The 20 " grind has begun

[JRM]

Hello All,

   I finally got around to start the grinding of my 20 inch mirror, The tile tool was made back in October, the back I ground flat, then

 a peice of 3/4 plywood 18" round was epoxied to the back, and to this another 3/4 by 20" round piece was glued and screwed to it,

making my title tool some 3" thick in total, with a 1" lip all around for easy handling of the lap.

Monday I spent three hours clearing the grooves between the tile,that was harder to do then I thought it would be..

I did a grid on the glass with a sharpy to see how contact was,two wets later at 120 grit the marks at the outer 3 inches were gone but

the inner still showing, I expected as much so all is good at this point.

Just to refresh everyones memory, the glass is pyrex, 2" thick, 20 inches round and already ground to F 4.5 at 220 grit,although

my test with flashlight showed curve at about F5.4

I have started my grinding with 120 grit for two reasons, first off is to get tile tool in good contact with mirror quickly, and secondly

I will be changing the F ratio to 5, so once I have all the sharpy lines gone on the glass I will retest focal lenght and go from there

to my desired focal lenght.

and also to let you know, all the grinding and polishing will be done with the tool on top. full size tool for grinding, pitch lap may be smaller,

I will cross that bridge when I get to it, for now I just want to get the glass ground out to 9 micron or so if possible befroe winter really

sets in here.

Tuesday I did 5 wets, 15 walks aroud the barrel, the sharpy lines are almost gone, got one problem to fix though, my grinding

stand is a 45 gallon plastic drum that is filled with water, it does not sit very good on the floor and moves more then I expected, will

change the stand soon enough, plastic does not have the rim like the steel barrels, lesson learned.

Today I first marked the tiles with a sharpie to see how contact was working out, three wets later the marks on glass are completly

gone, the ones on the tile too still need work so I will do more wets to achieve better contact between the two..

My wets consist of 1 teaspoon of grit per wet, plenty of water in a spray bottle to keep things wet, no pressure other then the weight of

the tool itself,the wets last for three trips around the barrel, with between 8 to 12 stops well going around and 6 to 8 strokes per stop, a stroke consist of one third center over center with a push forward and a pull back being one complete stroke.

 I plan to test focal lenght tomorrow before doing more grinding, will let you know where I am with it..and what is needed to get to

desired F ratio.

For what it is worth, the grinding is every bit as easy as when I did my 10", just heavier to lift tool off mirror when needed, also I am

suprised at the amount of water I go through during the wets, I will be filling the spay bottle at night so it can aclimate for next session of grindind, although I doubt it really matters at this point, just good practice in my opinion.

Rick M

[Ericgraham]

Good luck with your mirror. That's a big size for hand grinding and polishing, you're going to have some new muscles as well as a new telescope! Keep us posted on your progress

Eric

[crashtestdummy]

Good luck fella,will keep watching your progress as I'm just starting a 22" grind

[estwing]

yessssss! good luck just love these threads...lots of people on here to help along the way

[mapstar]

Yeah keep us posted as me and crash are doing a bit of grinding well 22" to be exact so not just a bit! I've got to the bevel up to now just not had the time but getting there

[JRM]

Okay All,

    I did a check on radius of curve with flashlight and white cardboard well holding the glass to get light in focus,

The radius of curve comes out to be about 188 inches, or F 4.7 there about, which sounds right for what the focal

ratio was suppose to be, not sure why the discrepancy from my last test ( F 5.4 ) thats 28 inches difference

in the radius, Hmmm

 No wets today, too tired, so until tomorrow  take care all.

Rick M

[clubjoker]

Watching with interest. It's a dream for me to do this myself one day.

Some photos would be fantastic

[JRM]

I got some pics to share...

This is to show how I made the lip on the tile tool, make for easy handling when removing

from the glass..

this one shows the lip better..

and this one shows a quarter on top the 20" glass disk

and finally the marks on tile tool after three wets

No wets tonight,will grind tomorrow till good contact is made between tile tool and glass,

have to make a better stand tomorrow as well, cheers

Rick M

[Rustysplit]

Nice one Rick. Just watch that you clean the lip out too when changing grits. Looking forward to following this.

