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Making twin 20" quartz mirrors

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#1 Gordon Waite

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Posted 14 February 2010 - 01:31 PM

A couple of months ago you might have seen a thread started by JadeSmith about his two 20" quartz blanks. His posting intrigued me, so I offered to make the two blanks into finished mirrors, keeping one for myself and sending one back for Jade. I promised to keep a diary of the process here for all to enjoy, as the blanks are ground, polished and figured into Newtonian primary mirrors. Jade liked the idea, and I soon found two huge boxes waiting on my front step!

Posted Image

I'll do my best to entertain you with this big adventure. Maybe you can all help keep my spirits up as the "challenges" unfold and we see if this turns out to be a comedy, a tragedy or a triumph.

Gordon Waite

#2 kfrederick

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Posted 14 February 2010 - 03:25 PM

Great are they pregenerated ? A 20 inch is a nice size .

#3 Gordon Waite

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Posted 14 February 2010 - 03:26 PM

Session #0: Here's a photo of one of the 20" mirror blanks, sitting on one of my turntables. The blank is sitting on three black hockey pucks.

Posted Image

The blanks have issues that make the fabrication a little interesting. There is a central hole, with different diameters on front and back. There is a little recess milled in the face edge. There are two holes drilled into the back of the mirror that extend toward the front face. And there are four finger grips milled into the edges of the blanks. This first session is just a look at metrics on the two blanks. They turn out to be very nearly identical:

Diameter: 20"
Weight: 41 pounds each
Thickness: 1.75"
Central hole: About an inch in diameter
Central hole: Over an inch and a half on the back

The two blind holes that are drilled through from the back can be seen through the face of the mirror. It would be nice to limit the sagitta on the mirror so that I don't risk grinding through and open up the two blind holes. If I go to f/4.5, that would be a sagitta in the center of .278". I'll see as the grinding progresses if it is possible to get all of the 180" radius without risk.

The faces of both blanks have had some milling work. There is a rim around the edge which is higher than the rest of the face, which appears to have been mildly milled or ground down. The face definitely isn't flat, and it seems to be a little uneven and not a surface of revolution. The back of the blank is actually nicer, but I don't want to deal with the larger central hole and the blind holes.

Gordon

#4 Gordon Waite

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Posted 14 February 2010 - 03:30 PM

are they pregenerated ?


No, the blanks are both close to flat. Roughly 44 cubic inches of quartz to hog out from each of the blanks.

Gordon

#5 Gordon Waite

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Posted 14 February 2010 - 05:14 PM

Here are links to some images that show the problematic characteristics of the 20" blanks...

Hole in center of face
Center hole in back side
Blind hole from back side
Blind hole as seen from the face of the blank
Finger grip on edge of blank
Notch milled in edge of face
Raised edge of face of blank

#6 Mirzam

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Posted 14 February 2010 - 05:22 PM

Hmmm. Could you fill all the holes with some epoxy mixed with glass particles to approximate the thermal and textural behavior of the blanks? Or maybe you already have another plan?

Jim

#7 Gordon Waite

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Posted 14 February 2010 - 05:56 PM

Hi, Jim,

Could you fill all the holes with some epoxy mixed with glass particles to approximate the thermal and textural behavior of the blanks? Or maybe you already have another plan?



I filled the center hole partway with dental plaster. I filled it from the front, going back to about 1/2" from the back of the blank. I just taped over the front of the hole, and put the blank face down on the turntable. Then poured in a bit of dental plaster. A half hour later it was hard and cold and ready for grinding. Here's a photo...

Posted Image

I'm going to leave everything else as is. The center hole is so small relative to any reasonable diagonal mirror size that any figuring defects due to the hole will be inconsequential. The plaster plug will let me grind with the mirror face up without letting all the juice drain down through the center.

#8 rboe

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Posted 14 February 2010 - 07:01 PM

That center hole should be able to play a nice role in a mirror mount. Not exactly a conical mirror but I bet you could do away with the strap.

#9 Mark Harry

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Posted 14 February 2010 - 07:07 PM

If the hole is exactly in the center, you could just bevel it, and put a piece of duct tape on the back with a thin layer of contact cement. I know it sounds crazy, but I've fiddled with perforated mirrors before. If your polishing regimen is up to snuff, it won't pose any issues at all. It has the advantages that there's absolutely nothing to effect strain while polishing. FWIW.
Mark

#10 Moggi1964

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Posted 14 February 2010 - 07:13 PM

20" quartz.......and TWO of them :bigshock: :bigshock: :bigshock:

I can't wait (or perhaps Waite) to see these two when they are complete.

