Friday, 20 January 2017

Two Part Knot Patch

This knot looked very dry and ugly, it also runs through the bow at a rather shallow angle. It looks sound enough on the belly, but how deep does the dry loose portion go? You can also see the heart wood swelling up through the sapwood on the back of the bow.

I was keen to have a good solid clean sapwood back on this bow. No point doing a load of work making a bow only to have it explode and to then look at it and say "Ah yes I see why it broke there!"
Click on the pics to see 'em full size for detail.

First stage was to rasp off the corner of the bow in a gentle scoop, this is where the patch will be, this exposes more of the dry loose knot and more heartwood. I don't want to take the scoop out much deeper, so I make a V groove into the dry knot and heart wood which I can fill with a wedge of sapwood. This groove still leaves some of the dry loose knot, which I dig out and fill with epoxy and Yew dust mix.

I view the bow as almost 3 layers 1/3 is belly which is in compression, and in this case has a good sound knot. The central third is pretty much neutral and not under so much strain so I don't mind filling it with Yew dust/Epoxy, but I certainly don't want a cavity or unsound wood. the outer 1/3 is IMO the most important and I want that to be good sound sapwood.
Having filled the little bit of remaining cavity, I then shaped a curved V of sapwood (a bit like a boat) and glued this into the V bound tight with rubber strapping. This was left overnight and then cleaned up to recreate the original gentle scoop. An off cut of sapwood was cut to fill the scoop keeing the growth rings in correct alignment and making it thin enough to flex and conform to the shape of the scoop. More rubber strapping and an overnight wait (the bow being brought indoors into the warm) and it was ready to be cleaned up.

The back of the bow will have more work to follow the growth rings and by the time it is finished the patch will be difficult to spot hopefully.

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