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The 20 " grind has begun


JRM
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Agreed Nigel,

   

     Parabolizing has begun, after cold pressing for 30 minutes I did  1/2 hour (6 tatb) of normal parabolizing stroke M.O.T., will test tomorrow

evening to see how things are going, now using the white ceo, a tad slower working, i figure what I am seeing is nothing to worry about, shoulld be

gone long before figuring is done, if not its back to a sphere and chalk it up as experience but thinking all will be fine...

Rick M

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Once you are nearer to fully parabolized it will be an awfull lot ( and a waste ) of work to return to a sphere just because of some zones however big. Don't forget that the 8 waves of glass have to be removed from the central inch or two only. To get back to a sphere you would have to remove those 8 waves from the rest of the mirror. This is why those old books talk about the "fatal Hyperbola" It is so much trouble that resorting to fine grinding is quicker than trying to polish out a deep hyperbola.

Your 20" f/5 will need small lap figuring as you are very unlikely to get fully figured with a full size lap because the amount of glass to be removed from adjacent zones is significantly different. Any size of lap can be used, I have even used my thumb for some narrow zones. Small laps can, themselves, introduce some zones so careful checking is needed and frequent return to larger laps helps keep things smooth.

Nigel

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Nigel,

 my plan is to sneak up on the parabol slowly with short sessions, lots of pressing and testing testing testing.

I do plan to make a smaller lap or two soon as well, I tested last night and things are going in the right direction

albeit slowly but thats exactly how I want it, this way I can keep track of things because we know a pitch lap can be

a funny thing to understand sometimes in how it works.

David Harbour told me when I made my 10" years ago to use my thumb to lower a t.u.e. issue when I was figuring,

it worked great, main thing is there is no rush so I will be testing more then anything and working out a strategy

before each session.

I believe it would take an awful mess of the figure to go back to sphere, best to avoid that, I have used star laps,

small laps, even a unchannelled fullsize lap well doing the 10" before and agree the fullsize 20" will not get me

to the parabol, but it can bring me close so long as I am careful, the white ceo is slower then the pink I was using

to polish out and I do intend to use the red rouge to slow things down even more once things get close, but I know 

figuring a 20" is going to be a challange,a learning experience for sure, I have some experience but when it comes

to the big ones I am but a greenhorn, your words of wisdom are appreciated, keep them coming, I will have questions

along the way and will be looking forward to your advice.

cold pressed today for 30 minutes then did two 15 minute sessions with a 20 minute press inbetween...

test tomorrow...

Rick M

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Reading with interest and apprehension about doing my 22".

I've not even cast the pitch lap yet but getting nearer to doing it with every day now that the first scope build is nearing its end.

Keep up the posts Rick

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been a busy week, I got some testing done, the curve is starting to take shape,need some more work but things

are moving in the right direction. have to get more action to the outer most portion of glass before it gets

to far behind, looking good so far.

Those rings I mentions in previous post are still there so I can say without a doubt they are coming from the bottom

of the glass just as I had thought.

going to make a smaller lap tomorrow after I do a figuring session.

Rick M

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did a 1/2 hour session Sunday and tested that evening, looking good,so I did

another 1/2 hour ( total figuring time) yesterday, repressing after each trip around, 

tested this morning and two lines with Ronchi shows an hour glass figure outside

of focus, time for a different stoke, outer area is in need of more work.

maybe a more central parabolzing stroke, will think about this, make a plan of action

and go from there..till then...

Rick M

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okay, looks like parabolizing strokes with less width to help bring more

correction to the edge is what is needed, testing shows correction

is needed from about the 70 - 75% zone outward and my edge is turned

up :)

this will be my next course of figuring tomorrow as I have to heat the

room up to a working temp of 73 degrees form the cooler 68 it is currently.

hopefully I can bring the edge to where it needs to be without turning it

down in the process, test more worry less is a good motto at this stage.

so its a session tomorrow , test the next day and I will let you know how

it went, till then...

