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Mirror Blank for Grinding, 18in f/4

DIY Equipment Optics Reflector Mirror Making
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#1 Dauntless Voyager

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Posted 04 December 2021 - 09:16 PM

What is recommended for a glass mirror blank for mirror grinding? I am not in the position where I can spend large sums of money for a premade blank. Are there any kinds of alternatives to a bought mirror blank? 

 

Thanks. 



#2 Keith Rivich

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Posted 04 December 2021 - 09:31 PM

Keep and eye on the classifieds. 18" scopes show up occasionally priced very low. 

 

Curious...do you have the equipment to grind  mirror? Sounds more expensive then buying a finished mirror. 


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#3 Dauntless Voyager

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Posted 04 December 2021 - 10:21 PM

Keep and eye on the classifieds. 18" scopes show up occasionally priced very low. 

 

Curious...do you have the equipment to grind  mirror? Sounds more expensive then buying a finished mirror. 

I do have the equipment. However, my question more less is whether or not tempered glass can be grinded. I would figure that tempered glass would not be suitable. For that reason, I trying to find where I can get annealed glass, perhaps from furniture, which I can use. 



#4 Augustus

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Posted 04 December 2021 - 11:15 PM

I do have the equipment. However, my question more less is whether or not tempered glass can be grinded. I would figure that tempered glass would not be suitable. For that reason, I trying to find where I can get annealed glass, perhaps from furniture, which I can use.


Didn’t you have an entire thread literally the other day where plenty of people explained this all to you?

#5 Dauntless Voyager

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Posted 05 December 2021 - 09:19 AM

Didn’t you have an entire thread literally the other day where plenty of people explained this all to you?

I would appreciate that if you don't have an appropriate response to the question which I have outlined, that you not respond. I am trying to learn a new skill. It is not for you to criticize or downtrodden somebody else. 


Edited by Dauntless Voyager, 05 December 2021 - 09:30 AM.


#6 Mike Lockwood

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Posted 05 December 2021 - 11:51 AM

I would appreciate that if you don't have an appropriate response to the question which I have outlined, that you not respond. I am trying to learn a new skill. It is not for you to criticize or downtrodden somebody else. 

A blunt but accurate response is not necessarily an inappropriate response.

 

There is, in fact, another thread going (6 days old) that covers this in detail, it's even on the first page, so easy to find:

  https://www.cloudyni...strain-pattern/

 

CN is a site where moderators actively try to keep redundant or duplicate threads from existing and confusing people, and they do this for free.  It's appropriate to try to make their job easier.  That's not criticism, it's advice.  There is also a good search function that can yield a lot of information.... if you use it.

 

As far as what is recommended, a properly annealed piece of borosilicate (Pyrex, Supremax, etc.) that is accurately machined is what I would recommend.

 

Here is a source in Germany, the glass is a bit thin in my opinion for an 18" but it is good quality:

  https://www.stathis-...materialeng.htm


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#7 Augustus

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Posted 05 December 2021 - 04:16 PM

A blunt but accurate response is not necessarily an inappropriate response.

There is, in fact, another thread going (6 days old) that covers this in detail, it's even on the first page, so easy to find:
https://www.cloudyni...strain-pattern/

CN is a site where moderators actively try to keep redundant or duplicate threads from existing and confusing people, and they do this for free. It's appropriate to try to make their job easier. That's not criticism, it's advice. There is also a good search function that can yield a lot of information.... if you use it.

As far as what is recommended, a properly annealed piece of borosilicate (Pyrex, Supremax, etc.) that is accurately machined is what I would recommend.

Here is a source in Germany, the glass is a bit thin in my opinion for an 18" but it is good quality:
https://www.stathis-...materialeng.htm


OP had a whole thread running about the exact same topic the other day. The general consensus is an 18” mirror is too big for a first project and a tempered glass table top won’t work.

#8 Dauntless Voyager

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Posted 05 December 2021 - 04:17 PM

OP had a whole thread running about the exact same topic the other day. The general consensus is an 18” mirror is too big for a first project and a tempered glass table top won’t work.

I am aware of that. Certain remarks were insufficient. 



#9 Augustus

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Posted 05 December 2021 - 04:20 PM

I am aware of that. Certain remarks were insufficient.

What exactly is insufficient? A lot of really knowledgeable folks explained what goes into mirror and scope making and what you’re getting into and Mike just provided a source for blanks that is probably the cheapest you’re going to find.

If making 18” mirrors more easily and cheaply than the current norms and with alternative materials was possible it would be commonplace. The unfortunate reality is that making big mirrors takes years of developing skills and the raw glass itself has to meet certain standards of quality to make a usable mirror and therefore is expensive. Trust me, I’m not happy about that either but that’s just reality.

Edited by Augustus, 05 December 2021 - 04:20 PM.


#10 Stephen Kennedy

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Posted 06 December 2021 - 04:24 AM

I would start with an 8" mirror and if that goes well, then try to make a 12" mirror.  If the 12" is a success, then you can start thinking about the 18" mirror.  one thing you have to consider is that an 18" blank is going to be so heavy that you will probably need some time of machine to rotate it while grinding and polishing. 



#11 Tom Stock

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Posted 06 December 2021 - 10:45 AM

"How to make a telescope" by Jean Texereau

 

My 16" mirror weighs 35 pounds.  Start with a 6" or 8" pyrex blank if you want to learn to grind a mirror.  


Edited by Tom Stock, 06 December 2021 - 10:45 AM.

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#12 MeridianStarGazer

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Posted 08 December 2021 - 05:38 PM

I do have the equipment. However, my question more less is whether or not tempered glass can be grinded. I would figure that tempered glass would not be suitable. For that reason, I trying to find where I can get annealed glass, perhaps from furniture, which I can use.

Tempered glass would explode in your face if you try to grind it. And are you thinking of 1/2" thick? 18" should be 1" thick or thicker. And lots of hogging.

What price range are you looking at?



...


And starting with a small mirror will build your skill faster. Also, my wrists can't lift an 18" mirror. I could not ship you one if I had one.

Edited by MeridianStarGazer, 08 December 2021 - 05:41 PM.

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#13 MeridianStarGazer

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Posted 08 December 2021 - 05:44 PM

I am aware of that. Certain remarks were insufficient.



What extra details do you need?

You might spend $$$ on grit hogging it correctly. If it is your first time, you won't grind optimally and will grind through $$$$$ of grit.

You also need to learn what good contact sounds like, before you are dealing with a big, heavy blank.
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