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Machining the Parts 4
11. Three holes, 120° apart, near each end of both tees, should be drilled and tapped for No. 6-32 screws (Fig. 80) which support the pipe or shafting centrally for the babbitting operation. Also drill two small holes near each end
Fig. 80. How shafting is centered in the tees for babbitting. of both tees, to allow the escape of air. The ends of the centering screws should be filed smooth to prevent their scoring the pipe. The ends of the 3/8" clamping screws (L, Fig. 72), which can be made or purchased, are filed or faced off flat.
12. The details of the babbitting operation have already been described in the making of the fixed mounting. About half the quantity of babbitt that was required there is needed for the portable Fig. 81. Pouring the babbitt for the declination axis bearing. mounting. The declination bearing is the first to be poured, and is assembled as shown in Fig. 81, after first coating the pipe with oil and lampblack. Be sure to insert the clamping screw. Be careful not to fill too much with the babbitt.
After cooling, remove the centering and clamping screws, screw a coupling onto the end of the nipple and drive it out with a few light blows of a hammer. That end of the tee adjacent to the flange should first be marked to identify it as the large end of the bearing.
13. Now screw the polar axis back into this tee until the previously drilled hole (operation 6) is in alignment; there a pin or screw should now be inserted. Coat the nipple with oil and lampblack and make up the assembly as in Fig. 82, ready for
babbitting. The previously marked face (operation 8) of the tee which is to house the polar bearing must be placed adjacent to the collar that fits against the screw joint, and while it is held centered the centering screws are brought up to just touch the polar axis,
Fig. 82. Polar axis bearing ready for babbitting. enough to keep the assem-
blage from shifting. The two bearings will thus be brought at right angles to each other with all reasonable accuracy. The second collar is brought up flush against the face of the tee, and held there by means of its own set screws. In preheating this
assembly, keep the torch flame away from direct contact with the babbitt of the declination bearing.
When the mounting is finally assembled, two wafers of leather, vellumoid, or some friction material are cut out and inserted before the clamping screw of each axis; these cushion the thrust of the screws and prevent their scoring the shafting. After the pipe nipples which constitute the declination and polar axes have been trued up in the lathe, under no circumstances should they be held in a vise, on account of the almost certain probability of their becoming deformed. Screw a fitting on the exposed threaded end, and apply the force to that. Use two pipe wrenches, cautiously, in removing the tightened fitting.
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