Re: [ATM] TDE (That Darned Edge)

[Mike Lockwood <melockwo@uiuc.edu>]

Jerry,

Jerry wrote:
> If you were experienced at using accented pressure and using the
> right pressure, I would suggest starting with very little pressure
> just a little inside the edge, and as you stroke a W inward to the
> 30% you increase pressure steadily as you go reaching max at 30%
> and then reverse back outward while reducing pressure.

I would like to clarify that a little more.

If I were to work the lap inward (I think you were talking about TOT
work) using that "pressure profile" (increasing the pressure while
moving inward), I suspect I would find that it very effectively dug a
hole near the center.  Using accented pressure near the center of the
mirror will change the shape much more quickly than near the edge, and
it must be done carefully, with knowledge of what can go wrong.

> A bit easier is to use the same pressure for the entire W. and some
> method of increasing wear in the inner zones such as working the W
> from the edge inward but when you reverse to go back outward only
> go 1/3 or 1/2 way from the 30% to the where you started at the
> outside of the W.  That will give more work near the rim of the
> hole.

Again, one must be careful because the center can deepen very quickly.

> The variable pressure is the superior method

I must disagree - there are few absolutes in mirror making, and I 
think this depends on the pitch hardness and the operator.  With soft 
pitch, using lots of pressure will make it flow quickly, and the 
"active area" of the lap will move inward rapidly.  I have seen this 
cause deepening of the center when de-correction of outer zones was 
the intended effect.

> I make what looks like an archery target that represents the zones
> of my test mask and seal it with plastic spray. I put it under the
> mirror and use it as a guide as I work zones.

That's a very good technique.  Once you understand where the active
area of the lap is, using that to aid you, you can attack nearly any
defect.

> I can't say that it is wise to do unless you are willing to
> practice it a bit. That is what mirror making is, but you don't
> have to learn it all on one mirror. It is a lot.

I can't count the number of times I have been "practicing" on a mirror
until the early morning hours.....  :)  Took me about 20 mirrors 
before I got to the point where I didn't feel I was practicing.

	Mike Lockwood

_______________________________________________
ATM mailing list http://www.atmlist.net/