I finally reached my target depth! I moved the markers on my dial indicator. The one at 0 is still the target depth for an 8" f-5 mirror, but considering the bevel around my glass I'll really only have a 7.8" mirror surface. So the slightly shallower mark is for a 7.8" f-5, and I'll be satisfied if I end up somewhere between these two, closer to the shallow one.
My tool unfortunately is still not worn all the way through (the white patches are parts that have not been ground yet because the tile surface was slightly uneven). This presents a problem because these parts are not smooth yet, and may have a slightly different hardness than the worn-through parts. I switched to 120/220 grit because I ran out of 60/90, and I know I have more than enough of the 120/220. Using this finer grit it took a while, but I was able to finally wear the tile tool down until it was uniformly smooth.
In order to do this and not deepen my radius of curvature I had to alternate grinding with the tile on top and the glass on top, which ultimately removed glass from the edges and center equally. I overcompensated a bit, though, and ended up too shallow. I'll need to correct this before I move on to even finer grits.
That make-shift spherometer looks pretty cool, and it will be infinitely more accurate than my make-shift feeler gauges. If I had bult one of those I wouldn't have over-shot my 8" f/5 (now an 8" f/4.3). You'll hit your target focal length within one inch very easily - and that's a pretty good level of tolerance at this stage of grinding.
ReplyDeleteAs you progress through fine grinding, your flashlight test become progressively more accurate, and I bet you can hit your target FL within a half-inch to a quarter-inch if you're diligent about it.
Good job!