A' X
This line will also pass through the center of the crescent. At the point of intersection of A' b with the safety roller we have one of the edges of the crescent. By placing our compass at the center of the crescent on the periphery of the roller and on the edge which we have just found, it follows that our compass will span the radius of the crescent. We now sweep the arc for the latter. thus also drawing in the remaining half of the crescent on the other side of A' X and bringing the crescent of sufficient depth that no possibility exists of the dart touching in or on the edges of it. We will now draw in the impulse roller and make it as light as possible consistent with strength. A hole is shown through the impulse roller to counterbalance the reduced weight at the crescent. When describing Fig. 219, we give instructions for finding the dimensions of crescent and position of guard pin for the single roller. We will find the length of horn: to do so we must closely follow directions given for Fig. 220. In locating the end of the horn, we must find the location of the center of the crescent and ruby pin after the edge of the crescent has passed the dart. From the point of intersection of A' b with the safety roller we transfer the radius of the crescent on the periphery of the safety roller towards the side against the bank, then draw a line from A' through the point so found. At point of intersection of this line with the real impulse circle r i r i we draw an arc radiating from the pallet center; the end of the horn will be located on this arc. In our drawing the arc spoken of coincides with the dart radius g g, As before pointed out, we gave particulars when treating on Fig. 220, therefore considered it unnecessary to further complicate the draft by the addition of all the constructional lines. We specified that the freedom between ruby pin and end of horn was to be 1 1/2 deg.; (these lines, which we do not show) are drawn from the pallet center. Having located the end of the horn on the side standing against the bank, we place the dividers on it and on the point of intersection of k A with gg--which in this case is on the point of the the dart,--and transfer this measurement along g g which will locate the end of the horn on the opposite side.
We have the acting edges of the fork on cc and have also found the position of the ends of the horns; their curvature is drawn in the following manner: We place our compasses on A and r i, spanning therefore the real impulse radius; the compass is now set on the acting edge of the fork and an arc swept with it which is then to be intersected by another arc swept from the end of the horn, on the same side of the fork. At the point of intersection of the arcs the compass is planted and the curvature of the horn drawn in, the same operation is to be repeated with the other horn. We will now draw in the sides of the horn of such a form that should the watch rebank, the side of the ruby pin will squarely strike the fork. If the back of the ruby pin strikes the fork there will be a greater tendency of breaking it and injuring the pivots on account of acting like a wedge. The fork and pallets are now drawn in as lightly as possible and of such form as to admit of their being readily poised. The banks are to be drawn at equal distances from the line of centers. In delineating the fork and roller action in any desired position, it must be remembered that the points of location of the real impulse radius, the end of horn, the dart or guard pin and crescent, must all be obtained when standing against the bank, and the arcs drawn which they describe; the parts are then located according to the angle at which they are removed from the banks. A Problem in the Lever Escapement. Among the problems of the detached lever escapement which often cause confusion, we wish to mention one in which the effect is that the actual lock, at the moment when the escape tooth drops on the locking plane, is different on each pallet, whereas it ought to be alike, in so far as angular lock is concerned. As regards the lock by linear measurement, in equidistant pallets, it is to be alike on each pallet, but, theoretically, not so in the circular pallet, as in the latter there are two locking circles, the larger one for the engaging and the smaller one for the disengaging pallet. If the angular lock in such pallet were alike, the linear lock would not be so. It would be greater on the engaging pallet, the difference depending on the width of the pallets; the wider they are, the greater is the difference.
It is a common practice to make the linear lock alike on each pallet, which has the effect of diminishing the locking angle on the engaging pallet, as it locks on the longer lever. This certainly diminishes the friction during unlocking on the pallet, which causes the greater resistance. On the other hand, the locking angle is decreased by increasing the lifting angle on the engaging pallet, thus creating greater friction during the lifting action. Having made these necessary explanations regarding the lock, we will now consider the effect described in our opening lines. Suppose we examine an escapement in which we find that the actual lock on the disengaging is much greater than on the engaging pallet. There can only be one cause for it, and that is, the lifting angles are dissimilar on the pallets, and no amount of shifting them in or out, or of altering their angular connection with the fork, will make any difference in the relative locking angles. There is but one remedy, and that is to grind the lifting plane of one of the pallets to the required angle. Which one are we to grind, and how are we to do it? The answer to the first question is, we can grind either one, as an examination of Fig. 225 will show. If we grind the disengaging pallet off to the dotted line N P, we will decrease the lock on it. Not having altered the discharging edge P, it follows that the lock on the engaging pallet has not been altered. On the other hand, if we grind the engaging pallet to the dotted line M O, it will be discharged earlier; thus making the lock on the disengaging pallet so much lighter; in the first case, then, increasing the lifting angle, in the latter, decreasing it.
We could also lessen the lift on one and increase it on the other, but it would be senseless to grind two pallets, if the proper effect can be obtained by grinding only one. Intelligent investigation will show which one to grind. As a cardinal principle, the lifting angles should be as small as possible. In a movement of good quality we can successfully employ a smaller angle than in one of low quality. This is the idea to be kept in view, but we will now expatiate upon it.
