Procedure for Altering Defective Draw

From The American Horologist magazine, July 1940

Procedure for Altering Defective Draw

By Carl Lundgren

In order to create draw, the teeth of the escape wheel are undercut and the locking faces of the pallets are inclined; so that the pressure of the teeth on the locking faces produces a drawing-in motion of the pallets toward the escape wheel. The ultimate action of this motion will be to bring the pallet fork against the banking pins and hold it there. The safety roller is then perfectly free and the pallet in position for the next vibration. 

Some watches have insufficient draw, especially some old single roller models. In these the pallet fork will not remain in contact with the banking pin; therefore, causing the guard pin to contact the roller table. If the watch doesn't stop, the rate will then be seriously effected. On the other opposite extreme, we have the watch that has surplus of draw. Many of the small models are affected with this serious defect. In these watches the balance will be found to have stopped with the roller jewel just entering the fork. In other words the impact of the balance was not sufficient to release the fork. This usually occurs on the discharge pallet, because most circular escapements have more draw on the discharge side.

A diamond charged lap facilitates the operation when it is necessary to grind the pallet slots. A copper lap mounted on a taper is used. After the diamond powder is rolled into the lap, the outer edge of the back of the lap is turned thin enough to allow the lap to enter the pallet slot without binding. The back edge of the lap is turned very thin, after the diamond powder is applied; in order to prevent the thin edge from bending while being charged.

In order to increase the draw the pallets are tilted or inclined toward the impulse and locking surfaces of the escape wheel. If it is necessary to decrease the draw the pallets should be inclined away from the locking and impulse surfaces of the escape wheel. It is not necessary to incline the pallets a great amount in order to secure the desired effect. It should be noted in the drawing that the pallet slot is ground at an angle. Sometimes just the one pallet can be moved or inclined; and the desired effect will be secured without appreciably altering the drop.