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6" F3 Mirror and Scope Build

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#26 m. allan noah

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Posted 16 December 2019 - 06:59 AM

While I'm a fan of cordless drills, they are just not intended for this kind of duty cycle. You would be better off with something more substantial, especially when you add the weight of the mirror, friction from the grinding, etc. There is a good chance your current drill won't be able to turn it. Also, that long threaded rod will act as a torsion spring, you should shorten it if possible.


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#27 Augustus

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Posted 16 December 2019 - 08:40 AM

While I'm a fan of cordless drills, they are just not intended for this kind of duty cycle. You would be better off with something more substantial, especially when you add the weight of the mirror, friction from the grinding, etc. There is a good chance your current drill won't be able to turn it. Also, that long threaded rod will act as a torsion spring, you should shorten it if possible.


Logan did it just fine with a weaker drill

#28 perfessor

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Posted 16 December 2019 - 11:57 AM

Logan did it just fine with a weaker drill

Maybe Logan got lucky.

 

Sometimes "Let's see how flimsy I can get away with" is a workable strategy.

 

Sometimes "Let's build it right the first time, so I don't have to build it over" is a better one.

 

Cheers,


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#29 Augustus

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Posted 16 December 2019 - 12:02 PM

Maybe Logan got lucky.

 

Sometimes "Let's see how flimsy I can get away with" is a workable strategy.

 

Sometimes "Let's build it right the first time, so I don't have to build it over" is a better one.

 

Cheers,

We do not all have the budgets to achieve the latter all the time.



#30 Aaron_tragle

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Posted 16 December 2019 - 02:44 PM

I can confirm that the drill is working, I've already ground out 3/1000ths of an inch.


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#31 DAVIDG

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Posted 16 December 2019 - 03:14 PM

  If the goal is to grind, polish and figure a 6" f/3 the standard manual techniques will work just fine. As research engineer and chemist  and someone who has taught mirror making for  35+ years now I have seen the goal get off track many times and the original project slowed down to  a crawl as another project blossoms from the first.. We seem to have that happening here with the construction of a grinding machining and all the details associated with it's construction. So you have to look at the  time required to build and perfect a grinding machine vs grinding this mirror by hand  using decade old  proven out techniques.  Also is a grinding machine going to result in better mirror ? I know for a fact that hogging  out an F/3 curve on a  6" blank will take  about 4 hours by hand.   From there it would take about another 2 to 3 hour to progress through the finer grits and be ready to polish.  So in  my opinion in less time then it would take to build a grinding machine I could be into polishing this mirror. How much  time will a grinding machine save if your still using standard glass or tile tool and grit ?  I believe you'll find not much. The time saving come when you use diamond bits and a Straumbuagh type machine or build a Sin table and use diamond coring bit on a drill press.  With those technique one can cut a F/3 curve in under 10 minutes. 

  In my opinion one would be better off building and learning to use the different test equipment that are  needed. There is an  old saying in mirror making that once you are  done polishing and start figuring "your 90% done and have 90% more to go " Figuring an F/3 parabola,  that is more like you have 200% more to go.  

 

                   Happy Holidays !

                     - Dave 


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#32 MGAR

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Posted 16 December 2019 - 05:32 PM

  " Figuring an F/3 parabola,  that is more like you have 200% more to go.  

bow.gif 

 

Well said Dave, figuring parabola took 50+ iterations on my 6" F4 and still wasn't happy. A few nasty words out loud and hung up the log book, star tested and sent it off for coating. lol.gif 

 

Good luck Aaron!

 

Gary



#33 Augustus

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Posted 16 December 2019 - 05:49 PM

Gary, that seems like an inane amount of time figuring - no offense, but have you considered you were being too aggressive, or perhaps the opposite? I spent maybe 25 hours from achieving a polish to a parabola with my 6" f/4.3 and most of that time was knocking down TDE, not parabolizing. Parabolizing for me was with super diluted cerox and I went really slow, literally counting the number of strokes as I went along. And I still ended up 1/6-1/8 of a wave overcorrected. If you are constantly bouncing back and forth between overcorrected and undercorrected, the problem is likely that you're either not testing often enough or being too aggressive.

 

I need to put that mirror back into a scope......

