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Old 17-12-2020, 06:51 AM
Stefan Buda
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Making a 310mm Zerodur planetary mirror

I took on the job of making an f/5 paraboloid mirror from a supplied Zerodur blank.
The problem is that the blank is 60mm thick and I never worked Zerodur before.
The blank weighs 11.3 kg and needs to be reduced to a thickness of no more than 40 mm to have any chance of becoming a good planetary mirror.

Instead of converting one third of it into ceramic powder, the plan is to salvage some of the surplus material. This is expensive stuff and would be nice to have some off cuts that I can use for other projects.
My largest diamond disc is 9" diameter with a hub diameter of 40 mm, and that allows me to cut to a depth of about 94 mm. Any suggestions?
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Old 17-12-2020, 10:18 AM
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Try talking to a granite supplier, they seem to be able to slice it for bench tops etc.
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Old 17-12-2020, 11:18 AM
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Maybe this could help finish the job?
https://www.diamondwiretec.com/home
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Old 17-12-2020, 03:52 PM
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Some stone and tile saws have a cut depth of 150mm . Maybe you can hire one.
Or the stone cutters can water jet cut.
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Old 18-12-2020, 10:18 AM
Stefan Buda
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Thanks for the suggestions.

About 20 years ago I had the same problem with a 16" Pyrex blank and I made a wooden support structure for it so that the stonemasons could cut it for me. After a few day I got a call that their machine did not have enough clearance to be able to do the job. At that time I decided that I wouldn't waste any more time with granite cutters and do it myself.
I still have the improvised machine and the tools I used for the job, so I will do the slicing myself, although the abrasive wire cutter idea is nice, I don't know if anyone in Melbourne has one.

As a first move I ground the side of the blank cylindrical because it was very rough, a bit oval and a bit barrel shaped. I also ground a 3mm chamfer on what will be the back edge.

This thing is hard, same as fused quartz. It took hour to get this far even though I removed less than a millimetre from the diameter.
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Old 18-12-2020, 04:56 PM
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Hi Stefan,

There are some 300-500mm diamond blades available that you could try, usually at reduced rotational speed compared to their smaller brothers. With glass (or similar) I suppose you want a continuous blade that may be difficult to find above 300mm diameter ( I found 1 at 300mm), but there are plenty of segmented blades (better material clearance for aggressive cutting) that are available in the 300 to 500mm diameter range. I suppose it goes without saying to be careful since you only have 1 piece of the Zerodur and these types of blades may induce some micro vibration which may not be good. On the other hand it may be just what you need. On problem is that as the size goes up so does the thickness of the blade (an hence heat generated).

Anyway here are a host of possible options. There is even a 300mm Husqvarna continuous edge diamond wheel listed in amongst the many at this link (also a 350mm EHWA with tiny gaps) ....
https://sydneytools.com.au/search?p=5&q=Diamond%20Blade

300mm Husqvarna continuous edge diamond wheel
https://sydneytools.com.au/product/h...-diamond-blade

350mm EHWA with tiny gaps
https://sydneytools.com.au/product/e...-diamond-blade


PFERD is a quality manufacturer I would check with them (1300 073373) about using their large diamaeter segmented wheels to carefully cut glass. (I'm fairly sure they'll say not to), but .... here's an example of a 350mm wheel for info....https://www.pferd.com.au/diamond-cut...se-segmented-t

Best
JA
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Old 19-12-2020, 07:00 AM
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Even if you found the right blade, I haven't seen any consumer level tile, brick, masonry cutting machines that would cope with something so tall and thin. At the least I'd think you're up for a workholding exercise.

My suggestion is to go with whatever worked last time. How did you do it?
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Old 19-12-2020, 08:57 AM
Stefan Buda
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Thanks JA,
That 12" disc looks good and I may buy one if I can figure what they mean by "10mm segment height".

Yes Tim, I did go ahead with the old way of doing it.

After I turned the blank over, I used various diamond tools to make a, 20mm deep, 80mm diameter pocket and then I used a small diamond coated disc to make a 20mm deep undercut a the bottom. After that my old 9" diamond disc was able to complete the cut.
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Old 19-12-2020, 09:08 AM
Top_oz (Tim)
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It just means the diamond edge is 10mm across (radially).
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Old 19-12-2020, 09:16 AM
Stefan Buda
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It also means that the picture shows a different product.
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Old 19-12-2020, 09:56 AM
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Quote:
Originally Posted by Stefan Buda View Post
It also means that the picture shows a different product.
Yes I agree. They appear to be using the image of their 230mm disc also for their 250mm and 300mm wheels if you look at the Sydney Tools link I referenced above and just changing the text. That being the case, the diamond distance between the inner and outer radius of the diamond annular section is "close" to 10mm, well more like 14-15mm if you scale off the dimensions taking the 230mm as a reference. Of course the best would be to go in to a store and ask, hopefully see one.

Have a look here/read the text > there are several blades specifically rated for glass /hard ceramic uses in the 300-400mm range as well as a 350mm variant of the same style of Husqvarna blade as earlier (last image bottom of page)....
https://www.udt.com.au/view/our-diam...carbide-blades

Best
JA

Last edited by JA; 19-12-2020 at 12:08 PM.
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Old 20-12-2020, 10:00 AM
Stefan Buda
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Thanks for the link. I bookmarked it because they seem to have some tools that I may need in the future. However I remembered that my machine's spindle has barely enough torque to drive the 9" blade, so I would be in trouble with a 12" one.
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  #13  
Old 20-12-2020, 09:51 PM
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Hello Stefan. Are you making the whole scope as well as the mirror?
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Old 20-12-2020, 10:14 PM
Stefan Buda
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Hello Stefan. Are you making the whole scope as well as the mirror?
Only the primary mirror.
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  #15  
Old 21-12-2020, 11:58 AM
sharpiel
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That's a shame for those of us who enjoy your builds.
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Old 22-12-2020, 01:10 PM
Stefan Buda
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Originally Posted by sharpiel View Post
That's a shame for those of us who enjoy your builds.
Not to worry, there are more projects to come, but at the moment I'm still making improvements to the manufacturing methods for my CDK250. Once that is finalized I'll start another phlog. Perhaps a spectrograph or a solid Schmidt camera.

But to get back to the Zerodur, I had to generate the curve in two cuts as my cup wheel is not large enough for this job. After that I used a large bronze ring to blend in the curve. The required sagitta is almost 4mm deep and at this stage it is within 0.1 of a mm.
I also gave a bit of a grind to the back face to make the original machining marks disappear.
The weight of the blank is down to 7kg, but it took me four days of work to get this far.
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  #17  
Old 22-12-2020, 02:26 PM
sharpiel
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Quote:
Originally Posted by Stefan Buda View Post
Not to worry, there are more projects to come, but at the moment I'm still making improvements to the manufacturing methods for my CDK250. Once that is finalized I'll start another phlog
Can’t wait. “Phlog” tho? So bad phonetically...
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  #18  
Old 22-12-2020, 03:38 PM
Stefan Buda
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“Phlog” tho? So bad phonetically...
Yes, that's why I like it. Sorry.
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Old 22-12-2020, 08:14 PM
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Again I am in awe of your ATM skills....the nice thing about Zerodur is: you don't get polishing/figuring "blisters" due thermal expansion of the substrate (or so I am told)

I expect the end result will be awesome
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Old 22-12-2020, 09:29 PM
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I expect the end result will be awesome
+1 to that!
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