28. _Common Causes of Extreme Isochronal Variation._

The most common causes of isochronal variation with which the repairer has to deal and which are often very destructive to position rates, as well as to general time keeping, may be found in the factor of, out of poise and uneven motive force, which is one of the elementary principles of adjusting. This feature should be thoroughly understood by all watchmakers, so that as good results as possible may be obtained from all watches above low grade, even though no test for adjustment is to be made.

When the balance is slightly out of poise and the motion is exactly one and one-fourth turn during the twenty-four hours, this out of poise will not affect the isochronism. When the motion varies and reaches approximately one and one-half turn during the first few hours after winding and then drops to one and one-quarter turn and finally to one turn or less during the latter part of the twenty-four hours, the poise error will have considerable effect. This factor is not perceptible in the flat positions, but shows up to the full extent in the vertical positions and the variation differs according to the location of the point that is heavy. For example, if the balance is heavy on the lower side when at rest, the watch will lose during the hours that the arc of motion is over one and one-fourth turn and will gain when the motion drops to one turn or less.

Should the heavy point be on the top side of balance the result will be reversed and the watch will gain when the motion is over one and one-fourth turn and will lose when it drops to one turn or less.

The total variation may be either seconds or minutes, depending upon the extent of the poise error and experiments will prove that serious isochronal variations can be traced to the simple cause of lack of poise and irregular motion in more instances than to any other cause.

The arc of one and one-fourth turn is the ideal motion, as slight poise errors are neutralized at this point, but very few watches will maintain this motion for twenty-four hours, therefore the poise must be as nearly perfect as possible. The nearest approach to even motion of modern watches is found in the fine Swiss grades equipped with stop work, which causes only the best part of the mainspring to be utilized.

Such watches also receive the most expert attention as to gearings of wheels and pinions and the train wheels are specially rounded up on their respective staffs. This latter feature has been adopted by at least two of the American manufacturers of fine watches during the past few years with considerable benefit in producing even motion and the use of lighter mainsprings. It should be definitely understood that these tests refer to the vertical positions of the watch only and that the horizontal positions are not affected in the same way by lack of poise.

CHAPTER VIII

RELATIVE PINNING POINTS OF THE HAIRSPRING

29. _Original Springing of Watches._

Theory and practice agree that different models of watches have important relative points of attachment of the spring to collet and stud. In the original springing and adjusting of high grade watches, these points receive careful consideration, and only a very small percentage ever require future alterations.

There are instances, however, where the original allowance of position variation has been considerable, also medium grades where no attention has been directed to pinning points and in which an occasional alteration may be required before a close position rate can be obtained.

30. _How Pinning Point Alterations are Made._

These alterations are generally made by breaking off or letting out a small section of the inner coil at the collet. In making such alterations a quarter of a coil broken away at the collet will have the same effect as will a quarter of a coil broken off at the outer end and will require less weighting of the balance to correct the mean time. It will also avoid breaking and remaking the over coil and the possible necessity of readjustment to temperature. Letting out the spring can be accomplished by unpinning and repinning the spring at collet with less of the coil entered in the pinhole. This is not a positive alteration, however, because very often the segment in the pinhole is as short as it can be with safety.

A more substantial correction is that of reforming the over coil in a manner that will cause the end holding the stud to be shifted further forward.

The method of obtaining this correction is ill.u.s.trated in Fig. 8. The broken line shows the original formation of the over coil with the stud on the line "B". The solid lines show the corrections with the stud shifted to the line A.

[Ill.u.s.tration: Fig. 8]

When the collet is turned to replace the spring in beat, the stud will be in its original location on the line "B."

This will cause the pinning point at collet to be shifted from "A" to "B" and bring it that much nearer to the horizontal line "C."

This alteration has the same effect as that of letting out the spring at the collet or of moving the stud forward on the over coil, with the advantage of eliminating any change in the mean time.

It should be definitely understood that the objective in making the above alterations and as ill.u.s.trated with the aid of the following cuts, is the relation of the pinning point at collet to the pinning point at stud, and that the change in length of the spring has no bearing on the matter whatever as far as the position rate is concerned.

