mapstar

Onto the polishing

Now the mirror was at the stage of polishing I required some more supplies. I trawled the internet for Cerium Oxide and came up with some from a well known auction site. The pitch for polishing was a different matter as it seems to be quite difficult to find a supplier that has some?  A visit to John though sorted that problem and 2kg of Gugolz #55 pitch was winging it's way home with me so I could make the polishing lap.

I made up a base for the lap from four 12mm plywood disc's of 17.5" diameter that I happened to have lying about. All glued together I sealed them with a coat of fibreglass resin.

Now actually melting and pouring pitch is a horrible business and not at all nice. I melted the pitch in a pan that I would dispose of later. The pitch is then heated slowly making sure not to overheat it as boiling drives off the volatile's which give the pitch it's unique consistency. Gordon Waite does quite a good video on this and a video writes a thousand better word's than I could so I'll just go through the basics.

The mirror is covered in tinfoil and the melted pitch is poured on top in the middle and slowly spreads out in an even layer. Then with this cooling the pitch lap base is pressed down onto it. The results are a perfect lap or in my case an absolute disaster! I had not quite got the blank level and the pitch started it's way towards one edge and I waited a little too long to press the base onto it as it hardly stuck! the lap wasn't exactly what I was hoping for and ended up a proper messy affair with pitch everywhere.

I then let it cool and chipped it all off to redo and hopefully get it right.

The second attempt was a success but as I've already said pitch is horrible stuff and just get's everywhere so glad I was in the garage doing all this. After cooling I cut facets in with a tenon saw finding out the blade doesn't clog if there is running water on it whilst cutting. I'm sure if there was a missus Mapstar living with me then she would be none too pleased with the use of the kitchen sink for this phase of the lap making! 

I then covered the mirror with grease proof paper and pressed the lap over night on top of the mirror to form a decent contact. Here's a shot of my rather messy lap cold pressing with oxide before polishing operations began 

The polishing compund comes in a couple of forms, Cerium Oxide the other being Regipol. I initially started out with Regipol optical polishing powder like this

After suspending it in filtered water (I used a brita water filter) I did my first half hour of polishing and below show's the lap wasn't making contact in the middle so more pressing with more weight

The lap eventually made full contact after warming it in water and pressing several time's. The bevel around the outside and facet channels were re-cut several times during this as they close up when the lap forms to the surface

After 12 hours the surface looked to be polishing but the centre was still hazy which I'm not sure can be seen in this shot? 

I had been polishing without any weight on the back of the lap so added 10Kg and continued. The lap become's quite a chore to push across the surface and I would polish for about an hour then press the lap overnight and carry on.

November by this stage was at an end.

When you first see your mirror polished it is a genuine WOW. I can count it as one of the most satisfying things I've ever done in life. It makes the hard work of the rest of the journey disappear. These two shot show the curvature of the surface with the fluorescent tube reflection. 

I inspected the surface throughout using a laser to test on the surface to make sure the micro pit's had all gone. I had picked up a couple of scratches near the edge but these would stay.  It had taken me nearly 22 hours to polish which probably would've been a lot quicker with weight on from the start so up to now well over 100 hours I had in this.

It now looked like a mirror so onto the testing 

Damian

Cheers Ron and Rick for the encouragement and kind comments.

Hopefully I have got all the details right and the thread make's sense in the way everything progresses.

To quote what a very wise person said "it's a 22" sub F4 mirror and a beast compared to the average mirror so must be treated with the respect it deserve's"

I'll get there in the end and maybe inspire others along the way.

Damian

After a short break I returned to the mirror.

Next stage was to work up through the different grades of grit.

Just on a safety note the glass is always worked wet as inhaling glass dust can result in Silicosis, a very serious lung disease, and to this end in the initial stages I also wore a dust mask. 

For those that have never really looked into the grinding process, when the rough curve is generated the 80 grit Silicone carbide chips away the surface creating a lot of damage. After this is complete the use of ever finer grits then repairs the damage and brings the surface back to a polish. 

