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  #61  
Old 19-09-2018, 08:42 AM
Kunama
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If it's worth doing, it's worth doing twice!
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  #62  
Old 24-09-2018, 09:30 AM
Billyboy78 (Bill)
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Originally Posted by Kunama View Post
If it's worth doing, it's worth doing twice!


No sooner started on planing the edge down when I decided the tool needed re-tiling. I tipped the new tiles upside down and glued the shiny side onto the old tile, figuring #46 grit will soon take care of the rough underside. I enlisted the help of missus and son to glue the 240 tiles on before the resin cured.

Once the tool had cured I started on chordal strokes (as in, moved the tool to get 3 1/2" overhang) with #46 grit, measuring sag and sphere at each 15 minute interval. I changed to #80 and moved the tool to the recommended 2 1/2" after I had ground 1.5mm from the edge zone. Now I'm on #220, sag is 8.7mm, sphere is steady on .025", but 1 huge pit in the middle. I'm hoping by the time that is gone I should be able to move on to #400.
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  #63  
Old 26-09-2018, 05:27 PM
Billyboy78 (Bill)
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Getting a bit hard to win here. That huge pit has turned into an airbubble! It showed itself after I planed that 1.5mm off the edge. Looks to be .5mm in diameter, not very big but there nonetheless. Have to go back to #80 again.
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  #64  
Old 26-09-2018, 05:33 PM
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That's no good!
I wonder how many more restarts it will take? (I'm hoping for 0!)
At least with the machine you can sit back and relax, infact I am grinding 240 grit right now!
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  #65  
Old 29-09-2018, 03:14 PM
Billyboy78 (Bill)
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So far so good. Airbubble gone, finished with #80, #220, midway through #500.
I had some suspect #400 and #600 and my guy sent me some good #500, so to save anymore messing about I'll spend extra time on the #500.
Don't you just love the silky smooth feel of the polished glass?
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  #66  
Old 29-09-2018, 04:44 PM
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Nice!
Yeah it is great!!!
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  #67  
Old 04-10-2018, 03:06 PM
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I'm on #1200 (5um) and keep getting persistent scratches, not very deep, like they come out after half hour of grinding. I'm thinking it might be the old grit sticking to (embedding) into the polyester resin that I'm using to seal/stick the tiles on (yes the resin is above the bottom of the tile) and despite scrubbing out the channels with a toothbrush they are dropping off and giving me grief. I'm going to give the tool another coat of resin to try and bury the monsters, but this means no more grinding for a couple of days while it cures.

There is an up side. The rest of the mirror has taken on a brilliant shine.
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  #68  
Old 11-10-2018, 05:20 PM
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Yay, fine grinding done. Make up the lap tomoz.
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  #69  
Old 11-10-2018, 05:27 PM
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Awesome!
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  #70  
Old 15-10-2018, 12:37 PM
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Lap #1 was a practice run.(koff koff). Turned out hard as a rock. Trip to the freezer, start again, this time with the full 100 mils of castor oil. I used GW's method of pouring a puddle on the mirror and settling the tool in the middle. Now it is channelled, scored and cold pressing. Let the fun begin.

In the meantime I hogged out the 8" secondary and made a 6" tool out of concrete ready for the tiles. Now that I know the 3.4fl of the primary I'll aim for f3 for the secondary.
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  #71  
Old 15-10-2018, 01:35 PM
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Wow 8 inch secondary!!!
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  #72  
Old 15-10-2018, 03:35 PM
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yeah, but its only 50 square inches, primary is 314 square inches, gives me 264 left.
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  #73  
Old 15-10-2018, 03:43 PM
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Cool, How'd you hog out the secondary so fast?
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  #74  
Old 15-10-2018, 04:04 PM
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Go back to post #30 and there is a pic of the concrete grinding tool on the angle grinder. Took about 10 minutes.
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  #75  
Old 15-10-2018, 05:09 PM
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Ah angle grinder. I should have done that for my 6 inch! Doh!
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  #76  
Old 15-10-2018, 09:43 PM
sharpiel (Les)
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Quote:
Originally Posted by Billyboy78 View Post
Lap #1 was a practice run.(koff koff). Turned out hard as a rock. Trip to the freezer, start again, this time with the full 100 mils of castor oil. I used GW's method of pouring a puddle on the mirror and settling the tool in the middle. Now it is channelled, scored and cold pressing. Let the fun begin.

In the meantime I hogged out the 8" secondary and made a 6" tool out of concrete ready for the tiles. Now that I know the 3.4fl of the primary I'll aim for f3 for the secondary.
It's just cat fishing without pics
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  #77  
Old 15-10-2018, 10:24 PM
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Can you share how you used the angle grinder to hog it out?
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  #78  
Old 19-10-2018, 10:52 AM
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Lap #3, (lap #2 still too hard) followed GW's instructions to the letter in preparing the lap. Lots of cerox, cold pressed for 2 hours, now I've locked it up and no amount of pushing/pulling (1 metre long lever) will move it. In my haste I was going to seal the back of the plaster tool for another day, so now if I toss it in the bath for a day I will ruin the plaster. Do not want to make another tool!

Remedy will be to seal the tool in situ with resin or enamel paint, then dunk it.
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  #79  
Old 19-10-2018, 10:57 AM
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sounds like you didnt use foil over the mirror?
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  #80  
Old 19-10-2018, 11:30 AM
Billyboy78 (Bill)
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Originally Posted by Lognic04 View Post
Can you share how you used the angle grinder to hog it out?
I just laid the glass on a rag and played a trickle of water onto it, then gently dug the edge of the cutter into the glass, working the middle of the glass more than the edge. In fact I didn't shave the edge at all, leaving about 2mm gap.

Because the cutter is 100mm diameter I couldn't lay it flat as I could with the 20" thus leaving a number of shallow score marks that will come out with #80 grit.
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