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Help mirror grinding question

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#1 pjmulka

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Posted 06 August 2022 - 11:11 PM

I started grinding a 10” mirror a few years ago but had to put it on hold while recovering from a stroke. My last log book entry before my stroke had the sag at a f/6.7 I recently started grinding again and just finished 120 grit sc. I’m not sure if maybe I had it wrong before or what but my sag measurement today was 0.067 which puts it at a f/9.3 as I don’t play in the NBA that’s a little long. My question is how much of a change can I expect if I stay MoT the whole time during 220 sc? I have a feeling I’m going to need to buy more 120 grit. But just wanted to double check before ordering. I’d like to be somewhere in the neighborhood of 6.5 to 7. This is going to be a mostly planetary scope under light polluted skies.

Thanks,
Pete.

#2 duck

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Posted 07 August 2022 - 12:28 AM

you need to go back to #80.  It'll save time over trying to get that much removed with #120.  8 ozs of #80 should be plenty.  If you ground out with #220, figure you could change your sag about double the particle size.  You might get down to f/8.7 


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#3 mconnelley

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Posted 07 August 2022 - 01:53 AM

Hello:

  Sounds like you have conflicting measurements.  I suggest the flashlight test to find the radius of curvature.  Prop the mirror up on edge, and get it thoroughly wet.  Hold a flashlight next to your head, with the light shining at the mirror.  Find the reflection of the light in the mirror, then move side to side.  If the reflection goes the same way as you do, you're inside the radius of curvature and move back.  If the reflection goes the opposite way as you do, you're outside the radius of curvature and move towards the mirror.  When we get to the place where you can't tell which way the reflection is going, you're at the radius of curvature.  Put the flash light down, and measure the distance to the mirror.  Divide by 2 and that's the focal length; no math or sags involved.  

 

Cheers

Mike


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#4 dogbiscuit

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Posted 07 August 2022 - 04:04 AM

 For a nice ROC finder and explanation of use look at post # 52 in this link.

https://www.cloudyni...rror-log/page-2



#5 pjmulka

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Posted 07 August 2022 - 12:36 PM

So the return image comes back to a different spot on the white card? The little hole + big hole in the image is the outgoing light not the return correct?

 For a nice ROC finder and explanation of use look at post # 52 in this link.

https://www.cloudyni...rror-log/page-2



#6 dogbiscuit

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Posted 07 August 2022 - 12:56 PM

For best accuracy put the image (return) near the outgoing.

 

The three different holes in my screen is just to have some options to see best focus.

The larger aperture lets more light through helping to find locate the return beam if not so close to focus. After finding the return the smaller aperture shows focus better than the large aperture.  The + is usually the one that shows focus best.


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#7 Don H

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Posted 07 August 2022 - 05:56 PM

Hello:

  Sounds like you have conflicting measurements.  I suggest the flashlight test to find the radius of curvature.  Prop the mirror up on edge, and get it thoroughly wet.  Hold a flashlight next to your head, with the light shining at the mirror.  Find the reflection of the light in the mirror, then move side to side.  If the reflection goes the same way as you do, you're inside the radius of curvature and move back.  If the reflection goes the opposite way as you do, you're outside the radius of curvature and move towards the mirror.  When we get to the place where you can't tell which way the reflection is going, you're at the radius of curvature.  Put the flash light down, and measure the distance to the mirror.  Divide by 2 and that's the focal length; no math or sags involved.  

 

Cheers

Mike

I like to do it just like this, although for some reason I found a night light bulb made it easier for me to get the ROC. A spray bottle makes keeping the mirror wet enough to reflect easier. An assistant standing next to the mirror can continue spraying if the mirror begins to dry before you get to the ROC, and the assistant can hold the end of the tape measure by the mirror.



#8 pjmulka

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Posted 07 August 2022 - 11:19 PM

I checked ROC with the method described in post # 3. and measured the ROC at 166” f/8.3 now when I used feeler gauges I was unable to find any thinner than 0.01” so the 0.067 could have been a bit low but none the less I’m still far enough away that I’ve ordered some grit to fix the issue. I would like to save an extra cleanup by correcting to 6.5 with 120 grit sc. If possible. My question then is should I

A. Use a chordal stroke with 120 grit sc until I’m close. 
B. Use 120 grit sc CoC mirror on top.

C. Go back to 80 grit sc. 

 

Thanks again for your time!

Pete.



#9 duck

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Posted 08 August 2022 - 10:39 AM

You'll spend more time grinding with #120 than is worth it.  When smoothing (#120 and finer), it's more efficient to occasionally flip the mirror/tool while in each grade.  If you flip, you maintain a constant R.   If you flip, you want to hit the R at #80.  When you hit the R, do 2 more wets with #80, then flip and do 2 more wets, using 1/3D C over C strokes with just a tad of sideways motion.  I think most beginners spend too much time at each grade when smoothing.  An hour at #120, flipping every wet wet.  45 minutes at #220, flipping every wet. 30 minutes is plenty there on.

 

This is all opinion.  I'd really like to hear comments from more experienced glass pushers.

