Re: [ATM] Stubborn TDE

[Jonathan Bsihop <jonathan_bishop@sbcglobal.net>]

David,

With the warmer temperatures the pitch is gettimg much softer. If you 
are trying to get to a sphere or cure TDE this can be a problem.

You may want to find some harder pitch, or if you prefer simply boil the 
pitch you have for an hour or two. Once you have remade your lap you 
will find it much easier to get back to a sphere using short strokes 
with a small amount of side to side variation.

Yes, you can get there using the lap you have, but it may take a lot of 
experimentation. In general with a harder lap you can get to a sphere in 
less than an hour, many times only 15-30 minutes of polishing. A hard 
lap may not be the best thing for parabolization though, unless you go 
sub-diameter (my preferred approach).

Jon Bishop

>>First, please tell us how hard your pitch is.  The easiest way to 
>>describe it is to tell us how long it takes your pitch channels to 
>>close (or require serious trimming) during normal polishing.  6 hrs is 
>>too long.  30 minutes ot too short.  2-4 hrs is best, in my experience.
>>    
>>
>
>With the warmer summer temperatures, it takes about 2-3 hours for the channels to close.  I am 
>amazed how much the softness can change with temperature, though.  In the spring (when I was 
>doing hour-long polishing sessions), I polished for more than 10 hours before I needed to trim 
>the channels.
>
>  
>
>>Second, please describe your polishing stroke.  I like to use moderate 
>>polishing pressure, which is still probably 15 lbs of downward 
>>pressure.  This depends on how the pitch is behaving, however.
>>    
>>
>
>I would say I am using moderate pressure, something like 10 lbs.  The stroke is COC or a narrow W
>(either one seemed to have the same effect) with a stroke length of 1 inch.
>
>  
>
>>Last, please tell us what the mirror is made of (plate? Pyrex?) and 
>>what the tool substrate is (what the pitch is poured on).
>>    
>>
>
>The mirror is Pyrex, 1 inch thick.  The tool substrate is dental stone, also 1 inch thick.  The
>tool is full-size (6" diameter).
>
>One thing I'm thinking now is maybe my 1 inch stroke is too short - so short it isn't doing
>much.  I might try increasing the stroke length to 1.5" (1/4 diameter).
>
>Jerry suggested I try doing accented pressure closer to the edge, since the TDE is narrow.
>But I think I got about as close to the edge as I can - about 1 mm.  I applied pressure with 
>my thumb at the very edge of the tool.  I tried various narrow and wide W strokes with 
>this type of accented pressure, to no avail.  I wonder, should I try a straight forward
>and backward stroke (rather than a W) when I apply accented pressure, with this narrow 
>of a TDE?  
>
>Thanks for all your feedback.
>
>David
>
>_______________________________________________
>ATM mailing list http://www.atmlist.net/
>
>  
>

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