Horological Work: Watch Dials
This type of work requires the very greatest precision and is done almost exclusively with the aid of powerful binocular microscopes mounted on the machine. Stopping and positioning must be accurate to within a "flea's eyebrow". We use the micrometers on the machine in conjunction with datum points to measure accurately to produce a high precision result on the dial.
A sunray watch dial sample cut in brass for a well known European manufacturer. The outer diameter of the dial is approximately 34mm.
In this example, we even machined the surface of the plain cartouches with horizontal lines, slightly finer than 35 lines per millimetre. We also machined the minute ring prior to cutting the minutes with circular lines, also just finer than 35 lines per millimetre. The minute ring is 0.75mm wide, the ratchet work channels are 0.6mm wide. The only uncut part of the blank in this image is the far lower right corner outside the outermost ratchet work circle.
For engine turning students, it should be noted that the entire piece was cut with no guide, using depth stops instead. Considerable care is necessary in the setting up, especially as the dial has to be transferred from the rose engine, where the circles and ratchet work were cut first, to the straight line machine where the job was finished, retaining very accurate concentricity and facing true.
The blank plate was stamped with the holes and glued to a steel disc with cyanoacrylate before centring on the rose engine. First the ratchet work was done, then the very fine circles 1/896 inch (approximately 0.028mm) apart on the minute ring and coarser circles, at 8/196 inch (0.2268mm) pitch were cut on the chapter ring.
On the straight line machine, first the cartouches were marked with the bottom of cut and edge of flange 0.3mm inside, feint tramlines to work to. Then the cartouches were cut, leaving the interior plain. Next the sunray was cut 120 cuts to the circle, then the minutes and hours at different depths. The diameter of the sunray is 20mm. Finally the catouches were engine turned with very fine straight lines approximately 0.028mm apart to ensure a perfect surface for printing the brand names.
A dial of this quality must be finished without even the hint of a burr remaining anywhere on it. The sunray must start and end as invisibly as possible. All the cutting must be clean and of precisely even depth without even a hint of a shade or streak. Errors are often more easy to see with the naked eye than with the microscope because of the averaging effect of a wider angle view on reflected light. The cutting tools must be ground and polished to the very finest quality.
How to layout a drawing of an engine turned watch dial design for the engine turner
A watch dial design. This is an edited drawing, to remove any confidential data, from an actual drawing supplied by a Swiss customer. The tolerance shown at bottom left is not our limit, we can usually manage within about ±0.015mm. The sunray pattern is not drawn in, just specified, top right of this image. The scale of 4:1 is what the paper image was, this image is around 6:1 on a 100 dpi screen. The measurements can confirm the exact scale.
This image shows the way that the above drawing relates to the actual engine turning, especially the cartouches. Lines from the drawing have been superimposed in red so that the relationship is clear. It is important to understand that the drawing shows the path taken by the tip of the escutcheon tool, and only the inner wide/shallow edge of the cut. The outer much steeper edge, most obviously visible in this image at the lower edge of the cartouche, brightly reflecting the light, will be broken by the engine turning and is not drawn in on the specification drawing.
Cartouches can be many shapes, always drawn on the layout with tramlines representing the path of the tool tip and the inside edge of the cartouche. An escutcheon tool is used, with the shallow angle on the inside, making a nice bevelled frame for the cartouche. See tool shapes.
The minute ring has been cut with 60 very fine radial lines, every fifth cut deeper for the hour. The background of the minute ring is cut with very fine circular lines, spacing about 0.03mm pitch. The cartouche has the same line spacing, cut horizontally straight line. Thus every visible part of the dial has been machined on the engine turning machines, rose engine and straight line.
By varying the depth of cut by very small amounts, different levels are created on the dial for the different areas, mostly created in an order of working so that the deepest areas are cut first, with shallower cuts able to exit into air cleanly, leaving crisp and clean edges to all sections. Very fine cutting and very shallow repeat passes of the tool on exit cuts ensure that no burrs are formed. The sunray above was cut inwards so that burrs would only form in the hole, and since the cuts are very fine at that point are minimal and easily removed.
Specification of the actual pattern is not normally very detailed, because we do most of the work ourselves in creating it. In the drawing layout image above, the specification is shown at top right; "Sunray lines, 240 cuts". For the pattern below, it would be ".1786 Crosshatch".
A cross hatch pattern we frequently use for watch dials. Sometimes known as basket weave.
This is a typical pattern used on parts of watch dials, such as the area within the chapter ring which in the above example is a sunray. Many of the patterns you will see in the pattern reference can be used, usually miniaturised, on dial centres, and for smaller inset dials and quadrants.
Partial specification image for a cross hatch pattern, .1786 crossing size, that we frequently use for watch dials. The cut direction is vertical in this image which can be deduced by the very slight filleting or rounding off of the pattern as the touch passes the peek of each zig zag on the pattern bar.
Contact us for more accurate measurement details, more information below.
0.01786 inch is half the distance from one peak of the bar to the next. The side of the small cross hatch square can be deduced by pythagoras since the diagonal is 0.03571 inch; it is approx 0.02525 inch, approx 0.641mm - considerably enlarged in this microscope image.
We can do this in various sizes, but it should be noted that the .1786 size illustrated above, with a 4x4 cross hatch design, was produced with a pattern bar made on the machine used for the engine turning to ensure the nearest perfect uniformity and accuracy of the pattern angles and repetition. The exact crossing was 1/56 inch, the pattern pitch at 32 div on a 64 tooth ratchet with 14tpi when cutting the pattern bar and touch. Accuracy of the bar is estimated at ± 0.0003 inch, smoothed by the multiple touch in the direction of the cut.
We are currently working with one of our horological customers on new technology for making even better pattern bars for sharper and longer wearing cross hatch and 90 degree square diamond barley patterns.
We not only manufacture our own pattern bars and multiple touches, we use a unique improvement on the Plant design with a much stronger exterior spring and hardened ball joint that helps to ensure consistent quality of work when cutting the sharpest zig zags like the one above.