If so heavy a pendulum is deemed undesirable, the bob would have to be made of cylindrical form, retaining the height, as necessary to compensation, and varying the diameter of the cylinder to suit the weight desired.
Fig. 7. Wood rod Fig. 8. Bob of metal casing
and lead bob.filled with shot.
Wood Rod and Lead Bob.—The wood should be clear, straight-grained and thoroughly dried, then given several coats of shellac varnish, well baked on. It may be either flat, oval or round in section, but is generally made round because the brass cap at the upper end, the lining for the crutch, and the ferrule for the adjusting screw at the lower end may then be readily made from tubing. For pendulums smaller than one second, the wood is generally hard, as it gives a firmer attachment of the metal parts.
| Inches. | |
| Length, top of suspension spring to bottom of bob | 44.5 |
| Length to bottom of nut | 45.25 |
| Diameter of bob | 2.0 |
| Length of bob | 10.5 |
| Weight of bob, 8 lbs. | |
| Acting length of suspension spring | 1.0 |
| Width of spring | .45 |
| Thickness | .008 |
| Diameter of rod | .5 |
The top of the rod should have a brass collar fixed on it by riveting through the rod and it should extend down the rod about three inches, so as to make a firm support for the slit to receive the lower clip of the suspension spring. The lower end should have a slit or a round hole drilled longitudinally three inches up the rod to receive the upper end of the adjusting screw and this should also fit snugly and be well pinned or riveted in place. [See Fig. 7]. A piece of thin brass tube about one inch in length is fitted over the rod where the crutch works.
In casting zinc and lead bobs, especially those of lens-shapes, the jeweler should not attempt to do the work himself, but should go to a pattern maker, explain carefully just what is wanted and have a pattern made, as such patterns must be larger than the casting in order to take care of the shrinkage due to cooling the molten metal. It will also be better to use an iron core, well coated with graphite when casting, as the core can be made smooth throughout and the exact shape of the pendulum rod, and there will then be no work to be done on the hole when the casting is made. The natural shrinkage of the metal on cooling will free the core, which can be easily driven out when the metal is cold and it will then leave a smooth, well shaped hole to which the rod can be fitted to work easily, but without shake. Lens-shaped bobs, particularly, should be cast flat, with register pins on the flask, so as to get both sides central with the hole, and be cast with a deep riser large enough to put considerable pressure of melted metal on the casting until it is chilled, so as to get a sound casting; it should be allowed to remain in the sand until thoroughly cold, for the same reason, as if cooled quickly the bob will have internal stresses which are liable to adjust themselves sometime after the pendulum is in the clock and thus upset the rate until such interior disturbances have ceased. Cylinders may be cast in a length of steel tubing, using a round steel core and driven out when cold.
If using oval or flat rods of wood, the adjusting screw should be flattened for about three inches at its upper end, wide enough to conform to the width of the rod; then saw a slot in the center of the rod, wide and deep enough to just fit the flattened part of the screw; heat the screw and apply shellac or lathe wax and press it firmly into the slot with the center of the screw in line with the center of the rod; after the wax is cold select a drill of the same size as the rivet wire; drill and rivet snugly through the rod, smooth everything carefully and the job is complete.
If by accident you have got the rod too small for the hole, so that there is any play, give the rod another coat of shellac varnish and after drying thoroughly, sand paper it down until it will fit properly.