle. This is the easiest sort of tillering to understand.

For any given material, there is a limit to how much it can be stretched, and how much it can be compressed, and it still returns to its original shape. These limits are expressed as percentages of the length. You need to keep each part of the limb within permissible percentages of elongation and compression. This is the soul of tillering.

Most historical prods are "deflexed". This is because they are generally too thick to work otherwise. If they started out straight, they would be overstressed by they time they were much more than half drawn. Many modern steel prods are thin enough that they can get away with being straight when relaxed.

A prod which is made of materials which permit greater elongation and compression than do wood or steel can be made without a deflex. Indeed horn-and-sinew prods flex in the opposite direction when unstrung.

Here is a link to series of book that are extremely good. They are about building bows on traditional materials. The authors don't care a bit for crossbows, and never so much as mention them, but the material is applicable none the less. These are the best, and most authoritative books ever written on the subject of bow making. The authors take great delight in "wringing the truth out of jive", and freely give the lie to much the contradictory nonsense that hitherto infested the literature of bowyery. If you have these books, you will need no others on bow making. These are the "required reading" of bowmaking.


http://www.amazon.com/Traditional-Bowyers-Bible-1/dp/1585740853
http://www.amazon.com/The-Traditional-Bowyers-Bible-Volume/dp/1585740861/ref=pd_bxgy_b_img_y
http://www.amazon.com/The-Traditional-Bowyers-Bible-Volume/dp/158574087X/ref=pd_bxgy_b_img_z

Mac

Mac, Thanks, I think I finally got it! :-)
And also thanks to Hotspur and Zardos (I wonder what kind of punch ) about the emphasis regarding the hole in the center.

ZigiMan

I hope I could show more progress but I can't due to working time limitation
Meanwhile I only have more dilemmas...
My next step is to find a model similar to my leaf spring and tiller the model as Mac suggested.
But while thinking about the shape of tillering the spring I got into a problem (that might be sounds funny) but I couldn't find any reference to it:

I am going to taper the spring like this, in order to keep the string as high as I can to prevent friction:



There will be only half of a tip that can hold the string.



So I am wondering what to do... Any suggestions?

ZigiMan

In the wikia article it says to make the other bulge from the scrap piece and weld it. I have welded spring steel many times at my job, but the weld wire is not spring steel. You have to V out the weld area really well and fill up the whole V with weld. Weld it with high amperage. Wrap a wet rag around all areas that you dont want to heat up and set the piece on a pad of wet rags when you are welding. There is less strain at the tips than the middle, so it might hold and hopefully you will keep the springs temper where you wrapped the wet rags. I was going to try this myself. By the way, v it on front and back.

Hi,

I just joined this forum because Im interested in all type of crossbows. I saw your sizing and shaping problems. I think the attached link can be helpful. I foud it pretty good. I also planned to build one medieval style crossbow using somekind of leaf spring. I would and will run with one of this size from the chart.

http://www.alcheminc.com/crossbow.html

Zardoz - Well, I'm a little bit concern about welding the piece. I'm afraid it won't hold. b.t.w: What did you mean by:

You have to V out the weld area really well and fill up the whole V with weld

?
I haven't found any reference regarding welding a the tips in the wiki.

David - Thanks for the info. I'm familiar with this shape. But in order to make it, a lot of material will get to lost, eventually I'll have a prod with less width, and it will be weaker.

Thanks guys... Thanks also to Rizzar who's been helping me in PM.

Well, after setting up a little workshop place:



I've decided to start with leaf spring #2 since #1 has a pyramid profile and the other two have holes not in the center.

I thought that I should measure the draw weight and length before I start the tillering. Since the hanging weight scale I have can only measure up to 25kg (56 pounds) I had to install a set of pulleys in order to divide the power and allow the weight to measure it. And also to draw it more easily.



I also improvised a ruler that can stand behind the spring:



And now, for the results! (tada!)

The brace height is just a littler bit above 11cm (4 1/3 inch):



and after pulling as much as I can, the height was: 31cm (12 2/5 inch):



and the draw weight is about 50kg (110 pound).

Since I don't know what to do with the tips for the string, I think I'm gonna tiller the spring much like ordinary prods, i.e., cut from but sides of each limb, enabling to create the tips.

Glade as always to get some inputs and opinions...

ZigiMan

Zigi,

That' an ingenious tillering system. It looks pretty convenient to use.

