First mirror grind advice

[Chris]

I also asked the guy at Galvoptics about their pitch. I mentioned that I read about someone struggling with it being too hard (I read this on Rustysplit's 14" grind thread). He said it's the same pitch they use and they adjust the hardness with Linseed oil depending on the ambient temperature. It's apparently quite sensitive to temp! I think RustySplit used linseed oil in the end, and dropped a drop of hot pitch in water so he could do a nail test. I'm guessing this involves digging your nail into a drop of pitch to see if it leaves a dent.

Anyway, I may get the quote back today. He did call back yesterday evening whilst I was working which confused my wife (forgot to mention I might be getting a call!).

[barkis]

There are pitfalls with Pitch laps Chris, too soft, or  too hard, either can have  detrimental effect
on the mirror, but It's a subject to cover when you are up and running,
If we fill your mind with too much Info. too early, it will just confuse.
How and where do you propose to carry out you work on the mirror?
Shed, Garage, or any other enclosure?

[Chris]

1 hour ago, barkis said:

There are pitfalls with Pitch laps Chris, too soft, or  too hard, either can have  detrimental effect
on the mirror, but It's a subject to cover when you are up and running,

True, I'm probably getting ahead of myself thinking of pitch laps right now, so I'll try and focus more on info to do with hogging out and initial measuring and testing

1 hour ago, barkis said:

How and where do you propose to carry out you work on the mirror?
Shed, Garage, or any other enclosure?

This is a very good question, I have no immediate answer despite giving it some thought.

I don't have a garage but do have a 5x7' shed full of bikes, kids garden toys etc. If it were warmer and stayed lighter later I could put the shed contents under a tarp in the garden and setup the shed specifically for grinding.

Despite the likely mess and noise I'm leaning towards doing it somewhere in the house (I'm subtly trying to get my wife onboard with this idea )

1) Kitchen/diner: Pros - small table and tiled floor so easy to clean. Cons - next to the lounge so noise could be a problem and the family will be wanting to use the kitchen!

2) Office: Pros - a bit more out the way. Cons - this is were my wife does her day job, although she works for the Royal Astronomical Society so it's all astronomy related right! lol Err? she might not see it this way...maybe I shouldn't suggest this 

3) Bedroom (Yep I said it!) Pros - well out the way on the third floor, and it has an on suite for getting water (I know I can't chuck slurry down the drain). Cons - its a bed room.

4) I have a massive loft with lighting but don't fancy carrying buckets of water up and down a ladder.

5) I could clear out my 5x3' obsy shed and use that, it's next to the outdoor tap too. Cons- it's freezing out side! and it's next to my neighbours living room so the noise could be an issue again. Plus I would need to setup lighting and it's not a big space.

The site in which to do this could be a problem Hmmmm?

 

[Chris]

to get around the mess I was planning on placing the glass blank onto a big circle of plastic sheet, so the slurry can be more easily cleaned up. 

[barkis]

23 minutes ago, Chris Lock said:

True, I'm probably getting ahead of myself thinking of pitch laps right now, so I'll try and focus more on info to do with hogging out and initial measuring and testing

This is a very good question, I have no immediate answer despite giving it some thought.

I don't have a garage but do have a 5x7' shed full of bikes, kids garden toys etc. If it were warmer and stayed lighter later I could have stick the shed contents under a tarp in the garden and setup the shed specifically.

Despite the likely mess and noise I'm leaning towards doing it somewhere in the house (I'm subtly trying to get my wife onboard with this idea )

1) Kitchen/diner: Pros - small table and tiled floor so easy to clean. Cons - next to the lounge so noise could be a problem and the family will be wanting to use the kitchen!

2) Office: Pros - a bit more out the way. Cons - this is were my wife does her day job, although she works for the Royal Astronomical Society so it's all astronomy related right! lol Err? she might not see it this way...maybe I shouldn't suggest this 

3) Bedroom (Yep I said it!) Pros - well out the way on the third floor, and it has an on suite for getting water (I know I can't chuck slurry down the drain). Cons - its a bed room.

4) I have a massive loft with lighting but don't fancy carrying buckets of water up and down a ladder.

