Plumbers' GEM

As an experiment I bought some regular plumbing stuff in a building market for the construction of a simple mounting. The various brass parts, meant for connecting copper water pipes, appear to be machined fairly accurately. The 22mm diameter connector and T-parts can be used as outer bearing surface. In order to accept 15mm copper pipe, a pipe converter 15-22mm can be soldered on. The plug fits snugly into the 22mm brass parts, and can as such function as a fairly play-free slide bearing.

The drawing above shows how the GEM is constructed.
Both the declination and hour axis are based on 15mm copper tube (red). The housing of the hour axis is made from 22mm copper tube (red). All the complex parts are brass: the bearing housings (yellow) and the bearings themselves (grey) which are soldered to the 15mm copper tube.
The business end of the declination axis is terminated with a 15mm brass endcap, with a 3/8" threading. This part is normally used to connect to a tap, that usually has an 3/8" outside threading. I chose the threaded endcap to facilitate screw-mounting a cradle that can hold a telescope tube.
The declination axis rotates in a housing made from a 15mm to 2x22mm T-piece. The bearings (grey) are cut from a converter piece normally used to mount 15mm pipes in 22mm connectors. The connector has to be cut in half to allow the 15mm pipe to pass through entirely. Finally, the construction is terminated with a half 15mm straight pipe connector, which can be fixed with a set-screw (preferred) or just soldered on. A counter-weight must be mounted on the far end of teh declination axis.
The hour axis is soldered in the 15mm end of the T-piece. Care must be taken to make this connection as square as possible! The bearings of the hour axis are made in a similar way to the declination axis. However, in order to withstand higher torque the housing length is extended with a 22mm pipe connecting the two 22mm straight pipe connectors.

It is amazing what tolerances can be reached with mass production of such parts. The result, using a vise as a temporary counter weight, is shown below: