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Equipment Discussions >> ATM, Optics and DIY Forum

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danjones
super member


Reged: 09/26/12

rough grinding trouble
      #5563354 - 12/09/12 11:48 PM Attachment (46 downloads)

I'm working on a 10" blank. It's dark tinted glass but is well annealed, plate glass, 1" thick. You can refer to my 'hogging' thread for additional pics and info if you like. I'm using a method I found online which that person was successful. I hogged the mirror out by hand using a sub diameter plaster tool using 60 grit. Went fairly quick. I hogged out to just a little more than required depth, planning to use TOT for the rest of grinding to save grit. The curve is coming out rough at this point, but I figured it would, but with an issue. Otherwise, it's seems to be pretty uniform, better if I know how to correct the issue. You can see in the pic, the two darker zones, these are high areas where the blank is basically flat. They are higher than the other two zones, which the curve in these two lower zones I think are coming along nicely. I'm now using a full sized plaster tool with glass tiles, using center over center strokes. The last grinding session I did didn't change the high zones much. May be a little. I grind now with 8 positions around the blank, 15 strokes per position, while rotating the blank and tool in opposit directions, one revolution of the blank equalling one wet. I'm thinking this consistentcy also keeps the high and low zones consistently high and low. I think I need to do some wets with focusing more pressure on the high zones to get em down to match the low zones. Or may be use the sub diameter tool on the high zones to lower them. Check out the pic. The darker areas are the high zones. Feedback please.

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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: danjones]
      #5563360 - 12/09/12 11:52 PM Attachment (44 downloads)

here's another pic of the blank along with the full sized tool i'm using.

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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: rough grinding trouble new [Re: danjones]
      #5563554 - 12/10/12 04:25 AM

Quote:

I'm now using a full sized plaster tool with glass tiles, using center over center strokes. The last grinding session I did didn't change the high zones much. May be a little.




How did you make the tool, that is did you match the curve to the mirror?
How long was the session that didn't change things much?

I suggest you just carry on with the glass tile tool. It will come right sooner or later. There may be (or may have been) high spots on the tool that have to wear down before real progress is made. Are all glass tiles showing wear? If you change back to the hogging tool you'll have to match the larger one later anyway.

Dave

Edited by dave brock (12/10/12 04:28 AM)


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ed_turco
Pooh-Bah
*****

Reged: 08/29/09

Loc: Lincoln, RI
Re: rough grinding trouble new [Re: dave brock]
      #5563731 - 12/10/12 08:41 AM

In all your grinding, always remember to take a little time to slow your strokes to 30 a minute; this means both back and forth. Going faster really reduces efficiency and can cause some unusual effects. Going slower means you can push harder.


Ed


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: rough grinding trouble new [Re: ed_turco]
      #5563822 - 12/10/12 09:38 AM

Since you're still experimenting with this you might not have gotten the right combination of pushing and bearing-down. If you're really getting it, all of those glass squares should be opaque and the same suface-roughness as the mirror-to-be. Don't stop!!!

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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: glennnnnnn]
      #5564720 - 12/10/12 07:01 PM

Ok. After thinking for a bit, I think I'm probably just not giving it enough time. After all, I've only done 6 or 8 wets. For wets in my case, I have the blank on top a bucket with a lid. I use little pieces of vacuum tubing with screws thru them to hold the blank. The rubber expands while tightening, therefor snugging the blank. There are 4 screws opposite of each other and labeled 1 thru 4. I pour on 80 grit, water, and grind 15 strokes center over center, 1/3 overhang per position, 8 positions around the bucket, rotating the bucket and tool in opposite directions. When rotating, I randomly rotate at various distances still keeping 8 positions until I make 1 revolution around the bucket. Revolution 2 begins at screw 2 with fresh grit, repeating thru each screw until done with screw 4. Once thru all screw postions, that is 1 wet for me. Then to make it fun, I do it all backwards, then end that session. I do feel that I'm staying consistent with the amount of pressure on the forward and reverse stroke. I'm thinking I just need to do some more wets and allow the tool to do it's work. After so many more wets, and have not begin to see the change, then may be I'm doing something wrong. Thanks for the advice. I think the wear on the tiles are progressing wear as it should. Would it be useful to do a sharpie test at this stage?

