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  #21  
Old 07-07-2018, 03:04 PM
Wavytone (Nick)
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Originally Posted by dave brock View Post
Carl Zambuto uses cerium oxide.
As do most if not all making mirrors commercially where minimum time is important.
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  #22  
Old 08-07-2018, 02:48 PM
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Lognic04 (Logan)
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Whered you get that blank?
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  #23  
Old 08-07-2018, 03:30 PM
Billyboy78 (Bill)
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Hi Logan
G James glass here in Brisbane. They have a water jet cutter. I've ordered it and paid but they are taking their time. I cancelled that 1st order for the 8" blank due to a personal problem. All go now. I pulled the fixed post turntable apart last thursday and finished it today, I felt bad that it was as rough as guts and somebody suggested all our work should be the best we can do. I also made up a number of concrete tools, I think casting plaster will be too soft for the machine,besides I wont need extra weight.
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  #24  
Old 13-07-2018, 02:56 PM
Billyboy78 (Bill)
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Glass has arrived! Starting on #46 grit with a 3" steel ring. Taking a break to knock up a depth gauge using an old spirit level and a dial gauge. I'm not going to worry about a spherometer at this point, maybe later.
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  #25  
Old 15-07-2018, 03:26 PM
Billyboy78 (Bill)
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3" ring is very slow. Ground a whole 1mm after an hour or so. 8mm to go. I've knocked up a 9" tile tool to get the machine to do a bit.
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  #26  
Old 19-07-2018, 10:08 AM
sharpiel (Les)
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We definitely need progress pictures.
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  #27  
Old 19-07-2018, 11:12 AM
Billyboy78 (Bill)
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Sorry, nothing much to see at the moment.

The #46 grit is only lasting about a minute on the tile tool, I'm forever recharging. I was looking for a diamond cutter to speed up the work and came across a diamond tool for shaping concrete that fits on my angle grinder. I gave it a 5 minute test and it looks like it may be the trick. The glass comes off in a dust so that may be a hazard, unless I take it outside and play water on it as I grind.

That depth gauge I made using a .001" dial gauge was useless, I couldn't get repeatable readings off it. I've now got a metric micrometer that I chopped up and using a lot better. The inflated figures on the old gauge were better to look at than this one.
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  #28  
Old 19-07-2018, 02:14 PM
Billyboy78 (Bill)
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Yay. Just over 1 hour and I'm down 5mm in the centre, 3mm mid and zero on the edge. Playing the hose on it is keeping the dust down. I plan to stop at 8mm and take the last mm down with tiles on account its got a couple of gouges and I would hate to put in another.
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  #29  
Old 20-07-2018, 01:50 PM
Billyboy78 (Bill)
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With 3 1/2 mm to go I stopped the heavy grinding to see how much of a hole in the centre I had. Did a bit with the 3" ring to tidy it up and then measured it with a 6" spherometer I knocked up. Turns out to be .005" variation over the face of the mirror (between .020 and .025) with 1" untouched around the edge so I'm pretty pleased with that.
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  #30  
Old 21-07-2018, 09:11 AM
Billyboy78 (Bill)
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Hopefully a pic will appear courtesy of my whizz kid daughter.

What you see is the glass, spherometer , concrete grinder, turntable driven by a couple of drill presses, and in the background parts of the GEM.
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  #31  
Old 21-07-2018, 09:52 AM
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Lognic04 (Logan)
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Awesome! I am interested in how you made the grinding stand!
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  #32  
Old 21-07-2018, 10:49 AM
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Hi Logan,
The story really goes back 12 - 14 yrs when I discussed 'the perfect scope' with the guys on STAR Astronomy in New Jersey. I had finished the 16" newt and was starting on a 12" nasmyth cassegrain which I never finished, but kept the idea in my head. Over the last 12 months plans were drawn up and I started to collect bits that might be useful, including buying cheapo drill presses from the sunday markets and bits off my Jeep that I wrecked. The spindle comes from the bracket that held the mechanical fan, and when sat on its end has 4 bolts sticking up that are attached to 3 pieces of 12mm plywood, 1 x 24" D and 2 x 23 1/2"D. There are 3 x 4" swivel wheels under the circumference to stop any flexure it might encounter. The accessory belt from the jeep sits on the smaller discs and attaches to the grooved pulley which is attached to the drill press. I measured the height of the pulley and cut the height of the drill press to suit. The drill presses are set to their slowest speed along with a connecting belt between the two. Current speed is 16rpm but this can be upped at any time by selecting different configurations of the belts. The pieces of Unistrut you may see beside the turntable are for the fixed post.

Hope I didn't bore you.
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  #33  
Old 21-07-2018, 02:56 PM
sharpiel (Les)
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Great pic. You could even rig the grinder on a mechanical arm to generate the required curve.
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  #34  
Old 21-07-2018, 03:10 PM
Wavytone (Nick)
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Ha... I was going say hang angle grinder at the end of a pendulum length = desired radius and let it swing !

Only snag is angle grinders donít play nice with water
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  #35  
Old 21-07-2018, 03:36 PM
AndrewJ
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Quote:
Only snag is angle grinders don’t play nice with water
What about using one of those flexible drill extensions in between.
Andrew
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  #36  
Old 21-07-2018, 04:19 PM
Billyboy78 (Bill)
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Quote:
Originally Posted by Wavytone View Post
Ha... I was going say hang angle grinder at the end of a pendulum length = desired radius and let it swing !

Only snag is angle grinders donít play nice with water
Been there, about 12yrs ago making the 12". I attached the angle grinder to a bit of 3x2 and hung it from the apex of my roof via a ball joint.(There is a pic of it in the Brisbane Astronomical Soc newsletter) Went ok for the first pass but despite running water over it the glass began to splinter and I ruined it.

I've never had a problem with water either then or now. You don't turn the tap on full blast, just a trickle over the glass. The water gets flung off once it contacts the blade, nowhere near the electrics.
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  #37  
Old 21-07-2018, 04:31 PM
Billyboy78 (Bill)
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Quote:
Originally Posted by sharpiel View Post
Great pic. You could even rig the grinder on a mechanical arm to generate the required curve.
I've seen one like that. The mirror was mounted vertical and the arm was horizontal and they made a pass, wound it in a bit and made a second pass.

When I made mine there was a lot of flexure, I needed 2 pieces of wood, one either side. If the arm was horizontal it would have to be braced to stop the arm sagging.
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  #38  
Old 21-07-2018, 04:43 PM
sharpiel (Les)
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Quote:
Originally Posted by Wavytone View Post
Ha... I was going say hang angle grinder at the end of a pendulum length = desired radius and let it swing !

Only snag is angle grinders donít play nice with water
I think the grinder would need to be solidly held. They do tend to kick and have a life of their own when given "freedom".

Lots of food for thought anyway!
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  #39  
Old 22-07-2018, 09:20 AM
Billyboy78 (Bill)
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Originally Posted by sharpiel View Post
I think the grinder would need to be solidly held. !
Yes, on the next one that's what I would like to do. I've stopped heavy grinding 1.5mm short of the total sag for that reason. I liked the idea of that plywood one posted earlier, that is doable.
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  #40  
Old 23-07-2018, 12:34 PM
Billyboy78 (Bill)
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Excuse me if I appear to skite, but I just gave the mirror a tickle with #220 then took her out into the sun to measure the FL, 1700mm, just shy of 1785 target. I've still got .5 mm to go on the sag and the sphere is not quite out to the edge. Gently gently we go with #80 grit.
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