Friday, July 2, 2010

Wooden helical Crayford focuser

I've done this project a while back, but in case anybody is interested, I am documenting it.  My Coulter 8" scope came with a friction focuser made of pipe fittings that Coulter described as "helical", I think because you aren't going to do very well just pushing the focuser tube in and out unless you give it a twist as well.  With practice, the focuser was usable for light eyepieces, but it really didn't work for something as heavy as a Hyperion 13mm (and there are eyepieces heavier than that).  So I decided to make a helical Crayford focuser, because I was too cheap to buy a KineOptics one.

The idea behind a helical Crayford is that you have ball bearings against which the draw tube rotates.  However, the ball bearings are slightly twisted so that their axes make a small angle with the angle of the tube.  Consequently, as you rotate the draw tube, it moves in and out.  If the ball bearing angle is larger, the movement is faster, and if it's smaller, the movement is slower.

I mounted the ball bearings on two T-shaped supports made by taking a #12 flat-head screw for the stem of the T, and JB Weld'ing a segment of 1/4" square rod to the head of the screw for the line.  I then JB Weld'ed a  skateboard ball bearing (I expect from VXB's ebay store) at each end of the T's arm, trying as best I could that everything would be square and the arms of equal length (not that I am very good at that sort of thing).  I then took a piece of 3/8" particle board (Baltic Birch plywood would have been better, but I was being cheap) and mounted three posts on it.  Two of the posts would sport the ball bearings, with the stem of the T going through the post, and with various nuts and spacers to hold the bearings in place.  The third post had a little flap adjustable with a thumbscrew.  The flap was made of a piece of bondable PTFE JB Weld'ed to a piece of metal, with the thumbscrew bearing against the metal, all as in my push-pull Crayford.  For the draw tube, I used the aluminum draw tube from the Coulter.  In the picture, the hole for the focuser tube wasn't cut yet.

Here's another picture, with a better view of the T-shaped ball bearing supports.  You can see the gray JB Weld all over the  place.  Two things to be careful of when gluing are (a) don't get any JB Weld on the bearing surface, because then movement will be bumpy and (b) ensure that the JB Weld stays on the inner ring of the ball bearings, so it doesn't stop the ball bearing from moving!  Note also that the posts are located in such a way that the focuser tube would fit snugly in.  At this point in the process, the posts are, I think, screwed in place.

Eventually, I unscrewed the posts, cut a hole of the focuser with one of my trusty cheap Harbor Freight hole saws, sanded the faux veneer off the particle board, glued the posts with JB Weld to the particle board, with screws for added strength.  Or something like that--I don't remember all the details.  (It's possible that at some point the posts were weakly affixed with craft glue to help with layout and then pushed off.)

I did have one worry, and that was that the posts might bow outward with time, thereby decreasing tension, and generally making the focuser not work too well.  So I JB Weld'ed some strips of hardwood that I had lying around in my scrap pile, joining the posts as if with a fence between fence posts.  I was generous with the JB Weld.

Finally, I cut the particle board down to size, removed the old focuser (cutting the sonotube around it, ripping it out, etc.--it was a messy procedure), and attached the particle board with screws and some hardwood rails to fit to the curvature of the tube.  I also painted the wood and JB Weld black.  It's pretty heavy, but somehow fits the home-made styling of the Coulter scope.

The focuser has had a bit of image shift, which has increased with age.  Lately, it wasn't doing too well, so I had a closer look at it.  It turned out that the T-shaped supports, under pressure, had rotated further, so that the angle between the axis of the ball bearings and the draw tube was too big for good movement.  I made the angle smaller, added washers under the nuts (did I forget? or maybe I just ran out of washers?), and tightened it up.  I am guessing they'll rotate away again.  If they do that, and maybe before that, I may replace the nuts with locknuts, and/or add some toothed washers.

The focuser weight is an issue--I've had to increase the counterweight--but that's part of the price of making stuff out of wood.  And the price is hard to beat: a couple of bucks for the bearings, and then some screws, washers, nuts, square rod, and various pieces of scrap.

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