Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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This is a thread for my 20" mirror grinding project. I hope to get lots of great advice and/or constructive criticism.
Here is the background...
The mirror is 20.25" dia, 1.22 thick quartz. It is being ground/polished/figured on a machine.
I am using is 15" diameter hydrostone porcelain tile tool that is about 2" thick (yeah, I went a little crazy).
The f ratio I am shooting for is somewhere between 3.5 and 4. The mirror has a curve generated on it (by our very own Mark Cowan).
I've started grinding with 80 grit and have been at it for about 2 hours now. The table is turning around 19 RPM. I have also placed a 25# weight on top of the tool while grinding for a total weight of about 50#.
It has taken a bit longer than I expected at this grit but I had some curve generation anomalies that needed to be removed.
Currently I am still grinding at 80 trying to get a constant RoC over the surface of the mirror.
I have a 4" diameter spherometer that measures to .0001 accuracy.
So my current question I have is how consistent from the edge to the center should my spherometer measure before I move on to 120 grit? i.e. should I have it exactly right across the surface?
As of now there is a .001 measurement difference between the edge and the center. Right now I am proceeding to try and get a consistent measurement before moving on.
Also, I suspect that the grind is taking longer because the grit is breaking down quicker than I think it is. I've never worked with quartz before, could anyone give some insight as to how long a given "wet" should last? To me it seems after 5 minutes the grit is history (even though it is still making a fair amount of noise).
Thanks...
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Brian....I've never used glass tiles for my tools..I use pennies...and coupled with the grit it takes me around a couple of hours on my machine to get to fine grinding. But I don't grind quartz either. I use my spherometer and make sure the measurement is nearly consistant all across the mirror before I go to fine grinding.
OH..and I do that during curve generation too.
Good luck with that big mirror. 
Rob
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A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Their actually tiles, not glass....
I moved the tool in a bit and added grit every 5 minutes and it quickly brought the curve into shape. The sound of the mud running on the tool/glass was fooling me. Indeed the grit is done in about 5 minutes.... The quartz makes quick work of it!
Everywhere on the surface now is within .0001. On to the next grit..
Thanks...
Quote:
Brian....I've never used glass tiles for my tools..I use pennies...and coupled with the grit it takes me around a couple of hours on my machine to get to fine grinding. But I don't grind quartz either. I use my spherometer and make sure the measurement is nearly consistant all across the mirror before I go to fine grinding.
OH..and I do that during curve generation too.
Good luck with that big mirror.
Rob
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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At each stage in the grinding process (beyond hogging out and sphericalizing), you are going to go through about 3X as much glass as the grit you are at.
So add up all the grit thicknesses you are going to go through and your tollerance is better than 3X that number ratioed by the amount of surface you are mesuring.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Brian,
A "wet" at 80 will last 30 seconds to 1 minute under good pressure. Pushing it beyond where it's noisy doesn't accomplish anything. With a water drip on the machine I add fresh grit every 30 seconds to 1 minute, depending on contact and how the mud behaves.
Best,
Mark
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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one test to do a fiew wets with something finer say 220 then see if it is fine grinding all over evenly
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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And Sharpie test if you want to know - recall how the generated blank looked when it arrived... 
Best,
Mark
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Mert
Carpal Tunnel
Reged: 08/31/05
Posts: 1803
Loc: Spain, Pamplona
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Any pics you can share with us?????
Soo nice to see these big projects coming along!
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Mert
42º49"N 1º38"W
3" Polarex refractor
6" F12 SW Maksutov,CS2-S
EQ6 + EQMOD 
SPC900NC/DFK21AU04.AS
My web-page don't laugh, I should make it better! 
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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So I've progressed thru 220 and my next grit is 320 SiC. The f ratio is coming in around f/3.9. I did have a chance to make it 3.5 during 80 grit but decided against it. So I backed the arm out a bit and have it at the "neutral point" for f/3.9.
My grit progression is going to be this...
80 SiC
120
220
320
500
25M AOX
15M AOX
9M AOX
Maybe 5M AOX, depending...
Before someone points it out, yes I know 320 SiC is about the same size as 25M AOX and that I will be "back tracking" thru the grits. I am doing this because it is quartz and on the recommendation of Mark (whose done many more mirrors than I ever will).
