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first time mirror grind ......


wurzelmike

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Hi Everyone,

I've finally started work on my mirror, a 6" pyrex blank which I bought *cough* several years ago.

I've a copy of Howards book, reserved from the local library, and a PDF copy of Texereau's book. But i'm planning on working mainly from the Stellaphane website, with help (hopefully :smiley: ) from here.

So far, I've worked through the grits 80, 120 and 220 and I've got a lovely looking surface. I can honestly say it's looking good, there's just one problem so far and it's kinda embarrassing :grin: .......it seems that it doesn't matter whether the tool is on top, or on bottom, whenever I grind, the focal length is shortened :rolleyes: and the curve deepens.

I haven't a clue why, as I was aiming for a f5 curve, and once I approached this and then changed over the tool/mirror, the curve just kept going deeper and deeper and.....well, I gave up on the 80 grit when the Sagitta was giving a f3.7 :shocked: curve.

I returned later, after thinking it through for a while, and switched to the 120 grit this got me back to a f4.1 curve for a while, but finished with a f3.9.

Just spent two hours with the 220 grit and finished up with a f3.5 curve :mad: It has a lovely finish, honest, and if I was aiming for that kinda curve I'll be really proud of the mirror. But right now I'm hoping for maybe something between f4-f5, I just don't know how to get there.

I've tried using the 'normal' stroke with the tool on top for most of the time. But added a few 'W' strokes to 'blend' things in a little.......clueless :grin:

Any suggestions, other than maybe finding a different hobby :tongue: would be really appreciated,

all the best,

Mike

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Not sure what's happening with the curve there - try longer strokes with the tool on top maybe? That should in theory grind the centre of the tool more and decrease the curve.

Good luck with it, anyway. :smiley:

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Good on you Mike. I need to get my dumbbell before I can actually start grinding my mirror. I can't offer any advice at this stage, due to utter lack of experience!! But encouragement I can do. I am sure either John or Nigel will be along soon to offer their help. I shall be watching with keen interest.

Good luck.

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Thanks for the prompt replies guys, I thought this was the simple part.

@Jim - I've started using longer stokes with the 220 grit. I had read that the most common mistake for a beginning to make, was to make the strokes too long, so maybe I was overcompensating. Not sure.

@ Rusty - thanks for the support, love to hear about your adventures with that dumbbell sometime :smiley:

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Yes, tool-on-top with longer strokes and you might as well go back to 80 grit as the sagittal difference between an f/5 and f/3.5 is 0.8mm which is the amount that will be needed to be removed from the edge. Make sure that you have, and maintain, a good edge bevel. I would try strokes at least 1/2D for a while. As you get nearer the curve you can shorten the strokes to 1/3D. 1/2D strokes will overwork the edge of the mirror and centre of the tool so going to 1/3D stroke will help smooth out the curve.

What tool are you using? If it is a solid disc of glass watch for the mismatch in the curves which can lead to the discs jamming together. At the slightest "tug" when you go centre over centre stop, clean down and use a fresh lot of grit.

Nigel

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Thanks Nigel, back to the 80 grit it is :smiley: It'll remove a little more pyrex from the mirror, and at this rate I'll end up with a light-weight 6" mirror :grin:

I'm using a concrete tool, with square glass tiles, and they've done a good job so far. Although I was thinking of adding a second layer and this seems to be a good time to do so.

Also, as the tiles don't reach all the way to the end of the disc, it seems I've been using an even shorter stroke than I intended. As it's only a six inches disc I'm sure that made a difference.

Finally, I've been wondering about the differing advice give on which stroke to use during the fine grinding stage. Stellaphane suggests the 'Normal' stroke, while Howard recommends the 'W' for all stages. Just wonder why the they disagree. And I'm kinda of thinking of using a combination of the two, say four Normal-strokes to one 'W'. Or shouldn't they be mixed?

Thanks for all the comments so far, off to order more grit :rolleyes:

Mike

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So, your tool's smaller than the mirror? That will be the reason for the failure to get the radius longer with a fairly short stroke and tool on top. If the tool will be smaller after you have added another layer of tiles I would tend to use the tool off centre from the mirror to get more action at the edges. Don't repeat strokes without moving the mirror/walking round it and rotating the tool. You have to move a sub-diameter tool around a bit more than a full size tool to keep things even over the mirror surface. One disadvantage of a tile tool is that you cannot see what sort of contact you have with the mirror. Also it will not stick like a solid tool can. Once you think that you have got to curve use the 120 grit for 5 mins and then check that you are reducing the 80 grit pits evenly over the surface with a hand lens/loupe. ( I use a 10x loupe, a 25mm simple eyepiece can do - might have to use it backwards. ) With a sub-diameter tool continue with the W stroke for the rest of the grits frequently checking with the lens that the surface is being ground evenly.

Nigel

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I wouldn't really call it a sub-diameter tool. I made it by covering the surface of the mirror in tin foil and made a dam around the mirror with cardboard, wrapped in more tin foil. So basically the disc itself is the same diameter as the Mirror, but as the tiles don't go all the way to the edge I'm assuming it's acting as a sub-diameter tool.

I'll try an upload some pics tomorrow, that should really clear things up :smiley:

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If you make a stroke centre-over-centre and there is a bit of edge not touched then yes it is sub-diameter, even if only just. I would still grind with the centre of the tool about 1" or so off centre for a while and see how that was reducing the curve and when near the target switch to a W stroke to smooth off.

