Primary Mirror Page
Rough Grinding:
Being a bit lazy I scoured the ATM Archive Pages looking for a way to hog out this big piece of glass in short order. What I eventually went with is an idea from another ATM to use a high speed drill, 4" diamond disk and a rotary table to literally grind away about 80% of the glass. Below are the pictures I took during this stage. The hogging out took 2 hours and the clean up consumed another 4 hours. The following information is an accelerated method I chose to use in order to hog out and rough grind a 25" aperture mirror.
Here is a picture showing the daunting size of this task!
Here is a picture of the blank siliconed to an indexing table. Notice the pennies used to space the blank off of the table. The motor is rotates at 20 rpm and one revolution of the mirror took about 30 seconds.
Here is a picture of the baffle which contained the pool of water used to cool the glass. It also contained the bulk of the glass particles. The inside of the baffle was sealed with a bead of silicone to prevent water from escaping.
Here is a close up of the first dig into the glass. At 2000 rpm the diamond wheel cut through the glass like it was butter. This view represents less than 1 second contact and about 0.035" deep. As the table turns I started grinding the inner 9" region from the outside in. After a couple of passes I extended this motion to about 15" moving towards the centre then evenutaly out to within 1/4" from the outer edge to the centre. The motion worked really well. I used a sponge to remove the water and glass particles every two rotations. I used a template 25" in diameter with one side flat and the other side cut to the radius of curvature. Using this template and the grinding wheel I was able to work the surface to the approximate shape I wanted. Rough grinding will even out the high and low spots to achieve roughly a spherical shape.
SAFETY Equipment must be worn. Glass particles are a very dangerous health hazard. I used a Heppa Filter mask specially designed for small solid particles (Home Depot $50). The mask must be worm during the clean up phase as well. Proper eye protection and/or a face shield is a must. I also used a large splash guard to catch the bulk of the flying water and glass particles.
Here is a picture of the hogged out mirror with a layer of 2" ceramic tiles used to make my grinding tool. The tool is 19" OD and is about 3/4 sub-diameter relative to the primary. I oiled the surface of the mirror in order to prevent adhesion of the plaster to the mirror. Worked really well! The table was first leveled to ensure a consistent thickness of the plaster. The baffle allowed the build up of a 2" thick tool. I attached a 1/2" plywood backing to the tool so I could screw some handles to the centre for grinding. I could not find dental plaster so I used CGC Durabond 90 plaster. It chemically sets in 90 minutes and has very little shrinkage.
Shown above is the compeletly hogged out mirror with a 0.270" sagitta.
Here is a picture of the completed tool. I sealed all of the exposed plaster with 3 coats of grout sealer. The acrylic sealer worked beautifully in my past projects is cheap and does not have the adverse health effects of two part epoxy. I used polyurethane to seal the plywood. Total cost was $25. If you chip the plaster then you must reseal it otherwise the particles of plaster may cause a scratch during fine grinding.
The total time spent in the hogging stage was 2 hours of glass removal and 4 hours of cleanup. During the 2 hours I removed about 70 cubic inches of glass. I am told that using conventional methods to hogging out a blank this size could take anywhere from 70 hrs for an experienced mirror grinder up to 200 or more hrs. I was VERY pleased with this method and hopefully other ATMs will tray some variation of this method.
Rough Grinding:
I used the tool shown above to smooth out the curve and complete the sagitta to the required 0.312" necessary for an f5 curve.
1) Smoothen out the curve. I started working the blank with mirror on bottom, tool on top to smooth out the curve from the deep gouges left over from the hand grinding operation. I spent about 1 hour with #36 grit to get a nice smooth curvature out to the edge of the glass. During this process I discovered that I dug too deep near the edge of the glass (in a couple of spots) so I had to spend an extra 2 hours rough grinding to remove the defect. The thumbnail below shows the defect and lets you see the aggressive pitting left over from from the #36 grit. Watch that #36 grit because it leaves nasty pits!
2) Complete the Sagitta. I flipped the mirror over and proceeded to use the standard hogging out stroke. The tool is now on the bottom and the centre of the mirror placed on top near the edge of the tool. I worked centre of the mirror over edge of the tool as I went around the mirror. I spent about 4 hours rough grinding the remainder of the sagitta for a total cutting depth of 0.042". This was very tiring work and I'm glad I did not use this approach for the entire depth! I switched to #60 grit with around 0.010" remaining on the sagitta. I then flipped the two so I could work the mirror on the bottom to remove the deep pits left over from the #36 grit which ended up taking about 1.5 hrs. I had some extra #80 grit which I used for 3 wets to speed up the pit removal of the #60. I then switched to #120 grit and spent another 2 hours removing the pits from the #80.
Here is a picture showing the full depth of the curve after rough grinding. The marble is 0.605" in diameter.
Here is a picture of the tool after rough grinding. In preparation for fine grinding I'll remove the plaster near the edge of the tool to ensure the plaster does not scratch the surface during the finer grits.
Hogging Summary:
The total working time in the rough grinding stage was about 11 hours. At the end of this stage I have a very nice smooth curve. I lost about 2 hours trying to remove the deep gouge from the hogging process but otherwise it went very well. If I make a large mirror again I will deepen the sagitta using a cast iron or steel tool Vs a tiled tool because I ended wearing down the edge tiles quickly.
Fine Grinding:
Not much to say here. I spend about 2 hours on each of the following grits (220, 320, 500). The 1000, 5u sizes were complete in about 45 minutes each. I used a small portable 50x microscope to inspect the surface before changing grit sizes.
Due to the thin glass I was very careful to make sure that I averaged out my motions very evenly to prevent astigmatism. I rotated the mirror every 10 minutes while walking around the stand every minute and a half. The mirror is supported on a large piece of carpet which was covered by a garbage bad to keep the contaminants down. That worked out really well.
The surface looks really good and the Sharpie test shows an even distrubution indicating a nice speherical surface. Sorry I do not have any pictures during this stage.
Oh, I almost forgot to mention my bonehead move. After finishing with the 220 grit I washed the mirror surface for inspection. I could not wait for it to dry (bone head move coming up) so I took a paper towel and used it to wipe down the mirror. I guess I should have expected it but I heard a loud high pitched sound. You guessed it. A scratch, one hell of a scratch actually.
My advice: In the Fine Grinding Stage DO NOT WIPE the mirror with anything. Dab don't wipe. Once polished the surface is quite durable. In Fine Grinding it is very delicate so be careful.
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Material Sources for Project:
Glass Components: Peek'r Technology
Aluminumizing: David Dunlop Observatory (Toronto, Canada) (planned for Primary)
MOONWARD Vacuum Coatings (planned for 8" flat)
