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

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

21^st century clockmaking uses an interesting mix of antique and modern tools. And,

being constantly tool-poor, clockmakers are always on the lookout for that rare utensil

which will speed up the restoration process. One of the modern tools that fit that

category is the computer. Using a program called computer-aided design or CAD¹,

clockmakers can design, alter, and manipulate a clock part, working out the bugs long

before the actual piece is made. The uses for CAD in clockmaking are limited only by

one’s imagination!

Drawing an Escapement:

As an exercise in the capabilities of a CAD program, let’s walk through laying out a dead

beat escapement as described by Ward L. Goodrich in his book, The Modern Clock ².

To get started, Goodrich says: “Draw a line, AB,… to serve as a basis for

measurements.” (fig. 1) Then, “…draw from some point C on this line a circle to

represent the diameter of our escape wheel.” (fig. 2)

Fig. 1: Step one in drawing

Fig. 2: Circle P representing the

Goodrich’s dead beat escapement.

circumference of a two inch diameter

Line AB.

escape wheel.

To follow Goodrich’s example, we will draw the pallets for a 30 tooth escape wheel

spanning 7 ½ teeth at a 90⁰ angle. “We now take 90⁰ and, dividing it, set off 45⁰ each

side of our center line and draw radii, R from the center to the circumference of our

circle; this marks the beginnings of our pallets.” (Fig. 3)

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Fig. 3: Radii R, 45⁰ away from line AB,

Fig. 4: Tangents T showing the pallet center

showing the amount of escape wheel the

distance at their intersection.

pallets will span.

“Now to find our pallet center distance we draw tangents, T (at right angles), from the

ends of these radii toward the line of centers. The point where they intersect on the line

of centers is the pallet center.”

(Fig. 4)

Giving each pallet two degrees of lift, “we draw a line two degrees inside the tangent, T

(towards the escape wheel center), from our pallet center on the entering pallet side and

another line from the pallet center two degrees outside of the tangent, T, on the exit pallet

side.”

(Fig. 5)

Fig. 5: Lines representing two degrees of lift on

Fig. 6: A circle, centered at the intersection of the tangents, T,

the entrance and exit pallets.

and meeting the point of intersection between tangents, T, and

radii, R. This circle represents the lock faces of the entrance

and exit pallets.

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Next, from the pallet center we draw a circle “cutting the tangents, T, and the radii, R,

where they intersect; this gives us the locking planes on which the teeth of the escape

wheel “run” (slide) during the excursions of the pendulum, if the escapement is to have

unequal lockings…” (Fig. 6)

Following Goodrich’s example gets a little fuzzy here, but sticking with an escapement of

unequal arm length, we can draw the lifting planes “at an angle of 60⁰ from the radii,

R…” to determine the pallet thickness. (Fig. 7) Finishing the outline of the pallets can

be done by drawing an arc from where the lifting plane intersects the lift reference line on

the entrance pallet and another where the lifting plane intersects the tangent line, T, on

the exit pallet.

(Fig. 8) This completes the escapement drawing and all that’s left is to

attach them to arms.

Fig. 7: The lift planes of pallets of unequal length,

Fig. 8: The edges of the pallets can be drawn from the

drawn at an angle of 60⁰ from radii, R.

intersections of the lift planes with the reference lines.

The accuracy of the CAD program makes laying out a set of pallets quick and easy. The

finished design can be printed to scale and used as a pattern for roughing out a set of

pallets.

(Fig. 9)

Fig. 9: The completed pallet design can be printed to scale and used as a

template.

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Another pallet design has the pallet arms the same length. The layout proceeds exactly

as before; producing line AB, circle P with center C for a 2” diameter 30 tooth escape

wheel, radii R, tangents T, and the two 2⁰ lift lines. The circle from the pallet center

now represents the mid-point of the pallets instead of the lock face. (Fig. 10)

Fig. 10: A circle, centered at the intersection of the

Fig. 11: Two circles representing the lock faces of the

tangents, T, and meeting the point of intersection

pallets, offset 3⁰ from radii, R. This will produce

between tangents, T, and radii, R. This circle

pallets which are too wide for the escape wheel,

represents the mid-point of pallets with equal arm

however.

length.

