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Further Suggestions - Portable Mountings
The amateur who has access to a lathe will, no doubt, contrive to machine the faces of the tee and wye, and so effect smoother performance. With careful planning of the whole operation, they can be made square with the axes. Added refinements, which are left to the discretion of the worker, are a worm-and-gear slow motion on the polar axis, and either a rack-and-pinion or a multiple-thread focusing arrangement on the eyepiece adapter tube. Note the provision for making latitude
correction in Fig. 72. It will pay to take a little time in accurately setting the four bolts (only three are visible in the diagram) in the concrete pier, as adjustment will be greatly simplified if they are exactly oriented. A method of locating true north will be found in Chapter XII.
Fig. 73. A rigid pipe mounting, with babbitted bearings. Parts are listed in Table I, under Portable "B."
A Portable Mounting. Table I lists sizes and descriptions of parts for both permanent and portable mounts. A 45° elbow with a tee, used as shown in Fig. 71, rather than the wye of Fig. 72, is preferred for the portable mount, as it dispenses
with the counterweight on the polar axis. The wye is more rigid, of course, and so is chosen for the fixed mounting. In thus dispensing with the counterweight, the radial thrust load on the polar bearing is increased and is in counter directions at the opposite ends. It may therefore be desirable to increase this bearing area by
using the parts listed under "B" in the table.
This is strongly recommended if an additional five or six pounds can be tolerated.
The gain is not only in an increased diameter of the polar-axis bearing, but also an increase in the length of both bearings, as a comparison of the proportions of the respective tees will show. The additional length in the bearings is an important factor in gaining added rigidity. The mounting, detailed in Fig. 73, and weighing but 17 pounds, is the one shown in Plate VI (page 136).
As the difference in cost is little, brass fittings are to be preferred. Brass is easier to machine than iron, will not rust, and has a more pleasing appearance. Also, the inside surfaces of the brass fittings can be tinned, thereby providing a secure anchor for the babbitt. The standard brass flange is not sufficiently heavy to support the saddle and tube adequately, but there is a similar fitting known as a flange union, consisting of two heavy flanges (3½" in diameter in the 1" pipe size) which comes as a unit, and it may be possible to pick up an odd one. Machining steps will be described following the discussion of the tripod.
The Tripod
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