[LVAS] Mirror Making - Part 3

Wed Mar 17 16:18:15 PDT 2010

Roger,

Thanks for the request! However, it'll have to wait until I finish writing them all. There are quite a few to go so it will take a while, but once they are all completed, I'd be glad to send you a set.

Fred

From: drivester at hotmail.com
To: lvas at lvlug.org
Date: Wed, 17 Mar 2010 13:35:37 +0000
Subject: Re: [LVAS] Mirror Making - Part 3

Fred, 

Would you be so kind as to send me all in sequence. I am going to forward to the the Charlotte and Raleigh groups, and maybe one more.

Excellent articles.... they for sure could help a person with an interest in grinding an astronomical mirror. It's really good to keep stuff like this alive, as this too is rapidly becoming a forgotten art.

Roger   

    The moon and stars to govern the night.....   Psalm 136:9

From: rayworth1969 at hotmail.com
To: lvas at lvlug.org
Date: Tue, 16 Mar 2010 21:42:27 -0500
Subject: [LVAS] Mirror Making - Part 3

All,

For the two of you that may actually be reading this, here is part three of my mirror making series. As before, I have also posted it on the web site Forum.

Fred

MIRROR MAKING – SETTING UP
            Once you’ve decided on a focal ratio, the next step is to hog out the bowl-shaped curve in one side of the glass. Here would be a good time to list the materials you will need. These components can be found in any standard mirror kit, but if you are the scrounging type, they are listed for your reference:
            Mirror blank: May be Pyrex, plate glass, black ceramic, etc.
            Tool: The tool is a matching blank that is often plate glass. However, some kit companies supply you with two Pyrex blanks so that if you ruin the main mirror blank, you can switch to the other.
            Grinding compounds: They consist of carborundum or aluminum oxide. A course of grits may include #80, #120, #180, #220, #320, #440. NOTE: There are many more variations of that grit spread, but that is one typical group.
            Polishing compound: In the old days, it was red jeweler’s rouge. Nowadays, though rouge is still available, you will most likely get cerium oxide or Barnesite. Both the new compounds polish the substrate (mirror) faster and cleaner. However, they also may polish too fast and get you into more trouble during the figuring More on that later.
            Pitch: The polishing is done with a pitch lap. A pitch lap is tar molded onto the tool. Most kits come with pure refined burgundy pitch. More on that later, also.
            The rest of the materials you’ll have to scrounge on your own. We’ll go into more specifics as they come up.
            To figure out the depth of the curve for our 8” f/7 mirror, you can do it the rocket science way and mathematically figure out the sagitta of the curve. The formula for that is in most mirror making books. Once you figure the depth, you will need a spherometer to measure the depth of the curve as you grind it.
            However, there is a much more low-tech method that is phenomenally cheaper and simpler. All you need is a bit of heavy kite string, a piece of stiff cardboard, and a pencil. The sagitta of the mirror is going to be twice the focal length of the mirror. For an 8” f/7 mirror, the target focal length is 8 X 7 or 56”. To figure the actual curve of the bowl shape in your mirror, the first thing to do is double that focal length. 56 X 2 = 112”. Now here is where the miracle of simplicity kicks in. Pound a nail down into a permanent structure, maybe an exposed floor stud in your garage. Tie one end of the kite string to this nail. Now, measure 112” along the string and tie a #2 pencil to it. Measure along the floor and mark a point 112” from the nail. Make sure the pencil sits as close to the 112” mark as possible. If you pull on the string to make it taught, the tip of the pencil should come straight down at the 112” mark. Now lay your stiff cardboard on the floor under the pencil. Make sure the piece of cardboard is at least 8’ wide, if not a bit more. Keeping the pencil tip as straight down as possible, draw an arc across the cardboard. That, my friends, is the curve you are going to try to match. Pick up the cardboard and take a sharp knife or heavy scissors and cut along the line and make as smooth of a curve as possible.
            With this template, you will be able to tell in an instant how the depth of the curve is progressing. Remember, it is only a rough estimate, as you will do more accurate measurements when you get close to the proper curve.
            Hogging out the mirror will give you plenty of upper body exercise. It is also a good idea to do it in the garage so the noisy grinding doesn’t annoy your family. I don’t recommend doing outside for a variety of reasons, including wind and dust.
            The whole mirror making process is going to be done on some kind of work stand. This is something that will not come in the kit. There are countless ways to make a stand, but trust me, the simplest and most reliable is to get hold of a 55 gallon oil barrel, preferably clean. The work stand must be stable and should not move or rock back and forth as it will seriously affect your grinding and polishing actions.
With a 55 gallon barrel, stand it up with the caps on top. Find the center point of the barrel top and mark it. Drill three holes into the top spaced 120° apart around a circle about two or three inches in from the edge. Purchase or scrounge three ¼-20 bolts about 3” long. Using mechanical fingers, slip a bolt from the underside of the barrel top into each hole. Now put a washer, lockwasher, and nut on the bolt and tighten it. Once you’ve done all three, you now have a way to fasten the working top to the barrel. Now cut a circle of ¾” plywood the same diameter as the barrel so that it sits down solidly on the rim with around a ¼” overhang. Mark holes for the studs and drill them out. Test fasten the top to the barrel. Make sure it fits centered over the rim of the barrel (the ¼” overhang). Now cut another circle the same size out of another piece of ¾” plywood. This piece will be the business end of the stand. Drill recess holes in the top piece to accommodate the nuts and stud ends of the bottom piece. It should sit flat against the bottom piece. The studs will also provide a key for placing the top piece on the bottom part. Once that is done. Take both pieces off.
Now position the barrel in an area where you can comfortably make complete circles around it. Also keep in mind the mess that will be dripping on the floor. It probably wouldn’t be a good idea to put it right in the path to the washing machine and dryer. Your significant other will not appreciate stepping around you or in that mess. Once the barrel is positioned, fill it with water and seal both caps. Now put the base top piece on and snug it down. Be careful not to go too tight and warp the wood. Once that is done, lay the top piece on it. Along the outer edges in the same pattern of 120° apart, drill pilot holes through both pieces and fasten the top piece to the bottom with heavy 1 ¼” wood screws. Keep in mind that the screws should not go deep enough to punch through the top of the barrel.
            The final step is to draw an 8” circle around the center point (a good idea to do this before you paint it). Using the same 120° pattern, mount three wooden clips (3/4” thick wood) around the diameter of the glass to hold it in place. Make sure the clips are not thicker than either the mirror or tool! A little slack is fine as you will be placing newspapers between the glass and the top to help contain the mess. You do NOT want to allow the clips hold the mirror too tight or it will pinch and distort the surface. Now that your work stand and top are set, you have the options of leaving it be, or if you want to get fancy, paint the working top piece with a waterproof epoxy. This will preserve the wood better, as it is going to get wet, often.
            In the next part, it will be time to start the rough grinding. 

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