The 8.75" f/6.7 Mirror Grind

[Chris]

Another hours grinding and bevelling done before work today. I decided to go back to using the 54% diameter tool as I can control it a bit better, I was finding the 0.5 kg weight a bit fiddly.

I'm now down to 1.3mm Sagitta on average, and the off center circle is no longer off centre!...Hurrah! Still some little hills and valleys that need grinding away but it's all going in the right direction I feel  

I'm just over half way through my 100 grit, so it should last about right (Good estimate of grit you gave me there Ron  )

As always, here's a saucy mirror pic 

[Chris]

Progress update, I've completed 3 more hour long grinding sessions since I last posted. I'm not going to lie progress is a bit slower than I predicted, 3 hours of grinding to go from a sagitta depth of 1.3mm to my current depth of 1.7mm. I've got about an hours worth of 100 grit remaining so will need to order some more. Fleabay seems to sell 80, 120, and 180 grits for rough grinding, so not wanting to go backwards I could either order some 120 grit, or 180 grit to go with the 100g of 180 grit I already have? Any thoughts on this are welcome?

 I was aiming to over shoot slightly from 2mm sagitta depth to 2.4mm to keep enough depth in the center when grinding out the edges into a sphere with a tile tool, but I think I'll call it 2.2mm as I was estimating to begin with.    

I'm easily pleased I know, but it's fun seeing which currency I can flick under a ruler laid across the mirror when checking sagitta depth. The 1.6mm thick 20 pence slides under with room to spare, but the 1.8mm 2 pence although on the verge doesn't quite fit under.

A few shots below to set the scene  

[Astrobits]

Getting the centre of your mirror down to your 2mm final target is all you need to do. Going to 2.2mm and then hoping to bring it back to 2mm will take very much longer, and much more 100 grit, than going from 2mm to 2.2mm at the centre. There is much more glass to be removed from around the edge than there is at the centre. Go to 2mm, make your tile tool and then work with the mirror on top using short strokes until every tile is in good contact. Then you can start to go with finer grits. With the mirror on top the centre will always get more work than the edge and the longer the stroke the more the centre will be deepened so keep the strokes short. If you are going to stick with 100 grit you can go straight to 180. 120 grit is not sufficiently different to 100 to make it worthwhile.  You will find that the pits from 100 grit can be removed with as few as 6-10 wets of 180 grit so once you have the curve each grit goes much faster.

Nigel

 

[Chris]

Hi Nigel, thanks for the advice that does make sense  To more decimal points I think the target Sagitta was something like 2.06mm, so maybe lets just overshoot by 0.04mm and call it 2.1mm, hows that sound? I'll of course wet the mirror at this point and try and fully illuminate it with a torch to calculate the radius of curvature thus the focal length as a double check, I think I'll be happy with anywhere from 1400 to 1500mm focal length.

I'm not sure I had my explaining hat on last night, so if you don't mind me double checking? I only have enough 100 grit remaining to get me down to about 1.8mm. I can't find 100 grit online but I can find 80,120, and 180 grits. Which would you use to hog out the extra 0.3mm, then bed in the tile tool to get full contact before moving to a finer grit? After that I have a bit of 180 grit and a whole lot of 400 grit next inline. 

The short strokes mirror on top, just to check are these center through center? and 1/3 diameter strokes? Just to check before I give it a try.

 

[Astrobits]

Working with the mirror on top your sagitta will naturally go deeper, you can't stop it. How quickly depends on the length of stroke and size of grit. It is very much easier to go deeper than to pull back from an over-deep sagitta so I would stop at 2.00mm, possibly even a little less, make up the tile tool and get it nicely ground to the mirror and then check your sagitta. I would use a very short stroke most of the time but just occasionally going a bit longer. I would aim for strokes of 1" - 1 1/2"  with a few of about 2"-3", all CoC, until you have got good contact between the tool and mirror. Any off centre strokes will also speed up the deepening of the sagitta.

