perfessor
sage
Reged: 12/30/07
Posts: 317
Loc: Northern Illinois
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Suppose, theoretically speaking of course, that one were to notice a mirror blank offered for sale - 1" thick, 22" diameter. That's really an unworkable size for me - too big, too thin, no way to use it as is.
But it could conceivably produce a pair of 11" blanks, with a couple 6" disks thrown in for good measure.
I have no idea how one might accomplish this. So it's probably unrealistic to think about.
I am curious, though.
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Tom
"Don't always know what I'm talkin about"
8" f/7
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AstroRealtor
professor emeritus
Reged: 03/26/08
Posts: 538
Loc: Terra Firma
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I think it can be cut with a waterjet, I'm going to look into it for an 8" diameter mirror that I want to trim to 7.5" Dia.
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Clear Skies, Jim
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Discovering: Messier objects, Deep sky treasures, NGCs, Double stars, & Nebulae of all kinds.
Televue 85 "Polaris" Mount
Custom Crafted Newtonian Dob 8" f7 w/Feathertouch
Celestron Cometron Reflector 114mm f8
Zeiss Diafun 8 X 30 Binos
Zeiss 10 X 56 Binos
 Aldrich Astronomical Society
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 1678
Loc: Kent, UK
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I'd put it up for a swap but if you want to cut it yourself old pipe/tin cans can be used with grit to trepan a disc out. Any glass firm can cut it for you, some use water jets which give a neat finish or a grit saw might eventually do it. (rather you than me) 
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
and a curious mind
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kfrederick
sage
Reged: 02/01/08
Posts: 212
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i had a thick piece of fiberglass cut with a cnc water jet the fiber glass board was 1.25 in thick and 26 inches diameter it took 5 minets and cost 40 dollars did a very nice job kevin
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AstroRealtor
professor emeritus
Reged: 03/26/08
Posts: 538
Loc: Terra Firma
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My question to the owner of the waterjet is`going to be about the already polished and coated mirror. Does`waterjet cutting affect the coating at all? I would assume it has to.
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Clear Skies, Jim
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Discovering: Messier objects, Deep sky treasures, NGCs, Double stars, & Nebulae of all kinds.
Televue 85 "Polaris" Mount
Custom Crafted Newtonian Dob 8" f7 w/Feathertouch
Celestron Cometron Reflector 114mm f8
Zeiss Diafun 8 X 30 Binos
Zeiss 10 X 56 Binos
Aldrich Astronomical Society
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CoreDump
member
Reged: 11/06/06
Posts: 34
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Hi,
It will probably ruins the coating and the figuring, water jet cutting uses additives (mostly abrasives) to watercut glass (pure water is only for wood, foam etc..).
Not to mention that triming by other means can release existing strain in the glass.
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perfessor
sage
Reged: 12/30/07
Posts: 317
Loc: Northern Illinois
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Well, I am talking about a blank here. At this point I'm just doing a "feasibility study," and it sounds like the next step is to find a place that does waterjet cutting, and ask them about it.
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Tom
"Don't always know what I'm talkin about"
8" f/7
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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I have cut disks of glass from a square plate sheet on a drill press using a homemade cutter. Using a router and a circle cutter guide base I first cut a disk of plywood the same diameter as the desired glass disk. A 1/2" diameter hole is bored into the center of the disk to accept a bolt which is later chucked in the drill press. I then wrapped a band of sheet galvanized metal around the outside of the wood disk to make the cutter. This height of this band should be about twice the thickness of the glass plus 3/4" for the thickness of the plywood. A few triangle shaped notches are cut out of the band to act as "teeth" with the abrasive that will do the actual cutting. The band can be drilled and screwed into the edge of the wood disk, or just held in place with a large hose clamp.
Set the drill press on it's lowest speed (< 200 rpm) and level the table at right angles to the chuck. Put a piece of soft wood between the table and the glass, anchor the glass in place with wooden wedges screwed into the wood to press against the edges of the glass. Make a dam out of plumbers putty or playdough on the surface of the glass around where the cutter disk will contact the glass to hold the abrasive slurry in place. Now start the drill press and lower the cutting disk. Slowly add a mix of #80 carbo and water, adding more carbo as required. If the cutter binds and stops moving reduce the downward pressure a bit. Cutting a 7" disk out of 3/4" glass took me about 45 minutes but "your mileage may vary" here. Rotate the glass under the cutter if possible (90 degree turn) about 1/2 way through. This will even out the cut. Some 'break out' fracture may occur at the point where the cutter makes it all the way though, backing off on the downward pressure toward the end may help. You have to bevel the disks anyway, so a bit of fracture flaking on the edge where the cutter breaks though isn't the end of the world here. I got two good disks for a blank and a tool on my first try.
