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Coring elliptical secondaries from military glass

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#1 ccaissie

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Posted 14 January 2020 - 09:54 PM

I have a bunch of military mirror assemblies...two per unit at 45 degrees which are mounted by through-fasteners to a metal base.  These are second surface mirrors, the front surface is flat to 4-5 fringes across the 4.25" square face. The 2nd surface looks silvered and is painted over.  I'm strippng the backing off one to assess for strain and flatness.

 

At 4.25" M.A. it looks like I could core out 3" m.a.secondaries.  The thickness is .68" so it's about 1:6 ratio.  I don't know the glass type but it is water clear and I expect it is a good crown glass. 

 

I've cored mirrors normal to the surface, but not at 45 degrees. I see sketches of setups with the glass invested in plaster...cover glass balsamed to protect the face, cored at 45 degrees with a rotating tube and grit.

 

Now that diamond coring bits are common, has anyone used them for this sort of project?  I have 10 individual mirror pieces.

 

The plan is to cut them out @45, block them on a silica runner, fine grind and figure them as a flat. Angle is not optically critical. Would be nice to make good secondaries.

 

What comes up typically when blocking pieces like this? I have Horne, but find no discussion of that detail.  I have mounting wax, blocking cement..

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#2 Gordon Waite

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Posted 14 January 2020 - 10:27 PM

Most diamond hole saws have a cutting depth of about an inch or a little less.  To cut your glass at a 45-degree angle, you would need a diamond tool with a 3" inside diameter, and a cutting depth of well over 3".  Watch out for that inside diameter, as most diamond hole saws are sold based on the outside diameter, which defines the size of the hole they will cut, not the size of the core that is left over.  If you find a series of diamond hole saws with that kind of cutting depth, please let us know!


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#3 TOMDEY

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Posted 14 January 2020 - 10:31 PM

So you are just using it for the glass? Why not just polish the fronts only to better than the current 4-5 fringes and ignore the backs entirely? A good orbital pitch polisher would flatten all of those to tiny fractional wave in just a few hours. I know, you don't have access to that machine... but these don't really need to revert to grinding.    Tom



#4 Mike Spooner

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Posted 14 January 2020 - 10:52 PM

Most diamond hole saws have a cutting depth of about an inch or a little less.  To cut your glass at a 45-degree angle, you would need a diamond tool with a 3" inside diameter, and a cutting depth of well over 3".  Watch out for that inside diameter, as most diamond hole saws are sold based on the outside diameter, which defines the size of the hole they will cut, not the size of the core that is left over.  If you find a series of diamond hole saws with that kind of cutting depth, please let us know!

 

https://www.ediamond...yABEgKnFPD_BwE 

 

There are long core drill bits for concrete (core samples, etc.). They should be okay for glass - not sure how smooth the cut would be.

 

Mike Spooner


Edited by Mike Spooner, 14 January 2020 - 10:54 PM.

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#5 Gordon Waite

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Posted 15 January 2020 - 12:19 AM

https://www.ediamond...yABEgKnFPD_BwE 

 

There are long core drill bits for concrete (core samples, etc.). They should be okay for glass - not sure how smooth the cut would be.

 

Mike Spooner

The hand-held core drill for a bit like that is going to cost you between $250 and $400.  Hand-holding the drill with a bit with walls that thick and long will be "interesting" to say the least!



#6 Mike Spooner

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Posted 15 January 2020 - 12:39 AM

The hand-held core drill for a bit like that is going to cost you between $250 and $400.  Hand-holding the drill with a bit with walls that thick and long will be "interesting" to say the least!

Might be better to clamp the bit and rotate the part with a turntable - not user friendly for small diagonals I agree. They are deeper than big box off the shelf stuff though!



#7 Jon Isaacs

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Posted 15 January 2020 - 01:08 AM

Might be better to clamp the bit and rotate the part with a turntable - not user friendly for small diagonals I agree. They are deeper than big box off the shelf stuff though!

A mill works..

 

Some years ago, I was testing concrete samples in dynamic tension.  The samples were 3.00" in diameter,  6.00" long with flat and parallel to within 0.0005". The samples were cored from a 6 inch diameter cast blank and then ground to diameter and length.. the was also a sub diameter gauge section.

 

Not a job for just any machinist.