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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: rough grinding trouble new [Re: danjones]
      #5565146 - 12/10/12 11:41 PM

Try it and see! Your sharpie is one of your best tools!
If you have a sphereometer you can actually find out how much the curve changes for even a single wet, but it will be "not much." This takes a long time, and you "have to work like a caveman" (John Dobson.)
You are probably appreciating that those guys are trying all sorts of shortcuts -like spincasting and slumping to bypass this stage of grinding out all that glass.


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: rough grinding trouble new [Re: danjones]
      #5565180 - 12/11/12 12:07 AM

Quote:

The rubber expands while tightening, therefor snugging the blank...... Would it be useful to do a sharpie test at this stage?




I'm a little worried with the term "snugging the blank". You should really have some room for the blank to move to avoid astigmatism.
At the risk of starting a riot here, I think the sharpie test is overrated and never use it. It will likely fail early in coarse grinding and later, with the finer grades you can "feel" when the mirror and tool match and are spherical.

Dave


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: rough grinding trouble new [Re: dave brock]
      #5567303 - 12/12/12 10:39 AM

Remembering your comments about trying to work with such symmetry and precision- that's important. But right now you just need to get that stuff out of there so the precision grinding can have some functional purpose. Since you're dealing with 2 surfaces that you want to make spherical practically ANY motion or direction will have that result. This includes rotating the tool (or the mirror) in circular or elliptical paths, and not straight lines. A grinding machine has the two surfaces rotating constantly with a weight bearing down to increase the effect.

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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: glennnnnnn]
      #5570170 - 12/13/12 11:58 PM

I'm not quite sure what you are saying Glen. Are you saying start using strokes other than straight forward and pull strokes? I have no problem doing this, just trying to learn. Thanks for the advice. 2 surfaces are correct. Just for fun, I really wanted to see a reflective test to see just how crazy it was. May have been too early for this test, but the anticipation was eating at me. I had to look. So I did a focal test. I used a long board, marked it out in inches to about 90 inches. I'm looking for a roc of 84 and fl of about 42. At one end of the board, I mounted the mirror upright. With the lights off, I wet the mirror and shined a bright led from the opposite end. I used paper to capture the recieving light and noted the best focus of the roc and fl. Roc was at 74, needing 84. FL was 55 needing 42. of course I know the edge is not ground out with a far from spherical service. Actually just doing the test was fun.

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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: danjones]
      #5570171 - 12/13/12 11:59 PM

I have noted on the tool that all the tiles have made contact, some more than others, and some much, leading me to believe that I may be going in the right direction.

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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: rough grinding trouble new [Re: danjones]
      #5570340 - 12/14/12 05:46 AM

Quote:

Roc was at 74, needing 84. FL was 55 needing 42.




Not sure what you mean here. FL is 1/2 ROC. With that test you are finding ROC so at 74" the FL is 37". How did you get 55" FL?

Dave


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: rough grinding trouble new [Re: dave brock]
      #5571010 - 12/14/12 03:18 PM

Yes. You can use other strokes- (circles, ellipses and figure 8's) but you should still keep an eye on the symmetry.
Grinding the way you are with such precise steps isn't how you're going to get that excess glass out of the way but when you get out to the edge you have a good regimen for accuracy.
Do you see any grinding machines that use straight strokes? The reason grinding the glass with grit works is because a spherical surface happens by default.
Use the sun for your focus testing. It's at the right distance and all you need is a convenient wall or piece of paper on a stick. I think its exciting to see that round perfect circle become sharper-edged and brighter. But right now any kind of focus test tells you nothing unless the mirror is ready for it. It won't be until you get all the way out to the edge.


