Thursday, 13 November 2014

Billet Tapered on the Thicknesser

I'm having another go at a Bamboo backed Yew target bow to meet BLBS and GNAS definitions of longbow, this time with the merest hint of reflex and no deflex.
I've found a couple of nicely matched billets with poor sapwood and roughed 'em down on the bandsaw.
I made an adjustable taper sled so that I could try out my DIY thicknesser.
It took some fiddling as I couldn't do a cut more than about 3 or 4mm at a time but the end result is pretty good.
The top pic shows the billet on the sled, which is made from a length of 2x2 sawn down the middle and joined back together with 2 screws. There is a spacer at the right end 42" from the left end. 42" is is 7 x 6". So, to get a 2mm every 6" taper, I put in a spacer 12mm thick at the open end.
You can see the bottom edge of the sled and the top edge of the billet (which is held down on the sled with double sided tape) are parallel and the taper of the sled ends up transferred to the billet.
Second pic shows the tapered billet alongside a second billet, not yet tapered.
the finish is much better than my first try outs as everything has been tightened up a bit and improved, the long sled slides nicely and the extra weight helps it run through smoothly. It needs good preparation with the bandsaw first, but gives a quicker better more accurate result than doing it by hand.
I've just measured the taper and it's 1.7mm every 6" which is fine. I usually reckon on between 1.5 and 2mm every 6" for a longbow.
Now I know it works well, I might rebuild it from better timber, I have a length of Ash off-cut from the big slab that forms the computer desk. The pine sled flexes a bit so the Ash would be better. I'll prob' experiment further with a better raise/lower mechanism for the table first tho'.

This follows my WAQAP philosophy of  development (Wrong As Quick As Possible!).
It's a light hearted piss-take of the "Right First Time" quality system, but there is an element of truth in it.
All the planning in the world takes time and you'll still meet unforseen problems. Sometimes it's better to get stuck in and get a feel for how something works and that will uncover the problems.
"If only we could foresee the unforeseen problems" as someone once said to me in a project meeting!

Update:- Explain more?
If you look at the post where I make the thicknesser, it's easy to think that the adjustable table will somehow produce a taper... it won't. If you push a piece of wood through a thicknesser (or sanding machine) it will come out an even thickness. The way to get a taper is to stick your bit of wood to a piece that already has a taper (the sled in my case) and push the two of them together through the thicknesser. If you look closely at the top pic you can see that the billet and sled cave come out with top and bottom face parallel. EG The pair have been pushed through and come out flat, but when the billet is taken off the sled it has the exact same taper as the sled, the thick end of the billet and the thin end of the sled when measured together is the same thickness as the thin end of the billet and the thick end of the sled..
The principal is that two identical tapers put top to tail end up being dead flat.
Dunno if that was what was needed?

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