Rick M

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Getting closer Rick and fingers crossed it all goes the right way.

I'm gonna be making a start on mine again at the end of the month all being well and hopefully polish it out without scratching it?

Keep up the reports as we're still here reading

Damian

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over the last few day have done three sessions with a total of 1 hour 45 minutes figuring time, I am pressing after every

trip around or so to keep good contact, everything is moving out towards the edge with the center starting to come

back up, will modify stroke next session , and the t.u.e. is still there, lets see what next session brings...

Rick M

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where to begin,

 after my last session I decided to take the lap outside and open up the channels, it was in the high 20's

(80 ish F) so I stayed in the shade, when I did a parobolizing session later that evening I had the

room heated to 75 degrees F, after 15 minutes I lost the center of the lap,I have heard of this

but it was a first for me, I am thinking the room was to hot, was doing center deepening strokes at the time, 

I tried pressing but could not get the center back,ended up with onion bags between lap and mirror with a 

very long press mirror on top, no weights,with temp down to 70 degees, but finally got full contact back and

now with a very nice micro fauceting from the onion bags,have since done a half hour of probably the best

contact yet, will test tomorrow and see how things went, but its onion bags between lap and mirror from now on...

Rick M

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Interesting find there Rick. Wonder if its been tried before I know they use the drainer grids sometimes to put facets in?

Should be casting my lap next week as tIe scope build is nearly there so hoping to start polishing.

Damian

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spent the last week remaking the fullsize lap, the old one was too hard so I added 25 milliliters of linseed oil and another ounce

of beeswax to the mix to soften thing, poured the 20" lap monday evening, spent tuesday and wednesday cutting channels and

pressing into shape, today after pressing I did 25 minutes of smoothing strokes c.o.c., will check surface in the morning to see

how things are going,hope to have things smooth soon, things are getting cooler here so time may be limited, here is a photo

of the new lap, squares are 2 x 2 and I micro faucetted with onion bags as well...and my room temp is now 65 degrees, as I said

cooler...

post-30729-0-96562000-1411080381_thumb.j

Rick M

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okay all,

currently head back to the sphere, the lap being to hard made a mess of the surfaces with a shallow zone out near the edge as well so its back to the sphere 

I go, will have to get more polishing compound as my supply is getting down, also trying to plan for polishing through the fall if possible, hate the thought

of waiting till next spring, will work something out.

Rick M

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Hopefully will be casting my lap late this week so will be making a start on the polishing.

Rich is progressing well with his polishing and getting decent contact with his lap.

Glass pusher (John Nichol) is running a seminar again so I'm going to try to get my mirror polished and attend as he will be concentrating more on testing. When its nearing completion I'll make a start on the scope build for it

Lap looks good Rick

Damian

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I feel for you Rik. When I did mine, I remember the huge difference that getting the lap soft enough made. The contact just becomes perfect and it seems to "suck" itself to the mirror.  Can you not sweet talk  your other half into letting you do the figuring in the house?

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Hi Alan,

   I am doing all my work in the basement, but with fall moving in I will real soon have to turn on the heat to

keep the better half and our kids happy, thats not the real issue though its the dust that gets sturred up with

the furnace blowing heat around, will work something out I am sure..

Rick M

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  • 2 weeks later...

still at it guys, took a couple weeks break from it, had to get the firewood done and such, also been looking for polishing powder as

my supply is running low, the prices are ot to bad for the stuff but the shipping stinks, cost more to ship a pound of polish then the

polish itself, oh well gotta have it, I did a total of 2 hours mot, coc over the last couple weeks and a session yesterday and today of

three trips each coc, tot, testing after each session, the zone is all but gone, couple more sessions and I will attempt to parabolize

once again,even though I am not posting much the work still goes on.

Scott, I hear ya but hope not to have to return to a sphere again, with luck.

as a side note, rain is forcasted for the eclipse this morning, go figure ...

