Jump to content

Fixed post mirror grinder advice

Velikovsky

By Velikovsky
in DIY Astronomer

Recommended Posts

Velikovsky

Velikovsky

Posted
Posted

Hi All

We've been having some idle discussions about grinding mirrors here. In particular, we've been discussing building a fixed post mirror grinding turntable for cutting mirrors up to about a 24inch diameter to spherical. Its only a pipe dream at the moment, but has anyone had experience building/using one?

There's some good looking ones on you-tube, and we've culled the following approximate numbers from listening / watching:

20 to 40rpm, (right speeds?)

1HP motor, (Is that enough?)

all step down done with pulleys & belts. (Are gears better?)

Any comments/advice welcome.

Cheers

Steve

Link to comment
Share on other sites
  • Replies 32
  • Created
  • Last Reply
Velikovsky

Velikovsky

Posted
  • Author
Posted

On the RPM I think Gordon Waite is king of this stuff and his rough grinder

seems to go at about 40, while his polishing seems slower. Should be interesting to see if anyone knows anything.

Edit happened because I pressed post instead of review :)

Link to comment
Share on other sites
barkis

barkis

Posted
Posted

That rotational speed seems a bit quick to me Steve.

By "cutting to spherical" Do you mean you intend to hog the mirror blank with sub diameter tools in a face up position?

At 20 rpm that disc will be spinning quickly, one revolution every 3 seconds, and will need to be well secured to the turntable. It will tend to throw the grit off too perhaps.

John, Glasspusher will have an input on this though, and may well have a different opinion of it.

Ron.

Link to comment
Share on other sites
Velikovsky

Velikovsky

Posted
  • Author
Posted

Barkis

Yes, we plan to hog while the mirror is face up with a smaller tool - probably 2/3 size. I've been doing some simulations (don't ask to see them, they're rubbish) but it seems pretty sound to me - you think that's a bad idea?

If we build this the plan is to "future proof" it with larger mirrors in mind, so the clamping for the mirror on the table will be "significant"...

Anyway, the sky ain't gona photo itself - gota go get set up.

Link to comment
Share on other sites
FraserClarke

FraserClarke

Posted
Posted

Gordon Waite certainly seems to get good results all the way through polishing on his fixed post machine. How much of that is his expert experience, I don't know...

Does seem an attractively simple method compared to standard over-arm machines.

Link to comment
Share on other sites
barkis

barkis

Posted
Posted
Barkis

Yes, we plan to hog while the mirror is face up with a smaller tool - probably 2/3 size. I've been doing some simulations (don't ask to see them, they're rubbish) but it seems pretty sound to me - you think that's a bad idea?

If we build this the plan is to "future proof" it with larger mirrors in mind, so the clamping for the mirror on the table will be "significant"...

Anyway, the sky ain't gona photo itself - gota go get set up.

I'm sure you'll do fine with the grinding Steve, I was just a little taken aback by the rotation speed, but as long as the mirror has three robust cleats in place to hold it, it shoudn't fly off.:(

Many years ago I made a machine to grind large blanks,but the largest I did was a 16" f4.5. The table speed was quite slow at 3 rpm, and a variable stroke length.

Like a fool,I broke it up about 3 years ago, now with all this mirror making going on, I'm starting to regret it :p.

Anyway, I'm looking forward to your progress reports mate, it is a great way to fill time, and quite pleasurable. The finished mirror is

a thing of beauty and no mistake. Good grinding.:)

Ron.

Link to comment
Share on other sites
Velikovsky

Velikovsky

Posted
  • Author
Posted

Well I got set up, Used my new illuminated reticule eyepiece to mess with drift alignment...That's not as easy as I thought it was! I got questions about that I'll ask elsewhere. But it went icy then clouded up then everything got really wet, so I broke camp, then it cleared up again, then it was 11pm, then there was this bottle of wine...

I hope this grinding machine goes better than tonight's scoping did.

Link to comment
Share on other sites
Glasspusher

Glasspusher

Posted
Posted

Basically there are two types of grinding/polishing machine. The ‘traditional’ type which involves the mirror rotating at a relatively slow speed, a few rpm for a big mirror (16-20 inches) with the tool moving across the mirror, from side to side, at a higher rpm depending on the size of the mirror and/or tool. This is the method that I use; one of my machines is shown in the first attachment.

An alternative, which seems very popular with home machine builders, involves rotating the mirror at a relatively high speed whilst the tool has minimal movement or is fixed (as in the case of Gordon W’s machine). The trick is to place a correctly sized tool in the appropriate position to ensure an even grind/polish. With both machines I am assuming the mirror is being worked face up with a sub-diameter tool or lap. Gordon Waite’s approach is interesting as he has dispensed with the ability to move the tool or lap during working. He is relying entirely on positioning the tool/lap in the correct position when working. The technique used by this type of machine is referred to as ‘spin’ polishing, as the mirror is turning at a relatively high speed during working.

