My crossbow project

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

Mac, I think you are right. It does seems like the middle is working harder. I tapered the model to 10mm at the tips and it looks like it flexes a little bit better:



(yeah, the left limb bends less, probably because the filing didn't go pretty well. also the model broke about a second after I took the picture :-( draw it too much...)

Ferdinand and Rizzar, the results are after I multiplied by 4. So your calculations are right Rizzar. About the un-tapered wood, yes it did stayed bend... and I was careful not to draw the un-tapered leaf spring too much...

The dilemma now is whether to rely on the model I made and taper the limbs to 10mm. I'm afraid that it'll be too much and then I won't have any way to fix it. On the other hand, maybe I should taper to 15mm and see how the flex is. Then if it's not good, I'll narrow it to 10mm. The only problem will be that the maybe narrowing from 15mm to 10mm won't go right since I won't have enough material to cut it straight. What do you think?

ZigiMan

Fast thought on that picture: do not continue with that bow model anymore. You won´t get any relieable information out of it.
The center fibres are absolutely done. left limb right after the center are compression cracks i´d bet.

In my opinion, you should taper yor leaf spring like you mentioned before and then have a look onto the tiller, when done properly (liniear taper) there should not be the possibility for the steel to behave strange (like wood). Wood is just not compareable to homogenic material like steel.



Thanks for the weight info, I am very happy to get some good calcs.

Rizzar

Rizzar, well, that model broke (and so are the previous two) but I got what I wanted from it.

Finally, I started the tapering of the spring leaf from 45mm at the center to 15mm at the tips.
I glue masking tape on the spring:



Made a half pattern of the tapering over a Bristol board, folded and cut it:



Glue it on the spring in order to draw the pattern over the masking tape:



BUT, since it turns out this way made the marks not accurate I deleted the lines and made the drawing again, this time directly on the masking tape:



I hold it firmly with clamps and cut it with an angle grinder:





Next I'm gonna check and correct the tapering and file the edges of the prod (I think now I can officially call it that way ).

ZigiMan

Your technique looks good so far.

I favored ago to make marking points with a pencil on the steel and span the masking tape directly between the points so i had nice straight lines.
Even when maschine grinding i had no trouble with that method and a very decent taper in the end.

Remember, fine work is important and taper should be symmetric by 1/10mm.

Rizzar

Rizzar wrote:
I favored ago to make marking points with a pencil on the steel and span the masking tape directly between the points so i had nice straight lines.
Even when maschine grinding i had no trouble with that method and a very decent taper in the end.

That's also a good way to mark... I'll keep that in mind for the next project.

I thinks I made a nice work for first time, although not so accurate. I will have to correct it a little bit.

Did some filing today - the edges and tips:



ZigiMan

For those who are interested, I build a drum brake forge in order to forge the parts for the crossbow...

Drum brake forge

ZigiMan

Made a small base for the prod. It also will be function as the reinforcement around the nice unwanted hole that comes along with spring leaf.











Later on, I will have to figure out how to camouflage the ugly nut on the front. Probably I'll use a piece of wood along with the bridle.

ZigiMan

U could always heat the bolt head and hammer it to look like a forged medieval pin. If wide enough u wont need the ring either.

@Ferdi - If I do that I won't be able to detach the prod from the tiller anymore. Anyway, I don't have a burner that I can forge the head of the bolt while it it's on the prod. I have to put it in my forge and then I can flatten it but then I won't be able to screw it to the base.
But when you wrote "ring" you gave me an idea: maybe I'll weld some kind of a ring or bridle to the head of the bolt. That way it won't look like modern bolt and I can use the bridle to screw it in...

I have some questions regarding strings that I couldn't find reliable answers to them. You folks probably will have good answers...

1) As I know, waxing the string is recommended but except from slipping the sting, does it has more benefits?

2) Does using a wax is a most or can I skip it?

3) The common wax material I read of is bee wax (correct me if I'm wrong) but I could find any in here. What are other waxing materials can be a substitute to bee wax?

4) Is there any important things I should know when using linen as string material?

And now, for my little progress - I've made a string jig:













ZigiMan