  If the goal is to grind, polish and figure a 6" f/3 the standard manual techniques will work just fine. As research engineer and chemist  and someone who has taught mirror making for  35+ years now I have seen the goal get off track many times and the original project slowed down to  a crawl as another project blossoms from the first.. We seem to have that happening here with the construction of a grinding machining and all the details associated with it's construction. So you have to look at the  time required to build and perfect a grinding machine vs grinding this mirror by hand  using decade old  proven out techniques.  Also is a grinding machine going to result in better mirror ? I know for a fact that hogging  out an F/3 curve on a  6" blank will take  about 4 hours by hand.   From there it would take about another 2 to 3 hour to progress through the finer grits and be ready to polish.  So in  my opinion in less time then it would take to build a grinding machine I could be into polishing this mirror. How much  time will a grinding machine save if your still using standard glass or tile tool and grit ?  I believe you'll find not much. The time saving come when you use diamond bits and a Straumbuagh type machine or build a Sin table and use diamond coring bit on a drill press.  With those technique one can cut a F/3 curve in under 10 minutes. 

  In my opinion one would be better off building and learning to use the different test equipment that are  needed. There is an  old saying in mirror making that once you are  done polishing and start figuring "your 90% done and have 90% more to go " Figuring an F/3 parabola,  that is more like you have 200% more to go.  

 

                   Happy Holidays !

                     - Dave 

4 hours hogging and then 3 for all the other grits? At f/3? I've never heard of someone doing an f/8 that fast. Maybe I'm just missing something but I spent a lot more time than that just going from #120 to 9 micron on my 6" f/4.3 which was already pregenerated. Probably at least 10 hours.

 

At least the machine saves manual labor and allows one to multitask and do something else productive while grinding..... can't exactly do that if you're hogging by hand. I don't know about Aaron but a lot of young folks have rather little free time. If one can do homework while running this thing with the only upkeep needing to be sprinkling grit/water every once in a while that alone justifies the machine's construction.



#34 Aaron_tragle

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Posted 16 December 2019 - 05:51 PM

At least the machine saves manual labor and allows one to multitask and do something else productive while grinding..... can't exactly do that if you're hogging by hand. I don't know about Aaron but a lot of young folks have rather little free time. If one can do homework while running this thing with the only upkeep needing to be sprinkling grit/water every once in a while that alone justifies the machine's construction.


This is one of the reasons for the machine, but also I have never done woodworking before other than pinewood derby cars and I didn’t have the materials and grits to start grinding until 2 days after the machine was finished.
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#35 MGAR

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Posted 16 December 2019 - 06:41 PM

Gary, that seems like an inane amount of time figuring - no offense, but have you considered you were being too aggressive, or perhaps the opposite?

No offense taken ZL, I was too aggressive for my first mirror and like you my mirror ended up over corrected and will probably stay that way. I wanted a comet catcher and the fast F ratio was what I was after. I believe is took me 15 hours to rough out probably another 6 or 7 hours for the other grits.

 

Now I guess the good thing about a machine is if you decide on doing a bigger mirror you just need make a bigger arm.;-)

 

Gary 


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#36 Pinbout

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Posted 16 December 2019 - 06:43 PM

bow.gif

 

Well said Dave, figuring parabola took 50+ iterations on my 6" F4 and still wasn't happy. A few nasty words out loud and hung up the log book, star tested and sent it off for coating. lol.gif

 

Good luck Aaron!

 

Gary

but when they had the Delmarva mirror making classes, it was a lot easier to do with all the help... and easy tests.

 

wish they still had it. frown.gif


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#37 dogbiscuit

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Posted 16 December 2019 - 07:32 PM

DAVIDG gives good advice.

The machine is a diversion from mirror work, and will not be significant help for a 6" mirror.  For a first time mirror maker it is likely to be an impediment.  It would be a mistake to multitask, doing something else while the machine is running.  Bad things could happen... would happen.

f:3 is probably not the best choice to start with. It will make testing more difficult.   Be prepared for things to go wrong.  there is making good test equipment and learning to use it.

 

 

Get with an experienced mirror maker at the RAS.

Most would be mirror makers had gross misconceptions of what it takes to make a good mirror before they completed their first good mirror.  One of those RAS guys would probably be glad to help you.

 

Yes, a  6" f:3 can be rough ground in 4  hours by hand  (starting with 2 flat 6" disks).  Probably not going to happen for a  first time mirror maker. 


Edited by dogbiscuit, 16 December 2019 - 07:43 PM.


#38 DAVIDG

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Posted 16 December 2019 - 08:21 PM

Is y'all's goal to just get him to quit or stop posting?

 No the goal is to provide useful advice and information  from those that have many years of experience. Having made many many optics surfaces and helped many people make optics, what has consisting lead to success as been the test methods used.  

