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First mirror, 8" F/3.5-ish

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#1 chantepierre

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Posted 05 April 2020 - 09:12 AM

Hello,

 

I'm opening this post to ask for advice regarding my first mirror : it's a 8" Schott blank bought with abrasives, polishing compounds & pitch to a French artisan before the lockdown started.

I went through rough grinding without too much trouble and reached target sagitta last week, but will certainly need advice & criticism for the next steps.

 

Today I made a contact test at grit 230 : note that the thing looking like a chip is glass dust suspension after a quick clean that dried, and the white speckle on the second image is something that went off.

 

Vx3vm8I.jpg

 

It seems to be going okay-ish, but I'd rather have feedback on the way I'm doing it now that errors would be harsh : here's a video of 2 minutes of grinding.

 

https://www.youtube....h?v=ljYI88JGPE0

 

Thanks to anyone giving time for this, and anyone willing to help me get through polishing & figuring later on.

clear skies,

chantepierre.


Edited by chantepierre, 05 April 2020 - 10:10 AM.

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#2 rachnoman

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Posted 05 April 2020 - 10:16 AM

Hello,

 

Looks like you are in the fine grinding stages. As the compound gets finer like 400 grit, it is best to lighten up on pressure. Also the stroke should be a little shorter. The reason for this is so you don't roll the edge. This helps create a nice sphere before polishing.

 

Also, i don't recommend making such a fast mirror as a first attempt. It requires a bit of experience making slower mirrors first before you can go faster.

 

So far looks good.....

 

Dave.



#3 Pinbout

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Posted 05 April 2020 - 10:18 AM

 

Also the stroke should be a little shorter.

i thought his stroke was fine..

 

alternate tot, mot.



#4 chantepierre

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Posted 05 April 2020 - 10:51 AM

Rachnoman, Pinbout, thank you for your remarks.

I'll continue with less pressure and slightly shorter strokes.

 

Regarding figuring, I know it will be very hard. What I don't know is quantitatively how hard, since it's a first try, but with lockdown I have all the time in the world to try and do an acceptable mirror. What will be really lacking is the ability to get help in the physical world, so I'll try to take precise notes, pictures and videos when appropriate to give meaningful information to get advice.

 

If it ends up being too hard, I'll get professional help IRL when things are settled down a bit. Otherwise I'll do my best to be precise and attentive.


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#5 Pinbout

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Posted 05 April 2020 - 11:04 AM

 

Regarding figuring, I know it will be very hard.

it doesn't have to be...

 

the hard part is making sure your testing is accurate.

 

figuring is what it is... 

 

knowing when to stop at an accurate measurement.... that's a different story


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#6 dan_h

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Posted 05 April 2020 - 12:06 PM

Looks like you are making good progress. I magnified your picture to the extreme to see if I could detect a bevel on the glass.  Couldn't see one. I assume it's there just not showing from the angle the pics show.  You don't need a large bevel but you do need one to prevent nasty chips if you should bang the edge. 

 

dan



#7 Augustus

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Posted 05 April 2020 - 12:50 PM

Looks good to me!

My first mirror was just a hair above f/4 and it turned out pretty good. f/3.5 is definitely doable.



#8 chantepierre

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Posted 07 April 2020 - 11:27 AM

Some progress today : I moved to grit 400 in a water solution.

After five 8-minute wets, a new contact test shows that the center is a bit too deep, but I can't post a picture because it's too faint to be seen by my phone camera.

So I made an exxagerated simulation : 1 minute sharpie test

 

LcjH71g.jpg

 

Should I work mainly TOT for the next wets ? I stopped applying pressure other than the needed force to drive the disk.

Thanks, and clear skies

chantepierre


Edited by chantepierre, 07 April 2020 - 11:28 AM.

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#9 Pinbout

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Posted 07 April 2020 - 02:17 PM

 

hould I work mainly TOT for the next wets ? I

ToT does the edge

Mot does the middle



#10 davidc135

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Posted 07 April 2020 - 02:33 PM

I'd slow down compared to in the video and the centre contact will come. Tool is a little smaller so it could go on top. Not sure if it matters.  David


Edited by davidc135, 07 April 2020 - 02:35 PM.


#11 chantepierre

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Posted 11 April 2020 - 01:40 PM

Hello everyone, and thanks for your previous advice.

Contact has been solved since then from what I could see/feel/check with sharpie grids.
I finished with three hours at grit 600 alternating MOT/TOT.

 

Now is the time to prepare the pitch lap for polishing, well... I did not imagine how peculiar as a material pitch is.

We made a paper mold to make something akin to Texereau's advice, and I managed to save just enough squares to fill the surface of my varnished plaster lap.