5) I could clear out my 5x3' obsy shed and use that, it's next to the outdoor tap too. Cons- it's freezing out side! and it's next to my neighbours living room so the noise could be an issue again. Plus I would need to setup lighting and it's not a big space.

The site in which to do this could be a problem Hmmmm?

 

Shed would be the place I reckon Chris.  
I did my first mirror in my Shed, but one careless episode spurred me into being a lot more careful
than I had been. Whilst melting pitch on a portable Hot Plate,  I left it unattended, and went indoors for a reason I don't
recall. After a while, the wife who fortunately was looking out of the Kitchen window yelled at me.
There are flames and black smoke pouring out of the shed. I had forgot IO had left the plate switched on, and the gasses from the then  boiling pitch, had ignited.
Fortunately, most of the damage was due to smoke and soot, and a couple of hours work sorted that.  But, it focused my mind I tell you .

Later I got myself a 45 gallon  oil drum for a work top. I fitted a plywood section across the top of the drum, to which I attached three cleats to hold the disc
in situ, but free enough to facilitate rotating it occasionally. The disc could be either the mirror, or the grinding/ polishing tool. Sometimes it is necessary to alternate them
during both grinding and polishing stages.   Also, I drilled holes either side of the plywood support piece, to enable the sludge created to be washed down into the drum.
That way, everything could be kept clean. Contamination from previous grit particles, is to be stringently avoided, I used plastic sheet to cover the barrel top, prior to moving to 
the next fine grinding stage.

The above isn't mandatory, but I found it  a good way to do the work. Later, when I had my Garage built, I made a machine to to the hard work, 
and made provision for it to make it random, as a ritual stroke  process  is to be avoided if possible. The figuring had to be by hands though, the machine was not
sophisticated enough for that job. 

 

[Astrobits]

11 hours ago, Chris Lock said:

Whilst on the above subject, before work I spoke to a guy at Galvoptics about my plans and He's going to call me back with a quote on a 210mm 25mm plate glass blank. He was a friendly talkative kind of guy and said "you're not going to use a tile tool are you!?". Apparently they get hundreds of mirrors to coat through their doors, and they noticed that many of the ones produced by a tile tool exhibit scratches. I asked why this would be the case? he said because it's very difficult to remove all the previous grit size from a tile tool before proceeding with a finer grit. He said this is the case even if you scrub the tool. he recommended using a slightly sub diameter glass blank tool about 19mm thick, and said I should use it to both hog out and fine grind, followed by laying the pitch on it for polishing. Based on this info I asked for a quote on a glass tool also, but wondered what you thought of this? I must admit I like the sound of no sharp glass shards which could cut fingers.

 

 

Ahh yes. I realised that potential right at the start of my using tile tools. Normally I use hard pitch to stick the tiles onto the base. This always leaves narrow areas around each tile (where the pitch doesn't wet the glass tile) that will trap grit and inevitably release them at the wrong moment. My solution to that problem was/is to fill the gaps between the tiles with molten candle wax coming almost, but not quite, up to the top of the tiles. The wax will hold onto bits of grit that get pushed into it. At the end of each grit size you can then carefully scrape off the topmost layer of wax, where necessary, and re-heat the remaining wax ( or add more )  with a hot air gun  so that the surface melts and seals in any remaining grit. I have never had any scratches doing this.  If you ever want to do flats then you have little choice than to make a tiled assembly with the flat blank surrounded by 'wasters' and you hit the same problem with the same solution.

Certainly using a solid glass tool is the easiest ( and usually the most expensive ) option. It must be thick enough so that it allows the mirror, when the curved surface has been generated, to move over the tool as far as necessary without hitting the retaining blocks ( or the baseboard ) that are holding the tool in place. 19mm is probably the thinnest sensible tool thickness for your proposed mirror. I would do a CAD drawing to check this.

Making plaster ( other materials can be used ) tools is tedious and you do need two or more ( one for grinding and one one cast against the curved surface for the polishing tool ). However, having a tool for grinding separate from the polishing tool does give you the opportunity of going back to fine grinding, should you find this necessary, without stripping off the pitch from the solid glass blank and therefore having to re-make the polishing tool afterwards. If you do use a porous material to make tools then it needs to be thoroughly dried and sealed against water absorption so that it doesn't expand/ contract/crumble under it's influence.

