Fine Grinding a 20" Quartz Telescope Mirror
Вставка
- Опубліковано 28 сер 2024
- Gordon Waite shows some steps during fine grinding of the 20" mirror. As the grind progresses through the grades of abrasives, see the technique Gordon uses to change from 5-micron to 3-micron AlOx without interrupting the grinding process.
First is whether the mirror is at the radius that you want. If you need to change the radius, do that before going on to a finer level. Second, inspect the surface to make sure that grinding has done the job. With coarse abrasives, you can check for pits. If any are larger than average, keep going with the current abrasive. With finer abrasives, hold the mirror so light grazes, and see if it is reflected evenly across a diameter. Mirrors often grind inside to outside, or vice versa.
Very helpful time saving and scratch avoiding techniques.
your comment just made me realise that the abrasion depth is proportional to the radial distance from the center.
thank you for your input sir. i will investigate on the matter,particularly using ferromagnetic fluids + al2o3 mix around a cnc controlled electromagnetic mandrel.
oh my Just think if all human crafts were documented with videos? Earth would have a star ship by now. 😎 Thanks for the great stuff.
Lisa, I haven't done this so I could be missing something, perhaps Gordon will or someone with experience will chime in if I'm wrong. If you will go back and watch the tool making video he makes the tool on the mirror, so it has the same shape as the mirror. As the two grind together the edges of the tool are worn away faster than the center, and the center of the mirror is worn away faster than the edges giving it that concave shape that is desired. I suspect if the tool was to wear to the poin
a most elegant mechanism. Do i have to assume that the abrasive disk must have a diameter exactly 1/2 of the glass substrate diameter?the relative speeds of the disk will be calculated according to what formula?
...(continued) point that it had to be replaced, a second tool would also be made on the mirror's surface so as to duplicate the established curve.
I suppose when you have a machine for grinding but polishing is manual, that makes it worthwhile more often to go finer on the grinding.
Why can't you polish on the same machine you grind on? All of our machines can be used for both grinding and polishing. Going finer on the grind risks scratching, and doesn't take much polishing time away. We polish after 9-micron.
Hey there,
I'm curious if you've ever run 3 or 1 micron grit by hand. In the various chats and forums I'm in, I've seen people say that 3-1 micron should only be used with machines like what you've got. Do you suppose suspending in something other than water might make hand use easier?
Hi Gordon, I'm a bit lost on how you keep it concave whilst the pitch tool is so flat... is the pitch tool set at an angle or the tools placement, or even the slower speed at 1/4, that makes the curvature happen. I would love and explanation, can't seem to find an answer....To me it just seems it will just flatten out, its doing my head in. Thanks in advance.
First of all, thanks for the awesome videos - very informative and addictive :)
I'm thinking of building a turntable similar to yours but I'm still pondering on the appropriate wattage of the motor... how much kW is your motor so it's got enough torque not to struggle when you put weights on it?
Most of my motors are 3/4 hp or better. That will do up to a 25" mirror.
@@GordonWaite Thanks for the info! What about the VFDs, do you have any specific recommendations?
What additional steps would you take to parabolize the mirror?
you telescope guys should really start with reading on diy cnc machine before setting out to grind your lenses and mirror.
the whole grinding aspect can be made dead easy with cnc motors and modules.
and they are DIY,not industrial prices,DIY prices...
search for DIY cnc forums beofre you invest in any machinery for grinding.
@GordonWaite how many hours of work did it take to finish grinding?
How would you reduce a too deep sagitta with the fixed port method? Push the tool off the edge further?
Yes, you have to move the tool farther from the center of the mirror. Go in small steps.
interesting video Gordon, it looks like you have done this before. i am interested in making a reflector telescope myself and have been reading various things about it on the net. one thing i was wondering is why do you use glass? wouldn't some type of ceramic make a good substrate? thanks for any insight you could give me.
Glass is traditional, it’s reflective enough before the coating goes on to usefully test the curvature so you can do figuring by hand on it, and people know how it works. It’s also very available.
Technically, a super high temperature vitrified ceramic could work, but the word vitrified says it all - you basically would need a ceramic that’s been turned into glass.
The question is, what are you hoping to gain? Glass is super cheap for what it is, and Pyrex in particular (that is, borosilicate glass) has very good thermal properties and strength. It’d be a very expensive ceramic to better it - think, like, useful for a KeyHole or the Hubble where every pound costs 25 grand to send up, but not really viable for hobbyist astronomy on earth.
MASK!!! always wear protective mask for breathing when grinding quartz, its the most common form of crystalline silica and its extremely hazardous like aspestos, and relatively small exposures can cause silicosis, help to spread awareness through the lapidary community about the safe application of cutting grinding polishing and generally working quartz, by spreading this message might save some ones life. many thanks
This is wet grinding, there is no dust in the air.
the finished product is aluminized so why is quartz used. Why isn't say a cast aluminum disk used?
Michael Hursh Aluminum expands and contracts when it gets hotter or colder, and this destroys the precise shape of the mirror. Pyrex or borosilicate glass change much less than aluminum, and quartz changes even less.
3 micron AlOx? Isn't cerium oxide typically 3 microns?
Many types of Cerox do have a 3-micron average particle size. But aluminum oxide and cerium oxide are different materials and work in different ways. AlOx is an abrasive that is used with "loose abrasive" techniques. That is, the abrasive either rolls or slides between two pieces (a blank and a tool) and chips away the glass. Cerox is an entirely different process. With Cerox, the material embeds in the pitch substrate. When you move the glass over the pitch with the embedded Cerox, the particles "shave" the top of the glass instead of chipping it out. This isn't the whole story, as there are also chemistry effects during polishing.
Interesting. I jump straight from 5 micron AlOx to cerium oxide with my mirrors. Does 3 micron AlOx save time in polishing?
It does save some time, but only if you are using a fixed-post grinding machine. Otherwise it is prone to scratching, and it's hard to use without perfect contact, which is very difficult to achieve with hand work.
I am grinding by hand and getting scratches even at 9 micron. Very fine scratches though, which seem to go away after about 20 minutes with the 9 micron.
I find that big source of scratches are tiny chips of glass which are fractured from the bevel when working with tool on top, especially if the bevel was not fine ground and just made with a coarse-ish grinding stone.
Yes, the edge bevel is a source for problems. Keep a good bevel and things will be better! I use a diamond tool so the edge isn't quite so coarse.
Do you make a lot of money doing this?
Hi, Gary! I won't be ordering a Lambo any time soon! Our company, Waite Research, makes telescopes, mirrors and industrial optics, and it keeps us all very busy.
GordonWaite you say “us all”, so it’s more than a one man operation now? That’s genuinely impressive! Particularly for an industry which is so heavy in hobbyists-turned-pros.