 

Before heat-sticking the squares to the tool, and given that there's a 5mm difference in height between the smallest (9mm) and the tallest (14mm) : should I melt them again and try a direct pour ?

 

Here are pictures of the mold and of the current set of pitch "tiles". They're not stuck to the tool yet. I still have half of the pitch in its original container, only half has been poured to form squares.

 

EDIT : 5mm in height difference, after taking advantage of the 3.6mm sagitta, is brought back to 2mm P/V. So I stuck the tiles with a candle flame as shown by Texereau, heating longer for the bigger ones. That flattened them a bit more. I'll try a warm press overnight.

EDIT, #2 : Well, a warm mirror, silk paper, the tool and 40 kgs of books brought the tool to full contact in less than one hour. I’m impressed by the pitch. I’ll have to trim the channels tomorrow and polishing work will be able to begin.

 

Thank you again,

chantepierre


Edited by chantepierre, 11 April 2020 - 05:04 PM.

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#12 chantepierre

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Posted 14 April 2020 - 07:26 AM

Hello,

 

The "making a polisher" part has been sorted out, I'm about 5 hours in on the 10 I plan to conduct.

That raises a first question, namely : How do I know when those 10 hours were put in that the mirror's surface quality is good ?

 

Thank you,

chantepierre

 

P.S : I played with foucault (and with the mirror) to see how it works, not deducing anything at this moment though.

It's simply amazing, lots of you here must be used to this, but :

 

This is the small rectangular holes in my rolling curtains, one of those becoming a flat pinhole-like objective and forming the image of a building outside :

 

a5w3r3i.png

 

And this is the heat of my hand passing just under the air between camera and mirror :

 

MXnt86m.png

 

To me it's absolutely fantastic and enjoyable !

 

Do not take into account the black mark at 8 o'clock and the chip-like structure that expands from it : it's water residue.

The black mark at 5 o'clock is textile on which the mirror was resting on.

 

Clear skies !


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#13 ccaissie

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Posted 14 April 2020 - 12:30 PM

Having a good time, I see.  Yes it's fascinating.

 

Surprised you are going to polishing after 600 grit...nothing finer?  ok....will take longer.

 

Pitch should be same thickness edge to center.    If it varies....like making a lap on a flat tool and using it on a concave mirror, then it acts differently center to edge.  shouldn't matter much for gross polishing, but for figuring, it adds one more variable.

 

When you're fully polished out, remake the lap, I think.

 

oh, you'll want to slow down as you get well into polishing.  That fast stroking might turn the edge that you've so carefully sculpted.   Watch where the polish shows up first....should start in the center if MOT.


Edited by ccaissie, 14 April 2020 - 12:42 PM.


#14 chantepierre

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Posted 14 April 2020 - 01:06 PM

ccaissie, thank you for your advice.

 

Yes, my kit went from C600 to Cerium, but I have Opaline to add to the Cerium for the second half of the polishing stage and the figuring.

I managed to roughly even the thickness by pressing with 30kgs for two hours, that left me with a solid layer of pitch and no channels.

So I took a metal file and heated it with a blowtorch to make said channels. But since it still varies a bit, I'll follow your advice and pour a new lap when I'm polished out.

 

Speed-wise, surface tension makes it impossible to go that fast and Texereau says to "round" the movements a bit, so I slowed down.
Alternating 1 hour MOT / 1 hour TOT.


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#15 ccaissie

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Posted 14 April 2020 - 06:43 PM

ccaissie, thank you for your advice.

 

Yes, my kit went from C600 to Cerium, but I have Opaline to add to the Cerium for the second half of the polishing stage and the figuring.

I managed to roughly even the thickness by pressing with 30kgs for two hours, that left me with a solid layer of pitch and no channels.

So I took a metal file and heated it with a blowtorch to make said channels. But since it still varies a bit, I'll follow your advice and pour a new lap when I'm polished out.

 

Speed-wise, surface tension makes it impossible to go that fast and Texereau says to "round" the movements a bit, so I slowed down.
Alternating 1 hour MOT / 1 hour TOT.

All good.  I made pitch strips by putting down foil, and small wood strips...lots of dish detergent as a separation agent.  Cast them 1/4" thick gauged by the thickness of the strips.  For some mirrors I just lay the strips down...one-way channels, others I cut the strips into squares.  Same idea as what you are doing.  As long as you know where you're going, there's many avenues. 



#16 chantepierre

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Posted 15 April 2020 - 04:34 AM

More polish in, I re-read Mel Bartel's guide and decided to follow it. Switching to only MOT with short COC strokes.

