I've tried to let the pictures tell the story.
The thing to bear in mind when doing a repair is... you don't want to be doing it again, or making multiple patches to do the job if you can avoid it. Sometimes it may be necessary to do two which overlap, or in this case you'll see I've filled the groove of the central pith with yew dust epoxy mix. There is no point in patching over unsound material!
The procedure is:-
1. Rasp out the cracks and make a long flat area for the patch to cover all the discontinuities and suspect areas. In this case it exposed a line of pith so I cleaned that out using a chainsaw file with the end ground cleanly across at a slight angle to act like a round chisel/scraper.
2. Fill the pith groove, then rasp it all flat again.
3. Find a suitable bit of Yew for the patch. Easier said than done! The offcuts from that actual branch were two knotty, eventually I found an offcut which was relatively knot free and a reasonable match for grain. That is to say, the patch wants ideally to have the rings running the same way as the limb of the bow. It's not essential and I could have used a quarter sawn bit with the rings effectively running back to belly. It wouldn't match in so well, but would still look ok if done well.
4. Prepare the patch to size allowing plenty of length and a little extra thickness. Get the width about right as it needs to be flexible so it will pull down flat when glued. I run a cabinet rasp along the two faces to be glued holding it across the work like a draw knife, this gives fine grooves along the wood. There is plenty of argument about how smooth a surface to be glued should be. I'll let you make up you own mind.
5. The glue is mixed, I use Resintite which I apply to both surfaces, despite the instructions saying just one (again, make up your own mind)
6 The patch is held in place with masking tape to prevent it moving while the rubber strapping is applied. I only use one layer of strapping pulled tight. It's cut from rubber roofing sheet or old inner tubes, about 1" wide and wound on to overlap each turn slightly.
Because this patch is on a finished bow, I've protected the bow with plenty of masking tape. I've also strapped the bow down to a length of 2x1 pulling the tip down by about 1/2". This is to remove a bit of a deflex bend at the weak point which is being patched, it will make the bow more symmetrical and remove the little set which the weak area had taken on.
The strapped up patch is left overnight for the glue to cure. Watch out if you are doing it in cold conditions and bring it in somewhere warm to cure.
It's then a matter of taking off the strapping and carefully rasping araway the excess wood. It needs extra care at the ends, where the patch protrudes past the flattened area as it will not be glued down cleanly to the prepared surface, but may have glue loosely tacking it down to the belly of the bow which still has it's Danish oil finish. If you are not careful, this loose area can tear or splinter down into the patch. I use a cabinet rasp held gently. Of course you don't want to rasp into the parent wood or remove so much you are back to having a weak spot! The moral is slow and steady... you've put all the work in, don't screw up now!
I haven't finished it off yet, and I'll need to re-tiller the bow, but I'll post this for now as I've had someone asking about doing a patch.
I should have looked at this blog entry before I did the job! If only that were possible... I ignored my own advice and should possibly have rasped out more, I'll explain!
On the edge of the bow are a few black lines where rain or rot has got into the wood. The patch bridges over this area and I was hoping it would effectively sandwich the unstable wood between two bits of good solid stuff. The marginal wood is also on the edge and about half way between back and belly where it is theoretically neither in compression nor tension... Yeah right, someone tell that to the wood, now I've taken the bow to full draw a few times its bulging slightly where the black lines are. This is possibly due to the fact that areas which were previously distorting have now been strengthened, pushing the strain onto the next weakest link of the chain.
Anyhow, I probably couldn't have done it all as one patch anyway. It's frustrating, but a second patch should do the job. It just needs perseverance!
In the cold light of day, it simply needs doing, and the bow was an accident waiting to happen without this remedial action. a stitch in time saves nine... or more accurately two patches in time will save a smashed bow.
I've rasped off the offending corner of the limb, stopping just short of the sapwood at the edge. I've made a patch... much more difficult this time as it's curved incorporating some of the belly and some of the side like a long curved corner piece. Tricky to explain, but I'll post some pics tomorrow I've had enough for today!
If you look at the top picture (click on it to see it big), on the left, just where the blue of the G clamp meets the bow, you can make out the dark areas of poor wood on the side of the bow, there are 3 curved dark lines.