First Build

Ok so here goes, iv built this stock from plans available at Alchem and modified it to suit my taste. i haven't cut the slot for the prod yet as im not sure what distance from the nut to cut it to.Think im working a bit backwards but i intend to make a prod from car leaf spring. my question is how do i work out the dimensions for the prod at a given power stroke to get about 300lb draw weight. my intention is to hunt with this so would prefer the heavy draw weight. By the way the stock is made from Kiaat." alt=""/>" alt=""/>" alt=""/>

Looks nice...if I may offer a suggestion?

I would recommend using something other than wood for the nut. Say horn, born or delrin...

Thanks stonedog was toying with using aluminium with a hard steel plate for the trigger to push against is weight a problem with a nut or is ali ok

I can't recommend using aluminum for a trigger in a 300 lb. crossbow. You're going to have lots of wear on the distal end, where it makes contact with the steel sear. Mild steel will work out much better, and you don't really need a forge to work it. A barbeque pit with some charcoal briquettes will produce plenty of heat, particularly if you have an old hair-dryer for a blower. Heat it red-hot and bent to the shape you want... that can be done with a bench-vice, just be careful to avoid setting your shop afire.
As for the roller nut itself, I heartily recommend bone or antler... something with lots of ivory and little or no pith. Reinforce it with a bit of steel for a sear. Geezer.

Hi Geezer i have a peace of 10mm x 10mm thick mild steel for the trigger i wanted to build the rolling nut from ali, with a steel sear, i don't really thing i could get my hands on horn and not sure about bone, you wouldn't have any suggestions on prod dimensions would you, i was looking for about a 4in brace and have about 6 to 8 in of room for the power stroke.
thanks very much for the reps.

Just finished putting the deck on the tiller, its made from ( Dalbergia melanoxylon ) i no it as black ivory , a very hard wood with an awesome grain if you can see it. The groove for the bolt seems to be about right for a 10mm bolt which is what i was aiming for and the brass screws are counter suck, so they wont catch the string. there is still a fair amount of shaping to happen on the area past the nut socket, im just waiting for the glue to set.

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By the way if anyone would like a copy of the plans for this tiller let me no and il post them in .jpg format they can be printed on 3 A4 pages.

Looks good, I like the shape of it!



Todd

Thanks Todd we getting there,

That's coming along nicely. I really like the dark hardwood on top

I like the material choices you've made, and don't be surprised if I steal your tiller profile for an upcoming project! I built my first bow backwards, too: did a lot of woodworking before I had my hands on the prod. I got fairly lucky, hope you do, too. Even using an Alchem prod, there was some fine tuning to do on the prod socket angle after getting the prod spanned and checking out the fit.

Gnome

Well guys thanks for the encouragement so far, got the flu so haven't worked on the tiller again but thought id post the plans for this one, the are modified alchem plans which are free so i dont think anyone will have a problem with them. Help your self and do what you will with them.
Thanks Guys.

https://s1175.photobucket.com/albums/r625/weskuhn/Tiller%20Plans/

Well i just got the nut from the machine shop, friend of mine made it from tool steel, the shaft is 8mm and the hole throught the nut is 8.1 and there is no resistance on the shaft, was worried about it binding but that wont happen. just need to smooth off the edges and gun blue it and we away.

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Looks good so far, but all the roller nuts I have seen and the ones I have made are made with a curve where the string is held like a slight hook. I assume this helps to keep the string from prematurely jumping over the top of the hook/claw of the nut.



Besides all that how are you making out with obtaining a prod? Keep up the good work>



Todd

Todd i did think of that and if i need to il file the fingers to a hook shape, i picked up a new peace of spring steel 8mm thick x 50mm x 900 long for a prod, still not sure on dimensions tho, i did read the crossbow wiki and there are dimensions for a prod but its 900 long with a 400mm draw mine is only 12in including brace so still a bit lost.

Nut looks good. You will have to round out the outside edges of the fingers to keep from cutting bow string. My moose antler nut's fingers are not curved and I have not had any problems with string popping off of 600 lb bow. How wide is roller nut and how wide
between fingers? Most larger cities have a commercial spring shop
for trucks and auto leaf springs. They will usually put your un tempered prod in their furnace for hardening and tempering. They are also a good place to get un tempered spring stock of different
thickness. That stuff is much easier to work than a already hardened and tempered spring, plus it won't have any mico cracks or flaws in the steel. What weight prod are you going for? cheers fiddler49

Fiddler the nut is 39mm wide with 10mm fingers and 12mm between them for a 10mm bolt I have an unhardened peace of leaf spring and the company said they would heat treat with no charge , I'm going for around 300lb should give me good power but the prod dimensions are still a problem the peace of steel I have is 8mm thick by 50mm by 900 long I want to get the best power out of this thing so don't want to mess around to much

Well her is a few update pics, the nut is in and working a hundred percent and the prod is just about done ( 700mm long x 40mm deep in the middle and 8mm at the tips , the steel is 8mm thick, i do want to put some deflex into it so if anyone has ideas on how to do it without a roller or forge id gladly have a go. And if anyone would like to guess the prods draw weight id like to hear it. 3in brace with 9in power stroke.
Thanks guys