Rick M

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Thanks for the update Rick were all still here reading although I know how difficult it can be to get round to posting everything you do.

I have made a start last week with polishing and picked up a couple of small scratches which may polish out?

Good to see a shine on the surface after months of grinding which is satisfying to say the least.

The contact on the Lap wasn't great so lots more pressing.

I took a trip up to see John the other day once I'd got a couple of hours on it and we put the mirror under test.

Quite daunting after all these months but showed a good sphere although the middle looked raised but this was due to the lack of polishing in the area.

I am back to centre through strokes to rectify after pressing the Lap for a couple of days

Damian

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Hi all,

I am being drawn further and further towards making my own mirror and am loving this thread.

One question, though: at the moment I am planning to try making a 6" or 8" to get a feel for the process. Truth be told, I am thinking along almost silly sizes for the mirror - possibly 36" to build a Naysmith scope (why? I am fortunate to have the space to put it, I have just moved form a 6" to a 10" scope and have aperture fever, I have just come to the end of a three year DIY project at home and, like with many things I try, to see if I can...) and want to know how relevant the experience on an 8" would be for this? Reading from your experiences of pushing the tool around on 20" and similar size mirrors makes me think that a full size 36" tool could be very difficult to work. The Naysmith would use essentially Cass optics and would need to be pretty fast to keep sizes sensible so will need quite a bit of working!

I was wondering if I could make the mirror from concrete or porcelain and spin it while glazing to form the approximate parabola but I think the cost involved would be more than the cost of a glass blank for little benefit and a whole load of other issues I won't be able to ask other people for advice with. Not quite dismissed this as it is interesting but practicality will probably win.

I am a long way off starting the main project but want to get the preparation right as I don't particularly want to trash a 1m mirror! mapstar, I am heading over to your blog shortly but any advice you have picked up about the peculiarities of working your 560mm mirror would be hugely appreciated.

Thanks,

James.

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James.  I think you would be well advised to attempt a smaller (8"-12") mirror in the first instance. If you find you can't make a good one of these you might need to reconsider trying something larger, if you can it will give you confidence to upgrade. Although the basic tecniques are similar in both cases, a very large mirror is several magnitudes of greater difficulty. I also doubt whether anyone would use a full diameter tool for a 36" even with machine processing, a 20" tool should be adequate.   :smiley:

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Hi all,

I was wondering if I could make the mirror from concrete or porcelain and spin it while glazing to form the approximate parabola but I think the cost involved would be more than the cost of a glass blank for little benefit and a whole load of other issues I won't be able to ask other people for advice with. Not quite dismissed this as it is interesting but practicality will probably win.

Thanks,           James.

You will have to ask the potters James but I don't think you will be able to fire a large mirror without distortion, porcelain is mostly quite thin, high temperature firing and hard, segments may work, think dinner plates.  Potters do have a 'groggy' rough clay, lower temperature fire and stands a bit of rough (raku work) and supports used in the kilns - saggers, may be useful.  (perhaps this should have been in the other thread).   mick

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Thanks Mick, Peter,

to avoid taking over this thread, I will say thanks here and carry on with a smaller mirror for now - sounds like that is definitely the right approach in the short term.

Mick, I have ALMOST dismissed making the mirror from anything other than glass - the benefit : risk is all wrong for what I want to do.

If you have any other suggestions or comments, please PM me unless they fit naturally in this thread!

James.

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James,

I will be at the PSP and we can have a chat about mirrors and mirror making although I am no more than an amateur who has had help and guidance from someone with a wealth of experience to get this far.

Rich's (crashtestdummy) mirror grind blog has all the trials and tribulations of Rich's grind. All are different and have their own challenges.

I have well over 100hours work in mine to now with probably another 20 to go so time and dedication are very demanding.

36" is a massive challenge and one that I suspect could take years to bring to a successful conclusion.

Will be good to see you again at PSP and I look forward to chatting

Damian

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