[JRM]

For sure Alan, I had siliconed the inner section of the lip and plan to at least paint the inside lip and the rest of the plywood after a complete clean

and scrub down between each grit change. That is my only real concern with the lip, must insure no grit is there to contaminate at a later

stage of grinding.

Well that and the fear of the epoxy letting go.

I did seven more wets, 18 trips around the barrell and have marked a grid on the glass with sharpie to check for contact...

I will do more wets tomorrow to see how contact is but I have a question, my tile tool ,though it is 20 inches in diameter, the

tiles are only 18 inches in diameter for the most part, should I be making a slightly longer them 1/3 mirror diameter stroke?

instead of 6.5 inch strokes should I be doing 7 on average, I am thinking my stroke needs to be a tad longer.

I will read some on this just to be sure.

Did not make new stand as of yet but I found a tablesaw stand I have and thick it will work just fine, will keep you posted.

Rick M

[Sfarndell]

Hi Rick

Congrats on your progress so far! Large mirror making is so rewarding. I'm in the process of re-working my 18" x 1" mirror and experienced some of the concerns you posted above. I made a number of mistakes along the way, but this is my experience of the outcome of what finally went right which I hope helps you a little: I worked exclusively mirror on top and scrubbed my tool after each grit. There were loads of nooks and crannies on the side for large grit to hide in, even with vigorous scrubbing. My grit sequence was 80, 120, 240, 320,400,600,1000....then 400,600,800 and I spent between 60 and 90 minutes actual grinding time on each grit - 2 hours on the 800 grit.

With the MOT approach, I reduced my risk of scratches and actually only got scratches when I used 1000 grit due to clumping. On my regrind (went back to 400 grit), I chose to polish out from 800 grit (12"/66% lap) and apart from some trouble with the shape from a now-warped tile tool (another grinding how-not-to-do-it experience), I now have a nearly-spherical mirror with no visible scratches. 

My tool was about 17.5" in diameter. It was not anywhere near as uniform as yours which looks extremely well made. My fine grinding strokes were between 1/4 and 1/3 of the primary mirror diameter. I alternated the length every 2 trips "around the barrel" and used the sharpie/pencil test just before changing grits as well as after the 2nd/3rd wet of the new grit size just to make sure things were ok.  The length of the stroke while grinding will only really change the radius of curvature, and you are extremely unlikely to get a TDE until you start polishing so don't be too concerned unless you have a specific RoC in mind. I wasn't concerned about that, as long as my mirror was spherical and the faster the mirror the better for my project (cassegrain). Things looked ok until polishing, but I think I pressed too hard on the 800 grit and my mirror flexed, creating a slight channel at +-80% which you are not likely to get with a 2" mirror. Everything considered, this was a minor issue as by the time I'd fully polished out, (4-5 hours later) my current shape is now a slight turned up edge and a small hill/oblate sphere to the 50% zone.

Good luck with your project! I will be following your progress with much interest!

Scott

[JRM]

Thanks Scott,

   All my work will be done tool on top. the weight of the glass is 47 pounds, the tool is at most 30, I just have to be extra carful and insure complete cleanup

 between grits.. speaking of which, I did one wet and sharpie marks were all evenly faded almost completely, did one more wet and they are gone..

So I made my new grinding table with table saw stand, this brings my stand down from what was 40 inches to the top of the glass to a much better

30 inches, I am in the process of syphoning the water from the 45 gallon drum with a 1/4 inch piece of hose they use for the ice maker on a fridge

which is so slow watching grass grow would probably be quicker, but make do with what we got so they say.

 I will completely srcub the grinding stand top and tile tool, paint what needs painting to insure no grit left behind will fall off onto the surface of the

glass, then start on the 220 grit, hopefully by wednesday.keep you all posted.

