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Hi grinders,
For an interested party I said I would put on a brief log regarding time taken to go through the grind of a 22" float glass mirror blank.
As follows:
Grit #46 Sic. Using a 7"diameter 2.5kg barbell weight disc it took 4.75 hours (TOT) to hog out a sagitta of 5.8mm.(My aim for final sag depth 6.7mm(0.264")).
Grit #80 Sic.Continued with the 7" hogging tool for 3.75 hours (TOT) sagitta to 6.9mm (looks like an overshoot-but not so as you will see).
Grit #120 Sic. Changed to a larger tool 14" ceramic tile tool (50mm ply substrate).1 hour only (TOT). Sagitta back to 6.7mm due to the larger tool diameter moving the surface figure away from the hyperbolic formed by previous smaller diameter hogging tool. So now figure more spherical as action occuring simultaneously over a larger area of the blank.
Grit #220 Sic. Moved up to a full sized (22" diameter by 2.5" thick plywood) tool. This one overlaid with glass tiles (1" squares). Weight applied 15kg to facilitate grinding action.Very short strokes (TOT). Took 4.5 Hours to remove all pits from previous grits. In retrospect I should have spent another hour with #120 to take out all artifacts from #80. Sagitta remains constant at 6.7mm (my target sag). The full size glass tile tool further improves the evolving spherical figure appearing on the fledgling mirror. The sagitta dosent change much at this stage because tool and mirror diameters match and wear is across the whole area of the mirror.
Grit #400 Sic. Same 22" glass tile tool. 3 hours (TOT). All #220 pits gone. Sagitta still 6.7mm.
Grit #800 Sic. This is where the drama started. In all 13.5 hours was spent grinding with a mix of (TOT and MOT). For the first time part way through the grind I noticed scatches appearing. I would barely work one out when another (sometimes several others), would appear. I discovered the reason (towards the 8 hour point), why the scratching was occuring was because of the vacuum generated by mirror passing across the channels of the tool was drawing earlier grit particles up into the swarth. ( My strategy when changing down in grit size was to wash down the tool and scrub out the channels, but it can be difficult to get all of the artifacts from the previous level 100%. So I would paint in all the channels with a waterbased paint to attempt to capture such particles. unfortunatly the waterbased paint was inefective in the capture of all such unwanted particles. So I switched to an oil based paint mixed with a penetrating oil and painted the channels this eliminated the problem and no more scratches. Sagitta depth 6.7mm (target depth).
So in all time taken to fully grind this 22" mirror was 31 hours
I more recently ground a second 22" mirror in 20 hours
Stephen.
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