Thursday, 27 August 2015
Elm II Heat Treated
The weather has been appalling all week, at one point I had to take off my shoes and roll up my trousers to paddle back from the garage to the house, pausing to unblock the drainage gulley on the way!
The Elm warbow was up to about a 4" brace and drawing about 26" at 115# so I felt it was ready for heat treating, no point doing it too early as you just end up rasping off the heat treated wood.
I spent about 2 hours yesterday afternoon heat treating the belly with it strapped up straight. You can see how it's improved the shape compared with the chalk like on the garage floor done as a reference before the work.
I didn't try to force too much of the kink out of it, I just strapped it down with some rubber strapping. The heat was applied from the tips progressing slowly along the limb. Side cheeks of off-cut wood clamped in place, that way the worrying knot had time to warm up slowly and the bend was spread along the outer limb to direct the heat along the limbs (plenty of pics of this process elsewhere on the blog if you search for heat treating)
I put a little scrap of wood over the knot to help keep some of the heat off the epoxy/wood dust filling.
The heat has degraded the epoxy but not as deep as I'd feared, I can clean it up and re-fill it.
Damn it's just started raining again, and I've got to go up to the dentist later to have a cracked filling fixed... maybe I'll have to drive up there which is irritating as it's only a 15 minute walk to the dentist.
Update:- I've cleaned off the underbark layer from the back of the bow and narrowed the back a bit, aiming for a trapezium cross section (wide belly, narrow back) as most woods are stronger in tension than compression, also woods like Elm Hazel Ash need a wider belly... (others will assure you that Ash is a wonderful wood that needs no such allowance... personally I wouldn't trust it further than I could fling a grand piano, but it's still vastly better than having no wood at all!)
Anyhow, I put it back on the tiller, reverting to the slack string to see how it was bending... I'm glad I was cautious, the left (lower limb with the big knot) seems much stiffer, maybe it's the heat treated and re-filling that knot. I didn't pull it very hard (maybe 60-70#) I wanted to see how it moved before pulling it to full weight. It pays to proceed carefully after any major work. If I'd braced it and heaved it back to 115# it could have been catastrophic!
That limb was somewhat wider than the top limb so I've tidied it up, narrowing it more (mostly from the back), and running a fine cut of the spokeshave over the belly. You can feel and hear the change in the wood from the heat treating, some of the very outer surface is slightly charred, but basically it is definitely harder. It's looking a bit better and I've pulled it a bit more about 95#.