Before deciding to decrease the lift on the engaging pallet, the lifting angles must be closely examined when in action. If it is such that no light may be seen between the lifting plane of the pallet and heel of the tooth when the latter stands even with the locking edge of the pallet, then the lift must not be decreased. Were we to do so, the action would take place under adverse circumstances, as shown in Fig. 226. Further, when the lift on the engaging pallet is decreased then the inside drop is decreased as well; on the other hand, if the disengaging lift is decreased the outside drop will be slightly decreased, but not nearly as much as when the change is made on the engaging pallet. If the lift be increased on either pallet no change in the drop will occur, and no change will be made in the run, whereas if the lift be decreased then the run of the pallets to the banks after the drop has occurred, will be increased, and that, too, on the pallet which was not altered. Depending upon the more or less accurate construction of the escapement, it may or may not necessitate altering the angular connection of the pallets on the fork, or the "let off," as this is called, so as to equalize the run. A closer banking will also be required. This will alter the freedom of the ruby pin when passing the acting edge of the fork and place the guard point closer to the roller. If this particular alteration, that is, decreasing the lift, is made on a pallet with fork in one piece, either the fork itself must be slightly deflected, which is preferable, or the pallet which was ground off pulled out a trifle and the other one moved back a corresponding amount. As to the effect of these changes, depending upon the constructional faults, we may bring the pallets nearer to the theoretical requirements or tend to annihilate them, therefore we must observe that an accurate knowledge of all the functions of the escapement are necessary, if we would intelligently make a change. There are so many apparently natural operations which would destroy the theoretical advantages of certain constructions, and though the timepiece may give fair service to a person, whose requirements are not of the most exacting, it would be rendered unfit for accurate timing under those conditions. Having studied these points, the workman must decide by the particular escapement under his notice as to whether there is to be a decrease or an increase of lift.
To the second question, how are we to do it? The diamond laps for grinding are usually made of either copper or of soft steel. For this kind of a job we prefer the latter. Take a piece of soft steel about one and a half inches in diameter and at least one-sixteenth thick; bore a hole through the center of it to exactly fit the arbor chuck of the lathe, turn it up true in round and flat, then stone on both sides on a soft, flat cast iron block, with oil-stone dust and oil until smooth. For charging the lap we require a dead hard steel roller about one-half inch long by one-quarter of an inch in diameter. Through the center of it there is to be a good sized hole, through which a steel pin passes freely. The roller is held by the pin in a frame which is mounted in a stout handle.
Properly graded diamond dust may be procured from material dealers. It is sold in five grades. For general work use No. 3 for grinding and No. 5 for the polishing.
To charge the lap, take a small quantity of the powder, mix to a stift paste with thick oil and thoroughly roll it into the lap, resting the latter on a solid surface while doing so. To remove the oil, clean in benzine or alcohol. The lap for polishing should be of ivory and mounted in the same manner. The polishing powder is too fine to be rolled into the lap, so it will only be necessary to apply a small quantity mixed with oil to it. Water is generally applied to the lap when grinding stone, and oil in grinding hard steel. A method for holding the pallets which is suitable for the general workman is to fix them in the tool holder of the slide rest. To do so we require a little tool of which we show a plan in Fig. 227, and a side view in Fig. 228. H is a piece of steel of the form of the shank of the slide rest cutter. D, Fig. 228, is a steel table pivoted on it, by the shoulder screw S. C is a heavy spring screwed to the table D. A is a set screw for deflecting the spring C. The pallets are placed on the table D, the spring C is then clamped on them by means of the screw A. E is a circular groove in the table D. B is a set screw with threads in the holder H. By means of the latter screw the table D may be bound rigidly to H, or by slightly loosening it and working the milled screw head S, the table may be given a circular motion bounded by the banking of the ends of the groove E, on the screw B. It is important that the center of the screw S should coincide with the center of a pallet of ordinary thickness, when the latter is clamped on the table D. Suppose now, we intend decreasing the lift on the engaging pallet. They are to be clamped as shown in Fig. 227 with the shank H put into position in the tool holder of the slide rest. K, Fig. 227, is the diamond lap in position on the arbor chuck F which is mounted in the lathehead.
The tool holder, or the upper slide, is set at the angle desired, so that when the discharging edge O of the pallet touches the lap, the light between the latter and the locking edge M will equal the amount to be ground off. In grinding, a very light pressure only must be applied, the sound being our guide. Under no circumstances must the lap be allowed to get dry, and both lap and traverse slide must be kept in motion. Particular attention must be paid to these points, otherwise success will be out of the question. If the pallet is to be flat on the face it is held parallel to the lap through the set screw B. If, on the other hand, it is to have a rounded or convex face, the set screw B is loosened and a circular motion imparted to the pallet by slowly moving the screw S back and forth with one hand while the other is employed moving the traverse slide. The face of the pallet must nearly be ground off to the edge M; it is intended that the polishing should bring it up to it. When through grinding, we replace the steel lap by the ivory polishing lap which we charge as directed. We proceed the same as in grinding. When polished, the pallets are removed and thoroughly cleaned before being replaced in the watch.
What we have here said is also applicable to the Graham anchor for clocks.--Playtner.