Lots of pictures Gordon, it's not every day we get to see something like this.

What Focal length are you aiming for?

M

#11 jdownie

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Posted 14 February 2010 - 07:51 PM

100" or maybe 90".

#12 jgraham

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Posted 14 February 2010 - 08:06 PM

Neat! My 16.5" blank was right at 2" thick and being old-school I didn't want to make it any thinner so I generated the curve with a 9" tool while taking almost no glass off of the edge. Also, having had enough of figuring short focal length mirrors (some as short as f/3) I went with a fairly long f/6.7 (109" focal length). It has proven to be a wonderful scope!

Have fun!

#13 kfrederick

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Posted 14 February 2010 - 08:56 PM

I would grind the back flat.

#14 Gordon Waite

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Posted 14 February 2010 - 11:24 PM

I would grind the back flat.


You are right, but I don't have to with these blanks. They are already flat. I measured both at 13 random spots with my spherometer using a 5" head. Average deviation from the reference flat was .000052". All the deviations were concave. I don't know how these two blanks were machined, but whoever did it knew what they were doing! It's a beautiful piece of work.

#15 Gordon Waite

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Posted 14 February 2010 - 11:32 PM

That center hole should be able to play a nice role in a mirror mount. Not exactly a conical mirror but I bet you could do away with the strap.



An excellent point. I normally use a cable sling on my test stand, and that won't work given the slots milled in the edges of the blanks. I had experimented a lot with test stand supports before settling on the cable sling. But I'm going to take your suggestion to heart and knock together a lug to support the mirrors on the test stand using the center hole. I'll still put the cable sling under the bottom as a safety, though, but I won't let it carry any weight. Should be interesting to see if mounting by the center hole does as well against test-stand 'stig as the cable sling.

Thanks!

#16 kfrederick

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Posted 15 February 2010 - 01:37 PM

Maybe put the cable sling in the finger slots

#17 kfrederick

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Posted 15 February 2010 - 02:18 PM

MY first 20inch I made is a f5.25 when i did the math for my eyepieces the numbers are odd !! The numbers are 666 mag for the 4 mm 555 mag 4.8 nangler 444 6mm 333 8mm 222 for the 12mm 133 for the 20 and 66power with my 40 mm kevin frederick

#18 Gordon Waite

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Posted 15 February 2010 - 03:58 PM

Maybe put the cable sling in the finger slots



Easy enough to try your idea. I can compare that versus the support via the central hole. Thanks!

#19 Gary Fuchs

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Posted 15 February 2010 - 07:05 PM

Hi Gordon,

Nice (big) project, and nice documentation.

Gary

#20 BradleyB

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Posted 15 February 2010 - 07:11 PM

That may be because your f/l is 2 2/3 meters.

Brad


MY first 20inch I made is a f5.25 when i did the math for my eyepieces the numbers are odd !! The numbers are 666 mag for the 4 mm 555 mag 4.8 nangler 444 6mm 333 8mm 222 for the 12mm 133 for the 20 and 66power with my 40 mm kevin frederick



#21 JadeSmith

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Posted 15 February 2010 - 10:35 PM

Big glass....

This is going to fun.

:dob: :woohoo: :dob:

#22 Gordon Waite

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Posted 15 February 2010 - 11:33 PM

Let the insanity begin!

One of the two 20" blanks is pretty flat, while the other has a slight bit of concave curve to it. Normally I would rough cut the faces to depth on my diamond curve generator. But I only do that outdoors. And that means my generator is sitting under a couple of feet of snow in below-freezing temperatures. Welcome to New Jersey! Needless to say I'm not getting that wet and that cold. Second option is to ship the two blanks to a fab and have it done professionally. But that's $200 for shipping and $150 for the generating and it would take about two weeks for the round trip. It's not a happy thought, but I'm going to hog out these two blanks by hand on my powered turntable. Thus, the insanity reference above...

So now I need a grinding tool. For a job like this, I'll go with a ceramic tile tool, 10.5" in diameter, cast in dental stone. This will be a hogging tool, and I'll only use it to get from flat to something close to the right sagitta. When I get close to the correct depth, I'll pour a larger tool to use for "fixed-post" grinding on my powered turntable.

Why 10.5"? The smaller the tool, the higher the PSI. But if I make the tool slightly larger than 50% of the mirror diameter, I can do the hand hogging and then set up for spinning to get a good rough sphere using the same tool. So 10.5" is the smallest size for which I have a mold ring that is bigger than half of the 20" diameter on the blanks. I'll post images and a description of the process, but here's a photo of the finished 10.5" tool, ready to roll...