Nigel

 

[Chris]

3 hours ago, barkis said:

Shed would be the place I reckon Chris.  
I did my first mirror in my Shed, but one careless episode spurred me into being a lot more careful
than I had been. Whilst melting pitch on a portable Hot Plate,  I left it unattended, and went indoors for a reason I don't
recall. After a while, the wife who fortunately was looking out of the Kitchen window yelled at me.
There are flames and black smoke pouring out of the shed. I had forgot IO had left the plate switched on, and the gasses from the then  boiling pitch, had ignited.
Fortunately, most of the damage was due to smoke and soot, and a couple of hours work sorted that.  But, it focused my mind I tell you .

Later I got myself a 45 gallon  oil drum for a work top. I fitted a plywood section across the top of the drum, to which I attached three cleats to hold the disc
in situ, but free enough to facilitate rotating it occasionally. The disc could be either the mirror, or the grinding/ polishing tool. Sometimes it is necessary to alternate them
during both grinding and polishing stages.   Also, I drilled holes either side of the plywood support piece, to enable the sludge created to be washed down into the drum.
That way, everything could be kept clean. Contamination from previous grit particles, is to be stringently avoided, I used plastic sheet to cover the barrel top, prior to moving to 
the next fine grinding stage.

The above isn't mandatory, but I found it  a good way to do the work. Later, when I had my Garage built, I made a machine to to the hard work, 
and made provision for it to make it random, as a ritual stroke  process  is to be avoided if possible. The figuring had to be by hands though, the machine was not
sophisticated enough for that job. 

 

Crikey, that's a lesson to anyone getting into this game! I shan't be leaving pitch cooking unattended after hearing this. I'll take what you're saying on board. I'd need to clear the shed out and stick it all under a tarp like said, and get the electric hooked up for light and a heater. Should be good though once setup.

I'll look at putting a grinding table together, I don't think it will need to be that big for an 8". I guess the only big down side with the shed is it will limit the time I can work on the mirror, if I had found a way of dong it indoors I could get 30 mins here and there whilst the kiddies are playing etc, I would need them in ear shot. 

[barkis]

Mirrors have been created in all manner of places.
A shed is a luxury compared to some strange abodes .

 

[Chris]

2 minutes ago, barkis said:

Mirrors have been created in all manner of places.
A shed is a luxury compared to some strange abodes .

 

Well..I'll admit whilst racking my brains, I came up with the idea of grinding the mirror in the under stairs toilet....I burst out laughing at the thought, but I guess I could sit comfortably whilst I worked lol 

[Chris]

3 hours ago, Astrobits said:

Ahh yes. I realised that potential right at the start of my using tile tools. Normally I use hard pitch to stick the tiles onto the base. This always leaves narrow areas around each tile (where the pitch doesn't wet the glass tile) that will trap grit and inevitably release them at the wrong moment. My solution to that problem was/is to fill the gaps between the tiles with molten candle wax coming almost, but not quite, up to the top of the tiles. The wax will hold onto bits of grit that get pushed into it. At the end of each grit size you can then carefully scrape off the topmost layer of wax, where necessary, and re-heat the remaining wax ( or add more )  with a hot air gun  so that the surface melts and seals in any remaining grit. I have never had any scratches doing this.  If you ever want to do flats then you have little choice than to make a tiled assembly with the flat blank surrounded by 'wasters' and you hit the same problem with the same solution.

Certainly using a solid glass tool is the easiest ( and usually the most expensive ) option. It must be thick enough so that it allows the mirror, when the curved surface has been generated, to move over the tool as far as necessary without hitting the retaining blocks ( or the baseboard ) that are holding the tool in place. 19mm is probably the thinnest sensible tool thickness for your proposed mirror. I would do a CAD drawing to check this.

Making plaster ( other materials can be used ) tools is tedious and you do need two or more ( one for grinding and one one cast against the curved surface for the polishing tool ). However, having a tool for grinding separate from the polishing tool does give you the opportunity of going back to fine grinding, should you find this necessary, without stripping off the pitch from the solid glass blank and therefore having to re-make the polishing tool afterwards. If you do use a porous material to make tools then it needs to be thoroughly dried and sealed against water absorption so that it doesn't expand/ contract/crumble under it's influence.

Nigel

 

That's a great solution using the candle wax, I'll hold that in my memory banks  I think as it's my first mirror I'll do it the easiest (most expensive way), and if I get into the habit of making mirrors I'll need to look at these more cost effective methods so I can actually afford to make mirrors 

I

 

[Chris]

I've had my quote back from Galvoptics:

-210mm x25mm plate glass blank

-205mm x19mm plate glass tool (I guess I could rise this up higher/thicker if I have trouble whilst working mirror ontop)

- 120 grit 250g (they don't do 80 grit)

-180 grit 250g

-400 grit 125g

-smoothing 125g (guessing ali oxide?)

-polish/rouge 5 Ounce

Pitch 1kg

Price inc VAT and delivery £201.30

 

I can just about afford this, does it sound ok? I realise I could buy a mirror for this but I'm paying for a learning experience.

At f/7 will 120 grit do or shall I buy some 80 grit off fleabay?

Any thoughts? If this sounds ok I'll order it.

[barkis]

Chris, I will have a look in my Store cupboard where I kept most of my  Tubs and bottles of Grits, Alux, and I know I have Jeweller's Rouge, which is 
a very red, polishing agent, so called because it was used for polishing by Jewel profession.
Mind you it stains very badly, and your wife will be very upset if you get it on anywhere but the job you are doing .
Perhaps Cerium Oxide would be a better option for you.
Hold fire on your ordering until I establish what I do have, it may save you some money, as I won't want anything for it.
My mirror days are over, although I was tempted very much as I followed Damian's progress. Common sense prevailed though,
and I won't return to the grind . You will need a water spray container to keep the  grits lubed. 
Cheap enough at a Garden Centre Shop.

To take a tip out of Nigel's post, you could reinforce your 19mm tool blank with some of thosetiny square tiles that some folks use in
Bathroom or wash areas. They will give the tool added grinding power too.
Just a thought.

 

[barkis]

I might have some 100 grit, 120 is a bit slower, but it would mean less hard  work removing the 80 pits would leave behind.

[mapstar]

Hi Chris 

It sounds like you've an interesting project in the offing. There are quite a few 8" blanks about for around £60 although I think I know where there may be some 8.75" blanks for a similar price than that although I would have to enquire if they are still availabe.

I bought a lot of my grinding medium from minerals and water and also from UKGE as Nigel advised.

Cerium (1200grit) can be purchased through ebay and uk glass polishing is a good supplier of 2.5 micron stuff. 

Aalso you need boiled linseed oil to soften the pitch.

Hope that helps.

[Chris]

1 hour ago, barkis said:

Chris, I will have a look in my Store cupboard where I kept most of my  Tubs and bottles of Grits, Alux, and I know I have Jeweller's Rouge, which is 
a very red, polishing agent, so called because it was used for polishing by Jewel profession.
Mind you it stains very badly, and your wife will be very upset if you get it on anywhere but the job you are doing .
Perhaps Cerium Oxide would be a better option for you.
Hold fire on your ordering until I establish what I do have, it may save you some money, as I won't want anything for it.
My mirror days are over, although I was tempted very much as I followed Damian's progress. Common sense prevailed though,
and I won't return to the grind . You will need a water spray container to keep the  grits lubed. 
Cheap enough at a Garden Centre Shop.

To take a tip out of Nigel's post, you could reinforce your 19mm tool blank with some of thosetiny square tiles that some folks use in
Bathroom or wash areas. They will give the tool added grinding power too.
Just a thought.

 

1 hour ago, barkis said:

I might have some 100 grit, 120 is a bit slower, but it would mean less hard  work removing the 80 pits would leave behind.

 

That's very kind of you barkis, and very much appreciated  But only if you're 100% sure you're 100% sure you won't be returning to the grind?

Just in case, and I'm just checking because I can be a bit paranoid/neurotic sometimes lol, I didn't mean for my post to sound beggy at all? I'm just trying to get a feel for the quote being reasonable or not? I must admit I did expect it to be slightly less, but I guess the VAT and postage does rack it up. I think the 1kg of pitch was £30 odd alone. Makes you realise what an achievement it is for the Chinese to be churning out 1/4 wave mirrors at the price they do.

