Tillering

I've been E-mailing back and forth with a guy who's made his first bow, he had some problems tillering, but managed to make a working bow and has done a second already.

To try and help with tillering I shall post some pics as I proceed with the Ash bow.
I've been experimenting to try and superimose pics, havn't succeeded yet, but this cut & paste pic shows how the stave is starting to move from a taut string (but not actually braced) to 40 pounds pull, which is somewhere near the target weight.
It's important to get the bow moving evenly as early as possible, but of course this is easy to say.
At such a small deflection it's easy to think the centre of the bow isn't moving. By the time a fault is obvious it may be too late. You are trying to see the signs of a problem before they really become too obvious.
You can see the bow is bending slightly near the handle and along both limbs. The left limb has a slightly sharper bend at one point about half way along, and it looks slightly stiff near the handle.
The handle looks to be tilting very slightly down on the left side.

Nothing to worry about, but the trick is to stop it getting any worse, I shall mark a big W on that possible weak spot.
Now these effects are very slight, and I've actually re-written the previous paragraph drawing slightly different conclusions! This illustrates that it's a subtle process done a little at a time, it's a matter of stopping it going wrong, rather than actually doing it right.
Hmmm I don't know if that makes sense, but you are slowly removing wood, but taking care not to remove it from any suspicious areas. Each time I put it on the tiller and pull it back to 40 pounds I might not see any difference, or just a subtle change or maybe an extra half inch draw.
As the bow thins and comes slowly back each little bit of wood removed makes more difference. E.G 0.2mm removed from a 10mm thick bow makes more difference than 0.2mm removed from a 12mm thick bow.
If I keep pulling back to 40 pounds, and avoiding anything bad happening, eventually it will get to 40 lb at 28" draw and the job is done.
So it's a game of patience, which will amuse some of my familly as patience isn't my strong suit!