Posted Image

You'll notice this isn't your standard ceramic tile tool. This baby uses the tiles end-on. Each of the two blanks will require the removal of almost 44 cubic inches of quartz. You would grind through a dozen regular ceramic tools while turning all that quartz into powder. But one nice 10.5" tool with the tiles end on will do both mirrors and have tile to spare. The cost to make this tool is about $15 for the tiles, and about $15 for the dental stone. So the hogging tool comes in at $30, or only $15 per mirror. I'll show how I made it in my next post.

#23 Moggi1964

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Posted 15 February 2010 - 11:56 PM

By hand? That cold weather is affecting your mind, Gordon :)

I have a number for a chiropractor if you need it. LOL

Good luck, cannot wait to see the progress over time.

#24 Gordon Waite

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Posted 16 February 2010 - 12:25 AM

Long ago I took a 24" square piece of MDF and routed out a bunch of concentric circles. I tried to make the ID and the OD of the circles useful sizes. I find a lot of uses for these, and one of the best is to help me mold tile/plaster tools. I grabbed the 10.5" OD ring, and put it on a piece of self-adhesive shelf liner paper, and traced around the ring. When I peel off the backing, I can place the ceramic tiles on the paper within the circle, and they stick where I put them. And the tiles are easily heavy enough to hold down against the face of the mirror, and with the sticky paper they won't move when I pour the dental stone over and around them. Here's a photo of me tracing around the mold ring onto the sticky paper. (I'm doing this right on top of the mirror blank, which I have previously leveled.)

Posted Image

Next, I have to prep up my tiles. I start with standard "pool" tiles. These are ceramic floor tiles, rock hard all the way through. They are 7/8" square, and .16" thick. They come in 12x12 sheets with a web backing. Perfect for either normal tile tools, or you can pick them off the backing to make an edge-on tool like this one. I buy them from International Pool Tile in Georgia. They have white or black or checkerboard. You have to buy at least a case, and that's 30 sq. ft. per case. They run maybe $150 to $180 per case, or about $5 or $6 per 12" sheet. The price may have gone up, as I haven't bought any in a couple of years. (Last time I ordered I ended up with a lifetime supply!) Here's a photo of five individual black tiles...

Posted Image

Sorry... not such an exciting image. I'll continue in the next post.

#25 Gordon Waite

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Posted 16 February 2010 - 12:54 AM

The next step was to take groups of five tiles and put them together into a stack or cube. I slipped a little tiny rubber band around each group of five so they would hold together. The little rubber bands came from a "dollar" store, and there were about a million of them in the bag. Make sure the rubber band is close to the top of the stack. This makes a solid cube of ceramic tile, about 7/8" on a side. I also make up a few blocks using just three tiles, to fill in some of the corners where a block of five won't fit.

It's not important to completely fill the circle. And you want to keep the tiles within the circle, as you would like the cubes to be completely embedded, and not have the sharp edges exposed at the edge of the tool. So rubber band together blocks of five tiles, and put them in position on the sticky side of the adhesive paper. I used 1/8" cooking skewers to space the blocks and make neat and tidy rows. You want neat and tidy rows, because you really should route out the plaster between the tiles a little so your abrasive and water can be distributed and work right.

The photo below shows the final prep and positioning of the ceramic cubes on my sticky paper. I used 76 cubes, I believe, with a total of something like 368 individual tiles. BTW, making up the little cubes would be a good project for your teenager. Holding a stack and putting a miniature rubber band on it is nearly impossible if, like me, you're over 50.

Posted Image

One cool thing about using these ceramic cubes is that you can position them on the paper, and then press down on the top of them gently to align them vertically, and to press the ends down against the mirror, so they take the shape of the blank. You get a really nice grinding surface right off the bat this way.

The next step is to finish making the walls of the mold. I take the 10.5" ring, and I wrap a thin strip of plastic around the outside, using painter's tape to hold it together. I use plastic I bought at a sign maker's shop. It's thin but fairly stiff. Make sure your edges are really straight, because they need to mate up tight against the sticky paper and the mirror face. If there are gaps, the plaster will run out and ruin your work. Once you have the plastic wrapped around the MDF ring, just carefully put it over the assembly of ceramic cubes, with the bottom edge against the sticky paper. Here's what mine looked like:

Posted Image

Try to have a slight space between the cubes and the plastic wall. You don't want the cubes to touch the wall. Better to have fewer cubes. We're almost done. I'll finish in the next post.


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