[LVAS] Mirror Making - Part 3
Rob Lambert
scopegeek at gmail.com
Wed Mar 17 16:36:45 PDT 2010
Fred,
Do you have any photos to go along with the article. I'm considering
moving/copying this to the main website - some photos would be nice to go
with the articles.
Rob
On Wed, Mar 17, 2010 at 4:18 PM, Fred Rayworth <rayworth1969 at hotmail.com>wrote:
> 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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--
Rob Lambert
702-461-1390
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