I have been polishing a 20 inch borosilicate mirror on a fixed post machine using a 15 inch lap. I have been trying to get rid of a wide turned down edge using spin polishing. After a lot of hours of polishing the TDE is now only right at the edge of the mirror (I tested using a Ronchi screen.) But it shows no sign of going away after more hours of polishing.
I have now picked up some small shallow scratches, somehow small splinters of wood from the fixed posts seem to have got on to the mirror. I have to use some force to push in the bolts that hold the cross post that holds the spindle on the machine, this must have caused some splinters to fall on to the mirror. i will remove a bit of wood so this does not happen again.
I am planning to go back to fine grinding using 1000 grade grit, which I think is equivalent to 5 to 7 micron. Unfortunately I used the tool to make the pitch lap so will need to make a new one. I use dental plaster and make a penny tool.
My question is, using pennies (actually British 2 pence pieces) on a new tool with this size grit will I have problems getting the pennies to conform to the mirror? I pour the dental plaster on to the mirror using a dam and with aluminium foil underneath and then glue the pennies to the tool.
This time I will test with a Ronchi screen as soon as the mirror is polished out again, to make sure I don't have a turned down edge before I start figuring, which I didn't do last time.