The process would involve several grades of grit to achieve a polish. Each and every change would be followed by lot's of cleaning of the work area and tile tool

80 grit silicone carbide 

220 grit silicone carbide  

400 grit silicone carbide 

600 grit aluminium oxide 

1200 Cerium oxide  

Grit quality can be quite variable so the particle size is only an average of what you get dependant on the supplier 

Here's a picture of the surface after the curve generation with 80grit

The large chips or pits can clearly be seen without any magnification on the surface. Some of the 80 grit I recycled during the rough grind. Saved on quite a bit of grit usage

Next was the 220 grit and although a lot more difficult to spot, the pits are still visible naked eye although I used an up turned eye piece over the entire surface to spot larger one's which had to be ground out. A combination of strokes and stroke length was used from straight through the centre to W strokes and also chordal all the time pencil testing to maintain a sphere.  The grinding action gets a lot quieter at this stage

Here's a couple of pic's of the 220 grind during and on completion

The surface is still very opaque and has no shine to it at any angle. Next onto the 400 grit. At this stage the blank or mirror start's to take on a shine from a low angle which is quite a revelation

As before I used several random strokes and rotated the blank every time a trip around was complete in order to prevent any astigmatism occurring.  Next was the 600 grit aluminium oxide which is used due to it being a much softer and less aggressive abrasive. Here's a pic of some and has the consistency not much different to talc. I acquired this from John as the quality for this size is a bit hit and miss from other sources.

Here's the surface after the 600 Al Ox has been completed. The angle at which the shine was showing steadily increased

By this stage though I had been backwards and forwards through the grit's as it's so easy to pick a scratch up. But finally finished without scratching it

The surface really spurs you on at this point with it beginning to look like a mirror. I checked yet again with the up turned eye piece throughout and finally thought the tile tool was redundant, so onto the next stage which is polishing 

It was by now early October so coming up a  year since myself and Rich had received the blanks. During the process we have regularly exchanged experience, encouragement and tips to progress. Also constant contact with John along the way all of which has been invaluable. I can highly recommend this approach.

Damian

Back to it and on with the thread.

So as previously described I'd now got good contact with the tile tool and all was going well. I forgot to mention that throughout the mirror making process I was regularly rotating the blank on the grinding table to prevent any astigmatism. This is done entirely at random sometimes an Inch sometimes a 1/4 turn but always random.

Having stated earlier that I'd ground the rough Sagitta to a depth of 12mm with the theory that the tile tool should shallow this off again as the spherical surface is formed I measured the sagitta depth again. It was at 11.9mm 

So onto a bit of maths which I'd not explained in my earlier post about calculating the focal length and sagitta. I'm no maths teacher so bear with me 

The focal ratio was to be F3.9  

I'm a metric sort of chap so 

Focal length (FL) = dia of mirror x focal ratio 

FL =  560mm x 3.9

FL = 2184mm

Next was

Sagitta (S)= Radius of mirror squared (r2)  / 2 X Radius of curvature (R which is 2 x focal length)

S = r2  / 2 x R

So S= 280 squared / 2 x (2 x 2184)

S= 78400 / 2 x 4368

S = 78400 /  8736

S = 8.97mm 

This meant a lot of edge grinding to bring it back down to the desired sagitta and also a lot of bevelling! To do this the centre of the tool needs to be moved further out towards the edge as this is the part that does most of the work 

So off I went with the plan with chordal strokes interspersed with W stokes across the Mirror blank to try to maintain good contact

The overhang of the tool increases the weight on the edge thus grinding faster

After what seemed like an age (4 hours) I had removed 0.4mm of the edge and was down to 11.5mm (2.5mm to go!) so  more bevelling and edge grinding to follow

It was at this point that I reached probably the lowest part of my mirror making experience so far. My in experience showed and whilst grinding the edge I pushed for too much overhang and wobbled the tile tool. With a sickening clink I shelled (chipped) the side of the blank to a depth of about 2.5mm. It was a totally gutting experience to remove the tool and see the damage 

I left the blank for about three days to think before I had the heart to return to it. Several discussions later with John, Rich and others and I increased the bevel again to 5mm which went part way to moving the damage out. As others said if it was a finished mirror and you chipped it like that would you stop using it. Milk spilt time to mop up and get on with it. 

I did finally grind down the edge using a weight on the inside of the tool (near the centre of the blank) to balance it, but settled on F3.8 as the sagitta ended up at 9.2mm when I finally bedded the tool back in with 220 grade grit . Much harder task when it come's to figuring at the later stage. I also renewed the tile's on the tool due to them becoming that thin that the're was no facets left between them

New tile's glued a top old one's 

Here's another shot of the chip and what the surface looks like after the 80 grit

The 220 grit grind's much smoother and now the rough work is done it was all about repairing in finer and finer detail (working through the grit grades) all the damage to the surface that had occurred. here's a shot of the pencil test with 220 grit worked surface

Bedding in tiles

The blank was now just under 38mm thick and about 18kg. I'd spent 79 hours on the project to this point. Total grit usage 2 Kg of 60 grit 14 Kg of 80 grit and 2 Kg of 220 grit lots of water!

Not everything always goes the way you want it and more lessons learnt.

By the end of all this it was Mid June 2014 so time had flown and I had to leave the project for another one which was desperate for finishing.

I resumed again in September hopefully with more up's than downs this time

Damian

I made some sub diameter polishers from Dental Cement Damian.

