The 8.75" f/6.7 Mirror Grind

[Chris]

The next step was to bed in the tile tool using 80 grit, using mainly short 1-1.5" strokes center over center with the mirror on top.

 After 60-80 minutes work and a good dozen wets, I think I'm there. I marked the mirror with a pen cross, and added an extra marker pen cross for some reason, the marker ground/dissolved away quickly so not that good for testing mirror and tool contact, but the Bic pen stayed, I could see it through the back of the mirror, and this ground away completely by the last few wets. I could also see uniform bubbles across the tiles, again through the back of the mirror. I'm already finding it a nicer experience grinding with the mirror on top.

I'll measure my sagiti in a bit and see where I am with that, but for now I've just cleaned everything down and scrubbed the tile tool with a brush a few times to remove any grit. I may drip candle wax on the tile tool to seal any remaining grains that I can't see if moving to a finer grade grit. I will also replace the jig with a new 'better' one, and use a new sponge, clean clothes etc. 

[Astrobits]

Your edge bevel is difficult to see but looks a bit small to me. I would increase it before any more grinding.

Nigel

[Chris]

25 minutes ago, Astrobits said:

Your edge bevel is difficult to see but looks a bit small to me. I would increase it before any more grinding.

Nigel

Hi Nigel, I do a bit of bevel maintenance before each session to try and keep on top of it  I guess is was roughly 2mm before I bedded in the tile tool, but a bit shy of that now, although it doesn't seem to show at all on the pics unlike the bevel on the back of the mirror so maybe it's smaller than I thought? I'll bring it back upto a good 2mm before the next stage, thanks  

From what I've just written and your experience, would you agree that the tile tool looks to be bedded in, Nigel, or is difficult to tell? My gut feeling is to move to the next grade of grit at this stage. 

Plus, I'm not fully clear when I need to think about testing? should I test the sphere other than the pencil cross test?

[Astrobits]

1st check the curve.

If it is not as you want it, bevel and continue with 80 grit to your required curve ( allowing for the fact that the next grit will deepen the curve slightly).

If yes, bevel and progress to the next grit.

You can't do any more testing than checking the ROC during the grinding process.

Contact with the tiles looks good. Fortunately, tile tools don't get stuck the way solid tools can. When sealing with candle wax you will need a hot air gun as dropping hot wax onto the cold tool will create some grooves around the tiles that can hold grit. Blowing hot air warms up the tiles and the wax then forms a good upward curving meniscus that will not hold any grit.

Nigel

[Chris]

Cheers Nigel, I'll check the depth shortly   

And! put a hot air gun on my shopping list!

[Chris]

Just checked my sagita depth, and an old 2mm thick Spanish coin that I found just about slides between the ruler and the mirrors center, so I think I'm at 2mm.   

Just to triple check my sagita depth calculations so I don't end up with a Dob as tall as a house: Sagita (S)=r^2/2R where little 'r' is the radius of the mirror, and big 'R' is the radius of curvature, which itself is twice the focal length

I want a focal length of roughly 1500mm f/6.7 ish, so 'R' is twice the focal length thus 3000, and 2R = 6000. 'r' is just the radius of the mirror squared, so 111x111= 12,321

Thus.... 12,321/6000=2.05

Looking at the above I'm pretty much at depth so will now go to a finer grade grit. I have a small amount of 100 grit, a bit more of the 180 grit, then a big jump to a load of 400 fine grit.

Does this plan sound reasonable? Have a clean up of the 80 grit, use the remaining 100 grit for a few wets, then have a big clean down and change the jig, seal the tool with wax etc before moving to the 180 grit, and order a bit more 180 grit from the link below if it looks like I'm not removing the 100 grit pits by the end of what I have.  

http://www.ebay.co.uk/itm/SILICON-CARBIDE-180-GRIT-POWDER-CERIUM-OXIDE-LAPIDARY-STONE-TUMBLING-POLISHING-/261522359576?hash=item3ce3f28518:m:mHSw-yRxvQz73gYnICDoeWw

My thinking is that the 80 and 100 grits are so close in size that I probably shouldn't go too mad cleaning up between them, but it will take me a tiny step closer to the 180 grit so maybe worth bothering with?

 

[Astrobits]

Plan looks O. K. to me.

Nigel

[Chris]

This months update:

• Cleaned off the 80 grit as much as possible.
• Used the remaining 100 grit for a few wets.
• Cleaned off the 100 grit as much as possible.
• Sealed the tile tool with candle wax to trap any stray grits.
• Replaced things like the sponge, and built a new improved jig for holding the tile tool.
• Chamfer/edge maintenance with grind stone (see microscope pic at 20x) 
• Performed an hours grinding with the new 180 grit.
• Used low power microscope to examine the mirror surface for pits.
• Pits still present so ordered another 100g of 180 grit.
• Once arrived - another session of grinding with 180 grit.
• