The Use of Computer-Aided Design (CAD) in Clockmaking - part 2

And finally, the lifting planes can be drawn for the entrance and exit pallets. (Fig. 13)

Once the pallets have been designed, it is a simple matter to generate an escape wheel to
go with it. Then, in the CAD computer world, the pallets can be opened or closed to see
what effect that action has on the drops and locks. Varying the distance between the
pallet arbor and escape wheel center is just as easy to do and the changes there can be
measured as well. In other words, a CAD program can aid in visualizing escapement
theory.

Visualizing Escapement Theory:

Drawing an escapement with a CAD program is so fast and easy that slight variations can
be made to the escapement and then the effects of those variations can be observed. To
demonstrate how this can be accomplished, let’s consider how removing the wear out of
dead beat pallets can effect the escapement.

Using Goodrich’s layout of dead beat pallets of equal arm length for a 30 tooth escape
wheel, a slight reduction of the thickness of the pallets due to wear, say 0.010”, can be
easily rendered in the above model of Goodrich’s escapement in which the pallet lengths
are equal. (Fig. 14)

The results are easily seen and show a reduction of the lift face from 2

to 1.678

. This

would mean a corresponding reduction in the impulse of the pendulum! (Fig. 15)

Fig. 13: The entrance (left) and exit (right) lift faces of the pallets are drawn to give a 2

lift

angle.

As an apprentice, I was taught a method for maintaining or correcting the lift angles on
the pallets during the process of restoration.

This method is easy to use, adaptable, and

versatile. It involves measuring the length of the pallet arm to the middle of the pallet
(Fig. 16) and marking off a circle with a radius ½ that length. Striking a tangent line
from that circle is assumed to represent a 2

lift angle when the pallets are placed on the

diagram. (Fig. 17)

Fig. 14: CAD can help visualize the effect of wear
removal on a dead beat pallet. Here, 0.010” has been
removed from the lock face of the entrance pallet.

Fig. 15: Instead of a 2

lift, the pallet now has a

1.678

lift.

Fig. 16: The first step in restoring the lift angle is to
measure the pallet arm length.

Fig. 17: Laying out a circle ½ the pallet arm length,
and scribing a tangent, theoretically produces a 2

lift

angle.

A close-up inspection of the lift face of the entrance pallet and the 2

lift reference line

shows which part of the entrance lift face to remove in order to theoretically restore the
lift face to 2

. (Fig. 18) However, the CAD program proves the results to be off. (Fig.

19)

In this specific escapement, in order to restore the lift angle to 2

as laid out in Goodrich’s

example, it would be necessary to draw a reference tangent from a circle with radius
0.6247” instead of half the pallet arm length. (Fig. 20)

There are several conclusions we can draw from this exercise. It is apparent that some of
the clockmaking methods we’ve come to believe as “tried and true” may not actually
produce the results we were expecting. Also, this exercise has shown that just the act of
dressing the wear off of a dead beat pallet will change the lift angle, perhaps undesirably.
And finally, using a CAD program can help you “visualize” escapement theory and help
better appreciate escapement dynamics.

Fig. 18: A close-up inspection shows which part of
the pallet face to remove in order to restore the lift
face to a theoretical 2

lift angle.

Fig. 19: In this particular example, the method of
restoring the lift angle will actually produce a
result closer to 1.5

lift rather than 2

Fig. 20: Restoring the 2

lift angle on this pallet would require a tangent line

generated off of a circle with radius 0.6247” instead of 0.4977”.

Part Fabrication:

One of the most difficult tasks a clockmaker can face is having to accurately reproduce a
missing part for an antique clock. Whether it is an anchor, a lever, a piece for the case, or
something else, the task can become much simpler if an accurate drawing is available.
This is where a CAD program becomes a very valuable tool in the hands of a skillful
clockmaker. As an example of how CAD can ease the difficulty in accurately
reproducing unavailable parts, let’s look at a problem which came up in my shop
recently.

In for repair was a 1920’s Seth Thomas model 113A movement. This is a large, three
train, Westminster chime movement which can sometimes be a challenge even if all of
the parts are present. Unfortunately, this movement was missing one of the most
complicated levers Seth Thomas ever used; the chime sync lever. (Fig. 21)

I can just picture the scenario that led to this situation…The screw head, holding the lever
tension spring in place, breaks allowing the tension spring to fall off and become
misplaced. The sync lever therefore, remains in a constant “sync” position only partially
releasing the chime train at the hour. And one person’s solution? To throw away the
offending lever and pass the problem on to someone else! ??? Oh well, the damage is
done and it’s time to properly correct it.

Making the screw and tension spring is easy compared to making the missing lever but
with the help of the CAD program, making the missing lever becomes almost as easy.
The first step is to locate a picture or drawing of the missing lever, import it into the CAD
world, and scale it to size. (Fig. 22)

Fig. 21: A Seth Thomas model 113A movement which is missing some important pieces for the automatic
chime sync. A three-armed sync lever should be visible as well as the lever tension spring and screw. All
that’s left is the body of the screw (at arrow) still in the hole.

Content .. 1 2 3 ..