LOCATING THE CENTER OF THE BALANCE STAFF.

Somewhere on this line is located the center of the balance staff, and it is the problem in hand to locate or establish this center. Now, it is known the circles which define the peripheries of the escape wheel and the impulse roller intersect at _e e^2_. We can establish on our circle _A_ where these intersections take place by laying off twelve degrees, one-half of the impulse arc on each side of the line of centers _a f_ on this circle and establishing the points _e e^2_. These points _e e^2_ being located at the intersection of the circles _A_ and _B_, must be at the respective distances of 5" and 2" distance from the center of the circles _A B_; consequently, if we set our dividers at 2" and place one leg at _e_ and sweep the short arc _g^2_, and repeat this process when one leg of the dividers is set at _e^2_, the intersection of the short arcs _g_ and _g^2_ will locate the center of our balance staff. We have now our two centers established, whose peripheries are in the relation of 2 to 1.

To know, in the chronometer which we are supposed to be constructing, the exact distance apart at which to plant the hole jewels for our two mobiles, i.e., escape wheel and balance staff, we measure carefully on our drawing the distance from _a_ to _c_ (the latter we having just established) and make our statement in the rule of three, as follows: As (10) the diameter of drawn escape wheel is to our real escape wheel so is the measured distance on our drawing to the real distance in the chronometer we are constructing.

It is well to use great care in the large drawing to obtain great accuracy, and make said large drawing on a sheet of metal. This course is justified by the degree of perfection to which measuring tools have arrived in this day. It will be found on measurement of the arc of the circle _B_, embraced between the intersections _e e^2_, that it is about forty-eight degrees. How much of this we can utilize in our escapement will depend very much on the perfection and accuracy of construction.

[Ill.u.s.tration: Fig. 139]

We show at Fig. 140 three teeth of an escape wheel, together with the locking jewel _E_ and impulse jewel _D_. Now, while theoretically we could commence the impulse as soon as the impulse jewel _D_ was inside of the circle representing the periphery of the escape wheel, still, in practical construction, we must allow for contingencies. Before it is safe for the escape wheel to attack the impulse jewel, said jewel must be safely inside of said escape wheel periphery, in order that the attacking tooth shall act with certainty and its full effect. A good deal of thought and study can be bestowed to great advantage on the "action" of a chronometer escapement. Let us examine the conditions involved. We show in Fig. 140 the impulse jewel _D_ just pa.s.sing inside the circle of the periphery of the escape wheel. Now the attendant conditions are these: The escape wheel is locked fast and perfectly dead, and in the effort of unlocking it has to first turn backward against the effort of the mainspring; the power of force required for this effort is derived from the balance in which is stored up, so to speak, power from impulses imparted to the balance by former efforts of the escape wheel. In actual fact, the balance at the time the unlocking takes place is moving with nearly its greatest peripheral velocity and, as stated above, the escape wheel is at rest.

Here comes a very delicate problem as regards setting the unlocking or discharging jewel. Let us first suppose we set the discharging jewel so the locking jewel frees its tooth at the exact instant the impulse jewel is inside the periphery of the escape wheel. As just stated, the escape wheel is not only dead but actually moving back at the time the release takes place. Now, it is evident that the escape wheel requires an appreciable time to move forward and attack the impulse jewel, and during this appreciable time the impulse jewel has been moving forward inside of the arc _A A_, which represents the periphery of the escape wheel. The proper consideration of this problem is of more importance in chronometer making than we might at first thought have imagined, consequently, we shall dwell upon it at some length.

HOW TO SET THE DISCHARGING JEWEL.

