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Steel as bow materialEdit
if you're aiming at making a really high draw-weight crossbows, spring steel is probably the best - or at least the most convenient - material you can use. It has has several inherent advantages as a bow material:
- Really Cheap
- Readily available
- Takes no noticeable set
- The material is very uniform in quality, requiring little skill in making the bow
- Easy to make into very high draw-weight bows
- Virtually indestructible, unless seriously overstrained, e.g. due to bad bow design, overdrawing or dry-fires.
- Canting the bow's tips upwards (to reduce friction of the string and the stock) is very easy and does not make the bow any weaker
- Aggressive width tapering is easy and does not compromise the bow: for example, a 6mm thick and 40mm wide bow can be safely tapered to 6mm width at the tips. This is not true to the same extent with wood or glassfiber.
At this point it should be noted that modern spring steel is not exactly the same as the steel (or iron/steel combination) used in the Middle ages(england, wales and places around there). It is, however, close enough in looks and spirit for anybody except the extreme traditionalists.
The heavy weight of spring steel compared to some other bow materials would seem like a disadvantage. However, steel bows are much stiffer than, say, wooden bows of similar dimensions. While they weight a lot, they are capable of storing a lot more energy. For this reason steel bows can easily cast bolts at similar or higher velocities as wooden bows, if properly designed. That said, glassfiber has much better physical properties, even though as a fibrous material it does have its shortcomings.
There are a few important things you should note when using steel as a bow material. If you're working with hardened spring steel, you should make sure you do not overheat the steel. If you do, the steel will become softer at the spot that was overheated. The overheating point depends on the temperature at which the steel was tempered - with spring steels this is probably around 470 degrees celsius. For a good overview of the hardening and tempering process, see The Art of Blacksmithing (Bealer 1995: 149-155) or Wikipedia.
If you're making your bow out of non-hardened spring steel and sending for heat treatment afterwards, you should note that thin portions of the bow are likely to twist during hardening. To prevent this you should overbuild the bow (=leave the thin/narrow portions thick), have it heat treated and finally cut and grind it to shape.
You should not heat-treat the bow yourself, unless you have proper tools and skills to do the treatment properly. A poorly heat-treated bow will either
- Bend if not hardened throughout or tempered in too high temperature
- Break if not tempered thoroughly in high-enough temperature
Also note that any defects in the material used for the bow limits the safe draw length. These defects include things like badly corroded areas, rivet holes, dents and such, all of which are common in used leaf-springs.