Re:more ATM Grinding Machine Questions

[Bill Marriott <btk@pacbell.net>]

Peter D. Brooks wrote:
> 
> DROWESMI@aol.com wrote:
> >
> >   Suppose that the mirror is on top, so the spindle is driving the > > tool.  The 
>mirror is allowed to rotate on the pin but is not itself > > driven.  To get a nice 
>axially symmetrical surface, the mirror must > > rotate in a "random", i.e., long period 
>fashion wrt both the tool > > and to the stoke.  First, is this the correct anal
>ysis?

> Yes, this is my understanding.  You get something close to a > paraboloid, at least 
>close enough so that it doesn't take much work to > correct.  The rotation is induced by 
>the 'outside' edge of the mirror > seeing a different speed from the tool than the 
>inside edge.  Assuming > you are doing a center (more or less) to edge (more or less) 
>sweep, > you should see (presumably) rotation in the same direction.
> 

More 2 cents.

In general, this is correct as to how the top is driven by the bottom,
with the top powered in sweep only.

I have tried MOT a couple of times, and all I got were zones and a
turned edge!! However that does not mean it won't work for someone else.

What I have found to be true, initially passed on from Jerry Logan to
Bob Pfaff, to us on the list and probably buried in the archives from
1.5 to 2 yrs ago is the following. (I am somewhat quoting here, trying
to give credit where credit is due)

1) As previously mentioned, 60 rpm spindle, 1/5th eccentric speed, ie 12
rpm.

2) TOT, always

3)To generate a nice sphere (or at least as nice as you can get on any
particular machine, which has to do with bearing tightness, etc), have
the stroke set from center over center, to the left side only, approx
1/4 stroke, with a full size tool. Do not let the tool go to the right
of center. I have done this with a 75-80% tool with good results also
This is assuming that the spindle is going counterclockwise, which it
should.

4) I have been able to apply correction on the machine, by using a
subdiameter lap of 80%, with the eccentric turned way up (1/1 ratio).
This works the center. By pulling the pin forward, and slowing the
eccentric back down,you can acentuate the edge. Maybe more so with an
even smaller lap, but havn't had a chance to try that yet.

5)Please note that with #4, I've found that unless you have a REALLY
tight machine, the zoning from machine work takes a fair amount of hand
polishing to remove, and you don't want to run out of parabola before
the zoning is gone.  I reccomend 30-50% correction max on a machine,
you've saved your self lots of work already, and you want ALL remnants
of zoning gone by the time you reach full correction.

6)It maybe with the proper machine and techniques, full correction can
be machined in, since the big guys do it, I just havn't gotten to that
point yet.

Enough babbling

Later

Bill