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Machining the Parts
The following suggestions of step-by-step operations for making a machined mounting similar to the one shown in the frontispiece might be thought to be useful only to those having access to machine-shop facilities. It has been suggested, however, that an amateur not equipped to do the work himself might farm it out to a local machine shop. The idea is a practical one, and while it is difficult to assay the cost of such a project, the figure is not likely to be much in excess of 20 dollars.
After learning what is needed, the machinist can then either follow the steps outlined here or substitute other and better methods for achieving the desired results.
The parts used in this description are the same as those used in the mounting in the frontispiece and listed under "A" in Table I, but, as previously mentioned, substitution of the parts listed under "B" can be made if desired. On account of the small difference in diameter of pipe and fittings, the internal threads of the tees
must be bored out to create sufficient space for the babbitt.
The external and internal threading on pipe and fittings is tapered, so that a tight joint can always be made up, but in so doing several threads always remain exposed. Furthermore, the pipe is never perfectly round, and may have scorings on it from the fitter's wrench or vise jaws. None of this annoys the steam-fitter, but the telescope maker is going to utilize short lengths of this pipe (or long nipples, as they are called) for his polar and declination axes. To turn in bearings, these must have shafts that are truly round and smooth right up to the shoulders of the fittings to which they are joined.
The declination nipple screws into a heavy flange and the polar nipple into a tee, so the internal threads of both these fittings must be tapped deeper with a 1" pipe tap, to permit further entrance of the pipe threads. This should be done a little at a time, and the nipple tried frequently, until when made up tight with a wrench
only one or 1½ threads remain exposed.
In using the wrench, apply it near the opposite end of the nipple or, better, to a spare fitting such as a coupling which has been temporarily screwed on there. This will avoid scoring or bending that part of the pipe to be encased in the bearing. Two
pipe wrenches are convenient for working one against the other. Once the pipe has been rounded in the lathe, the wrench should never be applied directly to any part of it, for fear of deformation. Screw a spare coupling on the free end and apply the wrench to that.
machining the parts part2
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