Sunday, 10 November 2013

Warbow at 27" Video and Still

I'd put in a lot of work yesterday what with the bow and a bit of decorating. I was tempted to do more work on it, but experience told me to leave it alone and start fresh this morning by taking a video to see how it flexes. The improved tiller layout works well, I can sit on the floor with the tiller rope wrapped round a length of 2x1 and pull it in the manner of a rowing machine, the scale is about at eye level so I can exercise the bow and work it back to the desired weight. the video lets me study how it's flexing and look at the draw length and weight at my leisure.
It looks to me that the outer half of each limb is still stiff. They both have tricky dips and swirls at that point which make it difficult to simply reduce the thickness without careful consideration. Oddly the still taken from the video looks pretty much spot on 'arc of a circle', I think this shows the value of using video and watching it flex dynamically.
I'm in danger of arguing with myself here, as one of the guys from Primitive Archer was saying how he liked to get the tips moving first and right at the end of the draw you could see the middle starting to flex.
I said this was nonsense as the whole bow is subject to the load and it all must be moving together even if you can't actually see it. So in this case, the outer thirds are flexing but just less obviously than the centre.
How do I know they are flexing? Well the cambium popping off under the tension is a good indicator. If I wanted to measure or prove it I'd glue a couple of thin spills of cane sticking up from the back of the bow, say 6" long. I'd measure the distance between their tips, winch the bow back and repeat the measurement. I'm not actually going to do this, but if I was into maths I'd probably have spread sheets and graphs and all that stuff.
I can now string it manually using a stringer rather than having to winch it back, this is much quicker and more convenient.
When it's finished (assuming it doesn't explode) I'll measure the draw weight on the long string needed to get it brace height. This will be a useful indicator for the early stages of work if I make any more heavy bows.
Update:- I'm only working on the outer limbs and it's now 130# at 28" with a 6" brace height.
Due to some of the features I'm having to take wood off the back, with all the care and attention to following a ring where possible and maintaining an even thickness of sapwood...  Yeah, right, well it may be 3mm on one edge and 7mm on the other. All a bit nervy, but I can see the outers working a bit more. Slow and steady wins the day (insert your own platitude here... ).
If you review the earlier posts you'll see how the dip has slowly been worked around, it will end up being a fairly subtle feature on the finished bow, maybe even unnoticeable to the untutored eye.

Update2:-
That's it for the day, she's 130# at 29" now and I feel that's probably close enough for now, it's been scraped, sanded and given a wipe of Danish oil. That's shown up some of the tool marks which will come out with a bit more sanding. Just the last stage of finishing will probably bring it back another inch or so.
I may make a string for it now, as I'm not sure I'd want to risk loosing an arrow with the tillering string and toggle on there.


No comments:

Post a Comment