First mirror grind advice

[barkis]

32 minutes ago, Chris Lock said:

Hi Ron, the smoothing grades you have, would they potentially fit in well as the next step after the 600grit 9 micron aluminium oxide Damian is sending me? and is the 3rd smoothing the smallest micron hence the last to use? and finally just to check, these are these the grades to use with the pitch lap tool? Would anything finer be needed?

Sorry about pestering you with 4 questions in one go, Ron, but if the above checks out and you're still willing to let me have them? I may just need 80-100 grit and stuff for tooling to have a full kit to get started

Hi Chris.
I am busy preparing the grades for transporting now.   I have had to go up Town with the Good Lady this afternoon, domestic chores you know,
no way to escape them I'm afraid.

I will be sending 400 Carbo. my favourite smoother, I love the satin finish on that one. Of course that is not fine enough to start the polishing stage.
There is some 400 Alux too, which might give it a more sheeny look, you can use that or not, depends on how you see the surface  under an Eyepiece.
Always good to check all over for any pits too large for 400 to remove. An unlikely scenario that though.
The earlier smoothing powders are marked in Numbers,  (225 is the  1st. Smoothing)   (175 is the 2nd Smoothing)    ( 125 is the 3rd. Smoothing.)
That should take you down to where you can start the polishing stage.  The containers are clearly marked with the order of use, so you can't go wrong.
You should always thoroughly clean up, yourself, and the work area right after completing each grinding stage. I'm sure you know that, but a gentle reminder does no harm.
I can send you some 80 Grit, which I use in my Sand Blaster Cabinet. It isn't a Carborundum grit, but I used it to hog out a  6" f4 blank years ago. It breaks down
quicker than Carbo. but it should get you to where you need to be. Use it sparingly so it doesn't sludge up too much. Use Chordal Strokes initially, that will concentrate the 
grinding more in the centre of the mirror, As you near the depth, widen the strokes to 1/3rd W strokes. You could move down to 125 Grit to get you nearer the target, move down to 225, then 400. You can start fine grinding then.        
   
DON'T FORGET THE CLEANUPS AFTER EACH GRIT SIZE PRIOR TO CHANGEOVER.  .

What size and f/ratio have you decided on Chris. ? let me know please.  
Would you like me to include NE. Howard's book in the parcel too. It is not as In depth as Texeraux's book, but very capable of doing the job for you.
 

 

[Astrobits]

Before Chris gets totally confused perhaps I should make the following comments.

The different manufacturers of abrasive powders have different ways to describe them so it is easy to get confused as to which is finer/coarser. In some older books you might find that the some of the smoothing powders are graded as M303½, M304 and M305!! what a pain that was to relate them to other products.  Silicon carbide grit is usually graded in mesh size so the finer mesh has more holes per inch and thus smaller grit size so 320 is finer than 220. Aloxite is normally graded in terms of 0.1 micron so the grits labelled 225, 175, 125 are really 22.5, 17.5, 12.5 micron. I have usually polished after 12 micron Aloxite as going finer often results in scratching without the expected reduction in polishing time.

You can look at Newport Glass site ( USA not Wales ) which has a list of the quantities and sizes of powders that you need: http://www.newportglass.com/index.htm

Hope this helps

Nigel

 

[mapstar]

Hi Chris 

Thanks for being prompt 

This is how I'd work through the grades

80 grit

180-220 grit

400 grit 

600 grit alu oxide 

1200 grit cerium

You can add more steps if you like. There is plenty of cerium on the bay just make sure it is optical quality. 

Glad John was able to help but I do think it's his middle name 

[barkis]

Chris, those three smoothing grades were used by the Science Master at Trinity School in the City.
There was an Active Mirror making Group, who were active for a long period, and Although my only link with the School,
was with our Astro. Society Chairman, who was a Physics master at the School. It was he who supplied me with this material, together with 3 spare 6" blanks.
All that was left after the Mirror Making Group ended their projects.

