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A little tip on smaller mirrors...

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#1 ed_turco

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Posted 02 December 2012 - 09:33 AM

I've never said much about how I grind my diddly 4-8" mirrors, though I did my 12 this way. I adapted a large drill press so its table became a rotating spindle. Other ways would work too. I ran it about 30rpm and no accidents ever happened.

Here is the shocking news. On my 12" using 60 carbo, I roughed our my f/6 in 3 hours and put a 1/4" bevel on it in 45 minutes. The blank was cheap but had some issues on the sides.

Here is the true shocker, and sit down for this, each finer grade required on Pyrex took only 20 minutes. And the biggie is that I could skip grades. So the order on the 12" was #60, 80, 220, 500 carbo and 12 micron AlOx and 3 micron. Hadda be real careful with that tiny stuff, but no scratches. I always ran the 3 micron stuff for 40 minutes; is is obviously 4 times smaller than the 12 micron.

If you are nervous, you could use the 5 micron. For smaller mirrors, starting with #80 is fine.

I never ever try to polish on the machine. It is not safe and I like the sweet pull of the pitch when things are going right. I even get the slight aroma from the pitch too.

That's the story, folks. Please give it your consideration; I've definitely made over a hundred or more mirrors.

Ed Turco

#2 Pinbout

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Posted 02 December 2012 - 10:58 AM

was your tool ~75% dia of the mirror?

how did you get your drill to 30rpm, my long gone doall mill only went down to 50? :question:

#3 ed_turco

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Posted 02 December 2012 - 02:10 PM

I only use full sized tools. I changed the thingy inside to get those rpms down to 30.

Ed

#4 kfrederick

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Posted 02 December 2012 - 02:11 PM

Cool thanks ED. Keep the tips coming. :jump: Here is a tip channel your lap even numbers one way odd the other helps with zones .

#5 glennnnnnn

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Posted 03 December 2012 - 02:27 AM

Harbor Freight has a 15amp dial variable-speed control for DC motors like drill presses and routers that you just plug in to the power and plug your device into it.
If you also changed the thingy inside you could have good torque and exactly the right rpm.
Its called a Router Speed Control and costs $20.
-Glenn

#6 ed_turco

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Posted 03 December 2012 - 06:36 PM

Isn't anyone else listening? Sheesh!

#7 Ed D

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Posted 03 December 2012 - 06:40 PM

Ed, I'm listening and paying attention. I'm just very quiet most of the time, and work and family are keeping me busy. Thank you for sharing your knowledge with us.

Ed D

#8 kfrederick

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Posted 03 December 2012 - 07:38 PM

Ed a 1/4 inch bevel on your 12 inch is not that big 1/4 on each side you have a 11.5

#9 Gordon Rayner

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Posted 04 December 2012 - 12:20 AM

Re: Speed reduction

A "big" drill press does not have a DC motor, almost invariably. If it does, then $$$$ for a speed controller capable of handling a 1 or 2 hp. DC motor continuously, with good torque and not overheating or dumping lots of power into a resistor. Those little speed controllers from Harbor Freight, for smaller hand held DC drills and routers, are not suitable for the AC sychronous motors found on bench and column drill presses. Am I overlooking something?

Perhaps I have not kept up with tech changes, but speed control for AC motors implied $$$ not long ago. If that has changed, for a 1 hp or even a 1/2 hp, please let me know.

I have a 7 inch ( column edge to spindle axis) South Bend drill press, probably no younger than the 1970's. Well and solidly built, but the design had no quill lock. I made a lock which helps over a certain range.

The main problem is inability to reach low RPM. SB made an accessory which had a third cone of pulley wheels, and a second drive belt, but I do not have one of those . Countersinks and hole cutters run too fast and squack and chatter. Power tapping is out of the question, also for the reason that I have no motor reversing control.

Replacing the largest pulley wheel, and a belt change, would help, but it would require removal of the safety guard. Changing the AC motor from 1750(?) RPM to the 1170(?)RPM versions would help also ( $100 or more, no?)

How could 30 rpm be obtained with "changing the thingy inside"? Multiple intermediate pulleys and drive belts would not fit "inside" (??)

There is an old, 1930's(?) Sears/Atlas, slightly smaller than my south Bend, for sale at an estate sale nearby. But the drive setup looks to be the same as my South Bend , without any means for low speed metalworking .

#10 ed_turco

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Posted 04 December 2012 - 09:30 AM

It was one of the two 12.5" pyrex blanks I picked up for $25 each because there were chips down the sides of each. I've never seen anything quite like it.

In any case, with a 1/4" bevel, the clear aperture was 12".

These blanks played a great part in my article about my $300 telescope in S&T. Funny thing is that I made another f/6 with the tool and sold that mirror for $300, so my telescope was acually free ! LOL

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#11 glennnnnnn

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Posted 04 December 2012 - 09:58 AM

I found an old Craftsman 8" drill press and the speed control (with an SCR/TRIAC) works well on it, but for a shop-sized tool with AC motor you need something else. There's bound to be a solution if you look, although having a slower-speed motor would be a good start.

#12 Mark Harry

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Posted 04 December 2012 - 01:46 PM

"Free" scope, and even color-coordinated!
Mark

#13 ed_turco

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Posted 05 December 2012 - 09:27 AM

I was so much younger then ...

Ed

#14 droid

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Posted 05 December 2012 - 05:49 PM

I still have the sky and telescope article on the 12 inch scope, folded open to that page.When I read it I was amazed ,someone could make such a nice scope out of so little.
I unfortunately dont have the right stuff to build telescopes, I put up shelves perfectly level, and stuff rolls off one end, lol.

#15 ed_turco

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Posted 06 December 2012 - 06:43 PM

Well, using my machine, I can crank out a 6" complete, including figuring in a 24 hour period, allowing for a meal or two and an 8 hour sleep break.

#16 Dick Parker

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Posted 06 December 2012 - 07:02 PM

Ed -

I have seen a couple references to your article in Sky & T, but don't see any specifics. What month and year so i can look it up.

Thanks
Dick Parker

#17 ccaissie

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Posted 06 December 2012 - 11:28 PM

So those would be "fast" mirrors? :roflmao:

#18 droid

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Posted 07 December 2012 - 12:49 PM

April 1998 for the article on the 12 inch

#19 ed_turco

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Posted 07 December 2012 - 06:57 PM

Sorry, but I only do f/6 and above. I"ve done a few f/4's to 1/10 wave and that was that was that -- lesson learned!

#20 ed_turco

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Posted 07 December 2012 - 06:59 PM

Dick,


My articles are hidden in my signature...in the bio section. :)

#21 kfrederick

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Posted 07 December 2012 - 07:02 PM

Ed what was it about making F4 you did not like?






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