The lap has been greatly improved by aggressive trimming of the channels.

 

After a quick knife-edge view before/after, I see that there are two competing spheres, the one starting at center quickly expanding towards the edge.

I'll keep working like that to try to have a good sphere after the next hours.

Laser test gets better after each hour.

 

There must have been a problem with my Cerox solution though : I added a small drip of dish soap and wets lasted longer, but sometimes a thin layer of pitch went off the lap and stuck to the mirror, and the cerox solution left stains really hard to remove under water. As if a friction phenomenon put pitch on the mirror sometimes.

 

Since the only change was the addition of dish soap, I'll remove it for the next hours.


Edited by chantepierre, 15 April 2020 - 04:36 AM.


#17 chantepierre

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Posted 15 April 2020 - 10:13 AM

Well, I'm a bit confused...

I still plan to follow Mel's instructions because I'm not able to deduce anything anyway.

 

I just did 30mn press MOT, 1 hour MOT with 1/3D COC, rotate the mirror 22,5° counterclockwise every 8 strokes, rotate the tool clockwise a bit more than 30° every full turn of the mirror.

Laser is almost invisible now on the surface.

 

Here's a picture of the mirror and another of the lap in its current state.

 

wKxj7J2.jpg

 

There's something that looks very regular on the above picture... :/

 

bRiW1Ya.jpg

 

Lap and tool are the same size, plaster base molded on the mirror at the end of fine grinding, pitch is Gugolz 64.

It's 21°C in the room I polish in, with no air currents from outside.

 

I'm open to hearing criticism !

chantepierre

 

Edit : Aaron gave me advice, and it seems that my precise measurements of angles in the last hour introduced this.


Edited by chantepierre, 15 April 2020 - 10:21 AM.

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#18 Aaron_tragle

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Posted 15 April 2020 - 10:29 AM

I am on my first mirrors at the moment, so take this with a grain of salt. The first thing that pops out to me is the measurement of exact angles for mirror rotations and an exact number of strokes before rotation. It seemed backwards to me at first, but randomness is key. Instead of rotating every 8 strokes, try a range of maybe 6 to 12 strokes. Even better if you listen to music while polishing and know the song just rotate on a beat in the song. Instead of perfect rotations just sort of rotate it a bit in the hand, but make sure to vary it quite a bit. I will sometimes slightly shuffle it in my hand and other times I will fully rotate it using both hands with every possible rotation in between being a possibility. 

 

Next thing is that pitch lap looks too orderly to me, I'd take it outside with a knife and just hack at it for a good minute or two creating tons of little cuts in random directions. If you have a brass brush, brush the lap with that to get micro facets and that lap would be perfect. Good luck to you!

 

Aaron



#19 dogbiscuit

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Posted 15 April 2020 - 12:14 PM

Made a movie of grinding.

Make one of polishing.

 

There is much I would say about the lap and the ripple, but it is not so important right now. the lap will work for polishing.

 

Ok... just a little about the ripple...

It has been my experience that relatively wide channels and resulting smaller facets tend to make zones and ripple.  Also a new lap sometimes will do that.

 

The center of the tool is in a channel, in fact it is at the crossroads of two channel.  Best if center of the tool is on a facet but not centered on that facet. 

 

Your turning and stroking might have something to do with the ripple but I'm pretty sure it is the wide channels and maybe a little bit because it's new.  New doesn't last long so don't worry about that. 

 

There is no need to intentionally add randomness. Occasional intentional variation can be useful.

 

Make a movie of your polishing and we can offer opinions.

 

 

 

 



#20 Pinbout

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Posted 15 April 2020 - 12:44 PM

Well, I'm a bit confused...

I still plan to follow Mel's instructions because I'm not able to deduce anything anyway.

 

I just did 30mn press MOT, 1 hour MOT with 1/3D COC, rotate the mirror 22,5° counterclockwise every 8 strokes, rotate the tool clockwise a bit more than 30° every full turn of the mirror.

Laser is almost invisible now on the surface.

 

Here's a picture of the mirror and another of the lap in its current state.

 

wKxj7J2.jpg

 

There's something that looks very regular on the above picture... :/

 

bRiW1Ya.jpg

 

Lap and tool are the same size, plaster base molded on the mirror at the end of fine grinding, pitch is Gugolz 64.

It's 21°C in the room I polish in, with no air currents from outside.

 

I'm open to hearing criticism !

chantepierre

 

Edit : Aaron gave me advice, and it seems that my precise measurements of angles in the last hour introduced this.

I always warm press by soaking the pitch lap and mirror in hot water, then press with a mesh... let booth cool then have at it.

 

doing figure 8's will get rid of your rings and ripples.