It was reasonably cheap from a local dental technicians workshop in the town, so I made a 16" full size polishing lap

for a later project, a 16" f3.5. I never did finish that job. It was a thin mirror, and I made the mistake of grinding the back surface

with 400 grit to tidy it up. Unfortunately, doing so relieved  stresses in the glass that allowed it to badly distort the lovely sphere

I had on it. It would have taken fine grinding and re polishing to get it back to a sphere, the position where I always liked to start the figuring process.

It disheartened me too much to want to rectify it.

It was stupid mistake to make really.

Ron.

Such a shame Ron after all that work, I have been at this stage as my next post will reveal

How much did you pay for the blanks?

Hi there, the blanks way back then cost £480 but have since gone up to about the £550 area?

In financial terms the blank is only one part, you should also figure in the amount of grinding supplies below

Grinding table

Various grades of carborundum grit 60, 80, 220, 400

Aluminium oxide abrasive 600 grit

Cement

Tiles

Cerium oxide

Pitch

Plywood for laps and transport box

Test equipment

Postage

Mirror coating

And last but not least the time you have to put in to make it happen. 

Hi Damian,

Really enjoying this thread. Very well documented, that is a job in itself. I look forward to following this through to its conclusion. Keep up the excellent work and postings. Missed you at Galloway.

Ian

Cheers Ian

I've missed the Galloway gathering too could've done with getting up there!

 On to the next stage of the grind 

The rough curve had been generated and a quick measure on the edge showed the blank to be down to just over 41mm. 

This curve is non spherical so we have to make a tile tool to bring this to a sphere. The tile tool was to be 18" in diameter (about 3/4 diameter of mirror). It consisted of a cement base with mosaic tiles glued on the front face for the grinding. Sound a little strange at first, for a smaller mirror you would cast the tool the same size but this would be too unwieldy at this size for me to use so 3/4 diameter. 

I made a former to cast the cement disc which would fit on top of the mirror blank. Here's a few picture's of the progress from me making the former to casting the cement backing

I'd put packaging tape around the inside to help the cement release better. The former sit's on the greaseproof covered blank and when the cement is poured it forms to the rough curve that has been generated on the blank by the sub diameter tool (11" barbelll weight)

After scree-ding the excess off and giving the former a few taps with a block of wood to release any trapped air bubbles I left it to set. I used 15kg of cement for this. After I'd separated it from the former I sealed it with a coat of fibre glass resin

The mosaic tiles had arrived so it was the job of attaching them with yet more fibre glass resin. The convex surface of the tiles can be seen clearly

Once they were well and truly stuck it was out with the angle grinder and diamond disc to trim them up and put a nice edge bevel on them. 

So onto the grinding with the tile tool, the theory being to work two surfaces together one a rough convex surface and the other a concave surface of the same radius to eventually produce a spherical surface. My understanding of the mosaic tile's is that they create facets (gaps) between them which carry the grit across the surface each tile being a cutting surface to grind the glass away (Apologies if this an amateurish way of putting it!)

First of all the surfaces need to mate together and the first few wet's (carborundum and water) show on the tile tool how well these are mated together 

So the contact was not very good to start with. As things progressed it steadily got better. I used a variety of strokes at this point ranging from pushing the tool across the blank in a straight line through the centre to a W stroke across the blank in order to get the tool and blank to contact fully.

So finally all in contact so onto what is the pencil test. A grid is drawn on the surface of the blank to see if even contact was being maintained.

A trip then around the table and I inspected the lines to see if they were all grinding off at the same rate.

I did this several times until I knew that I'd good contact between the two by all the lines grinding off equally (took 5 hours). At this stage I must've been around this table a thousand times between the rough and tile tool grind.

More later but I was quite happy to this point 

Damian

Good luck with your project. I have nothing but admiration for you guys that take on builds like this. I just watch in awe.

Thanks again for all the encouragement guys. I'm not sure about the awe bit as the how mad must some people be?

Damian

Thanks for the encouragement guys.

Yes I hand chamfered the edge's and developed what's commonly known as Mirror bevellers elbow! Hard work

Losing weight is never interesting, stick headphones in and get dancing around that blank!

Yeah blank is how your mind ends up doing the hogging. If I ever do another I'll get it pre-generated. 

P.s. the blank lost weight a lot faster than I did 

Cheers Guys

I made a start on the front face by yet again flattening it making sure it was the same thickness all round. It was better doing this side as the ridges were no where near as pronounced as the other side. This time I started with 1Kg of  60 grit moving onto 80 grit to finish with. As can be seen it was a messy business!! 