I have prepared those three, together with a quantity of Aluminium Oxide 400, and a larger quantity of 400 Carbo.
Waiting to be boxed and dispatched.  
I've no doubt about these products, but If you feel you don't wish to risk them, then that's OK.
I can understand your head spinning a bit  with all the Posts you have had to digest., and some trepidation might be 
starting to creep in.
I'll let you decide yourself before I parcel these up.

[Chris]

Hi guys, I'm back from work and heavy eyed so will check out replies tomorrow

[Chris]

19 hours ago, barkis said:

Hi Chris.
I am busy preparing the grades for transporting now.   I have had to go up Town with the Good Lady this afternoon, domestic chores you know,
no way to escape them I'm afraid.

I will be sending 400 Carbo. my favourite smoother, I love the satin finish on that one. Of course that is not fine enough to start the polishing stage.
There is some 400 Alux too, which might give it a more sheeny look, you can use that or not, depends on how you see the surface  under an Eyepiece.
Always good to check all over for any pits too large for 400 to remove. An unlikely scenario that though.
The earlier smoothing powders are marked in Numbers,  (225 is the  1st. Smoothing)   (175 is the 2nd Smoothing)    ( 125 is the 3rd. Smoothing.)
That should take you down to where you can start the polishing stage.  The containers are clearly marked with the order of use, so you can't go wrong.
You should always thoroughly clean up, yourself, and the work area right after completing each grinding stage. I'm sure you know that, but a gentle reminder does no harm.
I can send you some 80 Grit, which I use in my Sand Blaster Cabinet. It isn't a Carborundum grit, but I used it to hog out a  6" f4 blank years ago. It breaks down
quicker than Carbo. but it should get you to where you need to be. Use it sparingly so it doesn't sludge up too much. Use Chordal Strokes initially, that will concentrate the 
grinding more in the centre of the mirror, As you near the depth, widen the strokes to 1/3rd W strokes. You could move down to 125 Grit to get you nearer the target, move down to 225, then 400. You can start fine grinding then.        
   
DON'T FORGET THE CLEANUPS AFTER EACH GRIT SIZE PRIOR TO CHANGEOVER.  .

What size and f/ratio have you decided on Chris. ? let me know please.  
Would you like me to include NE. Howard's book in the parcel too. It is not as In depth as Texeraux's book, but very capable of doing the job for you.
 

 

Thanks Ron, and no worries, I'm well versed in family chores/duties. Thanks for sorting all that out for me it's very much appreciated  I'm going to have to call it the Chris Lock mirror sponsored by barkis, mapstar and John Nichol!

I understand a bit more where your grits fit to the scheme of things, cheers for clearing that up, Ron  I'll be VERY careful not to cross contaminate grit sizes like you say - I'll completely clean everything including my clothes and myself. I have a low power illuminated binocular microscope which should good to check for pits and fine scratches, plus a 25mm Plossl reversed.

Damian is very kindly including a John Nichol DVD and ebook with his parcel, so I might be ok on the book front, great of you to offer though.

Yes please for the 80 grit, I'm sure that will be fine not being carbo if you managed an f/4 with it and I'm only going down to f/6 f/7. That's very kind of you again

with a 223mm 8.75" mirror I'll be aiming for f/6.75 in order to give me a focal length of around 1500mm. This is so it will give a good eyepiece position close to zenith, plus the slow f/ratio will be kind on my basic eyepieces and hopefully an easier challenge for a first time mirror maker.

I'll PM you regarding postage money Ron

I'm going to take the girls to the hardware shop now and try and pick up some bits for making a mirror blank/grinding tool 'holder/jig' in order to aid in keeping everything still whilst I'm grinding and turning the tool/blank. I'll pick up a carpet square to rest the mirror on too. I'll probably clamp the whole contraption to a small table....ooh and pick up a spray bottle. I'll leave the tooling supplies for now as I'm still undecided.

