The 20 " grind has begun

[JRM]

Okay folks,

 320 grit is done, well for all intent and purposes, I just have to spend some time tomorrow going over the surface with the loupe

and insuring no big pits remain.

I did 5 wets today with a total of 12 trips around, all C.O.C. strokes T.O.T.

 One teaspoon of grit per wet except for the last one, I used only half or so a teaspoon and did two trips as a last wet,

These last 5 wets I did more stops around the barrel so to speak, some 24 to 28 stops per wet, the grinding is going very smoothly

so far, I am getting the mixture right but still think to much grit is being used,oh well better more then not enough.

I took pics but it appears they turned out blurred, will try again with next grind session.

Alan, I agree about strokes and focal, did my 10" with T.O.T  and had my focal lenght right on 60 ", little more fun with the

bigger size but yes easily managable.

tomorrow will tell if I am truly done 320 or not, if so then its on to mixing the grit in a watery slurry.tomorrow tells the tale...

Rick M

[estwing]

Good progress nice and steady, hope it's not grinding you down:)

[JRM]

      It is offical, 320 is done, found an incandesent light and did the glacing look and surface looks good, I also went over the whole

surface with the light and a loupe everything looks great,so now its a complete clean up scheduled for tomorrow and get everything

ready for saturday, the start of 500 grit and mixing up the slurry.

When I did my 10" it was 220s, 320s, 25m, 9m and 5m and I do have 25m but will skip it and go to 500 as that is roughly 20m and falls

almost half way between the 320 (approx-30m) and the 12m.

Will pick up a wet stone tomorrow as well and redo the bevel before going on to the finer grits and I best get the mirror holding stand

done soon for testing as well.

Estwing...I have notice a sweat being worked up after the last couple sessions, I blame it on the wood heat , but really its not bad at

all, different story after the grinding is done and it is time to polish out.. that may be a chore, will cross that bridge when the time

comes

Rick M

P.S. working on getting more pics in future, maybe even a video or two, will give a better insight into the hand grinding and

polishing process of large mirror making... will see what I can get setup.

[JRM]

Okay,

  Been under the weather the last couple days but finally got around to regrinding the bevel with a honing stone, if you have never done this before well,

you grind at a 45 degree or there abouts angle to the glass going along the edge as you circle the glass,just when you think things are going fine....

you guessed it the darn thing slips and your going across the glass surface instead of the edge....input favorite curse words here...

did I say I hate beveling the edge,did same darn think with my 10inch mirror.... well I got lucky, after everything dried there is no  scratch or gouge to be

seen...really thanking my lucky stars and then some. the bevel is done.

Did all the cleaning, washing and scrubing of stand, glass and tile tool, everything is ready for the next stage (500s), Hope to start tomorrow evening, 

until then....

Rick M

[Rustysplit]

  You had me worried for you there Rik! I thought you were gonna say that you had knocked a seashell chip out of the edge, phew!! Quick, go and buy a lottery ticket as your luck is running with you at the moment.

[Astrobits]

For those who will be making their own mirror, do the final edge bevel after the first coarse grind. Don't leave it until the finer grits have been done. The finer grits remove very little of the edge and the bevelling can always produce those oyster chips from the surface with one careless stroke. Or you drop the stone and get a big scratch/chip. Then it's back to coarse grinding to remove them.

Rick was lucky this time.

Nigel

[JRM]

I totally agree Nigel, 

The bevel was good when I started and would probably have been fine till finish,

just did not want to chance it and have to redo at an even finer stage then 320,

my attempt to lenghten focal was only reason I went at the bevel again, and yes

lady luck was definitely on my side for that fowl up.

Rick M

[starman345]

Video would be great Rick, I'd love to see that 47 lb piece of glass. Thanks Nigel for that edge bevel advice, I didn't know that and will be doing it on my grind.

[mapstar]

For those who will be making their own mirror, do the final edge bevel after the first coarse grind. Don't leave it until the finer grits have been done. The finer grits remove very little of the edge and the bevelling can always produce those oyster chips from the surface with one careless stroke. Or you drop the stone and get a big scratch/chip. Then it's back to coarse grinding to remove them.

Rick was lucky this time.