[Ill.u.s.tration: Fig. 140]

Theoretically, the escape-wheel tooth should encounter the impulse jewel at the time--instant--both are moving with the same velocity. It is evident then that there can be no special rule given for this, i.e., how to set the discharging jewel so it will free the tooth at exactly the proper instant, from the fact that one chronometer train may be much slower in getting to move forward from said train being heavy and clumsy in construction. Let us make an experiment with a real chronometer in ill.u.s.tration of our problem. To do so we remove our balance spring and place the balance in position. If we start the balance revolving in the direction of the arrow _y_, Fig. 140, it will cause the escapement to be unlocked and the balance to turn rapidly in one direction and with increasing velocity until, in fact, the escape wheel has but very little effect on the impulse jewel; in fact, we could, by applying some outside source of power--like blowing with a blow pipe on the balance--cause the impulse jewel to pa.s.s in advance of the escape wheel; that is, the escape-wheel tooth would not be able to catch the impulse jewel during the entire impulse arc. Let us suppose, now, we set our unlocking or discharging jewel in advance, that is, so the escapement is really unlocked a little before the setting parts are in the positions and relations shown in Fig. 141. Under the new conditions the escape wheel would commence to move and get sufficient velocity on it to act on the impulse jewel as soon as it was inside of the periphery of the escape wheel. If the balance was turned slowly now the tooth of the escape wheel would not encounter the impulse jewel at all, but fall into the pa.s.sing hollow _n_; but if we give the balance a high velocity, the tooth would again encounter and act upon the jewel in the proper manner.

Experienced adjusters of chronometers can tell by listening if the escape-wheel tooth attacks the impulse jewel properly, i.e., when both are moving with similar velocities. The true sound indicating correct action is only given when the balance has its maximum arc of vibration, which should be about 1 revolutions, or perform an arc of 225 degrees on each excursion.

Fig. 142 is a side view of Fig. 141 seen in the direction of the arrow _y_. We have mentioned a chariot to which the detent is attached, but we shall make no attempt to show it in the accompanying drawings, as it really has no relation to the problem in hand; i.e., explaining the action of the chronometer escapement, as the chariot relates entirely to the convenience of setting and adjusting the relation of the second parts. The size, or better, say, the inside diameter of the pipe at _C_, Fig. 143, which holds the locking jewel, should be about one-third of a tooth s.p.a.ce, and the jewel made to fit perfectly. Usually, jewelmakers have a tendency to make this jewel too frail, cutting away the jewel back of the releasing angle (_n_, Fig. 143) too much.

A GOOD FORM OF LOCKING STONE.

A very practical form for a locking stone is shown in transverse section at Fig. 143. In construction it is a piece of ruby, or, better, sapphire cut to coincide to its axis of crystallization, into first a solid cylinder nicely fitting the pipe _C_ and finished with an after-grinding, cutting away four-tenths of the cylinder, as shown at _I_, Fig. 143. Here the line _m_ represents the locking face of the jewel and the line _o_ the clearance to free the escaping tooth, the angle at _n_ being about fifty-four degrees. This angle (_n_) should leave the rounding of the stone intact, that is, the rounding of the angle should be left and not made after the flat faces _m o_ are ground and polished. The circular s.p.a.ce at _I_ is filled with an aluminum pin. The sizes shown are of about the right relative proportions; but we feel it well to repeat the statement made previously, to the effect that the detent to a chronometer cannot well be made too light.

[Ill.u.s.tration: Fig. 141]

[Ill.u.s.tration: Fig. 142]

[Ill.u.s.tration: Fig. 143]

The so-called gold spring shown at _H_, Figs. 141 and 142, should also be as light as is consistent with due strength and can be made of the composite metal used for gold filled goods, as the only real benefit to be derived from employing gold is to avoid the necessity of applying oil to any part of the escapement. If such gold metal is employed, after hammering to obtain the greatest possible elasticity to the spring, the gold is filed away, except where the spring is acted upon by the discharging jewel _h_. We have previously mentioned the importance of avoiding wide, flat contacts between all acting surfaces, like where the gold spring rests on the horn of the detent at _p_; also where the detent banks on the banking screw, shown at _G_, Fig. 142. Under this principle the impact of the face of the discharging jewel with the end of the gold spring should be confined to as small a surface as is consistent with what will not produce abrasive action. The gold spring is shaped as shown at Fig. 142 and loses, in a measure, under the pipe of the locking jewel, a little more than one-half of the pipe below the blade of the detent being cut away, as shown in Fig. 143, where the lines _r r_ show the extent of the part of the pipe which banks against the banking screw _G_. In this place even, only the curved surface of the outside of the pipe touches the screw _G_, again avoiding contact of broad surfaces.