 

https://www.youtube....salR23dE&t=156s

 

 

also while MoT watch the cerox pull out of the channels

 

https://www.youtube....W8rBwA&index=96



#21 davidc135

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Posted 15 April 2020 - 01:29 PM

The facets of your lap are too well centred. That and a CoC stroke could explain the zones. Too much speed could explain the dimples. But if you are still polishing out the last pits there's little harm. For figuring there should be a new lap. Have the exact centre of the lap within a corner of a facet.  David


Edited by davidc135, 15 April 2020 - 01:32 PM.

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#22 chantepierre

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Posted 16 April 2020 - 08:29 AM

dogbiscuit, thank you.

Here's a video of polishing after a press : https://www.youtube....h?v=CXuxThiEyxI

By watching it, I see my tool swings maybe too much on those foam pads.

 

pinbout, I followed your advice. Yesterday was the return of the mosquitoes and thus installation of mosquito nets. This came perfect to microfacet the lap.

 

davidc135, I slowed down again, but will make a new lap following your advice (decentering) and others (narrower channels) for figuring.

 

Here's the current state of the lap :

 

5HySo8r.jpg

 

Here's a KE after 20 minutes polishing as in the video (couldn't manage more at the room was getting hotter and hotter).

Stains are water. I did not manage to see the dimples as contrasted as yesterday, which would indicate progress ?

Red laser is now mostly invisible from center to 50%.

 

Mu2T92W.jpg

 

Have a nice day,

chantepierre


Edited by chantepierre, 16 April 2020 - 08:32 AM.


#23 Pinbout

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Posted 16 April 2020 - 09:07 AM

Make the movements more fluid. Too jerky. Don’t be timid.

 

most likely that’s causing your lumps.

 

do some figure 8’s to loosen up your stroke and smooth the mirror 

 

didn’t see a mark on the back of your mirror that looks like a starting mark, just some scribbling on it.

 

 

and those 2 huge chips are causing your rings - use a soldering iron and fix them if your not going to make a new lap.


Edited by Pinbout, 16 April 2020 - 09:10 AM.


#24 dogbiscuit

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Posted 16 April 2020 - 10:15 AM

T

 

dogbiscuit, thank you.

Here's a video of polishing after a press : https://www.youtube....h?v=CXuxThiEyxI

By watching it, I see my tool swings maybe too much on those foam pads.

 

The pad is ok.  A little rocking with stroke is not a problem.

I recommend cutting the pad in a circle exactly the diameter of the mirror/tool so it fits between the cleats without bunching at the cleat.  Or cut the pad ~1/2" larger than the mirror with cutouts at the cleats so it fits without bunching around the cleats.

 

Over time the facets will spread out and narrow the channels.  That will reduce the ripple (lap pattern in the mirror surface). When you trim the channels just trim for depth but let them get narrower.

 

The center over center (COC) stroke also enhances the formation of ripple.

Using W strokes can reduce the ripple.

Typical polishing strokes are ~1/3 D COC, but it is ok to add some side to side motion to make a W pattern.  Width of the W can be between 1/4 D and 1/3 D and occasionally 1/2 D. Same stroke length and width of W for full turns of the mirror.  About 7 strokes across the W is good, and it is ok to turn the mirror at the sides of the W and stroke another W back the other way.

 

So that gives you 4 different stroke lengths and widths you can do COC 1/4  1/3  1/2 and its ok if stroke length is different than width of the W.  That makes 12 combinations of length and width.

 

The variety of different size patterns will smooth out the ripple.  The ripple is not really important while polishing, but it will be good for you to practice doing different size patterns each done same number of strokes, same stroke length, same width, for a full turn (or two) of the mirror and switch to another size pattern.  In the beginning it takes a little concentration to avoid drifting away from the intended pattern as you work.  In time it becomes habit.  Figuring involves using different size and shaped patterns to change the mirrors shape.  It will make figuring easier if you have developed the habit of maintaining an intended stroke.

 

I think you are turning the mirror in your hands ok.  I'll look more closely at the video to be sure.

 

I don't know your workspace limitations, but there is a definite advantage if you can stand at and walk around the work table while stroking.  This allows continuous stroking without need to stop and turn the tool, an makes it easier to get into a rhythm of stroking and stepping around the work.



#25 chantepierre

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Posted 16 April 2020 - 11:14 AM

Thank you pinbout and dogbiscuit for your answers.
I redid the lap, which can be seen here :

https://i.ibb.co/QJW...-F4-C43-E05.png

Dogbiscuit, I’ll study your answer in a detailed fashion tonight and apply those behaviors. Thanks a lot for that much insight.


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