360mm First time mirror grind.

[Rustysplit]

I will see how stable it is when filled with sand and water. If still a little lively, then as mentioned earlier, I will simply bolt a stirling board disc about 4ft diameter to it so that I will actually be standing on it as I shuffle round

I think I have sourced a suitable plate weight, from Steve (Swampthing), but thanks for the link. Some other useful stuff there....

[Ajohn]

I am a model engineer. Cast iron prices have gone through the roof. It wouldn't surprise me if a bar bell weight worked out a lot cheaper. I was amazed how much a chunk for chuck back plate costs now. Tile and plaster is probably the cheapest of the lot.

I use a fish tank that's just over a metre long standing on end with a 2gal bucket of water in it to keep in place. The water is handy too.

John

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[Rustysplit]

Thanks to Steve, Swampthing, I was able to make a start rough grinding tonight. In my haste to get on, I did not fully read the label on the tub of ali oxide that I picked up from Machinemart. I thought it was 80 grit. Turns out to be 80/120 grit?? So not quite as course as I thought, especially as 80grit seems to make up a tiny percentage of it. Nevermind, it will take longer but be smoother.

The grinding stand works perfectly. With a full 25 litres of water in the base can, it is as solid as a rock. So after about an hour and a half tonight, I now have a curve depth of just over 1mm. Slow going, but I only need 4.5mm, so 1/4 the way there

Here is a quick pic after tonights labours....

[Glasspusher]

Well done Alan, I am following your thread with interest.

John

[Ajohn]

That's not bad going really and interesting. Ali oxide breaks down more slowly than silicon carbide so even though it's a little softer it might work out quicker in the long run. Super coarse silicone carbide leaves such deep scratches some people feel it's best not to use it.

I think the coarsest ali oxide that can be obtained easily is 60/80.

You will probably find mirror on top deepens the mirror more quickly but switching to mirror underneath during fine grinding will give a more accurate figure - if your using a full sized tool. Personally I would find some softish carpet tile / similar carpet and have a disc of that under the mirror for the latter stages - it will even up the support,

John

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[Astrobits]

Once you have the curve required on both the tool and mirror it really doesn't matter whether the tool is on top or the mirror. You will no longer be trying to correct the curves as the grits get finer but just remove the pits from the previous, coarser grits. You don't need to use a long stroke once the finer grits are in action. I always shorten my stroke to no more than 1/3D, often more like 1/4D. If you do use tool-on-top then as John says put some soft carpet under the mirror and don't forget to rotate the mirror on that carpet ( or any other material ) frequently to avoid any astigmatism that might result from patterns in the support from the carpet. I tried bubble wrap once and without rotating the mirror got triangular astigmatism on a 40mm thick mirror . Yes, bubble wrap has got a triangular pattern to it, and yes, it got through 40mm thick glass!!! Added a few hours polishing it out

Nigel

[Glasspusher]

You will find that grinding 50% of the time mirror on top (MOT) and 50% of the time tool on top (TOT) will give a more uniform grind and will stop the radius of curvature wandering.

John

[Glasspusher]

Nigel, without doubt bubble pack is the worst thing to sit a mirror during polishing. It is not made to any standards and is inherently uneven, as you say carpet fits the bill better,

John

[Rustysplit]

Ah. Before I go too much further with the rough grinding, should I have the blank sat on something like carpet? At the moment it is directly on the flat ply disc in the above pic. This however seems to be very flat and the wet blank sticks to it like stink with hydraulic lock.

[Glasspusher]

I don't think you need to worry too much at this stage, I would say carry on as you are. Astigmatism (a situation in which the mirror does not have a figure of revolution) can appear during fine grinding...I would suggest that is when you need to start paying attention to any flexure that might be happening in the glass. As Nigel says, rotate the blank relative to it's support, every 10-15 mins should do. The carpet does not really provide a support as any uneveness in the ginding table will be transmitted throught the carpet, it does however allow easy rotation of the mirror blank relative to the support. This is what helps prevent astigmatism which results from an uneven support.

John

[upahill]

Looks like an interesting thread, looking forward to trying this myself.

How thick is the blank?

[fwm891]

Ah. Before I go too much further with the rough grinding, should I have the blank sat on something like carpet? At the moment it is directly on the flat ply disc in the above pic. This however seems to be very flat and the wet blank sticks to it like stink with hydraulic lock.

I think you should put at least a few layers of wet newspaper - or better as suggested by others - carpet even at this stage - you must rotate the disc regularly (in an irregular manner!). It you put astigmatism into your mirror with coarse grit it will be even harder to remove with fine grits....

[Rustysplit]

Thanks for all the replies. I have indeed been rotating the blank every time that I wash off the slurry.