We show the gold spring separate at Fig. 144. A slight torsion or twist is given to the gold spring to cause it to bend with a true curvature in the act of allowing the discharging pallet to pa.s.s back after unlocking.

If the gold spring is filed and stoned to the right flexure, that is, the thinnest point properly placed or, say, located, the gold spring will not continue in contact with the discharging pallet any longer time or through a greater arc than during the process of unlocking. To make this statement better understood, let us suppose the weakest part of the gold spring _H_ is opposite the arrow _y_, Fig. 141, it will readily be understood the contact of the discharging stone _h_ would continue longer than if the point of greatest (or easiest) flexure was nearer to the pipe _C_. If the end _D^2_ of the horn of the detent is as near as it should be to the discharging stone there need be no fear but the escapement will be unlocked. The horn _D^2_ of the detent should be bent until five degrees of angular motion of the balance will unlock the escape, and the contact of discharging jewel _h_ should be made without engaging friction. This condition can be determined by observing if the jewel seems to slide up (toward the pipe _C_) on the gold spring after contact. Some adjusters set the jewel _J_, Figs. 143 and 141, in such a way that the tooth rests close to the base; such adjusters claiming this course has a tendency to avoid c.o.c.kling or buckling of the detent spring _E_. Such adjusters also set the impulse jewel slightly oblique, so as to lean on the opposite angle of the tooth. Our advice is to set both stones in places corresponding to the axis of the balance staff, and the escape-wheel mobiles.

THE DETENT SPRING.

[Ill.u.s.tration: Fig. 144]

It will be noticed we have made the detent spring _E_ pretty wide and extended it well above the blade of the detent. By shaping the detent in this way nearly all the tendency of the spring _E_ to c.o.c.kle is annulled. We would beg to add to what we said in regard to setting jewels obliquely. We are unable to understand the advantage of wide-faced stones and deep teeth when we do not take advantage of the wide surfaces which we a.s.sert are important. We guarantee that with a detent and spring made as we show, there will be no tendency to c.o.c.kle, or if there is, it will be too feeble to even display itself. Those who have had extended experience with chronometers cannot fail to have noticed a gummy secretion which acc.u.mulates on the impulse and discharging stones of a chronometer, although no oil is ever applied to them. We imagine this coating is derived from the oil applied to the pivots, which certainly evaporates, pa.s.ses into vapor, or the remaining oil could not become gummy. We would advise, when setting jewels (we mean the locking, impulse and discharging jewels), to employ no more sh.e.l.lac than is absolutely necessary, depending chiefly on metallic contact for security.

DETAILS OF CONSTRUCTION.

We will now say a few words about the number of beats to the hour for a box or marine chronometer to make to give the best results. Experience shows that slow but most perfect construction has settled that 14,400, or four vibrations of the balance to a second, as the proper number, the weight of balance, including balance proper and movable weights, to be about 5 pennyweights, and the compensating curb about 1-2/10" in diameter. The escape wheel, 55/100" in diameter and recessed so as to be as light as possible, should have sufficient strength to perform its functions properly. The thickness or, more properly, the face extent of the tooth, measured in the direction of the axis of the escape wheel, should be about 1/20". The recessing should extend half way up the radial back of the tooth at _t_. The curvature of the back of the teeth is produced with the same radii as the impulse roller. To locate the center from which the arc which defines the back of the teeth is swept, we halve the s.p.a.ce between the teeth _A^2_ and _a^4_ and establish the point _n_, Fig. 141, and with our dividers set to sweep the circle representing the impulse roller, we sweep an arc pa.s.sing the point of the tooth _A^3_ and _u_, thus locating the center _w_. From the center _k_ of the escape wheel we sweep a complete circle, a portion of which is represented by the arc _w v_. For delineating other teeth we set one leg of our dividers to agree with the point of the tooth and the other leg on the circle _w v_ and produce an arc like _z u_.

ORIGINAL DESIGNING OF THE ESCAPEMENT.

