The Detent
The counterpoise of the pivoted detent affords a ready means of poising it, so that the question of a long or short detent has not the same importance as it has in the spring detent. In the spring detent the spring must sustain all the weight of the outer end so that it is advisable with this form to make it as short as may be safely done. With a long spring detent, of course, there will be more metal to be sustained by the spring, so that it must be left thicker, and this will make the work performed by the balance so much the greater, which is a detriment when held in such a position that the weight of the detent acts to increase or decrease the unlocking resistance. Saunier in his Modern Horology, article 850 says: "In the case of the marine chronometer as now made, it has been experimentally ascertained that the acting length of the spring detent should not be less than the diameter of the escape wheel, nor more than the radius of the balance. A pivoted detent, measured from the center of motion to the extremity of the auxiliary spring, should be much shorter (by rather more than one-third this amount): for otherwise its weight, being increased by that of the counterpoise, would render the action of the escapement uncertain, especially in pocket chronometers beating 18,000 or 21,600 vibrations per hour." It is difficult to see the logic of his reasoning regarding the pivoted detent. The only hypothesis on which his reasoning would seem to apply would be where the counterpoise was added merely as an ornament, without due regard to the poising of the detent, a thing which is very necessary, in this respect having an advantage over the spring detent. Making a new or replacing a broken detent is about one of the most difficult jobs the watchmaker has to undertake, especially in the latter case, when its location and proportions are already determined. In making a detent it will result beneficially if the measurements are made directly onto a steel plate sufficiently large that all the various proportions may be made directly upon it rather than making them on a separate plate and then transferring them to the piece of steel from which the detent is to be made. The selection of the steel from which the detent is to be made is very important, as it must be made as light as possible and yet sufficiently rigid to withstand the impact of the locking. The conditions above enumerated can only be satisfied by using steel of a very superior quality, and in this respect almost any steel may be improved by carefully hammering it, which will make it more dense. Another reason for using a plate from which to make the detent is that the holes may be drilled more accurately at right angles to the plane of the detent. When all the points of the detent are located, drill all the holes and rough it approximately and harden, drawing it to a spring temper of dark blue. If a lead or cyanide bath be at hand for heating, only ordinary care will be required in hardening, but if a blow pipe is used great care will be necessary to avoid overheating, which would ruin it. When replacing a broken detent the centers of the different parts should be marked upon the plate, which has been carefully smoothed, and by mounting the impulse pallet upon a well made arbor a hole is to be drilled into which this arbor will accurately fit, the same being done with the escape pinion. By this method the face of the locking stone may be accurately located. In the spring detent the corners where the spring joins the body of the detent to the foot, should be left slightly rounded, not taking special care to have them extremely sharp, which would make them liable to be broken. The amount of this rounding should be slight, almost an imperceptible amount being quite sufficient. After the hardening process the forming may be proceeded with by using a fine file, which should be new and sharp, that the metal may not be in any way strained or injured, finishing off with carborundum powder, and a tin slip if to be left unpolished, or if to be polished, using a zinc slip and diamantine. Mr. Britten says "The locking pallet should not be perfectly upright. It should lean a little from the center of the wheel, and a little towards the foot of the detent, so that the locking takes place at the root of the stone, and then the action of locking and unlocking does not tend so much to buckle the detent. The face of the impulse pallet, too, should be slightly inclined so that it bears on the upper part of the wheel teeth. By this means the impulse pallet will not mark the wheel in the same spot as the locking pallet."
If the above theory be true why is it necessary to go to such expense and care that each part be accurately made, but that the above reasoning is incorrect we cite a few facts below and endeavor to show why the locking and impulse pallets should be set perfectly upright, taking extraordinary care that they be so set. By setting the locking stone upright there can be no tendency to buckle the detent, for the pressure will come equally upon the top and bottom of the locking pallet, and the wear being distributed equally upon the surfaces, the wear will not be so great. With the pivoted detent the locking pallet may be set very near on a radial line so that it will have just a small amount of draw, as, if the detent be accurately poised, the force of the locking spring will be sufficient to prevent the escape wheel becoming unlocked at the wrong time, but with the spring detent a perceptible draw is necessary, ten degrees being sufficient when well constructed.