One thing I have noticed, and did not think about at the start, is the amount of crud that falls off the edge of the blank . I think I will router out a larger base board to go under the blank to catch it. I have to keep cleaning the floor up. Just as well it is on a concrete floor in a lean to type utility space.

Another hour tonight, and now just under 2mm depth.

upahill, the blank is 25mm thick.

[estwing]

great progress Alan...

[Astrobits]

Nigel, without doubt bubble pack is the worst thing to sit a mirror during polishing. It is not made to any standards and is inherently uneven, as you say carpet fits the bill better,

John

Tell me about it

We learn from our mistakes. It was many years ago and now I have thought about it I didn't figure the astigmatism out after all but sold the 20" mirror with the astigmatism as a flux collector not as an imaging objective. (The customer was made well aware of the poor figure).

As it happens I have successfully used bubble wrap for mirror cushioning since then ( didn't have any carpet to hand ), making sure that the mirror is rotated frequently and irregularly.

Nigel

[Ajohn]

I think texereau mentions a method of avoiding problems - vary the steps round mirror. Vary the stroke. Even go round in the opposite direction etc. Wonderful book. He used to run a part time school in Paris when not fixing observatory mirrors. He used to get people to work on each others mirrors to make sure the strokes and steps were random. A mix of straight, w and figure 8 strokes tend to do that anyway as does trying to do these while taking varying steps round the the mirror.

I feel that you will have to do the finer work and polishing mirror up because you will bend the blank with the mirror on top. Then there is pressing the lap.

Personally I would watch out for the base board you are using warping due to it getting wet. One of the reasons for rotating the mirror is avoid problems with it flexing due to the support.

John

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[fwm891]

Lay newspapers or poly sheet under your mirror to catch the spent grits, then throw the newspapers away when moving to a finer grit grade. Avoids a lot of contamination and keeps the floor clean!

[Rustysplit]

After about 6/7 hrs using the 80/120 grit, here is where I am...

I am a gnats whatsit shy of 3mm deep. This would have been quicker with the coarser grit I guess, but at least I should have a smoother finish to start fine grinding from. I have found that the slurry really affects the cutting ability of the grit. I now add about 2 teaspoons of grit to the centre hole in the weight, give a good spray with water, and grind going in one direction round the blank until it becomes worn out. I then use a wet sponge to wipe off the slurry, add another dose of grit and water, and grind again until worn out, going the other way round. I then turn the blank around and repeat. As suggested, I have also been altering the size of steps.

[Ajohn]

That finish looks like it will save you a lot of time fine grinding to me Alan. I aim to make something in the region of 300mm after I have suceeded or given up on an F3 220mm so will buy myself a tube of shot blasting grit. I'm not a fan of big scopes really. 300mm would be huge for me.

Some people use sharp sand, make a machine and sit swigging cans of beer throwing another table spoonful on every now and again - documentary on the beeb a while ago, No water at all.

I think any one of the links I posted would have mentioned wiping off spent grit etc.

John

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[Astrobits]

You might have saved some time in the finer grits but you probably lost that time on the coarse grit stage. Swings and roundabouts The remains of the grit and the ground glass produced does make a thick mush that gets in the way of grinding so frequent cleaning is normal.

Haven't considered sand as an abrasive. As glass is mainly sand it depends on whether the sand grinds the mirror or the mirror grinds the sand, either way it will probably take a very long time to get the curve you want. O.K. if you have nothing else to do I suppose

Nigel

[Rustysplit]

Getting closer. Just over 3.5mm now. Another bash tomorrow night and wednesday night and I think hogging will be done

[crashtestdummy]

wish i had the confidence to do my own mirror for my planned next scope in the 18"-20" region.big respect to everyone who has managed this

[upahill]

Out of curiosity how are you measuring your depth? Also, any idea how much grit you have used so far?

Sorry for all the questions - its an inspiring thread.

[Rustysplit]

Don't give me any respect yet, Crashtestdummy. I have barely started it. The tricky stuff is yet to come...

upahill, I have not measured how much grit I have used yet. It will seem a lot though as I have used too fine a grit really for initial hogging out. I should have used either 80 or even 60 grit. I am actually using 80/120 grit, so going is slow, but finish is nice. I am measuring the depth by laying a straight edge across the diameter and using twist drills of known sizes to determine where I am at. If it slips under the straight edge, I am at least that depth. Only 2 drill bits to go now, the 4mm and the 4.5mm.

[upahill]

Ahhh good idea, I think I might eBay for a dial indicator soon and get it ready for measuring.

Just noticed the saw is propped up on a piece of wood which explains why the curve didnt look even lol

Good working, following with great interest!