I have decided to go with the dual speed Helical Crayford as proposed by Pierre Lemay on cloudy nights
Page 1 of 4 - Introducing the Dual Speed Helical Crayford Focuser - posted in ATM, Optics and DIY Forum: INTRODUCTION Over the years I have built a number of focusers, including: Threaded Helical, Conventional Crayford and Helical Crayford (HC) focusers. Since many of my telescopes are...
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I will be attempting to refine it down to the smallest package I can.
The main problem is I cannot find any stainless tube for the focussing tube with sufficient material to allow me to bore it out to 31.75mm to fit an eyepiece into and still leave sufficient material for rigid support.
I wont use alloy as I've found with the other helical crayfords I've built that long term usage mars the surface with the ball races.
In a blinding flash I suddenly realised why not just place a top collar of aluminium on a length of 1.25" stainless of 1.2mm thickness to hold the eyepiece, theres heaps of that size available.
So thats what I will do.
All that agonising regarding drawing a circle and dividing it equally was to get the bearings in the correct place and alignment and to allow projections of various axis so I can drill the holes for the bearing shafts at the correct angles.
Quick code for the sketches
Magenta = eyepiece tube, it is pressed against 4 the inclined roller bearings so a screwing motion moves it up or down.
Red = 4x6mm roller bearings
Gold = 6mm shaft with a 6 start by 1mm thread, the gold knob turns it pushing the white plastic block against the magenta tube thus turning it a small amount for fine focus.
White = Delrin block that is pushed against the magenta tube thus pressing the tube against the 4 roller bearings.
Blue = various fixing screws
Grey = aluminium 12mm plate to hold everything together
Green = cast aluminium tube that the grey plate is screwed to then is screwed into the top right angled diagonal that protrudes from the side of the rotating top cage.