“If the pallet arms are to be of equal length,” Goodrich continues, “the arc for the

entering pallet is drawn three degrees below (outside) the radius, R, while that on the exit

pallet is drawn three degrees above (inside) the exit radius.” (Fig 11) Goodrich errs here

since this will produce pallets which have an overall thickness of 6⁰ and wouldn’t leave

any room for drops or tooth tips. (6⁰ x 2 pallets x 30 spaces = 360⁰) I suspect Goodrich

means for the total pallet thickness to be only 3⁰ which would necessitate drawing arcs

1.5⁰ below and 1.5⁰ above the radius, R.

(Fig. 12)

Fig. 12: Two circles representing the lock faces of the pallets, offset 1.5⁰ from radii, R. This

will produce pallets which are the proper thickness for the escape wheel and leave room for

drops and tooth tips.

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

Fig. 13: The entrance (left) and exit (right) lift faces of the pallets are drawn to give a 2⁰ lift

angle.

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 2⁰ to 1.678⁰. This

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

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Fig. 14: CAD can help visualize the effect of wear

Fig. 15: Instead of a 2⁰ lift, the pallet now has a

removal on a dead beat pallet. Here, 0.010” has been

1.678⁰ lift.

removed from the lock face of the entrance pallet.

As an apprentice, I was taught a method for maintaining or correcting the lift angles on

the pallets during the process of restoration.³ 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 2⁰ lift angle when the pallets are placed on the

diagram. (Fig. 17)

Fig. 16: The first step in restoring the lift angle is to

Fig. 17: Laying out a circle ½ the pallet arm length,

measure the pallet arm length.

and scribing a tangent, theoretically produces a 2⁰ lift

angle.

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A close-up inspection of the lift face of the entrance pallet and the 2⁰ lift reference line

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

lift face to 2⁰.

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

19)

Fig. 18: A close-up inspection shows which part of

Fig. 19: In this particular example, the method of

the pallet face to remove in order to restore the lift

restoring the lift angle will actually produce a

face to a theoretical 2⁰ lift angle.

result closer to 1.5⁰ lift rather than 2⁰.

In this specific escapement, in order to restore the lift angle to 2⁰ 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)

Fig. 20: Restoring the 2⁰ lift angle on this pallet would require a tangent line

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

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.

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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)

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.

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)

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Fig. 22: An imported image⁴ is scaled in the CAD program and

several measurements taken to ensure accuracy. The needed

spring and lever can be clearly seen in the picture (circled).

Once the image is scaled, it is a simple process to trace the outline of the lever, leaving

extra material to account for bends and unseen portions. Reference lines, where the

bends are located, will aid in the final shaping of the lever. (Fig. 23)

Fig. 23: The lever and spring can be easily traced,

Fig. 24: The tracings can then be printed to scale

adding extra material to account for bends.

and used as templates for making the spring and

sync lever.

Once the lever and spring have been traced, they can be printed to scale and used as

templates. (Fig. 24) In the case of the sync lever, the template can be cut out and glued

to a suitable piece of brass and then the brass machined to the trace lines. (Fig. 25)

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Fig. 25: The scaled template can be glued to a piece of

Fig. 26: Only minor adjustments and polishing is

brass to aid in the fabrication process.

left to do. The project went much quicker and easier

using CAD.

CAD made the fabrication of a lever with three acting arms a routine task. (Fig. 26)

There was no “starting over” or wasted time with “trial and error”. The spring and lever

function just as they should and, in addition, bear a close resemblance to their original

counterparts.

(Fig. 27)

Fig. 27: The fabricated parts look and function just like they’re

supposed to.

These are but a few of the uses of CAD and an in-depth study would take volumes. It is

like any other tool and becomes more valuable with use. The more one becomes familiar

with its capabilities, the more diverse CAD uses will become. In the future, using CAD

as a tool in clockmaking is sure to become as commonplace as using a screwdriver.

Whether it is used for design, theory, fabrication, or some as yet unknown purpose, CAD

will save time and money for the 21^st century clockmaker as it would have for Goodrich

had it been available in 1950.

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