As you need more grits I would save the 100 and buy some 80 ( not a retrograde step, you are trying to remove glass at the moment and 80 does the job more quickly ) and use that until the tile tool and mirror are ground in and close to ( but less than ) your target. Then use the remaining 100, 120 and 180 to finalise your sagitta getting a bit closer with each one, using 1/3 dia strokes CoC.. There is a bit of a gap to the 400 so you will find it a bit slow removing the 180 pits.

My normal sequence was 80, 120, 220 Carbide grits and then onto Aluminium oxide powders. The curve would not be finalised until the 220 grit size. The Al oxide is a 'softer' abrasive and leaves a much nicer surface ready for polishing.

Nigel

[Chris]

Thanks for that Nigel, in that case I'll indeed stop at 1.9mm-2mm. Hopefully the remaining bumps in the glass will have ground out by then, they are getting better but I can still feel them when I run my finger over the ground surface. I'll order some 80 grit now and follow your steps

 I'll also look at a grit size to go between 180 and 400 to help speed up removing the 180 pits, I'd better triple check what I have first. 

Cheers  

Edit: 3x100g packets of 80 grit ordered for now.

[Chris]

" Working with a sub diameter tool tends to produce a curve that is deeper in the middle than at the edge. For this reason the curve should be ground a little deeper than calculated as when work begins with a full sized tool the curve will even out resulting in a shallower centre."

The above is why I thought I needed to go deeper than 2mm, but then again it makes no reference to having the mirror on top when work with the full size tool begins, so maybe it meant tool on top? 

Just been doing some back reading trying to get my head around things. 

[johnfosteruk]

Good luck getting your head around it Chris and I'm glad you're making progress. Good luck, not that it looks like you need it.

[Astrobits]

It is fairly obvious that the tool will only work on the areas that it touches. It is therefore quite possible to generate an irregular curve with a sub-diameter tool. It is not a fault of sub-diameter tools, it is up to the operator to guide the tool appropriately to generate the required surface. It is therefore useful to be able to check the curve as you go along, not just at the centre but also towards the edge. One way is to draw and cut out your required curve in card and test the profile of the mirror with that. Using a full sized tool avoids this necessity as the smooth travel of the mirror over tool indicates a near spherical surface.

None of my mirrors up to 10" ever saw a sub-diameter tool and conversely none of my 16" and larger mirrors ever saw a full size tool of any type ( grinding, polishing and figuring ). Look on the web at the large mirrors being worked on and all will have a very small tool on top of the mirrors whatever the stage they are at.  So it is perfectly possible to make a mirror, with sub-diameter tools, to the accuracy the mirror maker wants, and for the larger mirrors it becomes a necessity.

At the moment you are simply removing the bulk of the glass with a steel tool. Steel works more quickly than, and doesn't wear away so much as, the tile tool that you will shortly be using. For my repetitive mirrors I made a full diameter steel tool to generate 8" f/6 mirrors and I would start with 80 grit. After 1 1/2 hours grinding the curve would be about 1/4" from the edge of the mirror. Then I would switch to 120 grit and take the curve to about 2mm from the edge ( 1/2 hour or so ) when a switch to 220 grit would take the curve to the bevel ( another 1/2 hour or so ). Steel tools are impressively fast.

Nigel

[Chris]

22 hours ago, Astrobits said:

It is fairly obvious that the tool will only work on the areas that it touches. It is therefore quite possible to generate an irregular curve with a sub-diameter tool. It is not a fault of sub-diameter tools, it is up to the operator to guide the tool appropriately to generate the required surface. It is therefore useful to be able to check the curve as you go along, not just at the centre but also towards the edge. One way is to draw and cut out your required curve in card and test the profile of the mirror with that. Using a full sized tool avoids this necessity as the smooth travel of the mirror over tool indicates a near spherical surface.

None of my mirrors up to 10" ever saw a sub-diameter tool and conversely none of my 16" and larger mirrors ever saw a full size tool of any type ( grinding, polishing and figuring ). Look on the web at the large mirrors being worked on and all will have a very small tool on top of the mirrors whatever the stage they are at.  So it is perfectly possible to make a mirror, with sub-diameter tools, to the accuracy the mirror maker wants, and for the larger mirrors it becomes a necessity.

