A Practical Course of Instruction in The Science of Horology
By Orville R. Hagans and D. L. Thompson
25. The Millimeter Gauge
A millimeter gauge, shown in Fig. >16, is often used by the horologist for taking measurements of parts. Its scale is based on the metric system of measurements of lengths, which was developed as the need for an international system of measurements became imperative. The unit of length in this system is the meter, which is equal to the ten-millionth part of the distance from the equator to the North Pole.
The metric system of measurements is a decimal one, the meter being divided into 10 decimenters; a decimeter into 10 centimeters; and a centimeter into 10 millimeters. Or, one meter equals 1,000 millimeters; one decimeter equals 100 millimeters; and one centimeter equals 10 millimeters.
The scale is divided into 10 centimeters, these being numbered from 1 to 10 in both directions, and further divided into 100 millimeters. The sliding jaw, Fig. 16, a, is arranged so that measurements can be made between the jaws a and b, as in measuring diameters and lengths; between the points of the jaws at c, as for measuring the thickness say of a mainspring barrel bottom; over the outside of the points at d, as for measuring say the inside diameter of a mainspring barrel; and, with the sliding jaw moved to the other end of the scale, between the end of the gauge and point e, for measuring lengths of staffs, pinions, etc.
When measuring over the jaw points marked d, two millimeters must be added to the reading of the scale as the width of the two points is two millimeters, the true reading of the scale being between the jaws.
The small scale on the sliding jaw, which is called a vernier, is provided for measurements in tenths of a millimeter. This scale consists of 9 millimeters divided into ten parts. With the jaws closed, the line marked 0 on the large scale coincides with the line marked 0 on the small one. If the sliding jaw is moved so that the second line on the small scale is in line with the second line on the large scale, the space between the jaws then measure .1 millimeter. If moved so that the third lines on each scale coincide, the space measures .2 mm, and so on, until when the tenth line on each scale is in alignment, the space measures one full millimeter. Further movement of the sliding jaw would divide the second millimeter in the same manner, etc. To retain a measurement on the gauge, the sliding jaw can be locked in position with the locking lever, f.
The millimeter gauge is handy for making ordinary measurements, where a plain pair of calipers would do as well, but its smallest measurement is .1 mm, which is much too large for fine work for which there is need of a gauge that will measure to .01 mm.
In the following text, all measurements will be stated in millimeters, and if the student wishes to convert them into inches, he has only to divide them by 25.4 which is the approximate number of millimeters per inch. The exact equivalent of a meter in inches is 39.37039.
26. The Metric Micrometer
As much of the Horologist's work is fitting parts of extremely small dimensions, he has need of a gauge that will accurately measure them. The metric-micrometer, shown in Fig. 17, meets his requirements nicely in making measurements, such as diameter, thickness, and length, of parts or portions of parts.
The body of this gauge, Fig. 17, a, is made of a heavy U-shaped bar of steel, and points, band b, between which the measurements are to be made, are of hardened and tempered steel. The pitch of the screw, which controls the movement of the movable point, is .5 mm, and, therefore, one turn of the knurled nut, c, will separate the points .5 mm. The scale on the barrel, d, is usually marked in full millimeters, although some are marked in .5 mm also, and two full turns of the nut will be required to separate the points 1 mm. The nut is marked . with 50 divisions, so 1/50 of a turn (one division) will separate the points .01 mm (1/50 of .5 mm).
To read the scale of the micrometer shown in the illustration, count the number of full divisions showing on the barrel and then read the fraction, or decimal part, of a mm. from the number of the division on the nut which is nearest in alignment with the straight line on the barrel. Thus, the scale shows the separation of the points to be five full mm. on the barrel and five divisions on the nut, which is 5.05 mm.
In using this gauge to measure an inside diameter, say of a mainspring barrel, it is necessary to first measure this diameter with inside calipers and then measure the spread of the calipers, To measure the thickness of a section of a part, say of a recess· in a watch plate in which the points of the gauge are too large to fit, a short length of metal can be inserted between the part and the movable point and an overall dimension taken, after which the length of the piece of metal is to be measured and this deducted from the overall dimension. It is important to use a light touch in adjusting the points in making measurements. They must not be screwed up tightly against the part being measured, but just so that they touch lightly. The reason for this is that there is a small amount of back-lash of the screw in its hole to permit it to turn freely. If the points are set tightly against the work being measured, the backlash is taken up and when the points are removed from the work they will move closer together by a very small amount, say of several thousandths of a millimeter. Another reason is that the U-shaped body of the gauge would spring slightly and when the points were removed from the work the body would spring back to normal. This would balance up the back-lash of the screw to a certain extent, but not equal it. Such care in adjusting the points is, of course, only necessary in making the finest of measurements.
