Tuesday, May 11, 2010

6" F/5 Dobsonian: Wooden Crayford focuser

Update: I now prefer the much simpler design of this focuser.

Like I said, I hate making focusers.  If I were making this for myself, I'd probably stick with a push-pull Crayford like on my travel scope.  But I'm making this for a friend, so I made a more normal Crayford.  The draw tube is aluminum with 1.5" outer and 1.25" inner diameter.  Actually, my eyepieces and laser collimator didn't fit when I got it, so I had to sand the inside of the tube.  The method was to take a screwdriver, wrap some sandpaper and foam around the handle, affix it at one end with duct tape, and spin the screwdriver with a drill.  Took a while, but eventually I got the bore large enough.

The main part of the focuser is a particl board plate, with two 5/16" dowels along the sides to fit the curvature of the tube, and three posts.  The two of the posts on the right are, I think, oak, and the one on the left is cut from 1"x1" poplar square rod, and they all have little plywood thingies behind them to keep them from falling over.  They are both glued and screwed to the plate.  The two posts on the right get ball bearings (the cheap ones for skateboards that one can buy on ebay, with wood screws JB Weld'ed inside them), and the one on the left has a hole  cut out in it for a holder for the focusing shaft.  There are two screws on the holder for adjusting tension.

The non-fixed parts consist of a little H-shaped poplar thingy that fits in the slit in the left post.  It has some V-shaped holes cut in it and lined with bondable PTFE.  Between the H-shaped thingy and its post there is a folded rubber thing to add some spring, cut from a bicycle inner tube.  The adjustment screws on the post bear against the rubber.  The shaft is 1/4" stainless steel.  The knobs are cut from pine boards.  They're not as straight as they could be, but they're light.  I cut the knobs by first drilling about 1/3" deep into a pine board with a 1.25" paddle bit.  Then I cut the pieces out with a larger (1.5" or 1.75") hole saw.  Result: cylinders with a recessed hole that lightens it and makes it look better.  I then put a bolt through each, and spun it against sandpaper on a drill.  The shaft then fits in the holes that the hole saw mandrel made.  I put some PTFE rings on the shaft, and I filed the ends of the shaft to make it rougher and less round, and glued the knobs in placed with, of course, JB Weld.  (I use JB Weld for most bonds other than wood-wood.  I use Titebond II for most wood-wood joins.  Except that the dowels at the bottom of the focuser, to make the focuser hug the tube, were attached with Gorilla Glue, because I wanted a glue that would (a) dry quickly and (b) make little gussets.)

Motion is delightfully smooth.  The next major step will be mounting the secondary.

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