I guess it wouldn't hurt for the hogging out to be a little slower with 120 if it meant less problems with pits later on, and also because the sagita can't be that deep for an f/7 mirror. I probably wouldn't be saying this if I was doing a bigger f/4 

[barkis]

Chris, I've had a good look and the only grade I can find is a quantity of  400 Carborundum. You are welcome to it
If you want it. 

There are some Jars that came from a local School Science department years ago.
The Physics Teacher was a member of Border Astronomical Society, and he passed
them on to me when the making of Mirrors ceased there.

They are labelled   (1st Smoothing  225. )  ( 2nd, Smoothing 175)   (3rd. Smoothing  125).
There is a good quantity in each container.. Yours If you want them.
 

[Chris]

2 minutes ago, barkis said:

Chris, I've had a good look and the only grade I can find is a quantity of  400 Carborundum. You are welcome to it
If you want it. 

There are some Jars that came from a local School Science department years ago.
The Physics Teacher was a member of Border Astronomical Society, and he passed
them on to me when the making of Mirrors ceased there.

They are labelled   (1st Smoothing  225. )  ( 2nd, Smoothing 175)   (3rd. Smoothing  125).
There is a good quantity in each container.. Yours If you want them.
 

Thanks for looking, really great of you to do that  I think the kit supplies 125g of 400 grit, not sure how far this will go. Can I take you up on the offer if it looks like I'm going to run out of 400 when I get into the finer grinding? I'll of course pay P&P  

[barkis]

Having read through the thread, I see there is ls lots of input from folks, which is good.
There is a chance of some confusion creeping in to your mind. I don't want to seem critical,
it is great that some willing help is flowing in, which was expected really. Everybody wants to help.
You need to take a step back, and re think what you wish to do
I thought the price for the kit was a bit over the top. Have a look at Damian's post, regarding the possibility of 
obtaining blanks from another source. The rest of what's needed is obtainable from many places, so have a rethink on 
that price again, personally I wouldn't commit to that, but It's your project, and you mustn't be too influenced
by us telling you this and that, helpful as it is.  You said yourself you could almost buy a 200mm Dob. for that price, and as much as I appreciate you
wanting to make your own, you have to be sensible about it too.. There is no rush to do it. Time is on your side.

I'm sure somebody somewhere has blanks of various sizes they don't need. You could do a want Ad. you might get lucky.

 

[barkis]

OK Chris.  If you are going ahead with this. PM me your home Address, and I'll post this stuff off to you.
I'm going to remove the contents from the Glass bottles to reduce weight, I will carefully transfer them into
Plastic containers suitably labelled. And don't worry, I will be careful to avoid contamination. 
I will handle the finer ones, and lead up to the rest one by one .

[Chris]

28 minutes ago, barkis said:

Having read through the thread, I see there is ls lots of input from folks, which is good.
There is a chance of some confusion creeping in to your mind. I don't want to seem critical,
it is great that some willing help is flowing in, which was expected really. Everybody wants to help.
You need to take a step back, and re think what you wish to do
I thought the price for the kit was a bit over the top. Have a look at Damian's post, regarding the possibility of 
obtaining blanks from another source. The rest of what's needed is obtainable from many places, so have a rethink on 
that price again, personally I wouldn't commit to that, but It's your project, and you mustn't be too influenced
by us telling you this and that, helpful as it is.  You said yourself you could almost buy a 200mm Dob. for that price, and as much as I appreciate you
wanting to make your own, you have to be sensible about it too.. There is no rush to do it. Time is on your side.

I'm sure somebody somewhere has blanks of various sizes they don't need. You could do a want Ad. you might get lucky.

 

 

22 minutes ago, barkis said:

OK Chris.  If you are going ahead with this. PM me your home Address, and I'll post this stuff off to you.
I'm going to remove the contents from the Glass bottles to reduce weight, I will carefully transfer them into
Plastic containers suitably labelled. And don't worry, I will be careful to avoid contamination. 
I will handle the finer ones, and lead up to the rest one by one .

Hi, sorry for the gaps in replying guys (cooking, cleaning, kids etc, I'll probably get more time after 9:30pm when they are all in bed)

I'm kind of basing my answers on the assumption I will be going ahead with the Galvoptics kit as my local supplier hasn't got back to me with a quote after a week. The only thing is I did get a bit of a shock when the guy at Galvoptics told me the price, and I know you guys think the same now.

