14: Changing the Design

         Drawings could be superimposed for comparison, such as this comparison between the equidistant impulse and lock designs. It reveals that, in the equidistant impulse design, the exit pallet's impulse face does not exactly fit between the lines: while the exit pallet is identical to the entry pallet, it is not a mirror image of the entry pallet. The entry pallet does not have a perfect fit either, but the difference is very small and inconsequential. The result of the unequal fit on both sides is that the exit pallet releases the escape tooth a little prematurely, causing the design to be out of angle: the angle on the left side to the drop-lock position is one degree greater than on the right side. This problem is corrected in Chapter 22.

         To combine the advantages of equidistant impulse and equidistant lock, watchmakers have designed an escapement half way between the two, called the Semi-Tangental Escapement. Here, the angle between lines (2) and (23) and between lines (9) and (21) is 2º, so rotate lines (2) and (9) counterclockwise by 2º to get lines (23) and (21). Rotate lines (12) and (14) clockwise by 2º to get lines (22) and (24). Draw the entry pallet's impulse face from the point where lines (23) and (20) intersect to the point where lines (24) and (19) intersect. Draw the exit pallet's impulse face from the point where lines (21) and (17) intersect to the point where lines (22) and (18) intersect. Rotate lines (23) and (21) clockwise by 15º to draw the locking faces of the pallets.