Nigel

Thanks for the advice

I've done a decent bevel for the initial grinding so will renew on my 22" with a nice fine 220grit grind before moving onto the finer grinding and polishing.

I am nearly finished with flattening the blank back but not had time to post anything about starting and progressing my blank/mirror.

It is virtually flat with about a 1/4mm concave at the most so hopefully it will be about right.Next stage is the 400grit to finish it then I can hog out the curve to F3.9

[mapstar]

For those who will be making their own mirror, do the final edge bevel after the first coarse grind. Don't leave it until the finer grits have been done. The finer grits remove very little of the edge and the bevelling can always produce those oyster chips from the surface with one careless stroke. Or you drop the stone and get a big scratch/chip. Then it's back to coarse grinding to remove them.

Rick was lucky this time.

Nigel

Thanks for the advice

I've done a decent bevel for the initial grinding so will renew on my 22" with a nice fine 220grit grind before moving onto the finer grinding and polishing.

I am nearly finished with flattening the blank back but not had time to post anything about starting and progressing my blank/mirror.

It is virtually flat with about a 1/4mm concave at the most so hopefully it will be about right.Next stage is the 400grit to finish it then I can hog out the curve to F3.9

Damian

[estwing]

F3.9...sounds good mate

[mapstar]

F3.9...sounds good mate

Cheers Calvin

I'm hoping the mirror and scope will be finished for the peak star party this year ;-D

[JRM]

Got 2 hours in today,7 wets with a slurry mix, made of 5 teaspoons 500 s with 5 ounces of water,also the occasional spray

to keep thing from getting dry.

The wets are lasting longer, easily 3 trip with some 36 stops in those 3 trip around, i do one trip with 16,second trip 12 and

the third trip with 8 stops,these trips get mixed up with each wet to be sure and keep things mixed up.

Once I got started I did not stop till the batch that I mixed was all gone, did not want to leave it set overnight for fear

of clumping even though it has to be shook up before using, so will have to mix next batch by the time I want or have to

put into the grind for that day.

checked the surface of the glass after it dried and it looks good,there was no sticking during the session so I know the surface

is in good shape.will do another session of 500 then its time to test to know exactly where the surface is.. but

thats it for now, take care..

Rick M

[Sfarndell]

Sounds like you're making great progress Rick! keep the posts coming!

[JRM]

Ty Scott, New progress report, grinding wise, 500s is done.

  Yesterday I did a grid on the glass with sharpie to see how quick it would be removed when I did a wet, mixed up

3 teaspoons 500s with 5 ounces water today and did 4 wets, 10 trips around, sharpie marks were gone before first

trip around was completed,did 2 trips on first two wets and 3 trips for the last two wets.

Will build a new more robust mirror stand tomorrow and pull out the foucault tester and check the surface, so long as

the glass is spherical in shape with no zones or such it will be on to 12m next week.

tried to get more pic today but they did not turn out, will have to use the bigger cam next time around.

till then....

Rick M

[JRM]

just to update,

 I did test and the true radius of curve is 207.5 inches, foal lenght is 103.75 inches or F- 5.18, that was easy enough,

light moves opposite direction when outside radius and same diection when inside,when it blinks thats the R.O.C.

  I use a side marker light of a car but think I need to upgrade to an L.E.D light, I had though the light would have

refected back after 500 grit without wetting the glass, that is why I am thinking light is not strong enough.

will be starting 12 Micron this week.

  I was looking over my grits and found the 9 micron that came with the glass when I bought it is marked as 9 U,

does anyone know what the U stands for ? 

Thanks

Rick M.

[starman345]

I'm pretty sure that would be the Greek letter for micron, does the U have a long tail?   I ordered some grit recently and found it confusing as some places have different names for the same thing. Gotgrit has a grit table in their links section that helped me figure it out.

[JRM]

I did 4 wets yesterday,12 micron mixed with water in a squeeze bottle, 3/4 ounce grit to 7 ounces water, nd another 6 more wets today.

a total of 27 trips around the barrell, cleaned surface and let dry to check. its amazing how much faster the surface dries out as you

get down to the smaller grit sizes.I found a sleekish kind of scratch, about 4 inches in and some 5 inches long, nothing deep but will continue

with the 12 micron to remove.

