NavList:

Hi Frank,

    That worm and rack "jump" is also quite noticeable even on a metal sextant if you don't engage them carefully, it would be wise to exercise some caution before you decide to set the index arm. (Do not release the clamp suddenly since on frequent occasions both teeth on the worm and rack may not be resting on the neibouring grooves, instead they may be clinging right against each other over their tips. If you proceed and begin adjusting the micrometer drum, causing the teeth to detact from each other, then the strong clamp spring would press and slam the teeth into the next grooves, producing considerable wear over the adjoining teeth,even worse on a plastic one like Davis). What you should do is to gently release the clamp,maybe move the drum a little bit just to let the teeth slow slide into the grooves and let go of the clamp until you felt a firm and concrete gearing over the worm and rack.

    FYI, Worm and racks on certain sextants(a well-used one for example) may have developed gearing gaps in between, you should always rotate the micrometer drum in one direction(from measuring index error all the way till you've finished your sights),just to minimize the associated errors produced by these gaps.