Thought I would post a progress update but first I want to give a shout out of thanks to Ali.j for his tip on changing strings. (see his video tip https://www.youtube.com/watch?feature=player_embedded&v=P4uu3y7jo1w) When I thought about it I realized that the technique would work for the first string by using a longer bow string to start with. I just used an old recurve bowstring I had in the shop. It is long enough to just slip right on once the power cam strings are on and they are fairly easy to do by hand. A bit like using a bastard string to brace a conventional crossbow.
So the first big problem was related to the issue Hermit and I talked about above. The power cam string is anchored to the axle to the side of the cams. This causes an unbalanced load on the limbs and results in a bit of twist. Not a huge problem if it is not too much because the strings should follow the deep groove on the outside of the cams. But it is a much bigger problem for my two part power cam that consists of two round cams. As the cam rotates, the power cam string must transfer from the small round cam to the large cam. You see that here:
The string is on the small cam to start and as cam set rotates from pulling the main string back, the power cam string will be picked up by the larger power cam but there is about a 2"gap.
Unless the whole cam is twisted!
Then the string will miss the cam groove and slip off to the side.
So my experiment showed why a single piece cam if better even if harder to make. With a single cam, the string will continuously track the groove.
Here is how much limb twist I started with:
So I thought about how to take the twist out of the limb and came up with two possibilities. First, make the top limbs weaker so the offset load would pull the limb straight. The second is to change the angle of the base mount a bit to counter the twist angle. I ended up doing a little of both. Weaking the top limbs helped a lot but it still had a little twist so I angled the wood below the base mount a bit. Now it looks like this:
The double thickness limbs kind of work. They haven't exploded yet but the strength is less than 1/2 what I was looking for. I expect the glass content of the blanks I started with is too low. But I will continue withe these for now and think about replacements. Also, I knew my main string was a bit short and I was only getting 18"of total draw, so I made a temporary string a 4 inches longer for a test to see if it would give me the full 20" draw I designed it for. That worked perfect so tonight I will make the real one. Starting on the stock today!
More to come,