[Ill.u.s.tration: Fig. 22]

It will be seen that this gives a positive "draw" of twelve degrees to the entrance pallet; that is, counting to the line _B f'_. In this escapement as delineated there is perfect tangential locking. If the locking face of the entrance-pallet stone at _C_ was made to conform to the radial line _A b_, the lock of the tooth _D_ at _o_ would be "dead"; that is, absolutely neutral. The tooth _D_ would press the pallet _C_ in the direction of the arrow _x_, toward the center of the pallet staff _B_, with no tendency on the part of the pallet to turn on its axis _B_.

Theoretically, the pallet with the locking face cut to coincide with the line _A b_ would resist movement on the center _B_ in either direction indicated by the double-headed arrow _y_.

A pallet at _C_ with a circular locking face made to conform to the arc _g_, would permit movement in the direction of the double-headed arrow _y_ with only mechanical effort enough to overcome friction. But it is evident on inspection that a locking face on the line _A b_ would cause a retrograde motion of the escape wheel, and consequent resistance, if said pallet was moved in either direction indicated by the double-headed arrow _y_. Precisely the same conditions obtain at the point _u_, which holds the same relations to the exit pallet as the point _o_ does to the entrance pallet _C_.

ANGULAR MOTION OF ESCAPE WHEEL DETERMINED.

The arc (three and a half degrees) of the circle _a_ embraced between the radial lines _A b_ and _A e_ determines the angular motion of the escape wheel utilized by the escape-wheel tooth. To establish and define the extent of angular motion of the escape wheel utilized by the pallet, we lay off seven degrees on the arc _a_ from the point _o_ and establish the point _n_, and through the point _n_, from _B_ as a center, we sweep the short arc _n'_. Now somewhere on this arc _n'_ will be located the inner angle of the entrance pallet. With a carefully-made drawing, having the escape wheel 10" in diameter, it will be seen that the arc _a_ separates considerably from the line, _B f'_ where it crosses the arc _n'_.

It will be remembered that when drawing the ratchet-tooth lever escapement a measurement of eight and a half degrees was made on the arc _n'_ down from its intersection with the pitch circle, and thus the inner angle of the pallet was located. In the present instance the addendum line _w_ becomes the controlling arc, and it will be further noticed on the large drawing that the line _B h_ at its intersection with the arc _n'_ approaches nearer to the arc _w_ than does the line _B f'_ to the pitch circle _a_; consequently, the inner angle of the pallet should not in this instance be carried down on the arc _n'_ so far to correct the error as in the ratchet tooth.

Reason tells us that if we measure ten degrees down on the arc _n'_ from its intersection with the addendum circle _w_ we must define the position of the inner angle of the entrance pallet. We name the point so established the point _r_. The outer angle of this pallet is located at the intersection of the radial line _A b_ with the line _B i_; said intersection we name the point _v_. Draw a line from the point _v_ to the point _r_, and we define the impulse face of the entrance pallet; and the angular motion obtained from it as relates to the pallet staff embraces six degrees.

Measured on the arc _l_, the entire ten degrees of angular motion is as follows: Two and a half degrees from the impulse face of the tooth, and indicated between the lines _B h_ and _B f_; one and a half degrees lock between the lines _B f'_ and _B i_; six degrees impulse from pallet face, entrance between the lines _B i_ and _B j_.

A DEPARTURE FROM FORMER PRACTICES.

Grossmann and Britten, in all their delineations of the club-tooth escapement, show the exit pallet as disengaged. To vary from this beaten track we will draw our exit pallet as locked. There are other reasons which prompt us to do this, one of which is, pupils are apt to fall into a rut and only learn to do things a certain way, and that way just as they are instructed.

To ill.u.s.trate, the writer has met several students of the lever escapement who could make drawings of either club or ratchet-tooth escapement with the lock on the entrance pallet; but when required to draw a pallet as ill.u.s.trated at Fig. 23, could not do it correctly.

Occasionally one could do it, but the instances were rare. A still greater poser was to request them to delineate a pallet and tooth when the action of escaping was one-half or one-third performed; and it is easy to understand that only by such studies the master workman can thoroughly comprehend the complications involved in the club-tooth lever escapement.

AN APT ILl.u.s.tRATION.

As an ill.u.s.tration: Two draughtsmen, employed by two competing watch factories, each designs a club-tooth escapement. We will further suppose the trains and mainspring power used by each concern to be precisely alike. But in practice the escapement of the watches made by one factory would "set," that is, if you stopped the balance dead still, with the pin in the fork, the watch would not start of itself; while the escapement designed by the other draughtsman would not "set"--stop the balance dead as often as you choose, the watch would start of itself.

Yet even to experienced workmen the escape wheels and pallets _looked_ exactly alike. Of course, there was a difference, and still none of the text-books make mention of it.