 

There's considerable time to be saved in clean up if you make some adjustments.  Primarily, you need to figure out how you can use a garden hose spray nozzle to wash down the table.  If you can modify your table to put a catch basin under it, that's a huge step forward.  On my machine, I have a cast iron table with a catch basin under it.  I put a layer of 4mil plastic on the table.  Then I put a sponge rubber pad.  The another layer of 4mil plastic.    After a grade, I blow the whole thing off with the water nozzle.  Then change to new plastic.  It takes 20 minutes at the most.  I'm getting lazier, though.  I just had to grind out a polished surface, going back to 9 micron.  I didn't even change the plastic.  Just blew it down with the nozzle.


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#10 pjmulka

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Posted 08 August 2022 - 07:26 PM

I bought a new set of feeler gauges today and measured the sag at 0.076 f/8.2 so very close to what I got from the flashlight test. I’d like to shoot for f/7

#11 mconnelley

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Posted 09 August 2022 - 04:19 PM

Hello:

  There isn't a big difference between 80 and 120 grit, so I'd just stick with the 120 grit and accept that grinding will take a bit longer.  I'd prefer that than to open up the 80 grit, do another cleaning, and risk contamination. 

 

   When I've done my recent mirrors, I "skipped" a few grits.  I went from 80 to 220 to 500 to 12 um then 5 um.  That's one way to speed up the process.  Also, the fewer grits that you open then the less opportunity for contamination.  Each grit is really a range of sizes, and generates a range of pits, chips, and sub-surface cracks for the next grit to clean up.  So it didn't make a lot of sense to me to go from 80 to 120 when they're so close, where as 80 to 220 is about a factor of 3 in size.

 

Cheers

Mike



#12 pjmulka

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Posted 11 August 2022 - 12:13 AM

I decided to use a 2nd tile tool I’ve been using to grind the back of my mirror as the tile tool I’ve been using is getting thin. Unfortunately after 1/4 pound of 80 grit sc I was unable to get it in good contact before I ran out. I’ve got about 5 tiles in the center that are lower then the rest. I’m not confident I can get it in good contact with 120 sc before I’ve ground a crazy amount astigmatism into the mirror. So I’m going to continue using my original tile tool and hope for the best. The good news is that I got my Sag back to where it needs to be and although the tiles on my original tool are getting low it is in good contact with my mirror. I’ll post a photo of my 2nd tile tool with the low spots.

#13 pjmulka

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Posted 11 August 2022 - 12:16 AM

Tool with low spots

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#14 ccaissie

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Posted 11 August 2022 - 06:37 AM

The new tool will cause you trouble unless it is in full contact... No #80 junk in the bottom of the bucket  to reclaim?  Some folks have tried masonry or other sharp sand....Engine Valve grinding compound could work...If you have a lot of 120 you might mate the new tool, but you've got a ways to go.

 

I wouldn't think you will grind  astigmatism in,  if you are even in tool rotation etc. 

 

If you are laready there with the old tool,  You may luck out....Remember to reverse MOT/TOT  for most efficient finishing. 


Edited by ccaissie, 11 August 2022 - 06:39 AM.


#15 dogbiscuit

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Posted 11 August 2022 - 10:10 AM

Depending on what sort of stroke you used with the 2nd tile tool your ROC may have changed significantly.  Better check that again.

 

If you used COC strokes the ROC is now longer than before.

Depending on how far off center and stroke length, chordal strokes might hold ROC or make it shorter or longer.

 

You say the first tile tool is in good contact but it probably isn't after having ground with the second tool. Swapping tools will probably cause more work and grit to get a good curve.  If the tiles on tool 1 were too thin to finish grinding, they are still too thin and even more grinding will be needed after having ground with tool 2. 

 

My recommendation is stay with tool #2 to complete the grind. 

 

I don't know how much grit you have so wouldn't know if you have enough 120 to finish but if not enough you would need to get more grit anyway, either 120 or 80. 

 

May as well go ahead with 120.

Use chordal stroke if your ROC is long (probably still is). Stay MOT until you have your desired ROC. 

 

Instead of feeler gauges, a #41 drill bit is right on sag for your desired 65" fl (130" ROC").

Or more commonly found in non-machinist households, a 3/32" drill bit is very close to your desired sag.  Put straight edge across mirror diameter and see if back end of the drill bit will just barely fit under.

Plenty close enough for rough grinding.

 

Chordal strokes dig at the mirror's center making a gull-wing like curve, shorter ROC in center, longer ROC in outer zones.  Even with proper sag much of the mirror will have a longer ROC.  Some COC strokes will be needed to round out the curve to spherical.

 

Chordal strokes to reach desired sagitta will not work much on the central tiles so you might get to desired sag and still have some central tiles not in contact.  That is OK.  When you go to COC strokes to make spherical you should notice better progress at getting all the tiles into contact.  Looks to me like by the time you get it to desired sag and spherical all tiles will be in contact.

If not, It is not necessary that every tile be in contact to get spherical and if not too much not in contact can continue on to the next grit.