 

Jon


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#8 jtsenghas

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Posted 15 January 2020 - 06:48 AM

For this job it may be easier in the end to simply rough cut the ellipses oversized with many straight cuts on a wet saw,  and then to fixture them at 45 degrees for grinding cylindrically.


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#9 JohnH

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Posted 15 January 2020 - 09:30 AM

Most of the ATM books show you how to make a coring bit. Using a length of pipe that has the right diameter you just have to make sure it's long enough to reach down the sides to cut a 45° slice out of a flat mirror it has flutes running up the side of it where you can add the grid turn away you go. One guy I know who did this made himself 7 in 1 and what he did was he embedded the whole thing in a plaster mold and once free of the molding the blank have been cut into an elliptical diagonal almost chip free

#10 dan_h

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Posted 15 January 2020 - 09:57 AM

https://www.ediamond...yABEgKnFPD_BwE 

 

There are long core drill bits for concrete (core samples, etc.). They should be okay for glass - not sure how smooth the cut would be.

 

Mike Spooner

I used something very similar to these drills in a previous life. We were drilling through concrete floors to install heavy equipment anchors.  A 3" core was remarkably smooth when lifted.   IIRC, it took about 20-30 minutes on a portable drill press to cut through a 6" floor.  Water fed with a garden hose    We managed to rent a drill press in just about every city we went too. 

 

Those drills linked to are about 20% of the cost we paid back in the 80's. 

 

dan


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#11 Mike Spooner

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Posted 15 January 2020 - 10:27 AM

I used something very similar to these drills in a previous life. We were drilling through concrete floors to install heavy equipment anchors.  A 3" core was remarkably smooth when lifted.   IIRC, it took about 20-30 minutes on a portable drill press to cut through a 6" floor.  Water fed with a garden hose    We managed to rent a drill press in just about every city we went too. 

 

Those drills linked to are about 20% of the cost we paid back in the 80's. 

 

dan

Hi Dan,

I just did a quick search and this company popped up so I copied the link. They looked pretty inexpensive to me also. My days of working at hydro electric dams is over so not up on current suppliers, etc.

 

Mike



#12 Lola Bruce

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Posted 15 January 2020 - 01:51 PM

You can rent the drill and motor. make all your set ups ahead of time as they rent by the hour and diamond used up. The interiors on the ones I have used are straight walled so no problem.

 

Bruce



#13 ccaissie

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Posted 15 January 2020 - 08:39 PM

I cut a 10" deep hole recently in my basement cellar wall, so know that diamond core drills are available.   I have a Bridgeport than can be set up to make the deep core and can hold it steady...water line to be designed.

 

My idea of making the elliptical pieces, cleaning, bevelling and blocking them and grinding them is because they are not likely the same thickness, and the issue of making them flat out to the edge would only come (in my hands at least) from ganging them up and working them as a 10" disk.  Maybe put in a few "wasters".   I've flattened 4" m.a secondaries, but the edge always takes a little hit.

 

When making some 8" flats, I used a precision straight edge, and with a strong light could detect a couple of waves of power before polishing.   

 

But we concur...Spooner's link to a 14" long core drill with a 3+" i.d. would work. Need to verify i.d.  Mounted on the Bridgeport would make it possible  I might be able to sandwich 5 pieces at a time in a fabricated metal box in plaster.

 

I think a true 1/10 wave secondary is a good thing to have....



#14 ed_turco

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Posted 16 January 2020 - 11:17 AM

Are those glass items still available?  I think your idea has some merit.



#15 ccaissie

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Posted 16 January 2020 - 05:11 PM

If I core out 10 secondaries, I might part with some.  But really why would anyone (other than hobbyist..me) want to put energy into making plain glass secondaries... I think United Lens will make up Borofloat secondary blanks of any size.



#16 Benach

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Posted 16 January 2020 - 05:37 PM

ccaissie: I made my own secondary mirror because of some ridiculous reasons that are irrelevant over here. I might be interested in a mirror without coring. If you have one available, please let me know.



#17 jtsenghas

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Posted 16 January 2020 - 06:21 PM

Actually,  if one sets up to core large secondary mirrors,  there is one distinct advantage.   One can change the angle by up to 15 degrees to make a customized low rider secondary that produces a round secondary obstruction and doesn't expose incoming light to edges (except for slight off axis light).


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