I am polishing a 10 inch blank and I have very little experience with the focault test (it is not intuitive to me I struggle interperating the results) and need some help.
My blank shows three zones all about equal radiis. The 0-35% zone and the 70-100% zone are spherical and have the same COC. The 35-70% zone is not spherical and has a longer focus than the inner and outer zone. When I am at the COC of the inner and outer zone the middle zone is inside of focus and the knife edge causes the shadow to grow from about the 40% in both directions towards 35 and 70 on the same side of the mirror as the KE. Using the outter most zone as the reference what this is telling me is that the middle zone has a less steep slope than the outer and the center zone has the same slope as the outer but is higher than the outer reference slope.
Close?
I am machine polishing and switched from pads to pitch but am not seeing much surface shape change as I polish. I am doing 30 - 45 minute sessions and have completed about 4 hours of polishing. 2 hrs with pads and 2 with pitch. The blank is polished to within about 1/8" of the edge. Should finish polishing out soon.
Thanks Dave
Sorry no Digital camera with a view screen or I would post some pics. Maybe I'll get one this weekend.
My blank shows three zones all about equal radiis. The 0-35% zone and the 70-100% zone are spherical and have the same COC. The 35-70% zone is not spherical and has a longer focus than the inner and outer zone. When I am at the COC of the inner and outer zone the middle zone is inside of focus and the knife edge causes the shadow to grow from about the 40% in both directions towards 35 and 70 on the same side of the mirror as the KE. Using the outter most zone as the reference what this is telling me is that the middle zone has a less steep slope than the outer and the center zone has the same slope as the outer but is higher than the outer reference slope.
Close?
I am machine polishing and switched from pads to pitch but am not seeing much surface shape change as I polish. I am doing 30 - 45 minute sessions and have completed about 4 hours of polishing. 2 hrs with pads and 2 with pitch. The blank is polished to within about 1/8" of the edge. Should finish polishing out soon.
Thanks Dave
Sorry no Digital camera with a view screen or I would post some pics. Maybe I'll get one this weekend.