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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: glennnnnnn]
      #5571766 - 12/14/12 11:22 PM

Cool. I think I got what you are saying. The only machine that uses a straight stroke is the mir-o-matic. the stroke is straight, but only on a turntable, but I get what you are saying. I'll try some different grinding strokes for several wets then determine how its coming. I knew the test wouldn't amount to much, but I got the idea of how it should work, which was my main goal, seeing any kind of reflectivity on the mirror was a bonus for me. fun stuff. thanks

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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: dave brock]
      #5571786 - 12/14/12 11:38 PM

Thanks for your comments. I'm learning alot here in the CN forum. It had been discussed previoulsy that because I used a sub diameter tool on this mirror, ( i won't next time), that it may have at this point already have two surfaces. My goal for the mirror is a 42 FL, and the ROC which is twice the FL of 84. I used a focal lenth test way too early i think, (just wanted to really learn how to do the test good for later), and came up with these measurements. knowing that i want a fl of 42, the closest focus i could get was 55. and with the papar out towards where the corresponding roc should be, the closest light reflection in this test was around the 77 mark. my thinking theory was that if the actual fl at this point was greater than the target, and the roc light reflection was shorter, this could correspond with the theory that there are at least 2 surfaces at this point in grinding. there are 2 high zones right now during rough grinding which i know of by seeing. I enjoy all the feedback while I learn about the process. Of course, i'm an amateur, and everything i just said is what i think is happening, coming to this conclusion from what i am learning from all you guys.

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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: danjones]
      #5571796 - 12/14/12 11:47 PM

Dave, you questioned the snugging of the blank. Just wanted to make sure I answered that. The blank is mounted on a 5 gal bucket. I tighten the screws down in pieces of rubber vacuum tubing, just enough to hold the blank in place while grinding. There is just a little give in it when you try to move it. Also, I do the grinding in my house right now, so the bucket sits on a good thick carpet, which I also notice lets it give a little. Feels okay to me. I was thinking of the astigmatsm when I decided to do it that way. Hoping thats all i need. plus, the bucket, and a little rubbermaid tub with the tool, spray bottle, and grit containers, it takes up little room in the house so the wife don't get mad at me for my weird telescope stuff in the house, lol.

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John Carruthers
Skiprat
*****

Reged: 02/02/07

Loc: Kent, UK
Re: rough grinding trouble new [Re: danjones]
      #5572149 - 12/15/12 08:23 AM

I had to grind in the garden but came into the unheated conservatory for figuring, and into the house for testing.
Have you tried a W stroke? centre over centre will get there but I find a W quicker, and it evens out any lumps/bumps. A tangential or W also lets you work on the edges more. Whatever stroke suits, each to his own.
I prefer a rigid base for the tool, I find any give or slop can lead to rocking at the ends of the strokes and edge problems.


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glennnnnnn
sage


Reged: 10/20/09

Loc: San Diego, CA
Re: rough grinding trouble new [Re: danjones]
      #5572155 - 12/15/12 08:29 AM

The mirror (or tool) shouldn't be too tight in the setup so it won't have any pinching that distorts the glass. The 5 gallon bucket is less than optimum, because it moves on the carpet. Some of the force of your effort in pushing the glass is being nullified by the shock-absorber on your floor! If you filled your bucket with gravel or concrete you would notice an improvement in your efforts.

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danjones
super member


Reged: 09/26/12

Re: rough grinding trouble new [Re: glennnnnnn]
      #5578201 - 12/19/12 12:12 AM

I guess I didn't mention that the bucket is full of drywall compuound. It's somewhat heavy. Thanks for the intersting feedback.

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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: rough grinding trouble new [Re: danjones]
      #5579079 - 12/19/12 02:48 PM Attachment (9 downloads)

Dan,
The image you posted starting the thread shows what you typically have as a result from using chordal strokes when roughing out the blank. A hole in the middle and flat edges. You can correct a bit with wider TOT strokes, high pressure and adding grit each 10 strokes or so.
Once you get to approximately the right depth, put saran wrap on the blank, make a rim all around, lay out a mosaic of tiles, and pour plaster over it. Then I take #120 grit to cut in the new tool and get the shape spherical. Sharpie is indeed your tool here.
Once you're spherical up to the very edge, you start inspecting pits and move down through finer grits when the current larger pits are gone.


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