Chapter IX: Case Fitting.
87. The way to fit the movement into the case changes significantly with the design of the case; therefore we must, in order to speak about it, decide which is the best design.
First we have the old English case with a fixed dome. It must be opened and the hands set from the front (i.e. from the dial side). In a case of this kind the movement is fastened by a hinge at XII and is held in place by a locking bolt at VI, which is pushed in by the thumb-nail. This method certainly results in a strong case, but it is very inconvenient for the owner who must open the back to wind it and must open the bezel to set the hands. A still worse fault of this design is the use of a bolt; if the thumb-nail slips over it the seconds and minute hands can be broken off. A case of this kind is meant to be used with a full plate movement, where the hands have to be set from the dial side, but a three-quarter plate or bridge movement should always have the hand setting square at the back (Art. 70).
88. For the latter types of watch the modern form of case is more suitable, where the movement is fastened into the case by means of a steady pin and one or two screws. Two screws are better. Swiss watches usually have three pins at the edge of the pillar plate at some distance from each other. The middle one is the thickest and in the form of a square; it goes partially into a small filing in the body of the case (whereby it prevents the movement rotating) and the upper side of the outer end is cut down sufficiently to fit under the case rim, as are the two side pins.
This method of attaching the movement, usually with only one dog screw, makes taking out and inserting the movement more difficult, particularly with the very thin cases which are used on so many Swiss watches.
89. Therefore I propose another plan, which is very easy and simple to make if the pillar plate and its shoulder fit exactly into the case. A hole is drilled into the rim of the case surrounding the pillar plate and into the plate. A pin is driven into the hole in the plate and shortened so that it enters the hole in the case without projecting on the outside; the pin both holds the movement in place and prevents it rotating. The attachment is completed by two dog screws, 120° from each other and from the pin, which lock onto two shoulders soldered into the body.
90. If possible the pin should be attached near the balance so that this most precious and delicate part of the movement is placed in position first and not exposed to any force that may be used to press the movement into the case. It is very important to carefully fit the movement to its case so that it goes in gently, without pressure and without clearance; because otherwise, particularly if the case is strong and the plate thin, the plate can easily bend enough to change the end shakes of the pinions.
91. I do not recommend attaching the dog screws onto the upper plate so that they lock above the case band, because this plate is too thin to give the screw threads enough hold, and if the screws are tightened firmly this fairly thin plate is likely to bend. The pillars form the pivotal points for movement and when the screws strive to lift the outside edge of the plate the internal part will pull downward by the same amount and the end shakes of the pinions will decrease.
92. In the modern case the movement can be accessed from the rear by opening the back and the hands are set from this side, so the owner of the watch need never open the bezel. With this kind of case the dial should be fastened with dog screws and not with pins, otherwise it would not be possible to remove the dial without first taking the movement from its case.
93. Recently Swiss watches have been made which have the heads of the case screws under the dial. This method does not offer any obvious advantage and causes unnecessary trouble for the repairer, who has to remove the hands and dial before the movement can be taken from the case.
94. Just like the winding square, the hand setting square should have a pipe to prevent foreign particles hanging on the key from entering the movement. We must take care that this pipe reaches up to the inside of the case back but not further; because then, in a strong case, a pressure would develop on the upper plate when the case is closed and this pressure is often sufficient to stop the watch by decreasing the end shakes of the arbors.
95. Cases in which the movement hinges offer an advantage for exact adjustment of the watch because the adjusting screws on the balance are more accessible; however this advantage is of no great importance.
96. It remains to say a word about the devices whose purpose is to protect the movement, or parts of it, against any dust which enters the case. The most perfect dust cap is that of the old English full plate watch, because it covers the whole movement without any exception. These caps are usually very well made and serve their purpose excellently. It was attempted, with some success, to protect the movement of the three-quarter plate watch in the same way; but the dust cap requires a higher case which is not entirely compatible with the modern watch. It was absolutely necessary to use it with the old watches which opened and closed by springs, and therefore were definitely not dust proof. However, through gradual progress in casemaking, cases now close more tightly than before and dust caps can be completely omitted. If they are made with some care, the case snaps are tight but can nevertheless be closed and opened very easily. For this purpose the snap must not be deeply undercut. The edge should be rounded off a little upward, so that the closing corner of the other part goes easily past its highest point (see Fig. 26). The better kinds of English case are usually made with much care and insight.
97. The circular dustcap, which surrounds the frame in full plate movements, avoids the problem of needing a higher case but it is also less effective. Why use it to protect the train against dust if, at same time, the balance, the balance spring and the oil-sinks on the upper plate remain exposed?