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  #21  
Old 07-10-2018, 03:18 PM
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Merlin66 (Ken)
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I agree with Rod...
Keep grinding until there are no visible pits when using a loupe.
Mark the rear of the mirror where the pits are seen with texta to help you check that area later....
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  #22  
Old 07-10-2018, 06:29 PM
Billyboy78 (Bill)
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Quote:
Originally Posted by Merlin66 View Post
I agree with Rod...
Keep grinding until there are no visible pits when using a loupe.
Mark the rear of the mirror where the pits are seen with texta to help you check that area later....
x 2
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  #23  
Old 07-10-2018, 06:40 PM
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Lognic04 (Logan)
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Thanks guys, I will continue.
Question, the pits seen larger towards the edge but definitely from 240 grit, like there are more anomalously big 240 grit pits towards the edge? The light reflection does *not* discolour either going towards the edge?
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  #24  
Old 07-10-2018, 06:52 PM
Rod
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Hi Logan

Not sure why some 240 grit pits are bigger than others. I think it more likely the grind is incomplete. Do what Ken suggested. Then you have a much more objective test of when to move to the next abrasive.

Rod
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  #25  
Old 07-10-2018, 07:03 PM
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I looked again and not as bad as I remembered.
I think I have been grinding too much towards the center and need to find the fixed post sweet spot.
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  #26  
Old 08-10-2018, 09:45 PM
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Ok, 240 grit pits are gone
Ugh, cleanup!!
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  #27  
Old 09-10-2018, 07:48 PM
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Started 25 micron!
Is my slurry too thick? The mirror needs water added every 30 seconds and dries out in under 2 minutes!!!
I'd say I'm at 1:10 ratio? Maybe 1:8?
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  #28  
Old 10-10-2018, 07:57 AM
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This thread-umentary need pictures at every step, otherwise it seems like the ramblings of a teenage astro-nerd...

Didn't happen if there's no piccies
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  #29  
Old 10-10-2018, 08:27 AM
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Lol, will take pics when I get home
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  #30  
Old 10-10-2018, 07:01 PM
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https://youtu.be/0AfBcW-CIrk

Short video of grinding
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  #31  
Old 10-10-2018, 07:35 PM
Billyboy78 (Bill)
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Looks good to me. Maybe just give it a spray with water so it doesn't dry out. If it does dry out it will leave scuff marks on the glass. Don't ask how I know.
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  #32  
Old 10-10-2018, 08:11 PM
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It seizes when completely wet too?

Grinding is going well, got 45 mins in today so 1hr total. Maybe a half hour to go? Does that sound reasonable for 240 > 25u/~500 grit? See pics.
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  #33  
Old 10-10-2018, 09:15 PM
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Quote:
Originally Posted by Lognic04 View Post
It seizes when completely wet too?
.
Hmm...try a sharpie test. Sounds like the discs may not be matching up and you're getting an air lock. Now is the easiest time to fix it before you get to the fine grits.
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  #34  
Old 10-10-2018, 09:24 PM
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Well it goes ok then I think if the mix gets too thin or some other variable it slowly slow down, locks then unlocks after 3 seconds stalled and makes a really harsh grinding sound until slurry added. Any thoughts?
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  #35  
Old 10-10-2018, 09:58 PM
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If it were mine I would ditch the spin for a bit and do some MOT just to make sure it is uniform and heading toward the sag target. Keep using the turntable, just ditch the fixed post.

If you have adjusted the overhang recently this will upset the spherical figure and may take a while to normalise, and lockup while it does its thing.
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  #36  
Old 10-10-2018, 10:15 PM
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Ahhh that explains it, I went from close to chordal to neutral pretty quick (only 40 mins to adjust at 240 grit)?
Also I moved the tool in a tad because there was too much removed at the edge and it definitely does lock more than before.
Damn, my tool is 75% so mot is out of the picture. I can try some tot? How do I use the subdiameter by hand?
Thanks
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  #37  
Old 11-10-2018, 05:24 AM
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Maybe just continue as before with the #240 and stay on it until the discs mate up, but do a sharpie test to see where you're at. What is the sag at the moment, you should be headed for a tad over 3mm. What are the channels like on the tool? They need to be around at least 1mm deep so it can 'breath', this too can cause lockup.

I recall 'in the old days' I was doing a 16", desperately trying to deepen the centre and ended up wearing the entire mirror too thin.
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  #38  
Old 11-10-2018, 07:59 AM
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Ahh, that maybe why - I have no channels!!
I will also do a sharpie test (will it ever grind off at 25u though?)
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  #39  
Old 11-10-2018, 08:57 AM
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Number 1 problem is the discs not mating. Because they are not mating and there are no channels, vacuum pockets are forming and locking the discs together. STAY ON #240 UNTIL (1) locking ceases and (2)sharpie test shows uniform mating AND (3)sag is close to 3mm and (4) there are no pits anywhere. (yes, I'm raising my voice)

Sharpie will grind off at 5um
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  #40  
Old 11-10-2018, 09:40 AM
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I think i should do a sharpie test now to see how they are mating, by the way at 240 grit there was never any locking? points 3) and 4) are already true I made sure this time there are no 80 pits (i did an extra 30mins to grind the last ones away)
Should i channel my tool?
Its not so much going back that i fear its the cleanup
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