The 22" mapstar mirror

[mapstar]

Thanks for the encouragement guys.

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

[Mark62]

Thanks for the encouragement guys.

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

As long as your brain dose not go the same way [emoji1]. It dose make interesting reading though. I can not wait for the talk it will run longer then Corry but a lot more interesting I'm sure [emoji6]

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[jabeoo1]

This is pure class mate. I am following this thread now. One day I would love to have a go at grinding a blank.  Lots of respect from me on this to you. 

[Astro Imp]

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

[mapstar]

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

[Soupy]

I know you and I know you are mad... But this is ridiculous. Only kidding mate the mirror is looking amazing and this thread should be a must read for all even remotely interested in telescope building and mirror grinding, or just the mechanics behind it all. Looking forward to the updates.

[wookie1965]

Brilliant write up again Damian, enjoying the build dont think i would do it i would either end up with a fruit bowl or an ash tray and no one smokes in our house. Gives a whole new meaning to the daily grind.

Edited February 23, 2015 by wookie1965

[mapstar]

 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

[Mark62]

Another interesting read Damian. It's a lot of hard work but when it's done u can be proud to own it [emoji106]

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[Uplooker]

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

[JamesF]

Very much enjoying it too.  I'm most impressed so far.

James

[mapstar]

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!

[barkis]

You're making good progress, and on track.

That grinding tool is looking A1.

Ron.

[kirkster501]

Most impressive indeed!

[jnb]

How much did you pay for the blanks?

[mapstar]

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. 

[jnb]

Sounds like a lot of projects I've done. Financially it makes no sense but it's interesting to do.

[barkis]

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.

[mapstar]

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

[mapstar]

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

[Stu]

Amazing stuff Damian, a crazy amount of work already! Looking forward to the next instalment [emoji3]

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[Soupy]

Great going Damian. Lots of trials and tribulations. But plenty of grit on the mirror and from you!

[Mark62]

I'll give you this Damian your a gluten for punishment. But doing a super job

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[mapstar]

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

[barkis]

You've done a great job Damian, and you can be mighty proud of this.

You are going to finish up with a superb Mirror, no doubt about that.

Still a ways to go, but you're under full sail now, and land is in sight.

I take my hat off to you, this is going to inspire more people to make their own too.

  :icon_salut:.

Ron.