Intro: Recurve Bow Failure and Success
My initial attempt to create a recurve bow stemmed from a Youtube challenge. At the time I had little experience shooting bows let alone creating one from scratch in the shop. I did a little bit of research across the internet and at a local archery shop and brought the vast knowledge I had acquired on the subject into the shop.
Sadly working within the rules of the competition for which I chose to make an attempt at making a recurve bow the initial attempt was unsuccessful. When I was nearing completion of the project I tested the limbs of the bow to get an idea of how strong the limbs were and how much flex the limbs had. The first limb I had tested, with the clearest knot-free plys used in the lamination process passed acceptably well. However, the second limb tested wound up snapping right along a knot which was in one of the plys in the lamination with a second knot on just below on the reverse ply in the lamination. A final third limb was attempted making sure to leave several inches between knots in any of the 3 plys used in the lamination. This too proved too weak to with stand the flex test for the limbs and snapped.
A second attempt was made using a clear section of maple. Additionally, using some of the suggestions in the comments in the original video, such as adding fiberglass to the limbs lead to a successful build for the bow limbs. Some variations in the build process were brought about also which just allowed the build to go more smoothly compared to the original failed build or were not applicable with the addition of fiberglass, such as in the shaping of the limbs.
Step 1: Selecting the Proper Wood
Wood is a very important component in a properly made recurve bow.
The obvious features in wood used to make a recurve bow would be knot-free wood. Knots or otherwise highly figured wood (which comes from grain direction changes) will have inherent weaknesses where the grain direction changes from long grain to end grain.
When preparing to build a recurve bow it is best to select a wood with a low modulus of elasticity and high modulus of rupture. These values are easily looked up on the Wood Database [referencing article: Bow Woods (From A Mathematical Perspective)].
Using the poplar from my yard which had a bow index of 6.39 [see referenced article] and many smaller knots was pretty much a guarantee that the project would fail. When switching to hard maple, while not in the ideal range, has a bow index of 8.64 which is still reasonably high. Additionally ensuring no knots were visible and using the quarter sawn side ensured straight grain.
Step 2: Fiberglass
Fiberglass on the bow limbs has a couple purposes.
The first set of limbs had small knots in the wood which created weak points. The effect from the weak points can be reduced, but not eliminated, through the use of fiberglass by creating an even pressure across the weak points in the limb, much like how the lamination process works.
The fiberglass also provides a level of protection against damage on the limbs since it provides a layer which is much more resilient compared to the wood used in the limb. The limb will be less likely to be damaged due to being dropped on hard objects, such as stones on the ground if the bow were dropped. Any damage to the wood fibers will create weak points in the limb of the bow.