For the present we will go on with delineating our exit pallet. The preliminaries are the same as with former drawings, the instructions for which we need not repeat. Previous to drawing the exit pallet, let us reason on the matter. The point _r_ in Fig. 23 is located at the intersection of pitch circle _a_ and the radial line _A c_; and this will also be the point at which the tooth _C_ will engage the locking face of the exit pallet.

This point likewise represents the advance angle of the engaging tooth.

Now if we measure on the arc _k_ (which represents the locking faces of both pallets) downward one and a half degrees, we establish the lock of the pallet _E_. To get this one and a half degrees defined on the arc _k_, we set the dividers at 5", and from _B_ as a center sweep the short arc _i_, and from the intersection of the arc _i_ with the line _B e_ we lay off on said arc _i_ one and a half degrees, and through the point so established draw the line _B f_.

Now the s.p.a.ce on the arc _k_ between the lines _B e_ and _B f_ defines the angular extent of the locking face. With the dividers set at 5" and one leg resting at the point _r_, we sweep the short arc _t_, and from the intersection of said arc with the line _A c_ we draw the line _n p_; but in doing so we extend it (the line) so that it intersects the line _B f_, and at said intersection is located the inner angle of the exit pallet. This intersection we will name the point _n_.

[Ill.u.s.tration: Fig. 23]

From the intersection of the line _B e_ with the arc _i_ we lay off two and a half degrees on said arc, and through the point so established we draw the line _B g_. The intersection of this line with the arc _k_ we name the point _z_. With one leg of our dividers set at _A_ we sweep the arc _l_ so it pa.s.ses through the point _z_. This last arc defines the addendum of the escape-wheel teeth. From the point _r_ on the arc _a_ we lay off three and a half degrees, and through the point so established draw the line _A j_.

LOCATING THE OUTER ANGLE OF THE IMPULSE PLANES.

The intersection of this line with the addendum arc _l_ locates the outer angle of the impulse planes of the teeth, and we name it the point _x_. From the point _r_ we lay off on the arc _a_ seven degrees and establish the point _v_, which defines the extent of the angular motion of the escape wheel utilized by pallet. Through the point _v_, from _B_ as a center, we sweep the short arc _m_. It will be evident on a moment's reflection that this arc _m_ must represent the path of movement of the outer angle of the exit pallet, and if we measure down ten degrees from the intersection of the arc _l_ with the arc _m_, the point so established (which we name the point _s_) must be the exact position of the outer angle of the pallet during locking. We have a measure of ten degrees on the arc _m_, between the lines _B g_ and _B h_, and by taking this s.p.a.ce in the dividers and setting one leg at the intersection of the arc _l_ with the arc _m_, and measuring down on _m_, we establish the point _s_. Drawing a line from point _n_ to point _s_ we define the impulse face of the pallet.

MAKING AN ESCAPEMENT MODEL.

[Ill.u.s.tration: Fig. 24]

It is next proposed we apply the theories we have been considering and make an enlarged model of an escapement, as shown at Figs. 24 and 25.

This model is supposed to have an escape wheel one-fifth the size of the 10" one we have been drawing. In the accompanying cuts are shown only the main plate and bridges in full lines, while the positions of the escape wheel and balance are indicated by the dotted circles _I B_. The cuts are to no precise scale, but were reduced from a full-size drawing for convenience in printing. We shall give exact dimensions, however, so there will be no difficulty in carrying out our instructions in construction.

[Ill.u.s.tration: Fig. 25]

Perhaps it would be as well to give a general description of the model before taking up the details. A reduced side view of the complete model is given at Fig. 26. In this cut the escapement model shown at Figs. 24 and 25 is sketched in a rough way at _R_, while _N_ shows a gla.s.s cover, and _M_ a wooden base of polished oak or walnut. This base is recessed on the lower side to receive an eight-day spring clock movement, which supplies the motive power for the model. This base is recessed on top to receive the main plate _A_, Fig. 24, and also to hold the gla.s.s shade _N_ in position. The base _M_ is 2" high and 8" diameter. The gla.s.s cover _N_ can have either a high and spherical top, as shown, or, as most people prefer, a flattened oval.

[Ill.u.s.tration: Fig. 26]

The main plate _A_ is of hard spring bra.s.s, 1/10" thick and 6" in diameter; in fact, a simple disk of the size named, with slightly rounded edges. The top plate, shown at _C_, Figs. 24 and 25, is 1/8"

thick and shaped as shown. This plate (_C_) is supported on two pillars " in diameter and 1" high. Fig. 25 is a side view of Fig. 24 seen in the direction of the arrow _p_. The c.o.c.k _D_ is also of 1/8" spring bra.s.s shaped as shown, and attached by the screw _f_ and steady pins _s s_ to the top plate _C_. The bridge _F G_ carries the top pivots of escape wheel and pallet staff, and is shaped as shown at the full outline. This bridge is supported on two pillars " high and " in diameter, one of which is shown at _E_, Fig. 25, and both at the dotted circles _E E'_, Fig. 24.

