Re: [ATM] Trying to reach sphere

[Ben McIlwain <cydeweys@gmail.com>]

Adam Krause wrote:
> Hi everyone,
> 
> I'm in the final stages of fine grinding and I want to be as close to
> spherical as possible before entering polishing.  I'm using 305 Emery
> (equivilant to 5 micron).  I've done the sharpie test and it is
> grinding away the edge before the center.  This perplexes me because
> I had reached a sphere a grit or two earlier, and I have been using
> tool on top 1/3 COC ever since, which is supposed to be what you use
> to solve "hole in center" contact problems.  Lately, my stroke has
> been getting shorter (about 1/4 COC TOT).  I have three questions:
> What else can I do to reach a sphere?  What is the effect of stroke
> length on grinding TOT.  Does the stroke length have the same effect
> whether it is TOT or MOT?

I'm so not an expert on this, so take my advice with a grain of salt,
but when I exited polishing and started final figuring I had a pretty
oblate curvature.  I think it's because my stroke was too short (1/4
center-over-center or even less).  I lengthened my stroke to a proper
1/3 center-over-center for a few 5-minute sessions and that got me to
spherical.  Don't go overboard because you can develop a turned-down
edge, but in my very limited experience, a too-short stroke will cause
oblateness.
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