At last I am ready to polish the mirrors. First job is make the lap.
I used plaster that took over a week to thoroughly dry then soaked it in tile grout sealant to prevent it from adsorbing water from the polishing.
Used some firm plastic sheet to make a dam around the tool and poured a hot mix of pine resin and caster oil in.
p2-poured.jpg
Just before it was poured I tested the consistency with a 30 second thumbnail press to see it it raised a small lip, it did so it should be soft enough to conform to the mirror (He hopes)
p1-thumbnail.jpg
then cut the facets and randomly score the lap then brush with a stainless wire brush to remove all particles.
p6-scored.jpg
coat the mirror in a very thick cerium oxide and water polishing compound layer
Lay the lap on top with a little weight and leave to cold press and hopefully conform to the mirror
p8-pressing.jpg
Panic stations arise when you cant separate the mirror and lap.
I will find out tomorrow.
Totally fantastic! You are doing things way beyond my pay grade!
Well, that was a bit of a fiasco.
1, the resin was too hard and didnt conform to the mirror surface after 12 hours.
2, the interface between the resin and the plaster tool started weeping and dribbling down the side.
Making a new ply tool base. I wont seal it with anything as I think (but dont know) that the sealant on the plaster may have still had sufficient volatiles left to soften the resin at the bond.
Ive melted it off the tool and will re-pour with more caster oil in the hope I can get it soft enough to conform.
Fingers crossed.
In the mean time I'm reading up on a Bath Interferometer http://gr5.org/bath/ and may make one, I only need 1 lens as I have all the other items on hand.
More work.
The rabbit hole is open.
I remade the lap with softer compound and all seems well so far.
Polishing has started, 2 hours and a reflection is seen.
I'm using a high grade cerium oxide.
I feel it is a little shinier at the edge that the centre but I haven't measured it yet.
2-hours.jpg
the sharpie marks are on the back of the glass
I'm still finding all this optics work fascinating. Keep posting about it, Charles.
To be honest the entire project has been amazing to me, and it has kept my interest for quite some time now. Thank you for continuing to post about it, Charles!
Thanks Adasha and Terry.
Now at 4 hours polishing and the reflection is much sharper.
4-hours.jpg
But there are now a few scratches showing up.
I'm really hoping they are left over from the grinding, they are very fine and dont show up in the photo.
If the polishing lap has 1 teenie weanie bit of grinding grit I will have to scrap it.
My fingers are crossed they will polish out and not get worse.
How many hours of polishing do you anticipate for this project?
Its difficult to say Terry, it usually works out to around one to two hours per inch of dia.
I'm hoping it will be under 24 hours per mirror but once I have the surface as smooth as I can measure I will switch to black rouge to get even smoother. (I hope)
I believe Carl Zambuto who is considered to be the best mirror maker takes even longer that that.
I find a 2 hour session of polishing at a time to be around my limit at present, my mind starts to wander or I want to do something else.
Some of the scratches at 10x magnification, they look very severe.
scratches.jpg
In the grand scheme of things they actually dont cause much of a problem with the finished mirror but personally they hurt like hell because I know they are there.
Theres three possible causes and remedies
1, left over from fine grinding, I stopped at 1200 grit, maybe I should have gone to 1600 or finer.
2, contaminated polishing lap, ditch it and pour another one.
3, contaminated polishing compound, mix a litre with de-minerilised water, allow to stand, decant the top and use that.
I think I will follow 2 and 3 and see what happens.
I did a quick test for a check, tester in the foreground, mirror on the test stand in the background, 100 line Ronchi screen.
test.jpg
It was virtually impossible to capture the image with the phone camera.
ronchi-1b.jpg
Still not polished out, the lines should be straight and sharper.
Some zones and possible a bit of turned edge.
I dont know what the bright streak just off centre is.
Quote from: savarin on Apr 25, 2024, 11:35 PMIn the mean time I'm reading up on a Bath Interferometer http://gr5.org/bath/ and may make one, I only need 1 lens as I have all the other items on hand.
Hey Savarin,
Not sure if this will work like a Bath Interferometer, but would be much easier and cheaper to make.
Here's the link to the "Applied Science" video about self-mixing laser diode: https://www.youtube.com/watch?v=MUdro-6u2Zg (https://www.youtube.com/watch?v=MUdro-6u2Zg)
I ran into this video a few years back and thought "That would make a cool and very accurate DRO".
Turns out I may well have been correct since I recently saw RLS makes and sells them.
According to the video, it's capable of measuring sub-micron distances.
Unfortunately I dont believe so but my knowledge is not sufficient to work it out.
The standard Bath is easy and simple to build and use.
Quick update.
I havnt started the Bath Interferometer yet.
Got the majority of the scratches polished out but a couple of obstinate ones were left so went back to 1000 grit.
That was going very well when the lap stuck and flung the mirror off the machine.
DONT PANIC
I actually caught it just before it hit the ground I dont know how but I did PHEW!
But I didnt catch the grinding lap it caused a very deep scratch about 30mm long and smashed so I had to make a new one.
Back to grinding at 800 to remove that scratch then 1000 grit then 1200 followed by 1600.
Then polishing again.
I wonder when I will be able to start on the second mirror.
OUCH!
:smiley_thebest: :smiley_flabbergasted: :smiley_sealed: :smiley_blackeye: The patience of Jobe!
Good news on the regrind. The new tool conformed in 30 seconds.
I plastered the mirror and tool with a sharpie pen,
before-regrind.jpg
800 grit small wag and overhang and this was the wear after 30 seconds only.
The grey smears are the grit slurry as I only wiped near the gouge.
after-30-seconds2.jpg
after-30-seconds.jpg
After 35 mins I almost removed the scratch from this (quite deep)
deep-scratch.jpg
To this
deep-scratch2.jpg
Removing it completely has taken 2 hours so far to get to a teeny pit thats almost invisible but does show up with a blue laser shone in from the edge.
blue-laser.jpg
The red laser didnt show anything.
So far there appears to be no change in focal ratio.
You're a wizard, Charles. Amazing stuff. I've never worked with glass before (successfully) so this is incredible to me.
Hell, there's even lasers involved!
This has been a remarkably long term project. I can't wait to see the final result.
Thanks Shawn but if I was a real wizard it would have been completed years ago, still I am getting there.The grinding and polishing is very tedious.
I can only imagine...
I'm still here for the ride. Still can't wait to see what these can really do. I certainly hope you figure out a way to attach a camera to them.
It's taking longer than it should because he is making and eating to many pavlova pies :smiley_crocodile:
He's taking Drama classes at the same tine He's at the part where they keep you guessing.
"Billy G"
Eating too many schnitzel maybe.
Oh, wait! That would be AUSTRIA, not AUSTRALIA!!
:headbang:
Enjoying the build, Charles!! Keep 'em coming!
All gouges and scratches removed, focal length has reduced from desired 72" to actual 70" but as long as I get the second mirror down to that theres no problem.
Did some major milling today. The base for the polishing lap was around 8mm higher on one side than the other so I had to mill it flat.
No milling machine, wont fit on the lathe so I milled it on the drill press.
This side must be milled down to the pencil line.
plaster-lap-raw.jpg
As its only plaster I could hold it by hand and move it around till I got it evenly flat.
plaster-lap.jpg
Now to pour the polishing compound on to it when it totally dry.
4 hours in the oven at 90'C didnt dry it fully.