To lay out the lower plate we draw the line _a a_ so it pa.s.ses through the center of _A_ at _m_. At 1.3" from one edge of _A_ we establish on the line _a_ the point _d_, which locates the center of the escape wheel. On the same line _a_ at 1.15" from _d_ we establish the point _b_, which represents the center of the pallet staff. At the distance of 1.16" from _b_ we establish the point _c_, which represents the center of the balance staff. To locate the pillars _H_, which support the top plate _C_, we set the dividers at 2.58", and from the center _m_ sweep the arc _n_.

From the intersection of this arc with the line _a_ (at _r_) we lay off on said arc _n_ 2.1" and establish the points _g g'_, which locate the center of the pillars _H H_. With the dividers set so one leg rests at the center _m_ and the other leg at the point _d_, we sweep the arc _t_.

With the dividers set at 1.33" we establish on the arc _t_, from the point _d_, the points _e e'_, which locate the position of the pillars _E E'_. The outside diameter of the balance _B_ is 3-5/8" with the rim 3/16" wide and 5/16" deep, with screws in the rim in imitation of the ordinary compensation balance.

Speaking of a balance of this kind suggests to the writer the trouble he experienced in procuring material for a model of this kind--for the balance, a pattern had to be made, then a casting made, then a machinist turned the casting up, as it was too large for an American lathe. A hairspring had to be specially made, inasmuch as a mainspring was too short, the coils too open and, more particularly, did not look well.

Pallet jewels had to be made, and lapidists have usually poor ideas of close measurements. Present-day conditions, however, will, no doubt, enable the workman to follow our instructions much more readily.

MAKING THE BRIDGES.

In case the reader makes the bridges _C_ and _F_, as shown in Fig. 27, he should locate small circles on them to indicate the position of the screws for securing these bridges to the pillars which support them, and also other small circles to indicate the position of the pivot holes _d b_ for the escape wheel and pallet staff. In practice it will be well to draw the line _a a_ through the center of the main plate _A_, as previously directed, and also establish the point _d_ as therein directed.

The pivot hole _d'_ for the escape wheel, and also the holes at _e e_ and _b_, are now drilled in the bridge _F_. These holes should be about 1/16" in diameter. The same sized hole is also drilled in the main plate _A_ at _d_. We now place a nicely-fitting steel pin in the hole _d'_ in the bridge _F_ and let it extend into the hole _d_ in the main plate. We clamp the bridge _F_ to _A_ so the hole _b_ comes central on the line _a_, and using the holes _e e_ in _F_ as guides, drill or mark the corresponding holes _e' e'_ and _b_ in the main plate for the pillars _E E'_ and the pallet staff.

[Ill.u.s.tration: Fig. 27]

This plan will insure the escape wheel and pallet staff being perfectly upright. The same course pursued with the plate _C_ will insure the balance being upright. The pillars which support the bridges are shaped as shown at Fig. 28, which shows a side view of one of the pillars which support the top plate or bridge _C_. The ends are turned to " in diameter and extend half through the plate, where they are held by screws, the same as in American movements.

[Ill.u.s.tration: Fig. 28]

The pillars (like _H_) can be riveted in the lower plate _A_, but we think most workmen will find it more satisfactory to employ screws, as shown at Fig. 29. The heads of such screws should be about 3/8" in diameter and nicely rounded, polished and blued. We would not advise jeweling the pivot holes, because there is but slight friction, except to the foot of the balance pivot, which should be jeweled with a plano-convex garnet.

[Ill.u.s.tration: Fig. 29]

IMITATION RUBIES FOR CAPPING THE TOP PIVOTS.

The top pivots to the escape wheel should be capped with imitation rubies for appearance sake only, letting the cap settings be red gold, or bra.s.s red gilded. If real twelve-karat gold is employed the cost will not be much, as the settings are only about 3/8" across and can be turned very thin, so they will really contain but very little gold. The reason why we recommend imitation ruby cap jewels for the upper holes, is that such jewels are much more brilliant than any real stone we can get for a moderate cost. Besides, there is no wear on them.

The pallet jewels are also best made of gla.s.s, as garnet or any red stone will look almost black in such large pieces. Red carnelian has a sort of brick-red color, which has a cheap appearance. There is a new phosphorus gla.s.s used by optical instrument makers which is intensely hard, and if colored ruby-red makes a beautiful pallet jewel, which will afford as much service as if real stones were used; they are no cheaper than carnelian pallets, but much richer looking. The prettiest cap for the balance is one of those foilback stones in imitation of a rose-cut diamond.

[Ill.u.s.tration: Fig. 30]

[Ill.u.s.tration: Fig. 31]