# Proper alloy for the prod (bow) of a crossbow.



## rogervan (Jul 19, 2011)

Hi,

I'm an avid tinkerer, and I have a craving to make a crossbow. The only thing that is stopping me is finding the right material for the prod.( I want an extra-heavy draw, use a crank, and make an extra-sturdy trigger system.) I am looking at steel, and preferably aluminum, of the proper alloy. I will make the parts strong enough in case you worry about that. I'm going to incorporate a polycarbonate shield in case something dangerously fast and heavy whips back at my face.

I canvassed the entire Internet for the answer, and got none. I'm convinced i reached some people who knew, but maybe for their personal feelings about my safety, gave no answer.

I've bought a few pieces that had extra-heavy modulus of elasticity, but they all failed. Each one took a permanent bend.

Thanks,

Roger


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## Hunter Dave (Jul 17, 2007)

Why not just pirate some limbs off of an old 70# compound with recurve-style limbs? You are going to pick up some serious poundage by shortening the "riser" section connecting the limbs and by eliminating the notch for the cam and the cam at the end of each limb. Of course, these kinds of modifications will be quite dangerous so be very careful in your experiments.

If you are fixed on steel limbs, you are looking for _spring_ steel. Some light trailers have leaf springs. You might try finding an old utility trailer or car and play around with the main leaf, i.e. the longest leaf in the leaf spring bundle. Bear in mind that grinding/cutting spring steel will probably cause it to lose its temper unless you are able to keep the spring very cool during the process. Making your own spring steel requires a knowledge of metallurgy which is beyond the scope of most garage projects.


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## tuckerd (Oct 27, 2010)

Here are some plans for a diy crossbow from a leaf spring. I'm going to try and make a modified version of this withing a couple of weeks.

http://www.vintageprojects.com/archery/crossbow-leaf-spring.pdf


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## rogervan (Jul 19, 2011)

Spring steel in the rectangular shape I want is impossible to find. Maybe I could use two pieces of straight round spring steel, maybe 5/8 inch each. I suppose it wouldn't hurt to weld the outside ends of the pieces with one inch long welds.

Yes I was checking out the resistance of the limbs in a couple of my spare bows. The sixty pound bow would be a candidate for beefing up, but it's old and the glass over one of the limbs crackles when I partially draw it. I can see a tiny crack starting in the plastic overlay. The other bow has too light a draw weight, but I was thinking about adding some layers of S-Glass or unidirectional carbon fiber over the overlay. I guess I'd start with the good limb from the first bow, using carbon fiber, to see if that lay-up can withstand the tension on the outside of the limb. IMHO there is a chance that carbon fiber has so little stretch that when it's pulled hard enough, that ply could either just snap off the limb in a long thin sheet, or it could create a sudden breaking point that would break the limb somewhere before it lets the limb bend far enough.

Thanks for the tip about the light-duty-trailer leaf springs. I wonder if a CNC machine could cut such hard steel. I also wonder if the water in the cutting jet would keep the workpiece cool enough.

Roger


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## rogervan (Jul 19, 2011)

Well, No one can get spring steel in the hardened condition, because it can't be machined. I have to buy it annealed, shape it with a cutter, bend it just right, and send it out to be hardened. That solves that mystery. I'll get back here when the project takes shape.

Roger


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## tuckerd (Oct 27, 2010)

Yeah, I think that is why people go the leaf spring route. I haven't tried it yet, but from what I've read you can use an angle grinds slowly and with very little pressure on the leaf spring without them losing their temper. I believe the trick is to do many light passes with as thin a cutting disc possible instead of fewer deep passes which will head up the metal more.
The limbs do need a taper of some sort so that they can function and be safe. An un-tapered steel limb will put a lot of stress on the point it attaches to the stock and could quite possibly break at that point after repeated use. I have not actually done this, but just passing on information that I have read in doing some research myself on the building a crossbow.


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## Veni Vidi Vici (Jan 23, 2011)

Modulus of elasticity is not what you need to evaluate in order to ensure that the material will not take a permanent set. Tensile yield strength is what matters. If you keep the maximum bending stress sufficiently below the yield strength you will be OK.

You may find some useful calculators at http://www.engineersedge.com/. Specifically, take a look at the Beam Deflection and Stress section.


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## Groo (Sep 17, 2012)

why not just get a leaf string from a lighter vehicle? like the front end of an old school snowmobile.


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