 

If you haven't already done so, several turns of masking tape around the edge of the tool will hold grit and water in the tool.  Water and grit would be held in the channels between the tiles and stroking would make a bubbling froth bringing grit up between tiles and mirror. When the bubbling froth abates add some water to get it going again.  Get much more grinding out of a charge of grit that way.  The top edge of the tape grinds away to the level of the edge tiles.  The tape usually will last to complete a grade of grit but if not it is easy enough to replace.


Edited by dogbiscuit, 11 August 2022 - 10:18 AM.

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#16 pjmulka

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Posted 11 August 2022 - 11:28 AM

Thanks so much! I have a plan now at least I’ll stick with tool # 2 .

#17 pjmulka

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Posted 15 August 2022 - 01:34 AM

I had a bit of a set back, I’ve been extra careful since my stroke but unfortunately my coordination isn’t what it used to be and I dropped my mirror into my tile tool. Took an pretty big chunk out of the side and just a bit of the edge. I beveled the crap out of it and managed to take the chip out of the edge. However I no longer have a perfectly round mirror. My question is, how will that effect me putting a good figure on the mirror? Am I going to be stuck with a poor performing mirror? It may be hard to see in the photo the offending area is at 5 o’clock.

 

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#18 dogbiscuit

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Posted 15 August 2022 - 01:48 AM

No problem.


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#19 pjmulka

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Posted 15 August 2022 - 02:01 AM

Wow, really…..that’s a huge relief!!!!

No problem.


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#20 pjmulka

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Posted 16 August 2022 - 03:01 PM

It’s been a while, what’s the mix ratio on micron grits?

#21 pjmulka

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Posted 22 August 2022 - 12:41 AM

Finished out 15 micron WAO today and did a clean up. My tile tool is on life support. There is a section of about 5 tiles that are flush with the epoxy. I attempted to cut out some epoxy to give even the smallest channel but a tile popped off which is about as thin as a couple pieces of paper. I’m praying it will hold on for 30-45 minutes of 9 micron and then I’ll have to spend a bit more time polishing. I’ll post some side by side photos of a fresh tile and the one that popped off the tool.
I have a philosophy when it comes to determining when it’s time to switch grits, when is doubt do 5 more wets. I may have to change that to, when in doubt do 2 more wets. I think my next mirror’s tile tool will start with a double layer of tile! I’ve almost completed my mirror-o-matic build so my next mirror will be bigger faster and spun on the machine. But I’m getting ahead of myself. With a bit of luck I’ll be pressing on a pitch lap tomorrow. Thanks to all for your time and advice.

Clear Skies,
Pete.

#22 Knight Sky

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Posted 22 August 2022 - 01:30 AM

It’s been a while, what’s the mix ratio on micron grits?

Hey Pete, a noob here myself, so I cannot advise you with a conviction as other experts can. I can share what I did for the ratio of water to grit on micron grits, if that's what your question was. And apologies for a late response.

 

I wouldn't say that there's a particularly precise ratio that I was aiming for, but as long as it was thin enough to spread across the tool for MoT and doesn't dry up too quickly. A drop of soap helps building the froth and keeps the grit in the working area of the tool for more efficient use rather than permanently settling down in the channels while doing MoT strokes.

 

You have already worked on the mirror after this post, so I am sure whatever method you have employed has worked for you.

 

Good luck finishing your fine grinding and hoping that your tile tool takes you to the fine grinding finishing line!

 

Please disregard my answer and forgive me if I completely misconstrued your question.


Edited by Knight Sky, 22 August 2022 - 01:46 AM.

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#23 pjmulka

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Posted 22 August 2022 - 02:29 AM

No yeah that indeed was my question. There are pages and pages on the “correct” water to cerium ratio but I found nothing on the micro grit mix other than it should look like skim milk…..as luck would have it I had some skim milk in the fridge and literally put some on my mirror and added WAO until it looked the same. :)

Hey Pete, a noob here myself, so I cannot advise you with a conviction as other experts can. I can share what I did for the ratio of water to grit on micron grits, if that's what your question was. And apologies for a late response.

 

I wouldn't say that there's a particularly precise ratio that I was aiming for, but as long as it was thin enough to spread across the tool for MoT and doesn't dry up too quickly. A drop of soap helps building the froth and keeps the grit in the working area of the tool for more efficient use rather than permanently settling down in the channels while doing MoT strokes.

 

You have already worked on the mirror after this post, so I am sure whatever method you have employed has worked for you.

 

Good luck finishing your fine grinding and hoping that your tile tool takes you to the fine grinding finishing line!

 

Please disregard my answer and forgive me if I completely misconstrued your question.


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#24 pjmulka

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Posted 22 August 2022 - 11:41 PM

Due to the concerns about my tile tool I decided to skip 9 micron and go straight to 5 micron. I just finished 5 micron, and have started cleaning up and getting ready for polishing. I’ll post some photos of my lap tomorrow.
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#25 Steve Dodds

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Posted 23 August 2022 - 09:34 AM

Due to the concerns about my tile tool I decided to skip 9 micron and go straight to 5 micron. I just finished 5 micron, and have started cleaning up and getting ready for polishing. I’ll post some photos of my lap tomorrow.

If you skip a grit you will have to go 4 times longer with the new grit.




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