ATM Grinding Machine Strokes

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

I've continued to play with the grind-o-tron, and using method 2 (see below) I have a 
raised center, minor TDE, and an area of no polish at the 70% zone about 1" wide (shower 
curtain flash polish).  The 8" test mirror is correctable, but the technique could 
backfire on the big mirror (see below).
- Tom Waineo's write up on elgin's does hint that an offset should be used as in method 1 
- does anybody use an offset for subdiameter tools?
- Is this turned edge which I noticed while doing method 1 a common problem or can it be 
avoided?
- 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 below reasonable? 
Chris Fuld
Toronto

I built a mammoth elgin grinding machine from Home Depot and hardware store parts - 
designed ultimately for a 41" 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.


Thanks!
Chris Fuld
Toronto