 

I big thank you to everyone that's helped so far I'll get to the other replies when I'm back from the shops, the help is very much appreciated

 

[Chris]

On 1/18/2017 at 15:50, Astrobits said:

Before Chris gets totally confused perhaps I should make the following comments.

The different manufacturers of abrasive powders have different ways to describe them so it is easy to get confused as to which is finer/coarser. In some older books you might find that the some of the smoothing powders are graded as M303½, M304 and M305!! what a pain that was to relate them to other products.  Silicon carbide grit is usually graded in mesh size so the finer mesh has more holes per inch and thus smaller grit size so 320 is finer than 220. Aloxite is normally graded in terms of 0.1 micron so the grits labelled 225, 175, 125 are really 22.5, 17.5, 12.5 micron. I have usually polished after 12 micron Aloxite as going finer often results in scratching without the expected reduction in polishing time.

You can look at Newport Glass site ( USA not Wales ) which has a list of the quantities and sizes of powders that you need: http://www.newportglass.com/index.htm

Hope this helps

Nigel

 

Hi NIgel, great link  I'll have to spend a bit more time checking that site out, but for now I've found this useful chart in a sub section:

http://www.newportglass.com/grit.htm

Useful to be able to translate grit size to microns!  I kind of wish it was all in microns to begin with as my brain seems to prefer that lol

With Ron sending me the 225, 175, and 125 among other things, I appreciate you translating that into 22.5, 17.5,and 12.5 microns as it makes me realise that the 9 micron stuff Damian is sending me comes after them! 

When I've got everything in front of me, I think it would be sensible for me to write a table of all I have converted to microns, then write the micron size on the tubs/packets. I can then have a physical table in front of me which will help reduce any confusion. I feel like I'm getting the feel for it though!

Ron is also going to send me some Rouge and sand blasters 80 grit, so I think I might be close to sorted in the grit and polish department

[Chris]

23 hours ago, mapstar said:

Hi Chris 

Thanks for being prompt 

This is how I'd work through the grades

80 grit

180-220 grit

400 grit 

600 grit alu oxide 

1200 grit cerium

You can add more steps if you like. There is plenty of cerium on the bay just make sure it is optical quality. 

Glad John was able to help but I do think it's his middle name 

Thanks Damian, the Newport chart goes down to 1000 grit which is 7 micron, so I'm guessing 1200grit Cerium must be 5 micron or less. Ron is sending me Rouge which probably replaces or is even finer than the 1200 grit, so I'm going to try the rouge at the polishing stage. He's warned me about the staining power of Rouge so I can keep things in check with SWMBO 

I think it's fantastic that someone so renowned in mirror making as John is also so helpful, great for those of us starting out

[mapstar]

The cerium I used Chris was  2.5 micron. Unlike rouge it does not stain hands and clothes and is not too hard to acquire. 

[Chris]

20 hours ago, barkis said:

Chris, those three smoothing grades were used by the Science Master at Trinity School in the City.
There was an Active Mirror making Group, who were active for a long period, and Although my only link with the School,
was with our Astro. Society Chairman, who was a Physics master at the School. It was he who supplied me with this material, together with 3 spare 6" blanks.
All that was left after the Mirror Making Group ended their projects.

I have prepared those three, together with a quantity of Aluminium Oxide 400, and a larger quantity of 400 Carbo.
Waiting to be boxed and dispatched.  
I've no doubt about these products, but If you feel you don't wish to risk them, then that's OK.
I can understand your head spinning a bit  with all the Posts you have had to digest., and some trepidation might be 
starting to creep in.
I'll let you decide yourself before I parcel these up.

Definitely happy Ron  it would be a shame for it not to get used. I think trepidation is what's stop me doing this the last 15 years, so I won't let it stop me know

[barkis]

A little Anecdote re David Sinden. On the subject of polishing mirrors, and Rouge for the use of.
David recocounted.

" This stuff is alive, I stood facing the walls tight up to the corner, and gingerly removed the container top, and very carefully removed a small
portion on a spatula. On turning around, I found a large area of the room  was covered red with the Bloody stuff."