I'm hoping to get back to Damian's post soon, and he's also PM'd me so I am/would be very interested in any other options.

I guess the galvoptics price is effectively for two blanks, so if they are say £60 a piece then I guess the grits are just a bit expensive. Think I've seen 80 grit on fleabay for 2 quid.

I'll take a step back and re-assess and talk to Damian before doing anything else  

[Chris]

3 hours ago, mapstar said:

Hi Chris 

It sounds like you've an interesting project in the offing. There are quite a few 8" blanks about for around £60 although I think I know where there may be some 8.75" blanks for a similar price than that although I would have to enquire if they are still availabe.

I bought a lot of my grinding medium from minerals and water and also from UKGE as Nigel advised.

Cerium (1200grit) can be purchased through ebay and uk glass polishing is a good supplier of 2.5 micron stuff. 

Aalso you need boiled linseed oil to soften the pitch.

Hope that helps.

Hi Damian, thanks, not as interesting as yours  Very impressed with both your mirror grind and scope build, really great work! I've a tab open for your mirror grind thread and I'm very much enjoying it so far...hoping to read some more tonight, very good read   

I must admit I either missed or forgot to chase up UKGE like Nigel mentioned, (sorry Nigel) I'll get on that and see what I can find out.

So I take it you need to boil the linseed oil at the same time as the pitch then add them together?

I'm taking a slight step back from the Galvoptics kit before I commit, is it still a bad price considering it's two blanks and 1kg of pitch? I am getting confused so think barkis is right in suggesting to step back a bit.

I'll go open your PM now whilst the kids are quiet lol

Thanks

 

[Chris]

@mapstar PM'd

Very interested

[Astrobits]

I don't know if Beacon Hill is still active as their price list has the date of January 2010 at the top. They might just have forgotten to update the heading.

In that list they offer the following:

Abrasive pack including grits, polishing powder and pitch for an 8"   £29

200mm dia x 25mm thick float glass   £39

200mm dia x 19mm thick tool    £37

Try them:    http://beaconhilltelescopes.org.uk/   

++44 (0)1507 363 381

Nigel

[Chris]

5 hours ago, Astrobits said:

I don't know if Beacon Hill is still active as their price list has the date of January 2010 at the top. They might just have forgotten to update the heading.

In that list they offer the following:

Abrasive pack including grits, polishing powder and pitch for an 8"   £29

200mm dia x 25mm thick float glass   £39

200mm dia x 19mm thick tool    £37

Try them:    http://beaconhilltelescopes.org.uk/   

++44 (0)1507 363 381

Nigel

Hi Nigel, I've had quite a bit of luck since my last post  Damian is very kindly going to send me some supplies left over from his massive 22" mirror at a great price, and he also very kindly put me onto John Nichol for a mirror blank.

John has just got back to me, and offered me a 8.75" low expansion blank at an amazing price too, I couldn't say yes fast enough!

With Damian's permision I've listed what supplies he has for me:

- half a tub of no.64 gugolz pitch 

-100g of 180grit

-1kg of 400grit

-Tub of boiled linseed oil

-300g 600grit 9 micron aluminium oxide

 

I just need to fill in the gaps, @barkis if I could take you up on your kind offer to help fill in these gaps, then all I need is some rough grit, a grinding table, spray bottle, and stuff for tooling. I have iron barbell weights of varying sizes which could be handy.

Things are looking very positive thanks to everyone here

 

[Chris]

20 hours ago, barkis said:

Chris, I've had a good look and the only grade I can find is a quantity of  400 Carborundum. You are welcome to it
If you want it. 

There are some Jars that came from a local School Science department years ago.
The Physics Teacher was a member of Border Astronomical Society, and he passed
them on to me when the making of Mirrors ceased there.

They are labelled   (1st Smoothing  225. )  ( 2nd, Smoothing 175)   (3rd. Smoothing  125).
There is a good quantity in each container.. Yours If you want them.
 

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, are these the grades to use with the pitch lap tool? or would anything finer be needed for the polishing? I see galvoptics have 50g 1 micron Cerium Oxide, not sure how this compares to the 3rd smoothing.

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