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I think the only way to put a permanent deflex in the prod is to get it red hot, bend it, re harden, then re temper. It's not that hard. I would
dig a shallow trench in some hard ground. Put a steel bow pipe 1 1/4" dia. x 30" long with 1/4" holes spaced every inch along it's length. Cap one end and put a gate valve on the other end. Lay blow pipe in trench. Fill trench with charcoal. Attach a small shop vac hose
to gate valve. This is now a genuine forge. Most shop vacs will blow too much air thats why the gate valve is used to adjust air flow. You could melt the prod with this set up. Break your charcoal into small
pieces. One lit sheet of paper and a bit of air will get it going in a few minutes. Re harden prod in used transmission oil. Temper in kitchen oven. Grind off sharp edges on prod before hand. I would also glue and tie a backing of belt leather on the back of the prod
for safety. cheers fiddler49

Thanks fiddler the prod is still soft and can be bent, my concern is to bend it evenly, once its finished il have it hardened by the guys who sold me the steel.

Cold bending can put micro cracks into spring steel. Not a good idea on a heavy prod plus very difficult to bend evenly cold. Charcoal is cheap, give it a try. fiddler49

First a "Hello to everybody!" since this ist my fist post here.


If you want to bend the prod evenly I would suggest you to build a press for it.
But i admit doing this procedure would be very complicated for just one prod.
Even for the press you would need a roller to get the right radius in the "pressing stamps".

Second thing is you should bend your steel hot, not cold, even if it is not yet hardened.
Normal spring steel is bent hot. The problem is to get it completely heated (btw fiddler nice idea with the heating pipe) and pressed in time.
If you can´t heat it in the whole or it would be more comfort for you, you could try it in little steps with a smaller press with a little danger of uneven bending.

There would still be the possibility to bend it with a forge and anvil, but to get it perfectly by hand I would assume you would need years of forging practice.

If you bend it cold, you could heavily stress the material which is in my opinion a terribly bad idea with a dynamic working item.

I could calculate the draw weight (roughly, with no warranty) but I personally have a poblem with the understanding of power stroke and brace height. Exactly does power stroke distance include brace height? If it does my formula says you could be around 180-190lbs at 9" with relatively low suspense (700N/mm²) on the steel.
Safety would benefit a lot but your speed would suffer (a lot) too.

If you add the brace height you would have approx 280lbs at 12" but at ~optimal suspense for correctly hardened springsteel (1000-1050N/mm²).

There is a point where the distance the prod end moves comes into relation to the movement of the string center. My calculations assume a factor of 3,3 since this is a common basis.
Again, there is no warranty for these calculations, the final data can only be measured at the produced prod.

Another problem is, it is extremely difficult to build a perfectly working prod to an existing tiller.
Other way would be much easier since you just need to know the prod data and work the tiller to optimum.

Apologies for some language difficulties but since this is not my native language im a bit rusted.

Greetings from Germany
Rizzar

Hi rizza thank you for putting it in numbers for me . Ok so how about not putting a deflex in the prod and just brace it at 3 inch , would the steel take the strain of a 12 in draw I do want as much power as I can get. Thanks for the reply

So, to be absolutely clear: the distance between the prod and the nut fingers is 12inch not 9 inch??
After re-reading the post I´m quite sure about this

(all calcs consider ~8mm on each end for string accomodation)

In this case with a brace height (included in 12") of 3 inch there would be a draw weight at about 270-280 lbs.
The steel (depends on exact type) would be near maximum suspense (safety included) if correctly handeled.
The calculation depends on a triangular shaped leaf-spring, but I dont think the measurements of your prod are all correct for an even distribution since the ends are going to be a bit thin. Some sources would say 16mm (40% of the middle) were optimum for even distribution in the whole prod. (Considering this aspect the formulas I use can not be 100% correct)
In your case I wouldn´t go to complete maximum tension for the prod. The thinner limbs should benefit in a greater speed (lower mass weight) a disadvantage of durability and a reduction of draw weight (edited) , so think about safety.

If you don´t make a deflex into the prod, the maximum draw would be ~24cm (9,45") at ~290#, if you exeed this (12" would be what I call exeeding) you´re going to get into trouble and I would prefer not to be in reach when you span it.
Another point is the string would be unter a certain tension the whole time and sring mounting would be quite hard, too.

Another interesting question, what kind of spanning device are you going to use?

Btw I like your tiller, but I would have preferred to glue the deck on the tiller, so you don´t have these screws all over.

Rizzar

Rizzar thank you I'm feeling better about the prod I made the tips so thin so not to slow the bow down to much I will b putting some kind of leather strap on the back of , I'm not keen to loose the top of mu head if the things break. I used screws on the deck and glue my intention is to take them out and put brass pins in there place so shouldn look to bad and with the other brass bits going on. Thanks for the help I'll keep posting pic so we can see how things r going