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    Re: SNO-M arc accuracy -- errors from one degree to the next?
    From: Richard Reed
    Date: 2010 May 6, 07:52 -0700

    "...This wasn't backlash but 'stiction'. I saw what was happening.."

    Bill Morris has given me what sounds like good advice on this:

    A lot of this information is on his web site here:


    I also await the print version of his book, also mentioned on his site.

    Bill wrote:
    "Although I used to recommend it, experience has now taught me that clock or instrument oil is not suitable for lubricating the rack. The conditions at clock pivots (journal bearings) are very different from the conditions at the rack of a sextant, which is effectively a segment of a worm gear."

    "The worm moves slowly over the rack with point, or at best, line contact, and a higher viscosity is required than clock oil can provide. I now use a single grade SAE30 oil, Mobil Agri Rotary oil and it seems to be just about right for the purpose, though the 5 litre bottle is going to take some time to use up..."

    "Another source of stiction may be what I call the "swing arm bearing". This is the vertical post about which the little chassis carrying the worm rotates. In the SNO-M the vertical shaft is tapered, like that of the index arm bearing. Like that bearing, the correct lubricant is grease, not oil. Again, the screw that holds it in place, via a washer, is used for taking up clearance until there is free rotation without shake. It should not be "firmed up" as you may not only introduce stick-slip, but may also screw its head off. The bearing is easy to get at. Remove the screw and washer (note the flat that prevents the washer from rotating and loosening the screw). Remove the screw and keeper washer on the front face of the index arm and you will be able to lilft out the swing arm chassis, probably losing the ball bearing under the leaf spring in the process (this too needs a little grease).

    Finally, an index arm bearing that is too tight can also lead to puzzling errors, usually because its screw has been overtightened . Grease is the appropriate lubricant. I use waterproof grease that is sold for outboard motor mountings."

    ----- about eccentricity, Bill had this advice:

    "I suggest a very careful examination of the worm, using a powerful hand lens. Also, have a look at the worm-rack interface as you rotate the worm. You may detect that the oil film varies as you do so, suggesting a bent shaft. If all else fails, you could dismantle the worm mechanism completely, clean everything thoroughly, lubricate and try again."

    ----- in my case, a bent shaft on the worm seems unlikely, so Bill had a theory.

    "Very poor bearings in the leadscrew of the lathe which produced the worm is another cause of sinusoidal errors.

    ...the cure is to make a new worm. [the machinist] will need a taper turning attachment on his lathe and be at home with screw-cutting on the lathe. Otherwise, it's simply a matter of copying the original.

    Finally, 30 seconds either way is not a huge error, and is just another error to allow for."

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