ATM - Grinding Machine Strokes

["." <cjf@idirect.com>]

I built a mammoth elgin grinding machine from Home Depot and hardware store parts - 
designed ultimately for a 41" pregenerated blank I currently have on the sidelines.  On 
the turntable now is an 8" blank and a 6" tool (tool on top) which I've been 
experimenting with for about a month. Due to the size of the big mirror and my general 
laziness, I'm working at my bottom end design speeds for the big mirror - 3 RPM and 60 
strokes per minute (back and forth)
1) I first tried an offset stroke arrangement as described in ATM 1 (subdiameter tools) - 
stroke was about 1/3 diameter.  I got a 1" turned edge as described in ATM 1.  The tool 
center swings an arc - the path of this arc  was offset from the mirror center.  The 
sharpe test guided me to an offset midway between the mirror center and edge of mirror 
(2" offset).
2) After reviewing all the thoughts on elgin use, I went back to 220 grit and ran the 
fine grinding sequence with a stroke in which the tool swings a path which passes through 
the center of the mirror.  The tool center starts centered over mirror center and  swings 
to right with an overhang of about 1.3" - roughly the right overhang for a classic hand 
stroke.  The result was a similar result as the original offset stroke.
Now for the Questions...
- Is this turned edge common to TOT machine grinding?
- Does the often-mentioned 0.25 diameter strokes for an elgin working from a tool/mirror 
centered position translate to a 1" or 2" tool displacement (for an 8"mirror)? (maybe my 
strokes are too long)
- Are the strokes described above reasonable? What variables have I overlooked?

Thanks!
Chris Fuld
Toronto