At the moment you are simply removing the bulk of the glass with a steel tool. Steel works more quickly than, and doesn't wear away so much as, the tile tool that you will shortly be using. For my repetitive mirrors I made a full diameter steel tool to generate 8" f/6 mirrors and I would start with 80 grit. After 1 1/2 hours grinding the curve would be about 1/4" from the edge of the mirror. Then I would switch to 120 grit and take the curve to about 2mm from the edge ( 1/2 hour or so ) when a switch to 220 grit would take the curve to the bevel ( another 1/2 hour or so ). Steel tools are impressively fast.

Nigel

Thanks for clearing that up Nigel. Yes, of course they must use sub diameter tools on really big mirrors, I know Damian needed to for his 22", I guess you'd need to be Bruce Banner on an angry day to work with a full size tool on the biggies! They wouldn't use sub diameter tools if they didn't work so that does make sense. I have worked CoC with about 1/2"- 3/4" overhang whilst turning the tool, mirror and myself, so I guess the tool must have worked all of the mirror fairly uniformly. It might be better than I think/worry! 

I could try cutting out a template, could be fiddly with such a shallow sagitta, I might give it a go though. Alternatively I could just stop worrying and crack on until I've bedded in the tile tool and started looking at Ronchi/faucault testing. I'm sure this will highlight what needs sorting, then of course I'll have to ask the question of how to sort it!  

Your steel tool and 80 grit does indeed sound very fast, it's slow and steady with the 100 grit and iron tool so far, it will be interesting to see how the 80 grit fares when it arrives.

Anyhow, I'll stop questioning and post again when down to depth   

[Chris]

Another 60 minutes of grinding with the new 80 grit and I'm down to somewhere between 1.9mm and 2mm which should do the job. 

I've made a start on the tile tool - Used a cereal packet covered in tape to build a wall around the blank, placed a grease proof paper circle over the blank to prevent sticking and poured a creamy mixture of mortar into the mould I had made.

I was advised by an ex builder in the DIY store that cement alone wouldn't set without something to bind too, so after explaining what I was upto he sorted me out with a a strong pre- made mortor that just needed water adding and a good stir. I'm a bit weary but will seal the tool well with fiber glass resin and see how the rough grinding goes with it...I can always make another if it doesn't work out.

I'll leave it a day to set and see how it looks  

[Astrobits]

It is amazing how flimsy the barrier can be when casting the tool base. I have used polythene from a dog biscuit bag as the barrier without any reinforcing When the mortar is set make sure that it will sit firmly on it's base before finishing it off. It is v. annoying if it rocks as you try to push the mirror over the tool. 

Nigel

[Chris]

Good tip Nigel, yes I'd better check for that once it's set  

[skybadger]

I've used cement quite happily for a tile tool. Went off without a problem. I also used the tiles set side on for longer grinding. Using 1cm square tiles worked really well.

Can't say I finished polishing it though..

Mike

[Chris]

Good to hear Mike, I'll try just pure cement for the next tool I make  

[Chris]

My wife kindly sanctioned an hour away from the kids to get a tad bit more done, so I quickly plucked some tiles from it's matting and mixed up some fiber glass resin and hardener so I could apply LOTS of it to the mortar disc and stick the individual tiles on. I may have a resin verses news paper situation once it's set....crazy stuff that resin I feel slightly high now! The tiles are as wonky as I am

[mapstar]

Looking good Chris and well done getting to this stage. This is where the fun begins 

[Chris]

54 minutes ago, mapstar said:

Looking good Chris and well done getting to this stage. This is where the fun begins 

Thanks Damian, getting there slowly but unsurely  

General question, shall I grind off the edges of the tiles, so they are flush with the circumference of the tool? or is it ok with some tile overhang? 