On delineating our chronometer escapement shown at Fig. 141 we have followed no text-book authority, but have drawn it according to such requirements as are essential to obtain the best results. An escapement of any kind is only a machine, and merely requires in its construction a combination of sound mechanical principles. Neither Saunier nor Britten, in their works, give instructions for drawing this escapement which will bear close a.n.a.lysis. It is not our intention, however, to criticise these authors, except we can present better methods and give correct systems.

TANGENTIAL LOCKINGS.

It has been a matter of great contention with makers of chronometer and also lever escapements as to the advantages of "tangential lockings." By this term is meant a locking the same as is shown at _C_, Fig. 141, and means a detent planted at right angles to a line radial to the escape-wheel axis, said radial line pa.s.sing through the point of the escape-wheel tooth resting on the locking jewel. In escapements not set tangential, the detent is pushed forward in the direction of the arrow _x_ about half a tooth s.p.a.ce. Britten, in his "Hand-Book," gives a drawing of such an escapement. We claim the chief advantage of tangential locking to lie in the action of the escape-wheel teeth, both on the impulse stone and also on the locking stone of the detent.

Saunier, in his "Modern Horology," gives the inclination of the front fan of the escape-wheel teeth as being at an angle of twenty-seven degrees to a radial line. Britten says twenty degrees, and also employs a non-tangential locking.

Our drawing is on an angle of twenty-eight degrees, which is as low as is safe, as we shall proceed to demonstrate. For establishing the angle of an escape-wheel tooth we draw the line _C d_, from the point of the escape-wheel tooth resting on the locking stone shown at _C_ at an angle of twenty-eight degrees to radial line _C k_. We have already discussed how to locate and plant the center of the balance staff.

We shall not show in this drawing the angular motion of the escape wheel, but delineate at the radial lines _c e_ and _c f_ of the arc of the balance during the extent of its implication with the periphery of the escape wheel, which arc is one of about forty-eight degrees. Of this angle but forty-three degrees is attempted to be utilized for the purpose of impulse, five degrees being allowed for the impulse jewel to pa.s.s inside of the arc of periphery of the escape wheel before the locking jewel releases the tooth of the escape wheel resting upon it. At this point it is supposed the escape wheel attacks the impulse jewel, because, as we just explained, the locking jewel has released the tooth engaging it. Now, if the train had no weight, no inertia to overcome, the escape wheel tooth _A^2_ would move forward and attack the impulse pallet instantly; but, in fact, as we have already explained, there will be an appreciable time elapse before the tooth overtakes the rapidly-moving impulse jewel. It will, of course, be understood that the reference letters used herein refer to the ill.u.s.trations that have appeared on preceding pages.

If we reason carefully on the matter, we will readily comprehend that we can move the locking jewel, i.e., set it so the unlocking will take place in reality before the impulse jewel has pa.s.sed through the entire five degrees of arc embraced between the radial lines _c e_ and _c g_, Fig. 141, and yet have the tooth attack the jewel after the five degrees of arc. In practice it is safe to set the discharging jewel _h_ so the release of the held tooth _A^1_ will take place as soon as the tooth _A^2_ is inside the princ.i.p.al line of the escape wheel. As we previously explained, the contact between _A^2_ and the impulse jewel _i_ would not in reality occur until the said jewel _i_ had fully pa.s.sed through the arc (five degrees) embraced between the radial lines _c e_ and _c g_.