I've also rigged up a dripper to drip water on the mirror as it is grinding. I used a standard IV drip bag, poured the saline out and refilled with water and some dawn. I know this is not revolutionary but it is cheap and works REALLY well.
BTW, just wondering if anyone uses dawn mixed with water during grinding? It make such a huge difference in the action of the tool.
Sharpie and spherometer tests confirm the mirror is being ground evenly.
I am still amazed how quickly the SiC is consumed. The last mirror I did (27" over 10 years ago) was basically plate glass and I got longer times before having to put grit on. With this quartz blank I find myself putting grit on every 1-2 minutes (if not more frequently).
I've included 3 pictures below. The tile tool, the generated blank with tiles on it with my "bib" with some #12 copper wire holding it up so it doesn't touch the back of the center tiles and the mirror on the turntable ground thru 220.
Quote:
And Sharpie test if you want to know - recall how the generated blank looked when it arrived...
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Tile Tool
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Mirror ground thru 220
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
I am still amazed how quickly the SiC is consumed.
Looking good. I'd recommend skipping the 5 micron, though.
Just think how much fun it'd be hogging out the 5 lbs or so of quartz dust from the initial generation with SiC instead of diamond tooling!
Best,
Mark
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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Yeah, I am thinking of skipping the 5 micron. Since I have a machine I can let it polish all day. The only reason I would/have used 5 micron is to shorten polishing time.
BTW If you had to guess, how many pounds of 6O grit SiC would it take to hog out a quartz blank from a flat? 10 pounds? more?
Quote:
Quote:
I am still amazed how quickly the SiC is consumed.
Looking good. I'd recommend skipping the 5 micron, though.
Just think how much fun it'd be hogging out the 5 lbs or so of quartz dust from the initial generation with SiC instead of diamond tooling!
Best,
Mark
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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how flat is the back i checked my 14.7 quartz blank it is NOT flat .
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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I don't really know. I know it was blanchard ground and my spherometer measures pretty consistantly across the back.
Geez... I hope it is flat. I have been using it as a reference to zero out my spherometer indicator.
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JohnH
professor emeritus
Reged: 10/04/05
Posts: 734
Loc: vancouver near the wilds of B...
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Quote:
I don't really know. I know it was blanchard ground and my spherometer measures pretty consistantly across the back.
Geez... I hope it is flat. I have been using it as a reference to zero out my spherometer indicator.
You need to use a proper reference surface to zero a spherometer. Most recommend using a granite plate made for just that purpose
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Brian Engel
member
Reged: 08/25/09
Posts: 93
Loc: Cincinnati,Oh
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So I am grinding at 500 SiC right now and having some trouble with the action of the tool. The tool seems to be mostly floating and not doing much work. Every so often I will get good action (from the sound) but mostly it seems like it is floating.
Any advice? Do I need more weight? The tool plus the weight I put on top of it is around 50 pounds.
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 666
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i fine grind with NO weight .
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Quote:
Yeah, I am thinking of skipping the 5 micron. Since I have a machine I can let it polish all day. The only reason I would/have used 5 micron is to shorten polishing time.
Any changes in polishing time are negligible, compared to the potential for sleeks from the tile tool at this stage. Your very thick tool base might keep that at bay, but I quit using anything below 12 micron years ago.
Quote:
BTW If you had to guess, how many pounds of 6O grit SiC would it take to hog out a quartz blank from a flat? 10 pounds? more?
Something like that. The removal rate is not very high even with SiC on quartz. I know that manually hogging Pyrex with SiC used a lot of grit...
Quote:
So I am grinding at 500 SiC right now and having some trouble with the action of the tool. The tool seems to be mostly floating and not doing much work. Every so often I will get good action (from the sound) but mostly it seems like it is floating.
Well, it could be, but you don't get much sound from 500 anyway. If the tool is Hydrostone don't let it dry out, if it does dry out soak it in water for a couple hours. It will change curve on you if it dries out.
If you let it run a while with a drip and then add some 500 you should hear and see it pick up action immediately. You might want to try the Sharpie test and see how it's working.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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I'm pretty good at economising the grits. When I ground a 20, I used about 7+ pounds to get an F/5.1 curve on it.
M.
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So many projects, so little time!
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