This took 5 hours in total, the 60 grit cutting a lot faster. I then renewed the bevel on both sides which was another hour

Once the front was flat, I started the hogging out progress after preparing the 10Kg 11" barbell weight by grinding the middle off to leave the ring around the outer edge

I'd worked out that the sagitta for F3.9 was going to end up at roughly 9mm  so a lot of glass to remove. I used a centre through stroke testing with a straight edge as I went.

I needed to take the sagitta deeper as the barbell would create a non spherical curve. A tile tool is used then to bring it to a spherical curve and would grind the edge down (in theory) so lots more grinding later and another 1kg of 60 grit, 9 kg of 80 grit 12 gallons of water and I was at the depth I required. Here's a few photo's of it progressing.

A total of 41 hours later I had a depth of 12mm. Just out of interest I weighed it at 18.7Kg. Wish dieting was as easy!

Just to say hogging has to be one of the most boring things in the world!!!!!!!!!!!!! 

I re-bevelled several time's during this process

So next I was ready for the tile tool

Damian

nice one fella. look foward to your next bit. just like your big dob build off

Cheers Faulksy just wish it was finished like the scope was before writing.

I look forward to it!

Is the mirror going in a Mapstar special custom truss Dob?

Hopefully, although it's not finished before putting a thread together unlike the scope build.

I've been on with this whilst doing the scope builds so its been a bit of a slog!

Yet to start the scope wood work but that's another thread although won't be as long as the last one ;-)

Great stuff Damian. It will be a real labour of love once finished, something to be proud of.

Cheers Stu, hopefully you'll get to see it by the end of this year!

Wowsers mate it's really going to be a sense of achievement when this is complete!

Cheers Calv

Notice you added the word "when"! I'm getting there but it's slow going as I'll get to in the thread

Hi Ron

I was initially aiming for F3.9 but as I'll be detailing later this changed to nearer F3.7

It's an ambitious goal but with time and dedication its steadily progressed. There have been plenty of ups and downs

Hi all,

I began this journey way back in Oct 2013 so it has taken a while to get to where I am at present but thought it about time I put my experiences down.

Wanting a larger scope than the 8" I had and after discussing with my good friend Rich (Crashtestdummy) we both decided to find out about mirror making. There is a mine field of information out there on the subject some of which is useful but not much in the way of practical experience except what I'd read about on here (Rustysplit 14" and Rick M 20")

So when a seminar was advertised on SGL by Glasspusher we both decided to book in and attend. 

As many know Glasspusher is John Nichol and he took us through a little bit of history on himself and mirror making in this country which is sadly not as prolific as it used to be, his experienced friend Jack also popped in to talk of the many years he'd spent at Grubb Parsons (

After an informative and enjoyable few hours with John we left with thoughts of big bits of glass and lots of work ahead (I mean lots) 

On returning it took us a couple of weeks to decide what size we were going to go for. Hunting for a glass supplier in this country leads to Spectraglass in Scotland and Rich ordered the two 22" blanks. Here's one with a cup for size!

The blank was 44mm thick and weighed in at 22kg! There was a small defect near the edge which I decided would be the back

Whilst waiting I'd put together a couple of grinding tables so we could make a start 

To make a start I had to bevel at 45 degrees the edges of the blank to about 5mm with a carborundum stone.  Each side took an hour! On a blank this size it is important when you turn it over you lift it completely or roll it on something flat. I didn't initially and caused some shelling of the surface. This would later be ground off but valuable lesson learnt.

Next was to flatten the back with the 80 grit carborundum. This was done using a steel ring about 12" in diameter. It was quite nerve racking due to the blank having concentric rings on the face I was flattening so each time it skipped over these really does not sound nice 

This took 11 hours in total so quite a while. I tested as I went with a straight edge and finally finished with 220grit although I think John is convinced I used 400grit as it's really smooth    Total grit usage up to this point was 2.5kg 80 grit and 0.1kg of 220grit. The blank had also lost weight, just under 1/2 kilo and 2mm from it's thickness so was 42mm.

Turning the blank over I then made a start on the curve generation using a barbell weight of approx 11". 

Damian

I had the 25, 15, 8 and 5mm BST's and for the balance of price and views I think they're very hard to beat and a fantastic upgrade to the standard eye pieces that come with most scope's.

Well designed and as you say extremely comfortable to use. I was using them in an 8" F5 newt.

Some have said the 25mm is the weakest of the range I found it nice to use.

Highly recommended from me

This is what i transport mine in,

It was actually made by the previous owner, although i do have the skills and equipment to do it.

I was planning on an update to match the scope transport box i made but it functions for now, and think with all the bits (12mm ply,hinges,catches,foam) it will cost in the region of £30-£40?

O.k. if you like a woodwork project just to give you an idea of the custom made option.