At this point we will explain why we drew the front fan of the escape-wheel teeth at the angle of twenty-eight degrees. If the fan of impulse jewel _i_ is set radial to the axis of the balance, the engagement of the tooth _A^2_ would be at a disadvantage if it took place prior to this jewel pa.s.sing through an arc of five degrees inside the periphery of the escape wheel. It will be evident on thought that if an escape-wheel tooth engaged the impulse stone before the five-degrees angle had pa.s.sed, the contact would not be on its flat face, but the tooth would strike the impulse jewel on its outer angle. A continued inspection will also reveal the fact that in order to have the point of the tooth engage the flat surface of the impulse pallet the impulse jewel must coincide with the radial line _c g_. If we seek to remedy this condition by setting the impulse jewel so the face is not radial, but inclined backward, we encounter a bad engaging friction, because, during the first part of the impulse action, the tooth has to slide up the face of the impulse jewel. All things considered, the best action is obtained with the impulse jewel set so the acting face is radial to the balance staff and the engagement takes place between the tooth and the impulse jewel when both are moving with equal velocities, i.e., when the balance is performing with an arc (or motion) of 1 revolutions or 225 degrees each way from a point of rest. Under such conditions the actual contact will not take place before some little time after the impulse jewel has pa.s.sed the five-degree arc between the lines _c e_ and _c g_.

THE DROP AND DRAW CONSIDERED.

Exactly how much drop must be allowed from the time the tooth leaves the impulse jewel before the locking tooth engages the locking jewel will depend in a great measure on the perfection of workmanship, but should in no instance be more than what is absolutely required to make the escapement safe. The amount of draw given to the locking stone _c_ is usually about twelve degrees to the radial line _k a_. Much of the perfection of the chronometer escapement will always depend on the skill of the escapement adjuster and not on the mechanical perfection of the parts.

The jewels all have to be set by hand after they are made, and the distance to which the impulse jewel protrudes beyond the periphery of the impulse roller is entirely a matter for hand and eye, but should never exceed 2/1000". After the locking jewel _c_ is set, we can set the foot _F_ of the detent _D_ forward or back, to perfect and correct the engagement of the escape-wheel teeth with the impulse roller _B_. If we set this too far forward, the tooth _A^3_ will encounter the roller while the tooth _A^2_ will be free.

We would beg to say here there is no escape wheel made which requires the same extreme accuracy as the chronometer, as the tooth s.p.a.ces and the equal radial extent of each tooth should be only limited by our powers toward perfection. It is usual to give the detent a locking of about two degrees; that is, it requires about two degrees to open it, counting the center of fluxion of the detent spring _E_ and five degrees of balance arc.

FITTING UP OF THE FOOT.

Several attempts have been made by chronometer makers to have the foot _F_ adjustable; that is, so it could be moved back and forth with a screw, but we have never known of anything satisfactory being accomplished in this direction. About the best way of fitting up the foot _F_ seems to be to provide it with two soft iron steady pins (shown at _j_) with corresponding holes in the chariot, said holes being conically enlarged so they (the pins) can be bent and manipulated so the detent not only stands in the proper position as regards the escape wheel, but also to give the detent spring _E_ the proper elastic force to return in time to afford a secure locking to the arresting tooth of the escape wheel after an impulse has been given.

If these pins _j_ are bent properly by the adjuster, whoever afterwards cleans the chronometer needs only to gently push the foot _F_ forward so as to cause the pins _j_ to take the correct positions as determined by the adjuster and set the screw _l_ up to hold the foot _F_ when all the other relations are as they should be, except such as we can control by the screw _G_, which prevents the locking jewel from entering too deeply into the escape wheel.

In addition to being a complete master of the technical part of his business, it is also desirable that the up-to-date workman should be familiar with the subject from a historical point of view. To aid in such an understanding of the matter we have translated from "L'Almanach de l'Horologerie et de la Bijouterie" the matter contained in the following chapter.

CHAPTER IV.

HISTORY OF ESCAPEMENTS.

It could not have been long after man first became cognizant of his reasoning faculties that he began to take more or less notice of the flight of time. The motion of the sun by day and of the moon and stars by night served to warn him of the recurring periods of light and darkness. By noting the position of these stellar bodies during his lonely vigils, he soon became proficient in roughly dividing up the cycle into sections, which he denominated the hours of the day and of the night. Primitive at first, his methods were simple, his needs few and his time abundant. Increase in numbers, multiplicity of duties, and division of occupation began to make it imperative that a more systematic following of these occupations should be inst.i.tuted, and with this end in view he contrived, by means of burning lights or by restricting the flowing of water or the falling of weights, to subdivide into convenient intervals and in a tolerably satisfactory manner the periods of light.