Nigel

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Thanks Nigel, I think you cleared that up :smiley:

As the tiles are now so thin, and I don't like the thought of playing around with a sub-diameter lap at the moment, I've decided to make a new one. Trying to keep things as simple as possible :grin:

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Thanks for asking Alan, things are going well here. With a new improved Mk2 tool, and the advice from Nigel above, I'm back with a f4.9 curve. I thought I'd never see that again with this mirror :grin: Feels good! But I'm in need of some more 220 grit and fresh pitch, tried phoning Beacon Hill telescopes but got put through to BT, not sure why :huh:

Maybe I'll try e-mailing them, although their website says they'll only answer urgent messages via e-mail - just see what happens I suppose. I'm playing around with mirror cell design and testing stuff at the moment. And of coarse following your thread with great interest :grin:

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  • 2 months later...

There's good and not so good news.

The not so good first, Beacon Hill no longer supplies mirror making kits, or grits, but Barry was very helpful with a few phone numbers. So I received a block of Pitch and some 180 grit from Galvoptics last week. Since then I've cracked on with things and I've gone through 7 grades, only one more left before polishing the glass begins. I have to say it's looking very pretty at the moment :grin:

For those who like to learn from the mistakes of others, I think the picture below explains why I first end up with a f3.5 curve on the blank. It's kinda obvious once you take a photo of it :rolleyes:

post-27253-0-78541600-1369081891_thumb.j

Thanks to everyone for digging me out of that hole, I like to think that the 'mark 2' version shows what a quick learner I am - rather than what a rubbish first attempt it was.

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Great news Mike. Glad you have got it sorted now. Don't forget to keep your edge bevelled, and be ruthlessly clean when you start polishing. It is very easy to pick up some crud from somewhere and put a gert big scratch in your lovely smooth surface. Trust me, I know!!

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  • 2 weeks later...

Finished the grinding stages, here's a couple of pics showing the mirror awaiting polishing - and the magic a little water on it's surface does.

I've spent most of the afternoon trying to build a 'Stellaphane type' Foucault tester, so I'll soon be pouring the lap and then it's all down hill :grin:

post-27253-0-72706900-1370209371_thumb.j

post-27253-0-10793800-1370209404_thumb.j

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Not sure that it is all downhill now. When you start polishing check frequently that you are polishing the whole surface evenly. If there is any imbalance in the polish you are getting then you must correct the problem immediately otherwise it is easy to end up with a turned edge or deep centre and you don't want either of those. The deep centre was often called a "Fatal Hyperbola" in some of the older literature and the recommended solution was to correct it with a sledgehammer ( or a bit of 80 grit ). Start with mirror-on-top and short strokes. If the centre is noticeably more polished after about 5-10 minutes re-press the lap and do a bit of tool-on-top to bring it back to even. If the edge is more polished then re-press and use a slightly longer stroke.

You will only need a pinhole and knife edge at first as you are aiming to get a spherical surface ready for parabolising. You can continue to make the Foucault tester while the lap is pressing between polishing stints.

Have fun!

Nigel

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Thanks for the advice Nigel, and yep I know there's sooooo much left to do.

I read about one guy who took 18 months to figure his 6" mirror :eek:

But it feels good the have completed one stage, hopefully without any hidden problems. There's nothing obvious wrong with the mirror so far, no chipped edges, etc, no pits left over from earlier grinding grits. I just hope it takes a polish and the only way to find that out is, I suppose, to start polishing :grin:

I've just poured a plaster of Paris base for the pitch, so I'll be spending this week checking out youtube videos and reading up a little on polishing, Can't believe the 'ultra thin mirrors' people are making, that's really pushing things to the extreme IMO, but really interesting. So much stuff around, and I won't even mention the Mirror-O-Matic :rolleyes:

Anyway, looking forward to polishing but fascinated by what others are doing

@MedicD - have to admit I'm using good old washing up liquid to clean the surface, the amount of research that's gone into developing that stuff is unbelievable

All the best, Mike

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It took me 9 months ( Yes, O.K. ) to make my first mirror but that was working only once a week and more time was spent quenching my thirst than actually working on the mirror :grin: .

From your pics I can see you have what appears to be a "Suprax" blank. I have made many mirrors from these and they come up nicely. ( "Suprax" is a Schott product, not quite "Pyrex" but similar, originally intended for reactor vessel windows. It has far fewer bubbles in than "Pyrex" )

When you have finished the polishing/figuring use just water to clean off, washing-up liquid can leave a surface deposit that could upset the aluminising.

Nigel

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Good luck with the polishing Mike, when that surface starts to shine, it is really satisfying!! As Nigel says, make sure to polish out evenly and fully. Also make sure that your pitch is not too hard before you pour it. My first lap was too hard and did not follow the changing shape of the mirror. I had to remake it with pitch that I softened with boiled linseed oil.Worked fine after that.To test it, heat the pitch, then drop a small blob into some cold water. You should be able to press your thumnail into it and leave an impression at room temp. I was leaving an impression in my thumbnail to start with!!

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  • 3 weeks later...

And make sure the room temperature is right,...... I spent a few cold sessions in the garage during May and thought I'd bought the wrong Cerium oxide because it was going so slow! Temperature was about 6C though! 18 degrees is so much faster, smoother and controllable for figuring!

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