Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Going back to fine grinding

  • Please log in to reply
5 replies to this topic

#1 david darksky

david darksky

    Explorer 1

  • -----
  • topic starter
  • Posts: 79
  • Joined: 25 Mar 2009

Posted 14 December 2019 - 06:28 AM

I have been polishing a 20 inch borosilicate mirror on a fixed post machine using a 15 inch lap. I have been trying to get rid of a wide turned down edge using spin polishing. After a lot of hours of polishing the TDE is now only right at the edge of the mirror (I tested using a Ronchi screen.) But it shows no sign of going away after more hours of polishing.

 

I have now picked up some small shallow scratches, somehow small splinters of wood from the fixed posts seem to have got on to the mirror. I have to use some force to push in the bolts that hold the cross post that holds the spindle on the machine, this must have caused some splinters to fall on to the mirror. i will remove a bit of wood so this does not happen again.

 

I am planning to go back to fine grinding using 1000 grade grit, which I think is equivalent to 5 to 7 micron. Unfortunately I used the tool to make the pitch lap so will need to make a new one. I use dental plaster and make a penny tool.

 

My question is, using pennies (actually British 2 pence pieces) on a new tool with this size grit will I have problems getting the pennies to conform to the mirror? I pour the dental plaster on to the mirror using a dam and with aluminium foil underneath and then glue the pennies to the tool.

 

This time I will test with a Ronchi screen as soon as the mirror is polished out again, to make sure I don't have a turned down edge before I start figuring, which I didn't do last time.

 

David


  • Stargazer likes this

#2 dave brock

dave brock

    Surveyor 1

  • -----
  • Posts: 1,924
  • Joined: 06 Jun 2008
  • Loc: Hamilton, New Zealand

Posted 14 December 2019 - 01:39 PM

Why not take the pitch off and re-use the existing tool?
You may want to remake the pitch lap anyway if it's contaminated.

Edited by dave brock, 14 December 2019 - 01:44 PM.

  • sopticals likes this

#3 Stargazer

Stargazer

    Vostok 1

  • *****
  • Posts: 155
  • Joined: 30 Jun 2006
  • Loc: NewBrunswick, Canada

Posted 14 December 2019 - 03:57 PM

Hi David,

 

 Are they scratches or sleeks? If sleeks they will polish out, as for the t.d.e. have you moved your quill out

a tad (very little) to see if that helps and are you sure it is t.d.e and not diffraction?

I would try a foucault test  on the edge and see what it shows to be sure.

 

As per the pence tool, if those are scratches you might want to go back further then the 1000 grit

to grind them out, it would be quicker to go back to 25M  and grind out the scratch and go back

up through the grits then to grind at 5 M with a new tool, would take some time to set a new tool

if even possible with 5 m.

 As per the edge,it does take a lot of work to fix the edge because you have to remove so much more

of the glass to get it back to where it should be.

 

I must say, I am by far no expert on this but I am working a 20" as well and have had some of the same

issues over time as well.

Hope it helps.



#4 ccaissie

ccaissie

    Surveyor 1

  • *****
  • Posts: 1,870
  • Joined: 13 Sep 2010
  • Loc: Whitefield, Maine

Posted 14 December 2019 - 09:50 PM

So, aside, just because it's a dream of mine..where is the best deal on a 20" x 2" Pyrex disk these days?



#5 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 23,244
  • Joined: 22 Feb 2010

Posted 14 December 2019 - 11:45 PM

go back to 25um and your back to 20hours polishing out, an inch an hour, and you can still have tde again

 

 

fix the edge and see how the scratches are polishing out.


  • Mike Spooner likes this

#6 david darksky

david darksky

    Explorer 1

  • -----
  • topic starter
  • Posts: 79
  • Joined: 25 Mar 2009

Posted 15 December 2019 - 03:46 PM

They are scratches. I will do some more spin polishing and see if there is any improvement.


  • Stargazer likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics