I don't like the distribution of pennies on the tool.
Sparse near the edge. Red circles in picture mark "missing pennies". Two problems with that.
The outer zones have less grinding surface than the more densely populated central zones. Might be why your central area is longer radius than outer zone. With COC strokes central zones grind faster because of more tool surface area there, grinding deeper but not much change of ROC in the center. Less contact in outer zone causes edge to grind slowly and it becomes relatively higher with shorter ROC than center. It is possible that you have reached your desired sagitta but the center being flatter than the edge you don't see it across the 3.56" diameter of your spherometer.
Take a measurement across the full diameter of the mirror. You might have your target depth but really only need to make the surface spherical.
There is not much support for the mirror when doing a chordal stroke with mirror center offset to the 65% - 70% zone causing a danger of tipping over the edge when mirror center is over one on the "missing pennies", so safe offset is limited, and you want that offset for shortening the ROC.
Your tool is what it is now. I wouldn't recommend adding pennies in the spaces now. They will be too high.
It might work to removed a couple of pennies in the central part of the tool so that overall distribution of pennies is more uniform. I don't know if the coin tool as is will work well. I think it might be too much trouble because of too many unknowns. My recommendation would be to cast a normal plaster and ceramic tile tool fully populated with tiles.
Cast on the mirror so tiles match mirror curve well and then it will only take a little while to grind into contact with 120 grit and make it spherical. Then you continue through the various grits in a normal manner.
You would do better using basic commonly used methods. Small tools, penny tools, and other oddball tools work much better if you have experience to know what is a good result and that particular tool can get to that result. I doubt there is anyone experienced at using a penny tool with a distribution of coins as on your tool, and that means no one can tell you from experience if you will be able to make a good sphere at each grade of fine grit so that you will be able to polish in a reasonable time. Your penny tool might work ok, or it might be a colossal waste of time.
Measure depth across the full 6" diameter and post your result.
If you put a straight edge across the mirror and measure depth at the center of the mirror.
Drill bits make a good gauge. Here are some sizes around your desired sagitta. Any would be close enough.
Put the back end under the straight edge just far enough under the straight edge to have full diameter under it.
1.9 mm 0.0748"
Edited by dogbiscuit, 24 March 2020 - 01:52 PM.