19. Making an upper balance cock.

8 02 2016

Tim White, from down under in London, very kindly agreed to provide a “Guest Post” about his making a chronometer balance cock. I have added some notes in blue. He is in the process of copying a Hamilton M21 chronometer, using some of the original dimensioned drawings.  Most chronometer upper balance cocks are in the form of a Z (Figure 1). Traditionally, the folds were made by cutting a 92 degree groove in the metal almost the whole way through, bending to 90 degrees and running solder into the groove (See Post 8, Figure 7), but manufacturers later probably used extrusions that were then finished to size and contour.

Fig 3.12.jpg

Figure 1: Common form of balance cock (from The Mariner’s Chronometer).

However,  the chronometer example supplied to the Hamilton Watch Co. in about 1940, for them to assess, was by Ulysse Nardin (Figure 2), who milled it from a solid block and attached it near the edge of the upper plate, so Hamiltons did too (Figure 3). The Nardin method seems to be somewhat over-thought, needing many operations and several settings to complete.

s-l1600 (1)

Figure 2: Ulysse Nardin balance cock.

Internal curves

Figure 3: Underside of Hamilton Watch Co. balance cock

Tim began with a piece of 1½ inch (38 mm) square brass bar which he faced slightly over length in order to mark out and drill 6.5 mm for the 1/8 inch cock retaining screw and counter-boring for the screw head. The hole and counter-bore for the upper balance pivot and its jewel must be accurately located relative to this hole. All other dimensions are “air fits”, that is to say that they do not need as precise positioning and dimensioning, since they are not in contact with other parts. Note that the curves marked with green lines in Figure 3 are not necessary for the part to function and could have been omitted as an economy measure.

Tim continues:

Put in the small 4 jaw and face down to within .005” (0.13 mm),  removing to measure. Remove using 2 jaws only. Clean and replace.

Tighten lightly, then applying tailstock barrel and apply strongish pressure, re-tighten 2 jaws quite strongly. Remove barrel and face .001” at a time measuring after each cut and clean between cuts. Take three final cuts stopping the lathe between cuts. (A depth micrometer, measuring against the face of the chuck, is handy for getting the correct thickness without having to remove the workpiece from the chuck. I use a lump of copper to tap the first face into contact with the face of the chuck.)

Next, I coated one side of block with marking blue and traced the outline of the original cock. It’s difficult to draw the cock from the plans as they are not completely dimensioned, as well as having centres that lie outside the block. In order to cut the 3 curves I made 3 discs of the right diameters attached to a mandrel. Referring to Figure 4, which is modified from the original Hamilton drawing, for curve A, I mounted the block in the machine vice of the vertical slide in the lathe and then aligned the edge of the disk with the scribed outline of the outside diameter. I replaced the disc with a fly cutter with an adjustable cutter and adjusted the tip of the cutter using a calliper until it had the correct diameter. I then cut the curve.

Copy of BALANCE COCK P5labelled

Figure 4 : Outline curves (Hamilton Watch Co., modified)

Next I machined the straight part B, Fig 4.  I then machined the curve C , Fig. 4, mounting the block on the 3.4in (86.36 mm) disc so that the curve marking is exactly lined up with the edge of the disk placing it by carefully marking, drilling and tapping holes so that it can be screwed to the disk. NB I made a counter weight which was weighed and machined until it was the same weight as the block. I then screwed it to the other side of the disc opposite the block and machined it at around 600 rpm (Figure 5 ).

BALANCE COCK 2

Figure 5: Set up for machining curve C

To machine curve D, Fig. 4,  I used the Sherline rotary table. The block could not be rotated by the lathe because the flat straight AB would hit the cutter. I attached the table to the vertical slide with the operating handle protruding towards me. I used the 2.8 in. (71.12 mm) disc (2 x 1.4 in) centred on the table using the table centre arbor and a spacer to plug into the disk. I secured the disk to the table using two sockets in the T slots. I then secured the block to the disk holding it with a small clamp while aligning the curve with the edge of the disk. I then centre popped through the two holes in the block using hardened centres of the correct diameter (transfer punches). I then drilled and tapped the holes and screwed the block to the disk.  Using a 7mm high helix end mill I then machined the short curve manually using small cuts of around .003in (0.08 mm) watching to ensure the mill never got near the flat area. This worked very well. The finish on all surfaces was good enough so that I could sand with 400 wet/dry and finish with 2000 followed by a Simichrome polish.

To machine the inside cuts I attached a round steel plate to the rotary table. In the middle of the disk (bored .4375” – 11.11 mm) I inserted a threaded pin screwed into the table centre. It has a protrusion which is the same diameter as the jewel hole. It protrudes only about 0.075in (1.90 mm) and is used to locate the jewel hole end during machining (Figure 6).

Central pin

Figure 6: Locating pin for jewel hole, and face plate.

The other end is secured by an M3 screw secured into the disk. The table is attached to the vertical slide. The cock is centred using a pin, which has a reduced diameter end .188in (4.77 mm) diameter. Its OD is 3/8 in (9.53 mm) and is held in the headstock spindle using a 3/8in collet. The vertical slide and cross slides are adjusted until the end of the pin goes into the jewel hole. The cross and vertical slides are then locked. Machining is carried out with a milling cutter held in the lathe spindle and the cut is put on by advancing the lathe saddle. I used a dial indicator to monitor cutting depth. I started the cutting at the jewel hole end and progressed outwards. This method worked very well with the M3 screw and centre pin securing the work piece. Figure 7 shows the workpiece secured to the rotary table, ready to be mounted on the vertical slide of the lathe. Originally, the part would have been machined on a vertical milling machine.

Rotary table 001

Figure 7: Work piece mounted on rotary table.

The curve E, Figure 4, is formed by making a filing button set-up using an OD of .400in (10.16 mm) and an ID of .188in (4.77 mm) to fit the hole. These hardened buttons, one each side of the part, were classically called “cheaters”. When the part is to size and shape, the file skids of the buttons.  I ground off any excess then filed the remainder using fine small files. Then I used 2000 grade wet and dry and polished the work with Simichrome.

Figure 8 shows the part after machining, with Figure 9 showing the finished part alongside a Hamilton Watch Co, original. The latter was made of nickel silver, similar to 60-40 brass with about 20 percent of the zinc substituted by nickel, to give a hard, corrosion resistant, white metal. Holes remain to be drilled for securing the balance spring, the upper balance jewels, and the steady pins which ensure that the cock is correctly located on the top plate.

Finish machined

Figure 8: Part finish machined.

BALANCE COCK  4_1

Figure 9: Hamilton original (left) alongside polished new part.

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