Also found two pits, only two so might be ones I um missed,,, will try a few more wets to see if they go away, if not im going back

to 500. the sleek should be gone in a wet or two of 12 micron, I believe it occured well removing tool from the glass after last wet which

may have gotten a bit dry at the end.my mistake.

 A cople more wets should tell the tale, sleek should be gone but the two pits tell if its n to 9m or back to 500.. tell then.....

P.S. I got a couple pics, will post them tomorrow.

[JRM]

Today I did a total of 4 wets and 12 trips around the barrel to finish up the batch of 12 micron I had mixed up in the squeeze bottle,and attempt to remove

the scratch and those two pits I had found, and am glad to report all are gone and 12 micron is done. will clean things up and ready everything for next grit 

size, think I will skip the 9 and go right to the 5 micron but if I start getting sleeks will go back to the 9 micron. had issues with sleeks at 5 micron when I did my

10 inch mirror years ago and ended up polishing out from 9 micron.

I am finding this grind to be faster then I expected really, less wets then I thought it would take,maybe it is because of the smaller size differences, when I 

did the 10 inch it was 220,320,25m,9m and 5m, this time around its 120,220,320,500 and now 12m, the smaller jumps I find are better, maybe a tad more work

but better just the same.

A wet for the 12 micron consisted of me first spraying water on glass to wet it, about 10 sprays, then squirting about 3/4th an ounce of 12 micron mix on to 

the glass and doing 3 trips around the barrel cleaning after each wet.

I ordered 2 Kgs of #64 gugloz pitch which arrived last week, so pitch is here for when things warm up and the wood fire is no longer needed.

Thinking polishing will not begin until May at the earliest.

5 micron here I come..

Rick M

[JRM]

Pic of glass with 12 micron grit on it,about 3/4 ounce, looks like milk.

pic of spent grit after the wet..

and here is a pic of the edge, 2 inch thick...

looks alot messier then it really is..

also found these two of grinding tile tool back flat..

and

that was a chore best done outside.

[Rustysplit]

Excellent progress Rick. It is interesting to see your total time so far. Shows how much time can be saved by getting a pre-generated curve blank. The hogging out was the longest task on my mirror, and by far the messiest!!

Good luck with the final grit. I bet you are itching to get on to the polishing stage?

[RCompassi]

Nice progress!

Keep up with the pics, please!

[RyanNehring]

Wow!  I admire your ambition and it looks incredible.  Hope I'm brave enough to try this someday!

[JRM]

Hi All,

      Mixed 4 pretty much level teaspoons of 5 micron with 5 ounces of water and 2 drops of dawn dish soap and proceeded to

do 5 wets and 13 trips around the barrel so far, cleaned everything up let surface dry and did a real close  inspection with flashlight,

5x loupe and a new pair of glass for me, everything looks great so far.

No edge sleeks to be seen, no bigger pits to be seen, smoothest ground glass surface I have ever seen.

will be doing another round of wets this evening only because it is the last of the grinding and I planned on doing extra wets with

the final grit to insure a very smooth surface,just have to worry about scratches but 4-5 more wets and a lot more time inspecting

before I say 5 micron is done, I have some 3 micron but thinking 5 micron is far enough by hand.

so until then...

Rick M

[JRM]

here is a pic of glass with charge of 5 micron grit (1ounce )

this one is after completing 5 micron wet

and one last look at fullsize tile tool

and the 20 inch glass (wet)

if you zoom in on the pic with the 5 micron charge you will see what looks like streaks in the charge,when

I first seen this the wet before and thought it was dirt that fell onto the surface so I set the tile tool back down

and was about to wipe the glass clean when I realized it was the dawn soap that was causing this, phew.... what

a relief that was, so it was grind on.

Did a total of 5 more wets and 14 trips around the barrel and can now say its official, grinding is done, bar a

real mess up during polishing.

I am glad thats done but hey it was far easier then I thought it would be. I have come to the conclusion that

grinding big glass is easier and quicker then grinding smaller glass.

Yes rustysplit I am itching to get polishing but I also know it will be a much tougher part of the mirror making

process but I'm up for the challenge. I may get a chance to polish this weekend, will keep you posted.

Rick M