The reader is urged to make the drawings for himself on a large scale, say, an escape wheel 10" pitch diameter. Such drawings will enable him to realize small errors which have been tolerated too much in drawings of this kind. The drawings, as they appear in the cut, are one-fourth the size recommended, and many of the lines fail to show points we desire to call attention to. As for instance, the pallet center at B is tangential to the pitch circle a from the point of tooth contact at f. To establish this point we draw the radial lines A c and A d from the escape-wheel center A, as shown, by laying off thirty degrees on each side of the intersection of the vertical line i (passing through the centers A B) with the arc a, and then laying off two and a half degrees on a and establishing the point f, and through f from the center A draw the radial line A f'. Through the point f we draw the tangent line b' b b'', and at the intersection of the line b with i we establish the center of our pallet staff at B. At two and a half degrees from the point c we lay off two and a half degrees to the right of said point and establish the point n, and draw the radial line A n n', which establishes the extent of the arc of angular motion of the escape wheel utilized by the pallet arm.
We have now come to the point where we must exercise our reasoning powers a little. We know the locking angle of the escape-wheel tooth passes on the arc a, and if we utilize the impulse face of the tooth for five degrees of pallet or lever motion we must shape it to this end. We draw the short arc k through the point n, knowing that the inner angle of the pallet stone must rest on this arc wherever it is situated. As, for instance, when the locking face of the pallet is engaged, the inner angle of the pallet stone must rest somewhere on this arc (k) inside of a, and the extreme outer angle of the impulse face of the tooth must part with the pallet on this arc k.
HOW TO LOCATE THE PALLET ACTION.
With the parts related to each other as shown in the cut, to establish where the inner angle of the pallet stone is located in the drawing, we measure down on the arc k five degrees from its intersection with a, and establish the point s. The line B b, Fig. 90, as the reader will see, does not coincide with the intersection of the arcs a and k, and to conveniently get at the proper location for the inner angle of our pallet stone, we draw the line B b', which passes through the point n located at the intersection of the arc a with the arc k. From B as a center we sweep the short arc j with any convenient radius of which we have a sixty-degree scale, and from the intersection of B b' with j we lay off five degrees and draw the line B s', which establishes the point s on the arc k. As stated above, we allow one degree for lock, which we establish on the arc o by laying off one degree on the arc j below its intersection with the line B b. We do not show this line in the drawing, from the fact that it comes so near to B b' that it would confuse the reader. Above the arc a on the arc k at five degrees from the point n we establish the point l, by laying off five degrees on the arc j above the intersection of the line B b with j.
The point l, Fig. 90, establishes where the outer angle of the tooth will pass the arc k to give five degrees of angular motion to the lever. From A as a center we sweep the arc m, passing through the point l. The intersection of the arc m with the line A h we call the point r, and by drawing the right line r f we delineate the impulse face of the tooth. On the arc o and one degree below its intersection with the line B b we establish the point t, and by drawing a right line from t to s we delineate the impulse face of our entrance pallet.
"ACTION" DRAWINGS.
One great fault with most of our text books on horology lies in the fact that when dealing with the detached lever escapement the drawings show only the position of the pallets when locked, and many of the conditions assumed are arrived at by mental processes, without making the proper drawings to show the actual relation of the parts at the time such conditions exist. For illustration, it is often urged that there is a time in the action of the club-tooth lever escapement action when the incline on the tooth and the incline on the pallet present parallel surfaces, and consequently endure excessive friction, especially if the oil is a little thickened.
We propose to make drawings to show the exact position and relation of the entrance pallet and tooth at three intervals viz: (1) Locked; (2) the position of the parts when the lever has performed one-half of its angular motion; (3) when half of the impulse face of the tooth has passed the pallet. The position of the entrance pallet when locked is sufficiently well shown in Fig. 90 to give a correct idea of the relations with the entrance pallet; and to conform to statement (2), as above. We will now delineate the entrance pallet, not in actual contact, however, with the pallet, because if we did so the lines we employed would become confused. The methods we use are such that we can delineate with absolute correctness either a pallet or tooth at any point in its angular motion.
We have previously given instructions for drawing the pallet locked; and to delineate the pallet after five degrees of angular motion, we have only to conceive that we substitute the line s' for the line b'. All angular motions and measurements for pallet actions are from the center of the pallet staff at B. As we desire to now delineate the entrance pallet, it has passed through five degrees of angular motion and the inner angle s now lies on the pitch circle of the escape wheel, the angular space between the lines b' s' being five degrees, the line b''[**note: check this against the diagram-most other lines nave a two-letter identification] reducing the impulse face to four degrees.