As it is now, the middle of the bow is flexing too much. Let's see how it looks when you begin narrowing the limbs.

Mac

Thanks Mac.

Before tillering the spring itself I've decided to do what Mac advised me and make a model.
I took a piece of wood and cut it to the measures of the spring:



Then I draw it to see how it flexes:



As Rizzar advised me:

Most medieval crossbows show a prod going from 100% center cross section
to around 35-40% cross section before the tips (with 10mm you will be
at 22% which influences the possible power stroke).

I tillered it like this:



The length of the leaf spring is 875mm (34 13/32inch)
The limbs are tapering from 45mm (1 25/32 inch) to 15mm at the tips(19/32 inch).
and draw it again:




Should I make the limbs narrower to make them flex more?

ZigiMan

Zigi,

It look to me like the middle is still working harder than the limbs. I would make the limbs taper more.

Mac

Zigi!

Nice device for tillering u have!
But i think u are making a calculating mistake with the drawweight.
The force u use is less because of the pulley blocks u use.
If i can see correctly u have to multiply by 4 meaning that the prod without tapering pulls 440 lbs wich does make sence.
The tapered prod should come out somewhere in the 300+ lbs range as Rizzar calculated. And i would not go thinner then 15mm, u risk braking or bendig the ends. Correct me if i'm wrong here guys, i would hate if it snapped off and swings back hitting u in the head!!
Wood doesnt flex the same as steel, my prod is 16mm at the tips and has a beautifull tiller shape i think. I draw from 75mm brace to 315mm. Powerfull shot!! Hope this helps u.

Hey Guys

I would say if you made a linear taper your wood definitely does not behave like some kind of homogenic material (comparable to steel).

Remember Ferdinand your prod is -a- thicker and -b- shorter than this one so you can not transfer draw weights.
I do not remember making calculations for this one, only stated that if Zigi stayed at this lenght hand drawing will be possible. (50kg seems for me a bit low at full draw, sounds more like +-30cm draw, remember with those long prods you really have to use a huge power stroke to use the full material strength)

Besides when giving measured draw weights you always should give information about the belonging brace height and draw length so these values provide some context.

Edit: Oh, just saw you provided draw lenght, Oops, shame on me.
Yes, 50kg is very close to my calculations sheet at around 30cm.
Your maximum draw lenght for a !!tapered!! prod is longer than that
I do not want to give the calcs for this prod since center hole makes it dangerous to completely go to the max. but a tip movement (!!) of ~100mm should be acceptable and be at some 75kg draw weight.


Greetings Rizzar

Rizzar wrote:Hey Guys

I would say if you made a linear taper your wood definitely does not behave like some kind of homogenic material (comparable to steel).

Remember Ferdinand your prod is -a- thicker and -b- shorter than this one so you can not transfer draw weights.
I do not remember making calculations for this one, only stated that if Zigi stayed at this lenght hand drawing will be possible. (50kg seems for me a bit low at full draw, sounds more like +-30cm draw, remember with those long prods you really have to use a huge power stroke to use the full material strength)

Besides when giving measured draw weights you always should give information about the belonging brace height and draw length so these values provide some context.

Edit: Oh, just saw you provided draw lenght, Oops, shame on me.
Yes, 50kg is very close to my calculations sheet at around 30cm.
Your maximum draw lenght for a !!tapered!! prod is longer than that
I do not want to give the calcs for this prod since center hole makes it dangerous to completely go to the max. but a tip movement (!!) of ~100mm should be acceptable and be at some 75kg draw weight.


Greetings Rizzar

Sorry Rizz, i assumed the automotive leafspring he used whas te same dimentions.
Anyway, stil qurious about that rigg, or did u take that in to ur measured draw weight!

I hope the 50kg are the real weight without the pulley system.
That would show me my formulas are very close.


About that wood: I would bet if you loosen it, it would already have a deflex from the first bending procedure. Bowyers rule #1, dont bend a untapered wood to its draw, best way to ruin wood.

And you should care about that rule with your steelbow, too.
There is no need for forceful bending of an unfinished (untapered) bow and in worst case you can destroy your material (wider limbs means less draw lenght and more tension on the steel).

About that Rig, you should be careful with your clamp, if the strength exeeds the capacity of the clamp, the head might loosen and snap/shoot in a direction typical for someone to have a look onto the prod. Imagine, would be a nice and painful surprise