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Cutting Cassegrain Hole

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#1 nypsirc

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Posted 19 June 2019 - 01:56 PM

I'm planning work to grind and polish my first cassegrain primary.  I have read in several books that the typical method is to use a hole cutter with water and carbide grit or a diamond hole saw (with water).  I have the ability to do this on my drill press but I was curious if anyone has had satisfactory results using a water jet. 

 

The water jet typically has a draft angle associated with it so I am skeptical of it's use for a cassegrain primary. Any input would be appreciated!


Edited by nypsirc, 19 June 2019 - 01:57 PM.


#2 DAVIDG

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Posted 19 June 2019 - 03:34 PM

 Get yourself a diamond hole saw from THK Diamond Tools https://www.ebay.com...orecat=15599558

   I have been using them for a couple of years now. They are inexpensive and work great. It will take about 15 minutes to core through 2" of glass.

 

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#3 Tucker512

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Posted 19 June 2019 - 04:05 PM

I have done a few mirrors using the THK hole saws on a drill press too, and it works great.  Goes pretty quickly.  I used a jig to center the mirror and go halfway from each side to avoid breakout.

 

Scott


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#4 DAVIDG

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Posted 19 June 2019 - 04:13 PM

 There is thread that shows it being done as well,  starts at message  #24.

 

       https://www.cloudyni.../?hl= gregorian

 

                       - Dave 


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#5 steveastrouk

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Posted 19 June 2019 - 04:32 PM

By means of a hijack. I acquired a 12" perforated blank, sort of polished, sort of F5.

 

How do I work it without the perforated plug wedged in ?

 

Steve



#6 dave brock

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Posted 19 June 2019 - 04:38 PM

With full size lap with pitch removed where the hole is and small lap that just fits between hole and edge of mirror.

#7 nypsirc

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Posted 19 June 2019 - 07:50 PM

Thanks everyone!

#8 gregj888

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Posted 19 June 2019 - 11:03 PM

On my 20" I used an Aluminum tube and 120 grit SiC.  I have the mirror on the machine turntable and the tube counter rotating on a gravity fed crossbar.



#9 Peter Nance

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Posted 20 June 2019 - 10:40 AM

By means of a hijack. I acquired a 12" perforated blank, sort of polished, sort of F5.

 

How do I work it without the perforated plug wedged in ?

 

Steve

Hi Steve,

Put the plug back in by slowly heating everything up and use blocking pitch to hold it in place.

This of course is assuming your going to start grinding. You will never get it put back exactly so don't bother if you want to polish.

Easier and you will get better results than trying to make a lap to fit in between the hole and edge.

Tape the front as best you can and pour from the bottom.

And yes I have done this a number of times and it works well.

Have fun,

Pete



#10 MitchAlsup

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Posted 20 June 2019 - 12:13 PM

Texereau states that one should partially trepan the central core from the back and fill with plaster of paris.

The target depth of the core is 1-2 mm less than the depth of the grind.

This enables one to grind, polish and figure as if there were no core.

And when the core is finally cut (from the front) it will disturb the figure the least.



#11 JohnH

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Posted 20 June 2019 - 09:52 PM

With full size lap with pitch removed where the hole is and small lap that just fits between hole and edge of mirror.

You can also plug the hole with a piece of glass cut with the same sized hole saw, then plastering or waxing it into place



#12 dave brock

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Posted 21 June 2019 - 01:13 AM

Only if you're grinding first. Steve has a polished blank.


Edited by dave brock, 21 June 2019 - 01:15 AM.



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