Rick

[JRM]

Yesterday I got the new stand set up, have some forty pounds weight on the shelf I made below for ballast, scrubed the platform completely, twice, also cleared grooves

between tiles and completely scrubed, washed and the repainted the plywood and lip  twice, washed mirror.... twice...( no such thing as overkill ) then placed mirror  

on top of stand with sheets of newspaper under it.

Today I went down stairs to do some wets to find I forgot to make new blocks to hold mirror in place, so I cut three unpainted blocks and secured mirror to stand,

sharpied the mirror surface and did two wets with 220 grit just to see where things stand,

I am pleased to say the new stand is much better at the lower height,the stand is solid with no movement, even placing the tool onto and removing it from

the mirror is much easier, after two wets sharpie lines are gone

I am finding the wets dry out fast though, the wood heat does not help matters either, will have to spend more time keeping things wet to prevent scratches and such.

I made and painted new block to secure mirror,going with three instead of the four I used for the 120 grit,will secure them in placelater, plan to do as many wets as I can with 220 grit tomorrow.. until then....

Rick M

[starman345]

Hi Rick, glad to see your project is underway.  Sounds like you are having fun. I'll be following along with interest.

[JRM]

Hi Brian,

  I was going to message you today, the grind is on yes, until 9 micron if I can get that far without sleeks or scratches happening, then

I will break for the winter...

Today I only had time for 5 wets with 220 grit, and I noticed a few things, one is the wets are only lasting for two trips around the barrell,

number two is without using a magnifier the surface looks as though it is all evenly ground, like I could be finished with 220 grit, but even if it

were done, I am not, will do more wets before I even begin to check pit sizes.

 I am finding it hard to turn the mirror one the table, its like the newspaper I have under it is sticking, I loosened the blocks off from the mirrors 

edge and no difference, might be first and last time for newspapers under mirror, need to find some carpet.

My biggest thing is the glass, When I ground my 10 inch it was plate glass, pyrex is a whole different feel to it,I like the way it

grinds, and feels, I find it to be a nicer smoother grind then the plate glass,I think this grinding will be easier then I first thought.

Rick M

[starman345]

Hi Rick,

Interesting about the Pyrex-glass comparison as far as grinding, I've never tried Pyrex but I may in the near future.  You spoke about your wets drying out quickly, I wonder if it is the cold we've had and the low humidity. I fired up the humidifier here this morning for the first time in a few years(-22 C here this morning), the wood stove has been humming along steady for about a week and it really dries things out quickly. Maybe your glass sticking to the newspapers is related too...I use carpet but if you are using wet papers it might be drying out. At 47 lbs that is one big chunk of glass to move around. 

Good luck on your project, I've already learned quite a few things about grinding from reading your thread. It's really interesting with the pictures and explanations.

[JRM]

The wood furnace is definitely the culprit for the drier air ,will have to make do as the winter storm is on....

the other thing is I was not cleaning between wets, I would just add more grit and water for three wets then clean up before

recharging with grit and water, makes for a better grind so long as you keep it wet. 

 Today I did 6 wets,cleaning after each wet, though the wets did last longer, I stopped after two trips around the barrell because I

did more steps around, instead of 8-10 steps I am now doing between 12-16 steps.

 I have to go out and get a new magifing loup to check pits later today after a few more wets of course.

Rick

[JRM]

Got y 5x magnifier loup today and did a check for pits, I did find some larger then the rest but for the most part the pits

are uniform in size, will do more wets to see if the few bigger pits change location or stay put, but figure no more then 5 wets

will be needed to finish 220... so I shall do 10 more.

 One another note... I did notice something I at first took for a huge pit but with more scutiny found it to be a tiny bubble, with

further investigation I found another even smaller bubble,in the whole glass there are only the two,they are tiny and about 3/8th

of an inch from the surface so they will not be any problem, they where found by glancing the light of a flashlight at an angle to the

 face of the glass, cant be noticed any other way, unless you know exactly where they are, but I am sure once the glass is polished

out they will be asy enough to see.. was definitly interesting to find them that way just the same..