Another alternative is illustrated by the second attachment. Here, the mirror is rotating at a slower speed as with a ‘conventional’ machine. The tool or lap is moved across the mirror using the arm which has a pin to engage the back of the tool or lap. With this type of arrangement a fixed speed for the table is Ok and would be a few rpm, it is not critical.

Regarding the motor, I would recommend a geared three phase motor of about ½ hp. The three phase motor can be run from the 240v mains (single phase) using a phase inverter, this device will enable the speed of the motor to be varied without lose of torque. My machines use a combination of pulleys and belts together with a geared motor, this arrangement works well.

John

post-15839-133877540753_thumb.jpg

post-15839-133877540766_thumb.jpg

Link to comment
Share on other sites
Velikovsky

Velikovsky

Posted
  • Author
Posted

Thx John

That's a nice looking machine. What size mirrors have you made so far, and did you work out a strehl rating for them?

As a matter of interest, is there another use for parabolic mirrors besides telescopes?

Just wondering.

Steve

Link to comment
Share on other sites
Glasspusher

Glasspusher

Posted
Posted

Steve,

I have made mirrors up to 24 inches in diameter, I would love to go bigger but the glass is not available in the UK. If anyone is sitting on a large unwanted blank (over 24 inches diameter), I would be interested to hear from you. You can forget about Strehl rating unless you have an interferometer and the experience to use it and analyse the results. A Strehl ratio derived from any other method of testing, eg knife edge and mask, is meaningless as the number of sample points on the mirrors surface is too small. I have seen mirrors advertised on ebay , and elsewhere, with high strehl ratings based on knife edge measurements.....these Strehl valves are meaningless. I do have an interferometer but it is by no means essential to make a good mirror.

Parabolic mirrors are used in a variety of applications like search lights, car headlights etc.. None of these need to be anywhere near the accuracy required by a telescope mirror.

John

Link to comment
Share on other sites
skybadger

skybadger

Posted
Posted

Steve

Have a look for the plans of the 'mirror-o-matic' by Denis Reichs. There are some pictures of mine on the website. The plans are sized for a 12" version or a 20" version but the 12" version can do up to 18" or so. It will do pretty much everything you want it to do but is not a hogging accelerator like, say, a diamond wheel on a hnd grinder swung from a ceiling joist.

HTH

Mike

Link to comment
Share on other sites
  • 2 weeks later...
Glasspusher

Glasspusher

Posted
Posted

Hi Ste,

Thanks for that...but I am afraid they are just too thin at 25mm. Ok for a 12 inch, possibly a 14 inch......and with great care a 16 inch. The global economy dicates that thick glass is no longer made in the UK, much too my regret. Keep looking though...

John

Link to comment
Share on other sites
  • 7 months later...
Velikovsky

Velikovsky

Posted
  • Author
Posted

Hi All

Thx for the various advice back then. I've been doing a lot of traveling, which is why I ain't been here for a while. Finally had some time to get building the last month or so and here is a couple of pictures.

We decided to go with just a turntable and to grind by hand. Seemed like you get more randomness that way.

Attached images are: The turntable x 2, making polishing tool x2 and warm pressing in the kitchen x2.

Polishing is going pretty well. We just got mark one of the Foucault test rig running today - was a bit pleased with that! There is a vid of first light on FIrst Light on our Foucault Tester - YouTube - pretty I thought. You can probably see the big remaining scratches - one right of center and one on the left just below center. Polishing continues.

I'll write this up in more detail when I have time but for now, its clear out (hmm, well, it was...).

Bye

Steve

post-23764-133877686001_thumb.jpg

post-23764-133877686036_thumb.jpg

post-23764-133877686039_thumb.jpg

post-23764-133877686042_thumb.jpg

post-23764-133877686045_thumb.jpg

post-23764-133877686047_thumb.jpg

Link to comment
Share on other sites
Velikovsky

Velikovsky

Posted
  • Author
Posted

Hi John

The scratches were contamination - I made the mistake of doing a little metalwork then not cleaning up enough before starting polishing. I have to step back to 3rd stage (303) for a rough scrub to get the scratches out or even to 202.

Do you know if its possible to use a pitch lap for coarser grinding? I could make a tool but my gut feeling is a tile tool will make bigger scratches when its bedding in at the start forcing me to use even rougher powder to cut those scratches.

Do you have to start again???

Steve

Link to comment
Share on other sites
Glasspusher

Glasspusher

Posted
Posted

Hi Steve, you cannot use a lap for grinding...it is too soft the abrasive grains would just sink into it resulting in no grinding. Despite your reservations, tile tools are the best option and should not produce bigger scratches during bedding in. What size mirror are you making and what tool did you use for initial grinding?

John

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.