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deepen curve with fix post grinding

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#1 Stargazer

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Posted 14 July 2019 - 09:02 AM

Hello All,

 

      When trying to deepen the curve on a 20" piece of glass with a

15" tile tool where would you say is the best place to set the tool, I

currently have it set with a 3/4" overhang and thing are progressing

in the right direction albeit slowly,would it be better to move it in to zero

overhang or am I looking at this all wrong?

 

   Just so you know I am trying to get to an F 4.5  from F 5 and have

moved the RoC. by almost 10" already, just hate to continue only to find

it would not go deep enough where it currently sits and really do not want

to waste grit. 

 

 words of wisdom would be appreciated.

Thanks.

 

 



#2 mark cowan

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Posted 14 July 2019 - 12:06 PM

As little overhang as possible so long as there is a bit - for fixed position grinding.  


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#3 Stargazer

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Posted 14 July 2019 - 02:08 PM

Okay then, I did move it in till it was barely overhanging on my last session,

nice to know I was right for a change lol.

test RoC then continue on. Thank you Mark much appreciated.



#4 Stargazer

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Posted 14 July 2019 - 04:39 PM

next question,

  My 80 grit did not last long, all the 120 I thought I had turns out to be

320, I have a pound of 220 left after trying to smooth out the pits from

80 grit well still trying to deepen the curve but doubt I can deepen curve

much more with a pound of 220 so I need to find some 120 grit.how

much 120 would it take to reduce the RoC on a 20" by about 10 to

12 inches more with fix post machine?

Holy cow the shipping prices to Canada are scary these days too,anyone

know of a supplier this side of the boarder?

this project is gonna break the bank, but until then...

 

Thanks



#5 mark cowan

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Posted 14 July 2019 - 06:56 PM

It takes a long time to do it that way.  I can't say how long but your experience should be a good guide.  Look for some lapidary (rock shop) grit perhaps.



#6 Stargazer

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Posted 14 July 2019 - 07:48 PM

I was thinking the same thing Mark, when I get some grit I will

do the hogging out cave man style as John Dobson use to say

about polishing, by hand.

will likely need a smaller grinding tool as well.



#7 Billyboy78

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Posted 14 July 2019 - 08:24 PM

I had to resort to manual grinding my 8" secondary of a 20" greg. I moved the tool all over the place but couldn't get the sag below 3.6mm. Small tool, heavy pressure on the centre zones, but use common sense. Grind a bit then smooth the surrounding areas. I admit I am using glass which moves a lot faster than Pyrex.


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