Re: [ATM] Open Question Concerning Pitch Laps

[Jonathan Bishop <jonathan_bishop@sbcglobal.net>]

Mike,

Mike Lockwood wrote:

> Jonathan,
>
> Jonathan Bishop wrote:
>
>> So I think we agree that to get back a spherical figure a hard lap 
>> will work better.
>
>
> Yes, but we want a parabola.  A sphere is NOT a necessary step in 
> gettting to a parabola.  Its importance is overstated in the 
> literature (okay, that's my opinion after making 20 mirrors).  Once 
> the mirror is polished out and has a good edge and no serious zones, 
> parabolization can (and should) begin, and the mirror maker should be 
> comfortable with testing at this point.

Actually, I really agree with you. I've always been surprised how people 
can parabolize with a full sized lap. Never had much luck with that. I 
seem to be able to get within 1 wave or so but then have a hard time 
controlling more precisely the results.

>
>> I know that most advanced ATMers will find that a little tedious, but 
>> for the beginner it is probably a good approach. In Bryan's case he 
>> actually went back to fine grinding to get to a sphere as he could 
>> not get there with his current setup, no matter what he tried. I was 
>> often tempted to do that myself until I switch to harder pitch.
>
>
> Here's what I'm getting at - not getting a sphere with medium softness 
> pitch is a symptom of a technique problem.  If that problem isn't 
> isolated, the lap may NEVER act like it should, no matter what the 
> hardness or technique.  There will always be unanswered questions.

What technique problem do you think Bryan had? Is there any lap which 
you would consider too soft or too hard? I know many have build pitch 
hardness testers to measure this. Or is what you are say is that there 
is a large range of pitch hardness which is workable?

>
>> I am sure many other beginners went the same route as Bryan. I blame 
>> this on the soft pitch, but probably the more advanced polishers 
>> could have dealt with that in some other way than hardenning the 
>> pitch. From what I saw all the advice that he was given did not help 
>> him though.
>
>
> I'm not sure we (list members) asked enough questions before Bryan 
> went back to grinding.  (We tend to jump to our own conclusions.) 
> That's why I came up with that giant list of questions.  I even missed 
> a few - I didn't ask about polishing speed.  Others made good points 
> about the laps getting "crusty" after use.
>
> Seems like a standardized list of questions would save time.  The 
> problem is, everyone's results can be unique.
>
>> On the more realistic side one nice thing about subdiameter laps is 
>> that the figure changes really slowly provided of course that you use 
>> light smooth strokes and maintain good contact. I know a lot of 
>> people have steered away from this approach, but it worked for me. I 
>> find it easy to understand. Simply polish where you want to remove 
>> glass.
>
>
> In large mirrors, subdiameter laps are often used to add correction, 
> rather than just tweak zones.  They are necessary for mirrors of 
> faster focal ratios.  The rules get much more complex when you're 
> working strongly non-spherical surfaces.

Exactly where I am heading!

>
>> BTW, are you saying that the sub diameter approach will get easier 
>> for large mirrors? That's good to know. So first I made a 4.25 and 
>> then an 8, for wich I used the 4.25 lap. Next I think should be 12 or 
>> 16, for which I can use both my 4.25 and 8 laps. Then for 32 I use... 
>> Seems like a nice progression!
>
>
> Go for a 16" - the 8" lap will come in handy.  Hopefully you won't 
> need the 4.25" lap.  I made a 16" F/4 with just an 8" lap for 
> figuring.  I learned tremendous amounts on that mirror.

Great to know. I have gotten out of the ATM mood for a while (new job, 
family, etc...). A nice 16" blank would be pretty tempting! F/4 would be 
pretty challenging. You probably chose that to make it shorter. What 
type of glass did you use? Thickness?

>
> Bryan, if you're still listening to all of this, you're hearing some 
> differing viewpoints.  We'll help you through the figuring process no 
> matter what route you take.  I still recommend sticking (no pun 
> intended) with the pitch that you already have.
>
>     Mike Lockwood
>
>
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>

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