[mapstar]

18 minutes ago, Chris Lock said:

Thanks Damian, getting there slowly but unsurely  

General question, shall I grind off the edges of the tiles, so they are flush with the circumference of the tool? or is it ok with some tile overhang? 

I ground them off chris as it's easier to handle and less likely to break one off mid grind.

[Chris]

4 minutes ago, mapstar said:

I ground them off chris as it's easier to handle and less likely to break one off mid grind.

Thanks Damian, that will be my next little job then  

[Astrobits]

1 hour ago, mapstar said:

I ground them off chris as it's easier to handle and less likely to break one off mid grind.

Agreed

 

Nigel

[barkis]

Just to sneak in on your thread  Chris. things are progressing very well, and under the close eyes of the other 
guys closely following your progress. That is a super disc you got from John, and soon to be doing it's work for you.
A point I want to mention, if it hasn't already been raised by one of the others.
That piece of carpet you are sitting your disc on, is it the original one, or have you been changing it for newer pieces as you have progressed ?
The reason for asking is in the interest of preventing any stray bits of the coarser grits that might be lurking down there, especially if it is the original 
pad.  It would be a pity to see any of the little beggars get loose and lodge itself onto the workpiece  in the polishing stages.
Even a broken down bit of 80 grit can make a nasty gouge in that ultra smooth surface. It would sicken you I'm sure.
Not being pedantic, just wary. Possibly unnecessarily, as you may have been specially careful  in your clean ups.
I'll pop in now and again to follow your progress, not to spy on things, I know you're in good hands for sure.
Good luck Matey.
You are doing a terrific job here.

 

[Chris]

19 hours ago, barkis said:

Just to sneak in on your thread  Chris. things are progressing very well, and under the close eyes of the other 
guys closely following your progress. That is a super disc you got from John, and soon to be doing it's work for you.
A point I want to mention, if it hasn't already been raised by one of the others.
That piece of carpet you are sitting your disc on, is it the original one, or have you been changing it for newer pieces as you have progressed ?
The reason for asking is in the interest of preventing any stray bits of the coarser grits that might be lurking down there, especially if it is the original 
pad.  It would be a pity to see any of the little beggars get loose and lodge itself onto the workpiece  in the polishing stages.
Even a broken down bit of 80 grit can make a nasty gouge in that ultra smooth surface. It would sicken you I'm sure.
Not being pedantic, just wary. Possibly unnecessarily, as you may have been specially careful  in your clean ups.
I'll pop in now and again to follow your progress, not to spy on things, I know you're in good hands for sure.
Good luck Matey.
You are doing a terrific job here.

 

Hi Ron, thanks very much for chiming in and your kind words of encouragement, much appreciated  Yes, John did me a very good turn on the blank for sure, I'll do my very best to do it some justice!

The carpet is the original but do not fear, I've gone from 100 grit to 80 grit so far as I've only just completed hogging out to my depth of 2mm. I will continue with the 80 grit until the tile tool is bedded in before completely replacing the carpet, jig, sponge, clothes etc. Then and only then will I proceed with a finer grit once I'm sure every grain of 80 grit is gone  

I'm hoping to get a bit more time on the grind soon, a few things have cropped up recently which have had to take priority, but I'm still as keen as ever. A big thank you to everyone for their help so far   

 

[Chris]

Here's the tile tool with the tile overhang ground away, I needed to use my huge angle grinder so it's not the neatest of jobs I'm afraid

Oh, and because I applied the resin with the disc laying on newspaper, lots of the paper had got stuck to the back of the disc creating an uneven surface so it didn't sit flat. I needed to sand the back of the disc flat again and re-seal it with resin, lesson learnt!

Talking of lesson learn't, I won't use mortor again, when you rub it the grit from the sand in it comes away, so I had to make sure the whole disc is really well sealed!

Anyway the tile tool is finally done, so the next step with be to bed in the tile tool with the mirror blank using the same 80 grit, mirror blank on top. 

[mapstar]

I used pure cement when I made mine Chris.

Keep up the good work we're all here following