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DAVIDG
Pooh-Bah
Reged: 12/02/04
Posts: 1423
Loc: Hockessin, De
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I've done it a couple of times. I use inexpensive tile saw to first cut the glass into squares, then cut the corners off to make it into a polygon. The rough shaped disk is then mounted on a turntable driven by my drill press. A diamond hone is then brought up against the edge and within 30 minutes or less the disk is edged perfectly round. Here is a link to a thread showing the edging process.
Lens edging
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 5870
Loc: Bremerton Washington
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Quote:
I have cut disks of glass from a square plate sheet on a drill press using a homemade cutter. Using a router and a circle cutter guide base I first cut a disk of plywood the same diameter as the desired glass disk. A 1/2" diameter hole is bored into the center of the disk to accept a bolt which is later chucked in the drill press. I then wrapped a band of sheet galvanized metal around the outside of the wood disk to make the cutter. This height of this band should be about twice the thickness of the glass plus 3/4" for the thickness of the plywood. A few triangle shaped notches are cut out of the band to act as "teeth" with the abrasive that will do the actual cutting. The band can be drilled and screwed into the edge of the wood disk, or just held in place with a large hose clamp.
Set the drill press on it's lowest speed (< 200 rpm) and level the table at right angles to the chuck. Put a piece of soft wood between the table and the glass, anchor the glass in place with wooden wedges screwed into the wood to press against the edges of the glass. Make a dam out of plumbers putty or playdough on the surface of the glass around where the cutter disk will contact the glass to hold the abrasive slurry in place. Now start the drill press and lower the cutting disk. Slowly add a mix of #80 carbo and water, adding more carbo as required. If the cutter binds and stops moving reduce the downward pressure a bit. Cutting a 7" disk out of 3/4" glass took me about 45 minutes but "your mileage may vary" here. Rotate the glass under the cutter if possible (90 degree turn) about 1/2 way through. This will even out the cut. Some 'break out' fracture may occur at the point where the cutter makes it all the way though, backing off on the downward pressure toward the end may help. You have to bevel the disks anyway, so a bit of fracture flaking on the edge where the cutter breaks though isn't the end of the world here. I got two good disks for a blank and a tool on my first try.
My blank cutting machine works on this principal..however, I can cut upto 24" blanks. I'm cutting a 13" right now.
I, however, hadn't used galvanized steal for banding...what benefit does this have..over steal flashing?
Rob
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www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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Sounds like a nice 22" piece of glass. 
A lttle sad if it never gets to look at the heavens. 
You would need a nice 27 point cell though.
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Gene Hunter
super member
Reged: 12/29/07
Posts: 194
Loc: SC
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hmmm , interesting thread. I have been toying with the idea of building a folded newtonian for astrophotography just to see how well it works. I wanted to build something like and 8" F4 folded newt for testing out the whole concept but I was not sure how to go about cutting the hole out of the middle of the glass.
Its a shame the mirror is ruined when you do this. I wonder if there is a method that does not ruin the figure of the mirror. I wanted to just buy a cheap chinese mirror for testing this so I dont spend a ton of money on the whole project. If it worked of course I would just refigure the mirror.
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Clear Skies
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1937
Loc: Boechout, Belgium
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Quote:
Well, I am talking about a blank here. At this point I'm just doing a "feasibility study," and it sounds like the next step is to find a place that does waterjet cutting, and ask them about it.
It works just great, I've done it on several occasions. Even with mirors that had been ready and figured. No problem there. Even at the very edge, close to the cut no strain was applied.
For smaller mirrors like cutting diagonals I always use a diamond saw runing at 11.000 rpm and LOTS of water.
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Chris
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10410
Loc: Amargosa Valley, NV, USA
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You can't get two full 11" blanks from a 22". There will be a thickness to whatever cutting tool you use, so the smaller blanks will be somewhat less than 11".
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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perfessor
sage
Reged: 12/30/07
Posts: 317
Loc: Northern Illinois
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Yeah, I was realizing that. Mainly, I'm trying to get the procedure understood. If I do "pull the trigger" on this deal, I'd figure I'd probably cut a 12", a 9", and two 6" blanks.
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Tom
"Don't always know what I'm talkin about"
8" f/7
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astrokido
sage
Reged: 06/09/08
Posts: 244
Loc: Phoenix, AZ
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You could probably sell the 22" blank, buy the blanks you need, AND keep the difference.
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- Gill C. - All opinions subject to a speed limit of 299,792,458 m/s unless noted otherwise.
Nikon D40 Bushnell 10x25 Zhumell 20x80 Celestron Cometron CO-100 binochair CN gallery
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