Thursday, 29 July 2010

A Change of Tack & Some Design Considerations

I'm going to let the Hawthorn bow 'rest'. It wasn't going to be the style I wanted.
I've pulled out two Ash staves cut in Feb 09. They are split as halves of a log 3" diameter at the top and 4" at the bottom.
I want to make something in the style of a Neolithic European Bow, (google 'Meare Heath bow') it will be a flat section similar to the Hazel bow on my website, but longer.
The Meare Heath bow has the outer curve of the log as it's back, the problem is that with a stave from a small diameter tree the geometry goes wrong.
Let's simplify the figures to illustrate the point.
Say we want the bow 3" wide at it's widest (a bit of an exageration as 2 1/2" is nearer the mark).
If we say the stave is split from a perfectly round 3" diameter tree, then the cross section of the stave will look like a semicircle 3" wide and 1 1/2" thick. So we have the width we want already but the problem is 1 1/2" is too thick and will give too much stiffness at that width!
So what's the answer? If we make it narrower it's no longer a flat bow and ends up being more like a

My solution is to de-crown the stave, that is to remove the grey area in the sketch below (it represents a cross section half way down a limb roughly to scale) by running a spokeshave along the centre of the back to an even depth thinning the cross section (into a sort of a trapezium, with back and belly parallel). The rectangle represents where the bow sits in the log. De-crowning has a couple of advantages

1. It's quick and easy.
2. It gives a nice flat even profile (logs just aren't perfectly circular).
3. The flatter profile distributes the stress more evenly across the back.
4. You can leave the handle area fully rounded for a nice grip, which also stiffens the handle, which is a good thing as it compensates for the handle being narrower than the rest of the limb (I mean you just couldn't grip a 3" wide handle!)

Now some people will say it's a bad idea because I'll be cutting through growth rings, which will weaken the back.
The answer to this is that the exposed rings (shown as red dots) will actually run up the length of the bow parallel to it's sides and that doesn't cause any problems. There will be a few rings cut across the limb where I leave the handle thick, but the bow isn't going to bend at the handle anyway!

The whole idea of not cutting through growth rings on the back of a bow is a laudable aim, however one has to temper these ideals with a good pinch of reality.
Anyhow, I've been marking out the best stave and I'll start work and post some pics later in the weekend.

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