A sense of humour, and great stories were part of the embodiment of Dave, a great personality.
I think he was warning against being careless with it, his exaggerated tale was meant to get the message home.

 

[Chris]

47 minutes ago, mapstar said:

The cerium I used Chris was  2.5 micron. Unlike rouge it does not stain hands and clothes and is not too hard to acquire. 

 

25 minutes ago, barkis said:

A little Anecdote re David Sinden. On the subject of polishing mirrors, and Rouge for the use of.
David recocounted.

" This stuff is alive, I stood facing the walls tight up to the corner, and gingerly removed the container top, and very carefully removed a small
portion on a spatula. On turning around, I found a large area of the room  was covered red with the Bloody stuff."

A sense of humour, and great stories were part of the embodiment of Dave, a great personality.
I think he was warning against being careless with it, his exaggerated tale was meant to get the message home.

 

Well, as it turns out Ron wasn't able to fit the Rouge in the parcel which is no biggy, (just grateful for all the other things!). Add to this the wonderful comedy words of David Sinden above, and Damian's words of logic...... I might just pick up some Cerium when I get to that stage 

[barkis]

35 minutes ago, Chris Lock said:

 

Well, as it turns out Ron wasn't able to fit the Rouge in the parcel which is no biggy, (just grateful for all the other things!). Add to this the wonderful comedy words of David Sinden above, and Damian's words of logic...... I might just pick up some Cerium when I get to that stage 

Hold It, I opened up the parcel again, and found a Niche  for the Rouge.
It's in a small bottle Jar with a Glass stopper. More than enough to polish two mirrors. 

This stuff as with Cerium and  the finer grinding powders are usually dispensed onto the
tool/lap  from a squeezie bottle, having been mixed with water.  The consistency creamy, but not too watery.
Don't let either dry out.  As the discs  become very very close together, it is important to keep them moving
relative to each other, or they will stick/ bind, and it can be pig to get the apart. If that happens, seek advice before you
do anything drastic. Never stop and leave then together during the fine grind stages.
During the polishing, the lap and mirror can be left overnight with polishing wet on them, but wrap the whole 
caboodle in wet towels, and they will be fine. It helps to keep the pitch lap and mirror in good contact.
All this Info. will be in the documentation you have anyway.

 

[Chris]

3 hours ago, barkis said:

Hold It, I opened up the parcel again, and found a Niche  for the Rouge.
It's in a small bottle Jar with a Glass stopper. More than enough to polish two mirrors. 

This stuff as with Cerium and  the finer grinding powders are usually dispensed onto the
tool/lap  from a squeezie bottle, having been mixed with water.  The consistency creamy, but not too watery.
Don't let either dry out.  As the discs  become very very close together, it is important to keep them moving
relative to each other, or they will stick/ bind, and it can be pig to get the apart. If that happens, seek advice before you
do anything drastic. Never stop and leave then together during the fine grind stages.
During the polishing, the lap and mirror can be left overnight with polishing wet on them, but wrap the whole 
caboodle in wet towels, and they will be fine. It helps to keep the pitch lap and mirror in good contact.
All this Info. will be in the documentation you have anyway.

 

Top man Ron! Rouge in suspension it is!  As for documentation, there is so much information to absorb for mirror making that the more times I'm told something the better I reckon  It might get my sinapses going! I'm sure I have a pretty bad memory for a 41 year old, so if it turns out in the end that I can make a mirror, I'm pretty sure most people can!  

[mapstar]

58 minutes ago, Chris Lock said:

 I'm sure I have a pretty bad memory for a 41 year old

I have too for 42 or is that 43? Cant remember! 

Grease proof paper over night between the lap and mirror keeps them from sticking but you're well away from that stage. 

[Chris]

7 hours ago, mapstar said:

I have too for 42 or is that 43? Cant remember! 

lol Glad it's not just me, and you did a 22" so I have no excuse! 

