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29953Re: CNC 4 inch focuser modification

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  • Tube Tim
    Feb 1, 2006
      >--- In tmboptical@yahoogroups.com, "Wayne G" <fomalhaut@...> wrote:
      > Hi Harvey,

      Hi Wayne,

      Not Harvey, but Tim here,

      > [WG: ] I've been looking into doing this as well to my own 152
      > for a few years now, as sometimes my rotating section can go from
      > turning very easy and smooth to being suddenly jammed up so tight
      > that no effort at forward movement helps and I end up knocking the
      > whole scope out of position and losing the object trying to get it
      > loose again.

      I've been thinking thru the rotating focuser for years myself.
      Without the fix, mine always spun smoothly, you just have to keep
      upward pressure on the drawtube housing.

      > I always intend to look into the matter 'later' but end up putting
      > the scope away and forgetting about it until the next time I
      > take it out!
      > :^)
      > Obviously, it would seem that the heart of the problem is that there
      > is nothing other than the three set screws riding in the channel
      > which both hold and guide the rotating sleeve in its motions.

      This is true as far as holding the sleeve from falling back. The
      sleeve and it's contact with the main flange are very well machined
      and precisely fit.

      > But I also hope/think it very likely that if I follow Thomas'
      > adjustment procedures recently published (thanks Tom), this will
      > probably either greatly ameliorate or even largely eliminate the
      > problem to any reasonable extent, which I will try some day soon
      > when I get the chance.

      TMB's adjustment is for the drawtube friction / centering, no bearing
      on the rotating part.

      > But there is still the likelihood of the problem returning again
      > later I fear. The key is in taking the weight off of the groove
      > walls and eliminating the friction between them and the screws
      > (which your modification accomplishes).

      Correct, you have to hold the sleeve up into the housing so the
      setscrews never contact metal groove.

      > However, as I do my own machining, I will be eventually settling on
      > some solution of my own choosing--- I suspect there is more than
      > one way to address the issue, and correct the deficiency so as to
      > entirely eliminate it from coming back again and further improving
      > the smoothness and stability of the rotating collar, which is
      > imminently superb when it is working well.

      Agreed the collar is super smooth. The fix makes the mechanism behave
      like this at all times without the need to hold the focuser from
      slipping onto the set screws. Imagine, loosen the lock screw, gently
      rotate to exact position, even with heavy load on focuser, then gently
      lock it down. A lot of force is not required.

      > To be honest, my own ideas to this problem have been to take an
      > entirely different approach to the one that you and Tim have chosen,
      > but we shall see once I have the parts all apart in my hands what I
      > will decide.

      I really wish you could come up with something that didn't require any
      machining. I've looked at this and it seems every thing requires some

      > In the meantime, as to your comments above, what I would ask and
      > propose here is for maybe you, Tim, or some other enterprising and
      > magnanimous person to put together a detailed description and
      > drawings/photos of the disassembly, reassembly, and any other
      > pertinent points, obstacles or hurdles for anyone undertaking such
      > an improvement/modification of their focuser to follow, and maybe
      > put it in the files section for all of us to refer to?

      I got the disassembly part done and posted. My one comment about
      reassembly would be to prevent stripping out the focuser housing when
      put the long threaded rods back into place. To start the thread
      easier, turn the rod CCW until you feel a 'click' of the threads
      lining up (in phase) and then start turning CW.

      > I've looked at Tim's pictures of the parts, both privately as well
      > as those recently published here, but they always leave me some
      > questions as to what the issues were in taking it apart.
      > Mechanically inclined as I am, I would feel better (as I think many
      > would who are considering this) knowing exactly what they are
      > getting into before taking a very expensive product like this apart.
      > I think it is the time now for a detailed and thorough article on
      > addressing this matter safely and with minimal risk, much like many
      > others have published articles on taking apart corrector plates on
      > SCTs and fixing loose primaries, mirror flop and focus shift. The
      > fact that you say above that the procedure is not very difficult
      > tells me that it is really quite simple once you see how the scope
      > is put together, which obviously was designed to be very
      > robust, as all TMB products have been!

      It's not to bad to take apart as long as one marks the orientation of
      everything. In fact if you don't touch the push screws on the lens
      cell it goes back to near perfect collimation. Shoot on my 229, I
      didn't mark the tube as to orientation of alignment with lens etc and
      it still came out real close. The tubes are obviously cut square, a
      huge help.

      > Can I compel you to be our volunteer to create such a document for
      > us? :-)
      > You wouldn't be recommending anything, just giving us the dry facts
      > of what is involved and a roadmap based on your own knowledge and
      > experience.

      Let me know what you guys / gals think of the paper. I can add to it
      if need be. I could even tear down my 152 again and take better pics.

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