Second, even if you could grind these figures, you'd probably need an active optical system like Keck's set of hundreds of servos which keep the mirror segments aligned as the telescope warms, cools, and changes orientation. Absolutely impossible for an amateur. Absolutely impossible for an amateur. I'm not convinced that you can't make a mirror cell that would hold everything stable, at least if you're not so concerned about mass. I mean, mechanically, a large unitary mirror is just a slab of glass holding a thin aluminum mirror still.
Thermal expansion is another thing.
It might be possible to use glue or pitch to keep the segments together during the process, and then reassemble them afterward in the same sequence. Absolutely impossible for an amateur. This would be difficult. However, as long as the mirror cell is sturdy enough, it might be possible to use manual controls. The segments might be collimated using a set of masks which would function to use one segment for a reference to collimate the others.