Grinding and Polishing Machine

  The basis of this machine came from Warren Fillmore's article in "Advanced Telescope Making Techniques", by Allan MacIntosh, where he gives a good argument for the "Elgin" type machine. However the variable speed drive is original and done mechanically.


  The 1/3 HP motor (blue) and first stage reduction is mounted on a platform  (white) that slides vertically on the ½" rod by means of the hand wheel (foreground under table) and cable to vary the speed.  The second stage wheel (the large horizontal plywood wheel), is mounted on a stub shaft  that  NORMALLY  freewheels inside the turntable shaft which is a 1" black pipe. Under the second stage wheel is a small V pulley that drives the eccentric shaft (back right) with approx. 3 to 1 reduction.  Under the eccentric (top end of this shaft) another V belt drives the turntable pipe shaft with approx. 4 to 1 reduction.  This belt can be removed and a pin put through holes in the turntable pipe shaft and the second stage stub shaft to lock them up for high speed. (the arrangement is similar to the back gears on a metal lath). Speed range is from about 150 RPM down to zero and even reverse if the platform is raised high enough.  A strong bungee holds the 1st stage platform against the second stage pulley. Speed can be adjusted while running, but eccentric throw adjustment requires that the machine be stopped.

       The over arm hinges on a 3/4" threaded rod and offset is made by turning the threaded rod with the white hand wheel at left (can be done while running).

         Looking down with the turntable and drip tray removed , the motor and first stage reduction are mounted on a platform that not only raises and lowers ,but pivots and is pulled forward by the bungee, the end of which can be seen at bottom right.   The first stage shaft is 1" and the bearing in the wood block is a trailer wheel bearing to take the thrust load.  The crank arm to the overarm shaft is at top.

The  turn table shaft is a 1" pipe.  A portion of the pipe was built up with weld then turned down to fit the bearing available  {scavenged from an electric motor repair shop}.  The hole in the maple board was done with an adjustable hole saw, saw cuts made either side, and the two bolts  tighten the bearing in  the hole.  A floor flange on the under side of the turntable threads onto the  top of the shaft. 

The eccentric shaft is also 1".  There are two sizes of pulleys on the under side of the eccentric disc which can be used to drive the turntable giving ratios of about 3 : 1 and 5 : 1.  All the large pulleys are made from 3/4" plywood with hard maple hubs.  The plywood discs were turned in a lathe using an old file as a turning tool, the end of which was ground to the shape of the V groove in V pulleys. The hubs were drilled and taped for 3/8" set screws.  The  top bearing on the eccentric shaft is a self aligning bearing of the type often used in farm machinery.  The plywood it is mounted on is pivoted and the bearing can be drawn away from the turntable shaft by a lag bolt to tighten the drive belt.  The pitman is oak with a sealed bearing in the near end.  The eccentric pin  can be loosened with the wooden knob and slide radially in or out in a slot in the top of the eccentric disc to adjust overarm throw.

A view of the underside showing the driving disc, a portion of the second stage wheel, and the drive belt to the bottom of the eccentric shaft pulley. Note the bungee that pulls the drive disc against the second stage wheel.  The second stage wheel is actually two pieces of  ½" plywood screwed together then turned  turned down to the appropriate size such that a v belt would just fit around it in the groove when reassembled.  It is the back of this V belt that forms the tire that the disc drives.  Not a great idea but I couldn't come up with anything better on the spur of the moment and it  is still working

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Last revised 28 Apr 2010