31. _Even Coil Hairsprings Very Incorrect for Some Models._

It is often supposed that hairsprings having exactly even coils are correct for close position and isochronal rating. Such springs do approximate the nearest correct relation in more instances than any other relation. They are precisely correct for very few models, however, and are very incorrect for many models, as will be seen through study of the following cuts showing the various points of attachment and the different results obtainable in each.

32. _How to Find the Correct Collet Pinning Point for Any Watch._

A very simple method of locating the proper point of attachment of the spring to collet is to face the train side of the movement and hold the balance stationary with a small twig, and with the pallet fork just midway between the two bankings.

[Ill.u.s.tration: Fig. 9]

Presume the existence of a vertical line through the center of hairspring and collet as shown at "A B" Fig. 9. Then presume a horizontal line as shown at "C D" on the same cut.

[Ill.u.s.tration: Fig. 10]

The proper pinning point is at the intersection of the collet and horizontal line; the spring may be either over or under even coils, depending entirely upon the location of the stud hole in the balance bridge as demonstrated by Figures 9, 10, 14, 15.

When the spring develops to the right from collet as shown in Fig. 9, for example, the proper point of attachment is on the right side of collet as shown at "E" Fig. 9, and also at "J" Fig. 14.

If it develops to the left as the springs of all fine Swiss watches do, the proper point of attachment is on the left side of collet as shown at "F" Fig. 10.

33. _Results in Vertical Position Rates Due to Changing the Pinning Point._

In either of the above instances the spring will develop upward as it leaves the collet. These points of attachment always produce a fast pendant up rate when compared to the opposite, or pendant down rate, and all high grade watches are originally fitted with springs conforming to this principle.

If these points of attachment were changed to the opposite side of collet so that the spring would develop downward as shown at "G" Fig.

11, and "H" Fig. 12, the results would be reversed and the pendant up rate would be slow in comparison to the pendant down rate.

[Ill.u.s.tration: Fig. 11]

This point of attachment in which the spring develops downward from the collet is generally known as the slow point among adjusters, and when a spring is pinned at either the slow or fast point the pendant right and left positions generally compare quite closely to each other in timing, provided that the poise and other conditions of the watch are correct.

If the pinning point was changed to the intersection of the collet and vertical line as shown in "I" Fig. 13, the pendant up and down rates would compare nearly equal to each other and the pendant right position would be slow compared to the pendant left position.

[Ill.u.s.tration: Fig. 12]

If it were pinned at the intersection of the collet and vertical line just opposite to that shown in Fig. 13, the pendant left position would be slow compared to the pendant right position.

[Ill.u.s.tration: Fig. 13]

The vertical points of attachment are seldom used, for the reason that the variation between the pendant right and left positions would be very difficult to control within close limits, due to the existence of the natural error. As these positions, together with the pendant up position are the most important of the four vertical positions, they are given preference, and the natural error is placed in the pendant down position where it will be the least detrimental to the performance of the watch.

34. _The Natural Position Error and Why it Cannot be Eliminated._

[Ill.u.s.tration: Fig. 14]

The natural error generally consists of from twelve to fifteen seconds in finely constructed watches, and exists because of the fact that it is impossible to perfectly poise a spiral spring. The location of the heavy point, however, may be shifted by changing the point of attachment at collet as described in No. 33, this Chapter. The nearest approximation of a poised spiral spring is probably attained through L. Lossier's inner terminal curve. Results are not positive, however, and any deviation from the required precision makes the curve valueless. It is possible to obtain perfect adjustment between three vertical quarter positions and the two horizontal positions, but all four quarter positions cannot be perfectly adjusted because the natural error will show up in one of them. Manufacturers of fine watches do not of course presume to supply perfect adjustment in the five positions. Some however, have considerably closer limits of allowance for variation than do others and it is logical to presume that a line of high grade watches having a five position allowance of six seconds from one position to any other would show better results than another line which had even a six position adjustment and an allowance of fifteen seconds from one position to any other.