Thursday, 8 March 2012

Roughing Out Figures and Technique

It occurred to me that I tend to show the bows once they are on the tiller, but don't really describe how I get to that point.

Here's a brief explanation.
I lay out a centreline along the stave trying to either avoid knots or get them on the centre line. I then mark the half way point and a line to give me about 30mm width there (depending on the power of the bow... but allow some extra for error). At the tip end I'll mark the width as 20mm, despite the finished bow being nearer 12mm wide. This gives room for temporary nocks and sideways adjustments for the string line and to take care of any sideways shift or flex.
It's then a matter of roughing it out on the bandsaw as close as I dare to that profile.
The sapwood on the back will be taken down to a suitable thickness and I'll then mark a rough thickness for the bow (allowing plenty of spare) say 30mm at the grip and 20 at the tip again, running a pencil along whilst a finger is following the back of the stave helps follow the contours. The excess from the belly is then taken off with the bandsaw. (An axe or drawknife will do just as well but it's harder work)

The previous few posts then get me to where I am now, I can't use the spokeshave much now because it snags and tears on the knots of the belly, it's not so bad on the sides as I work the bow down keeping a roughly square or rectangular profile (this make measurements easier)
At this point it looks like a roughly tapered bit of 2x1".

Right! Now I mark every 6" from the centre line and work out a reasonable taper based on experience (and measuring another of my bows) and rasp it down to these dimensions at each 6" interval (blending in to the adjacent points)to give a reasonably even taper. Once both limbs are done I can risk it on the tiller.
Here are some figures to show how it progresses. (I'm only working on thickness)
Roughed out thicknesses, as measured

Centre and then 6" intervals to the tip in mm, (to nearest 0.1)
29, 28.6, 29.1, 25.4, 22.8, 18.7, 16.5
Now you can see it's a bit fat near the grip and a bit thin where I've been working nearer the tip on that knot. Looking at one of my bows I think 25mm is a reasonable thickness at the point 6" from the grip and experience tells me that a taper of about 1.5-2mm every 6" along from that is about right.
So I write a list of dimensions starting from the centre (which I leave for now) and then to 25mm and 1.5 down after that. This gives
29, 25, 23.5, 22, 20.5, 19, 16.5 This ties in reasonably with the first figures near the tip.
I then work on the limb with a rasp starting at the point 6" from the grip and bring it down to about the right dimension. Here are my measurements after this exercise.
29, 25.1, 23.0, 22.9, 22.6, 18.6, 16.5 You can see I've not been too obsessive about it, but it's a lot closer. This is just the start and it will get worked down once it's on the tiller. As each area is rasped down it's blended into the adjacent areas you rely on eye and feel to keep the work even.
Hopefully the bow won't be too weak or have any nasty weak spots now. I've got to do the other limb, file some temporary nocks and get it on the tiller.
Working it down like this has allowed some of the knots to fall off the side or belly of the bow which is a relief! The pic shows how it looks like a bit of 2x1, the knot just about disappearing of the edge and a small pin knot dead centre. The width of the bow will get reduced a bit later on and the belly rounded.
Sorry if this has been a bit long winded, but hopefully it's useful.

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