25.01.2022 DIY 6" Synchronome style, turret clock, slave movement.

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Many years ago I sawed out a 30" turret clock, skeleton dial. Which I laid over a matt black painted disk of the same size. I wanted to maintain high legibility. Fussy dials can rob the hands of clarity. Making reading the time unnecessarily difficult.

Then I made a 6" Ø ratchet wheel, slave dial movement to drive the counter-weighted, aluminium hands. I fitted the movement with double locking like other heavy duty slaves. 

The double locking can be overcome for manual hand setting. By pressing the armature up to the electromagnet cores. Though hand movement is only possible in a forward direction.

The 120 tooth ratchet wheel was cut with a slitting saw on my Smart & Brown 'Sabel' lathe. Using a large, home made dividing disk in polycarbonate. I scribed the divisions on the disk with dividers and then used a small center drill in the drill stand to ensure accuracy. A pin on a stiff, sprung blade located the drilled holes in the dividing disk on the tail end of the headstock. 

 The idea was to minimize errors of tooth pitch, by the ratio of dividing wheel diameter, to wheel, tooth pitch. I cut the reduction [motionwork] wheels my a similar method.

 The twin electromagnet coils were wound on my lathe in back gear. While I wore hide gloves to maintain tension. The cores and armature were of mild steel. Which had been repeatedly annealed in a wood stove. By leaving the stove to cool overnight. After soaking the metal in red heat for most of the day. The bed of ashes slowed the loss of heat in the metal components.

  Despite my patient efforts the mild steel still retained some residual magnetism. Which could have been overcome with an insulating layer over the cores. Copper shim material works well. I did not try this at the time and the clock hands would eventually stop. With the armature held firmly onto the faces of the cores despite heavy, coil spring, driving power. 

 A [clock] slave works by an electromagnet[s] pulling back a drive pawl on a lever. This occurs during a short electrical impulse from a master clock. So that one tooth of the ratchet wheel is gathered. Then, when the magnetic pull ends, a spring pushes the ratchet wheel forwards, via the drive pawl, by one tooth. The drive pawl and a backstop click prevent overrun and backward movement. Most British master clocks of the first quarter of the 20th century impulsed twice a minute. Requiring a 120 tooth ratchet wheel to drive the minute hand on the same shaft. The minute hand steps forwards in half minute jerks. The hour hand is mounted on a pipe surrounding the minute shaft. 

 Soon afterwards I was offered a genuine, Synchronome No4 slave movement with a 4" ratchet wheel. Due to the residual magnetism problem I immediately retired the bigger [home made] slave. The Synchronome movement has since been working for many years. With almost perfect reliability. Its Achilles heel being ice forming in the hour pipe. I increased the spring pressure and oiled the hour pipe. No further stoppages have occurred.  The original skeleton dial was replaced by a larger [secondhand] GRP dial 3' in diameter. This is easily legible from up to 200 yards away.

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