Grind more tomorrow, shovelled to much snow today, what a storm, glad its over.

Rick

[JRM]

Hello Everyone...

   Today i placed a folded bedsheet, maybe eight layers thick under the glasss before grinding I had removed the newspapers two grinding sessions

ago but did not want to continue with nothing under the glass,the glass turns real nice now.

I did 11 wets,after doing 7 wets I sharpied the glass and the tile tool, after one trip around the barrel the marks on the glass where pretty much evenly faded

 if anything the outer edge looked a tad brighter, after completing another wet the lines were all gone on the glass and very evenly faded on the tile

tool other then 2 tiles..

After finding that the very edge was grinding out last I decide to do a slight W stroke instead of the C.O.C. and will continue the slight W stroke from

here on out, but being as I have changed stroke I will do more before changing grit size, my mag loup shows the pit size to be very uniform out to the edge,

but I want to insure uniformity with tool and glass before moving on to 320.

I just don't think three wets is enough because of the stroke change, anyone have any thoughts on this?

  Thanks

Rick M

[JRM]

Yesterday I did 4 wets with a narrow W stroke, and today I did 5 wets with Narrow W stroke first trip around then C.O.C stroke for the second trip around

the barrell of each wet and did an extra walk around for the last two wets, 220 grit is done.

Cleaned the glass,the grinding stand and cleaned the tile tool completely.I have repainted the wood and the lip of the tile tool so everything is ready for

the move to 320.

Some notes ....The wets with cleanup between each is about 1 hour for 5 wets, cleanup being wiping down the mirror and tile tool to remove the sludge

before recharging with grit and water..so thats 4 hours of 120 grit and  some 6.5 hours with 220, not including cleaning between grits of a good

1.5 hours each, bringing the total hours to almost 14 hours.

Did more wets then needed with 220 grit only because I want to make focal lenght longer so don't take these 33 wets as required, other then

trying to increase the focal lenght the 220 could have easily been done before 20 wets.

just a note..it takes about 50 squirts of water per wet, way more the when i did the 10",but as stated before the pyrex has a  feel different to the plate

glass I ground before...next grind is going to be 320 grit. until then...

Rick M

[JRM]

Hello All,

       Started 320 grit today, did 5 wets, 12 trips around the stand cleaning after each wet. find the wets are lasting longer and can now do

3 trips around before cleaning and recharge of grit.

Its not a lot of grit as some of you may think either, I am using a plastic teaspoon (never use steel) and at most  use 1 slightly more then level spoonful

per wet, the water is the same about 45 squirts.

I am doing a mix of strokes as well, one or two wets of narrow W then C.O.C, and vise versa, did another 5 wets, 15 trips around then

let surface dry to check with loupe, can see a half dozen large pits,(larger then the rest of the pits) plotted there location on the surface on

a piece of paper to see if they stay in same place after next session but for today I am done, 2.5 hours, 10 wets and 27 trips around

the stand, and the grind continues..

Rick

[Rustysplit]

Nice reports Rik. Sounds like you are making good progress. Go on, show us some pictures of the stages. Are you doing tool on top exclusively?

[JRM]

Hi All,

Thank You Alan,

  Yes, all the work will be tool on top, I am trying to lenghten the focal, I would preferr an F-5 so grinding tool on top

will benefit this goal. Once F-5 is reached, if it is reached, I can modify different strokes to maintain the F-5, its not

critical though, will be happy no matter what the final lenght is but closer to the F-5 is what I am shooting for. 

I did a sharpie test today , marks were gone before I finished second trip around, only had time for 3 wets, 8 trips

around, checked pits and those few I plotted are not there,but a couple showing up in other places,looking good I say.

 I am thinking next session with 320 will be it for that grit size, let ya know tomorrow.

Happy New Years.

Rick M

[starman345]

I'm following along with every post Rick,  Happy New Year to you and yours!

[Rustysplit]

I did my 14" fully tool on top. It was no real issue to use differing strokes to maintain the focal length.