Saturday, 20 April 2013

Belly Patch Bow Tillering

The bow with the belly patch is an ugly beast with dips and twists, not to mention the belly patch.
So why am I persevering with it?
The sapwood layer is lovely, thin and even. I haven't had to work it down at all and there are no knots. It's also a challenge and a good illustration of some of the problems.
It's difficult to show the twists and dips in a stave without a decent photographic studio and plenty of time on your hands. If you look back at some of the previous posts of this bow you'll see how awfull it looks.
It draws back ok, but the prob's are how to get a vaguely even tiller and how to get a good string line. As well as twist, the bow is trying to go slightly sideways.
One limb has a severe reflex then deflex dip which looks like it's stiff at one point and very weak at the dip. This shows worse on one side of the bow, so I've put the bow on the tiller the other way round and drawn a straight pencil line down the limb.
This helps to see the overall bend and to ignore the dips. Viewed from the other side I couldn't actually draw a straight line along the limb, it was just too wobbly.
As for the twist and sideways bend, the other pics show the advantage of leaving the tips wide and having temporary nocks. The string can be forced to one side to see if this improves the situation. If it looks better then it's a simple job to remove the excess from one side to create a new centre at the tip, if your tips are slimmed down then you are stuck with trying to do slight heat bends or weakening one edge to get the bow to come back across. Easing off the edges is tricky and it's easy to get in a muddle and weaken the wrong side. I won't event try to explain which side to ease off on a bend... if you can't work it out yourself, it's probably better to step away from the bow and have a cuppa.

Now I daresay this bow will end up still looking ugly, but if itn look balanced and shoot well, that's fine by me.

One last word about the horrible dip, it's deceptive, it looks like it bends too much, but in fact it will represent a slight weakness.
Consider a straight length of wire, say a piece of wire coat hanger.
If you try and push the two ends toward each other it's fairly rigid.
If you now bend a large but fairly gentle U bend in the middle (taking care not to work harden the wire) so that the ends still run in line but it has the U bend in the middle and repeat the test of pushing the ends towards each other it will now flex at the U bend.
So that bend has changed the geometry of how the forces act along the limb, similarly when the limb is bent. Obviously small dips and waggles don't make much difference but it's just something to be aware of... if it looks weaker... maybe it really is? Maybe it's just the dip? That's part of the joy, you could probably throw a vast amount of analysis and computer time at it and be no closer to the real answer, it's about feel, experience and of course some luck. Become your own expert and beware of those who pontificate from their armchair having never made a bow themselves.

Talking of which:-
I had the big bow brought over to look at, as the back had developed some unsightly streaks. Was it due to being shot in the rain? Were they potential cracks as suggested by an armchair expert? I was slightly concerned until the bow came out of it's bag. John had correctly thought it was probably just the remnants of the cambium and indeed he was right.
The colour and texture change was however quite pronounced, where there had been the faintest pinkish brown streaks a couple of weeks ago, they seemed to have swollen to look much more solid, almost like blotting paper! I don't know if that was due to moisture, or drying, UV light or the phase of the moon. It doesn't matter, it was obviously just the cambium and the back of the bow looked a good as ever. I suggested, it could be removed by going over gently with some wire wool and re finishing if he wanted, but he was happy for it to show the origins of the wood.
Quite right too.

I've done a little more and taken some more pics to illustrate a point about twist.
This shows why I leave the bow a fairly square cross section until quite late in the process if twisting is an issue.
You can effectively shape the cross section to suit how the bow has turned out.
In one pic you can see how I'm resting the rasp on the side of the bow as it was originally rouhed out and the handle of the rasp is high and not at all in line with the string. The next pic shows how I can just re shape that side face to line up with the string so the arrow will have a smooth passage round the side of the bow minimising the paradox.
the other pic shows how the string line is begining to look respectable.

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