While reading this chapter, please refer to the drawing in figure 1 below. First, draw three circles with diameters of 4 and 6 and 7.5 inches. Center them on
the page and draw a horizontal and a vertical line across the largest circle. Take the
vertical line (1) and rotate it counterclockwise by 30º to get line (2): this will be the
escape circle's radius for the first tooth. Rotate line (2) counterclockwise by 45º to get
line (3): this will be the impulse face of the escape tooth. I have shortened line (3) and
several other lines only in order to make this drawing easier to understand. Place line (3)
onto the point where line (2) intersects the six inch circle. Then rotate line (3) counterclockwise by another 45º to get line (4): this line will become the pallet circle's radius.
Place line (4) on the edge of the six inch circle at the point where the circle intersects
with lines (2) and (3). At the point where the vertical line (1) intersects with line (4),
rotate line (4) clockwise by 3º: this will determine the amount of lock that this design
will have. While the ideal amount of lock is recommended to be only 1º, I recommend 3º
in this drawing because it will make the lock easier to see during the simulation. It will
also make the simulation more forgiving if errors were made during the preparation of
the drawing. The best way to ensure that line (4) is rotated about the point where it intersects line (1) is to center line (4) on the page shown on your computer screen before
rotating, since line (1) has already been centered on the page.
Rotate line (2) clockwise by 24º to get line (6), and place it onto the point where
lines (2), (3), and (4) intersect. Line (6) will be the tooth's locking face, and the point
where it intersects the other three lines will be the entrance corner of the tooth.
Rotate line (2) counterclockwise by 24º, since there are 24º between each tooth of a 15 tooth escape wheel, to get line (7). Rotate line (7) clockwise by 24º to get line (8). Place line (8) onto the point where line (7) intersects the six inch circle. Line (8) will be the locking face of the next tooth. The locking faces of the teeth will appear to lean forwards by 24º.
Draw a three inch circle and place in onto the point where lines (3) and (5) intersect. This circle can be made larger or smaller, or stretched at will until the desired shape is achieved to trace the curve of the tooth. I recommend that you draw a relatively thin tooth that would allow the pallet to clear during the simulation. After the simulation, when you know how much clearance you have, you could draw the escape wheel again. Draw the first tooth over lines (3) and (8), and trace the curve over the ellipse, so that you get a tooth that looks like this:
Move all the lines and the two inner circles to one side, so that all you have on the
page is the 7.5 inch circle and the tooth in the same position.
Group the tooth and the circle, and rotate the group by 24º in whichever direction
you choose. Duplicate the group and rotate again. Duplicate the group again and rotate
again. Repeat this until you have 15 groups, all in the same place. Then ungroup all 15
groups, so that you end up with 15 teeth and 15 circles. Delete 14 of the circles, one by
one, until you have the complete wheel.
In order for the escape wheel to be true (or perfectly centered), it is necessary to rotate the tooth inside a circle that extends beyond the tooth. Since the tooth extends slightly beyond the edge of the six inch circle, a larger circle is needed for the drawing.