   I bet Gary used the Foucault test to make his mirror and in my experience that is the  reason why it can lead to many hours of frustration.  So my advice is to use proven techniques, multiple test methods to confirm the figure and to place the efforts were it makes the biggest difference. 

 

              - Dave 


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#39 MGAR

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Posted 16 December 2019 - 08:50 PM

   I bet Gary used the Foucault test to make his mirror and in my experience that is the  reason why it can lead to many hours of frustration.  So my advice is to use proven techniques, multiple test methods to confirm the figure and to place the efforts were it makes the biggest difference. 

You nailed it Dave! The zone mask and those pesky zones...

 

Gary


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#40 Aaron_tragle

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Posted 17 December 2019 - 12:57 AM

Alright just finished today's hogging, hogged through 0.21mm today. I didn't expect to chew through grit so fast, I just ordered 2lbs of 80 grit to hopefully finish out the mirror. Burned through almost 1/2lb today bawling.gif



#41 davidc135

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Posted 17 December 2019 - 03:46 AM

The sag is 3mm. Will 2lbs be enough?  David



#42 MGAR

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Posted 17 December 2019 - 04:05 AM

Good progress Aaron!

 

I see you make good use of Home Depot.grin.gif  Here is a pic of star test stand for my 6" F4. Just about everything came from the depot with exception the cookie pan. Worked so well I kept as my mount for the time being. I have a small stool I use with it in the field when I'm scanning the horizon for comets.

 

Gary

 

test_stand.jpg

 


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#43 Aaron_tragle

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Posted 17 December 2019 - 08:46 AM

The sag is 3mm. Will 2lbs be enough?  David

I am not sure honestly, I feel like I am going through grit way faster than I should be. Is using 3/8th of a pound for this little progress normal?



#44 Pinbout

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Posted 17 December 2019 - 09:24 AM

I am not sure honestly, I feel like I am going through grit way faster than I should be. Is using 3/8th of a pound for this little progress normal?

it is what it is...


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#45 Aaron_tragle

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Posted 17 December 2019 - 09:31 AM

it is what it is...

Would it make sense to get some 60 grit for hogging or just continue with 80? I forgot to mention I bought this mirror with a .0044" sagitta pregenerated.


Edited by Aaron_tragle, 17 December 2019 - 09:36 AM.

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#46 Pinbout

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Posted 17 December 2019 - 01:19 PM

sure

 

but understand the sub-surface fractures that can occur with really course grit.

 

you still may have to remove enough glass to get below any sub-surface fractures.

 

but yeah if its going to slow 60's good.


Edited by Pinbout, 17 December 2019 - 01:30 PM.

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#47 Aaron_tragle

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Posted 17 December 2019 - 01:25 PM

sure

 

but understand the sub-fractures that can occur with really course grit.

 

you still may have to remove enough glass to get below any sub-surface fractures.

 

but yeah if its going to slow 60's good.

Yeah with 6 hours of grinding with 5 kilos of weight I got 9/1000th of an inch ground and I still have 1/10th inch to go. Just ordered some 60 grit and 80 grit, I am just going to finish off what 80 I have today and then wait for the 60.



#48 Pinbout

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Posted 17 December 2019 - 02:04 PM

just thought I'd draw it up on autocad to see how much you need to go...

 

ha ha ha

 

you just barely broke the surface.

 

you may want 40...

 

a 6in f3 vs f5 grin.gif

 

f3 vs f5.JPG

 

 

how hard do you want a f3

 

while hogging you'll want to switch to finer grits before you actually hit f3.

 

or you may wind up with a f2.

 

wink.gif


Edited by Pinbout, 17 December 2019 - 02:06 PM.

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#49 Aaron_tragle

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Posted 17 December 2019 - 02:34 PM

just thought I'd draw it up on autocad to see how much you need to go...

 

ha ha ha

 

you just barely broke the surface.

 

you may want 40...

 

a 6in f3 vs f5 grin.gif

 

attachicon.giff3 vs f5.JPG

 

 

how hard do you want a f3

 

while hogging you'll want to switch to finer grits before you actually hit f3.

 

or you may wind up with a f2.

 

wink.gif

Haha I ordered 4lbs of 60, I plan on switching to 80 at around 120 then going finer and finer from there.



#50 Pinbout

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Posted 17 December 2019 - 02:58 PM

 

Haha I ordered 4lbs of 60, I plan on switching to 80 at around 120 then going finer and finer from there.

I'd switch to 80 before you actually hit f3 RoC...


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