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Tillering is the process of working a bow down evenly to reach the required draw weight at the required draw length and to ensure that bow limbs are balanced with respect to each other and ensuring that the "arc" of the drawn bow is even. The majority of the work here is simply removing wood from anywhere that is not bending enough, and *not* removing wood from places that bend too much. The final result is a bow that bends evenly throughout it's length. (Usually except for the handle section, although in some bows, even the handle section bends slightly.)

Initial Process
Initially, wood is rasped evenly from the length of each limb on the bow. After a small amount of wood has been removed, rest the end of the limb on the ground, grasp the other end of the stave in one hand, grasp the center of the bow and press against the bowgrip. The object is to get the limb starting to flex evenly. Once both limbs have started to flex about 5-6 inches forward from the vertical, we are ready to move on to the more precise tillering. Initial nocks are cut 1/2" in from the end of each limb, sloping at a 45 degree angle from back to belly, using something like a 5/32" circular rasp, pocketknife or 4mm chain saw sharpening file. With practice, floor testing the bow can be used to get to within 20-30 lb. of the desired weight, when starting it is advisable to be a bit more cautious. (Floor-testing is resting one end of the stave on the ground and grasping the handgrip and end of the upper limb. Putting pressure on the handgrip causes the limb resting on the floor to flex, the amount of flex is determined by the amount of pressure applied to the handgrip.)
Precise Tillering
The easiest way of doing this is to have a tiller stick and a pair of bowstrings. The first bowstring is a very heavy and very long one so the bow can be strung just by slipping the long string on without flexing the bow. The other bowstring is used later once the bow starts to flex evenly to about 12" or so.

The other alternative is to have a pulley rigged up in the workshop, so the bow can be drawn using a pulley and rope with the bow handle clamped down to the floor or bench, set up so that you can hold the rope and still stand back far enough to compare the developing curves. With a spring scale, this can also be used to determine the draw weight of the bow. It is also useful to trace the required curves on a section of wall or paper such that the developing bow can be compared against it. As long as both curves are graphed accurately, this helps to ensure that both limbs match perfectly when they are completed.

Offered by Brian.