[Ansteorra-archery] Bowyerthon materials Part 3: Tillering

[Sylvrfalcn at aol.com]

Friends,
  You've heard the term "tillering" in regards to bowmaking, but may not 
understand exactly what it means. Tillering is the process of gradually removing 
small amounts of material from the bow's limbs to even out or "balance" the 
bend of the bow. It's a little more involved than just ensuring both limbs bend 
the same, you also have to see to it that the limbs are bending gradually 
enough to properly distribute bending stress, that way too much stress doesn't 
build up in too small an area.
  Here's an example of what I'm talking about; If you take two green sticks 
of the same diameter, one short, and one long, you'll find it much easier to 
break the short one than it is to break the long one. The same amount of force 
that breaks the short stick will only cause the long one to bend. No matter how 
long the limbs of your bow are, if you tiller in such a way that one small 
area of the limb does all the bending, you just turned it into a "short stick", 
by concentrating the stress load into that area. Now you understand why it's 
important to have the limbs bend gradually.
  It gets even more complicated when you start deciding if you're going to 
make a working handle bow, or a non-working handle. A working handle bow is one 
that flexes through it's entire length, including the portion that runs 
through the handle. In a non-working handle bow, the center, or handle, section is 
rigid, and does not bend at all. Obviously, the approach to tillering the two 
styles is very different, and is more than I have room to cover here. But it 
highlights my point that tillering is more than just getting both limbs to bend 
the same amount.
  So what do you use to tiller a bow? Scrapers are the most common tool for 
gradually removing controlled amounts of wood from the limbs. A knife can be 
used as a scraper, so can cabinet scrapers, et cetera. Rudder bows sells an 
inexpensive scraper designed specifically for the job. I use a home made scraper I 
made from a big old metal cutting file. It gives a two handed grip like a 
drawknife, and with it I can produce an impressive pile of woodshavings lickety 
split.
  So how do you tiller? You always remove wood from the belly side only. 
During the rough shaping of the bow, it's important to make sure that the back 
(front side) of the bow is perfectly flat and as smooth as possible. Any wood 
removed after that, as in tillering, is taken off the belly side only. Tillering 
takes patience, because you're removing small amounts of wood at a time, and 
constantly checking and re-checking your progress on the tillering jig.
  Tillering jig? Yup, it's a fixture you place the bow on to "draw" the bow 
just enough to check the tiller. To do this you use a "tillering string" that's 
much longer than your finished bowstring. The whole point with a tillering 
string is to not have to bend the bow much, if at all, to get the string on. 
There are notches on the tillering jig to hold the string at different "draw 
lengths", so you can step back and look at how the limbs are bending. I can't 
stress enough how important it is to not draw an unfinished bow any further than 
you have to while working on it.  If the tiller's way off, and you draw it too 
far, you WILL damage the bow.  Don't worry about going out and making a 
tillering jig if you're coming to my class, we'll have tillering jigs and tillering 
strings ready for you.

More to come, cheers for now,
Robert of Yorkshire