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Making a Spring Detent

Finished Spring Detent
This page lays out my procedure for making spring detents for marine chronometers and chronometer watches.  In this particular example, it is my second detent for the same instrument.  The first detent (seen next to the prepared steel blank) had a polishing fault which resulted in the bend seen in the spring blade.  There was a nick which resulted in the spring cracking after about 10 trials before fitting it into the instrument.

Most of the time, "fatal" flaws will not show up until the after the detent is nearly completed.  This is because the blade itself gets polished down to .03 mm.  It must be polished flat and parallel.  Any mistakes at that stage are irreversible.  Also, hardening and tempering take place after all filing, drilling and tapping.  A mistake in hardening and tempering can result in several hours of lost work.  Too brittle, and the spring blade will break during final grinding and polishing.  Too soft, and the spring will not have enough strength to operate correctly.

The trick to making any complex part is to plan the work so that each section of the part has the strength to hold up to whatever machining or forming work to be conducted.  

After cutting a piece of oil hardening steel to size, file the four sides square and roughly to size of the detent.






Locate the position of the locking jewel and center punch it in preparation for drilling.










Drill the hole for the locking jewel.  Everything is taken from here, so be certain the hole is straight.  By drilling while the stock is oversized, you can file the sides parallel to the hole, and not be overly concerned with getting the hole dead center.









  Tap the hole and fit hardened steel screws at the top and bottom.  These screws will serve as the templates for filing the pipe round (the file simply slides over the hardened steel screws).













The locking jewel pipe, arm and detent horn filed to shape.










Check the dimensions of the detent.  The locking jewel pipe is over the inspection hole that is above the banking screw that adjusts the lock of the detent on the escape wheel.  The foot to be will reach the detent screw seen below the detent blank.





The spring blade has been "roughed in" to locate the pad for the passing spring.  Drill and tap the hole for the passing spring screw.








File the foot.








Drill and tap the hole hole for the detent screw.  Drill the hole for the locating pin.








After parting the detent from the blank, wrap in fine iron wire.  Coat with soap.  Harden to cherry red and quench in motor oil.  Be VERY careful when unwrapping the wire, the detent is glass hard and very thin in places.






Polish so you can watch the color of the foot and spring blade.  Temper the blade to blue.








Grind and polish the detent to size using ever finer grades of polishing powder or diamond paste.  The polishing "files" are made from steel, brass and copper.  By polishing on cork, the work is fully supported and finds its own level making it possible to maintain "flatness".





This is the detent after finishing with the diamond paste.  Clean thoroughly, wash your hands and work area.  Get a new cork.  Polish with diamantine on either a rockwood stick or an ivory polisher.





The finished detent.  A "black polish" is a bright polish that either looks bright white or black, depending on how it is turned to the light.




For shipping  or ordering information, contact:

Dewey Clark                   410.592.9998

Email Contact:         Historictimekeepers@gmail.com

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