The student is advised to secure one of these gauges, of good make, as its convenience and dependability more than justifies its cost.
27. Turning, Grinding, and Polishing Cylindrical Pivots
An important part of the horologist's work is the turning, grinding, and polishing of cylindrical pivots, called square-shouldered pivots, which are used on the arbors of the train wheels of watches and clocks.
The student has been made familiar with the shapes of most of the turning tools or gravers to be used and how to keep them sharp, which is most important if one expects to do good, fast work. The correct and incorrect positions of presenting the graver to the work are shown in Fig. 18. The correct position is shown in the figure at a, in which the point of the graver is presented directly to the center of rotation of the work. The tool-rest should be set close enough to the work to prevent the possibility of the graver being drawn between them, which would either break the point of the graver or spring the work out of true rotation.
If the graver is presented to the work perceptibly above the center of rotation, as shown in the figure at b, it will not cut but will merely burnish the metal and dull the point. If presented perceptibly below the center, as shown at c, it will cut too deep which will either break off the point or spring the work out of true. A graver will cut, after a fashion, if presented only slightly above or below the center of rotation of the work, but only with the attendant risks of spoiling the work.
The graver should be provided with a straight wooden handle of about 15 mm in diameter and 10 cm in length, in which a hole has been drilled centrally to such a depth as to allow only 3 cm of the graver to extend. It is to be held in the hand, in the same manner as holding a file, with the thumb placed close to the hand, which will give perfect control of the movements of the point. It should not be held in the manner used in holding a pencil, as control of the point is difficult in this case.
For first practice in turning, several cylindrical sections should be turned on brass rod. Place a rod of 5 mm in diameter in the number 50 chuck and allow it to extend for one centimeter (10 mm). Starting at the end, present the graver to the work in the manner shown in Fig. 19, which presents the point only, or as shown in Fig. 20, which presents the point and a small portion of the side, and turn roughly to a cylindrical section of 4 mm in diameter and 9 mm in length, after which the whole of the side may be presented to the work, as shown by the dotted lines in Fig. 20, and the irregular surface shaved down. Use a micrometer to check the diameter so that the turned section will be of the same diameter throughout its full length. This operation forms an oversize pivot, the shoulder of which should form a perfect right angle to the cylindrical section. To square up the shoulder, the graver is held in the position shown in Fig. 23 and at an angle to the work so as to slightly under-cut or dish the shoulder, after which the graver is held as shown in Fig. 22 and the cylindrical section continued to the face of the under-cut, thus removing any excess of metal or root in the angle and making a clean, square shoulder.
Next, measuring from the end, turn a cylindrical section of 3 mm in diameter and 5 mm in length, and square the shoulder as before. Then turn a section of 1 mm in diameter and 2 mm in length and square its shoulder.
This work should be continued on brass rod until perfectly cylindrical sections with square shoulders can be easily turned, after which the same operations should be practiced on iron rod until they can be done without difficulty.
Having mastered the above work on brass and iron, the turning of tempered steel can be taken up, which is more difficult and requires very sharp and hard gravers. Select a piece of staff-wire,which is tool-steel wire ready hardened and tempered to dark blue, or prepare a piece of mill-rod by hardening and tempering it to dark blue, of 2 mm in diameter and place it in a number 20 chuck, allowing it to extend for 5 mm. Turn a perfectly cylindrical section of .5 mm in diameter and 1.5 mm in length, square the shoulder and slightly under-cut it. Round the end, as shown in Fig. 23, and bevel the shoulder, as shown in Fig. 24. The beveling is done to reduce the diameter of the shoulder and thereby its friction with the plate or jewel, and. to prevent grit from getting between the shoulder and the plate, which would reduce the necessary end play of the arbor. The diameter of the shoulder should be only slightly greater in diameter than the pivot.
Correct and incorrect pivot lengths are shown in Fig. 25. The correct length is shown at a, where the end of the pivot comes nicely' through the hole. The pivot shown at b is too short and will wear to one side of a brass hole, making the ledge shown which reduces the required end-play of the arbor and prevents free action of the wheel. That shown at c is too long, and causes difficulty in assembling a watch or clock movement.
As the graver makes fine lines in the work, which under high magnification appear as ridges and valleys, the pivot must be ground to nearly finished size and then polished to reduce friction in the hole in which it turns. A pivot can be ground acceptably with an iron grinding slip and oilstone paste. A little of the paste is placed near the end of the slip, which is held under the pivot, as shown in Figs. 26 and 27, and moved back and forth endways for about 3 cm, with light pressure and slow and measured strokes, while the work is turning at moderate speed. To start, hold the edge of the slip against the shoulder of the pivot, and give the slip a slight side to side movement as the grinding progresses, replacing the paste between the slip and the work as required.