I had a dream about mirror grinding last night. One of my twins got me up at 4am and I eventually fell sleep on the sofa with her, but not for long! I started dreaming that a I was grinding a mirror, only the mirror had a pleated edge like a flower which made me wonder, and I was using a massively sub diameter tool and no grinding powder! At this point I realised it must be a dream and woke up.....had a dead arm from the sofa which might be realistic with mirror grinding...especially if I ever do a mirror as big as yours Damian lol  

I was reading your 22" mirror thread again last night, so I blame the dream on that   

[mapstar]

3 hours ago, Chris Lock said:

I was reading your 22" mirror thread again last night, so I blame the dream on that   

Sounds more like a nightmare than a dream and I think I'd had quite a few of those by the time I'd finished 

[SilverAstro]

On 18/01/2017 at 21:45, barkis said:

Chris, those three smoothing grades were used by the Science Master at Trinity School in the City.
There was an Active Mirror making Group,

Ohhhh ! That takes me back, it was called the Creighton School when I was there (well the boys bit was), no Astro Group but the whole school came to a halt when I informed the Chemistry teacher that there was a partial solar eclipse in progress ! It was a good school for the sciences. Good to know that it still has its uses !

Sorry for the interuption Chris, carry on

 

[barkis]

57 minutes ago, SilverAstro said:

Ohhhh ! That takes me back, it was called the Creighton School when I was there (well the boys bit was), no Astro Group but the whole school came to a halt when I informed the Chemistry teacher that there was a partial solar eclipse in progress ! It was a good school for the sciences. Good to know that it still has its uses !

Sorry for the interuption Chris, carry on

 

An old boy from way back,  It's good of you to mention that,  I'm sure Chris.   won't mind.  You turned the Clock back a ways there . 

 

[SilverAstro]

47 minutes ago, barkis said:

An old boy from way back,  It's good of you to mention that,  I'm sure Chris.   won't mind.  You turned the Clock back a ways there . 

Just a little , the words 'cat' & 'bag' come to mind !

 

[Chris]

4 hours ago, SilverAstro said:

Ohhhh ! That takes me back, it was called the Creighton School when I was there (well the boys bit was), no Astro Group but the whole school came to a halt when I informed the Chemistry teacher that there was a partial solar eclipse in progress ! It was a good school for the sciences. Good to know that it still has its uses !

Sorry for the interuption Chris, carry on

 

All good stuff mate, no interruption at all

[Astrobits]

Barkis is right to mention the tendency of mirrors and tools to lock together so this is my explanation of why they do and what to do to avoid it. ( This is all my theory, derived from practical experience, as I have never seen any explanations elsewhere).

When we grind the mirror on top of the solid tool ( locking doesn't happen with a tile tool ) there is a tendency for the mirror to take on a somewhat deeper curve than the tool. When this happens the mirror and tool fit together well when the mirror is off centre, as at the start of a stroke, but as the mirror goes centre over centre the mirror lifts away from the tool giving rise to a gap between the tool and mirror. If you are towards the end of a wet there is usually a thick slurry of finely ground glass and carbide which is a very effective sealant preventing any air getting into the gap and thus a vacuum is formed which causes the two bits of glass to lock together.

There are two bits of information that you get before this happens.

1. If your mirror has a clear base then you can see the abrasive mix and it will have some bubbles of air showing. At the start of your stroke these bubbles will be roughly round but when you are at maximum speed in the middle of your stroke they will elongate. The more they elongate then the better the match of your two surfaces. If they don't elongate by very much then you have mismatched curves. If the back of your mirror is obscured then you will not see this phenomenon.

2. when the two surfaces are mismatched you will feel a slight tug as you pass centre over centre. This is when the mirror is lifting and a differential pressure ( air on the outside and a vacuum on the inside ) is applying a force making it harder to push mirror over tool..

Should you see the air bubbles not elongating and/or that little tug then you are heading for a locked system and you should immediately remove the mirror from the tool and clean down both.

The next step is to correct the curves. You need to decrease the mirror curve and increase the tool curve. Continue with the same grit but place it in a narrow annulus at about the 70% zone. Do not put any in the middle of the tool. Place the mirror on the tool and use a VERY short stroke coc. No more than about 1" stroke. Do this for 2 or 3 wets and then go back to your previous stroke pattern. If the symptoms persist repeat the annulus of abrasive/short stroke treatment.

When the curves match there should be smooth movement of your mirror all the time.

Nigel

 

[Chris]

5 hours ago, Astrobits said:

Barkis is right to mention the tendency of mirrors and tools to lock together so this is my explanation of why they do and what to do to avoid it. ( This is all my theory, derived from practical experience, as I have never seen any explanations elsewhere).

When we grind the mirror on top of the solid tool ( locking doesn't happen with a tile tool ) there is a tendency for the mirror to take on a somewhat deeper curve than the tool. When this happens the mirror and tool fit together well when the mirror is off centre, as at the start of a stroke, but as the mirror goes centre over centre the mirror lifts away from the tool giving rise to a gap between the tool and mirror. If you are towards the end of a wet there is usually a thick slurry of finely ground glass and carbide which is a very effective sealant preventing any air getting into the gap and thus a vacuum is formed which causes the two bits of glass to lock together.

There are two bits of information that you get before this happens.

1. If your mirror has a clear base then you can see the abrasive mix and it will have some bubbles of air showing. At the start of your stroke these bubbles will be roughly round but when you are at maximum speed in the middle of your stroke they will elongate. The more they elongate then the better the match of your two surfaces. If they don't elongate by very much then you have mismatched curves. If the back of your mirror is obscured then you will not see this phenomenon.

2. when the two surfaces are mismatched you will feel a slight tug as you pass centre over centre. This is when the mirror is lifting and a differential pressure ( air on the outside and a vacuum on the inside ) is applying a force making it harder to push mirror over tool..

Should you see the air bubbles not elongating and/or that little tug then you are heading for a locked system and you should immediately remove the mirror from the tool and clean down both.

The next step is to correct the curves. You need to decrease the mirror curve and increase the tool curve. Continue with the same grit but place it in a narrow annulus at about the 70% zone. Do not put any in the middle of the tool. Place the mirror on the tool and use a VERY short stroke coc. No more than about 1" stroke. Do this for 2 or 3 wets and then go back to your previous stroke pattern. If the symptoms persist repeat the annulus of abrasive/short stroke treatment.

When the curves match there should be smooth movement of your mirror all the time.

Nigel

 

Hi Nigel, thanks for taking the time to explain this, I know it's theory but it makes perfect sense to me....I think  

So just to check, from the above am I right in thinking that mirror on top deepens the centre of the mirror, and tool on top deepens the edge, or in other words makes the centre more shallow? I'm not 100% sure this is right but think it might be, in which case would favouring working with the mirror on the bottom help prevent creating a space for a vacuum to develop? and thus help prevent lock up?

Great info on how to get out of a lockup, makes sense to deepen 70% out so there isn't a deep 'drop off' in the centre for a vacuum to form.... if that makes sense? 

EDIT: forgot to mention..the chap from galvoptics mentioned adding a touch of washing up liquid to help prevent lockup, what do you think?

[Chris]

Back from work and look what's arrived! Thanks John!  Quite excited now, what a chunk of glass! 222mm diameter and 38mm thick! It weighs a fair bit, can't imagine what Damian's 22" must weigh! 

Pound coin for size reference, not bling

A quick look over the mirror shows a definite side to work with, one side has bubbles close to the surface and the other side is bubble free as far as I can tell. It may be the glass of wine talking but it's a thing of beauty!

[Guest]

I use N E Howards books as a reference and it is excellent. There are second hand copies available through Abebooks.  There is also a huge amount of information on this website which I refer to all the time -

https://stellafane.org/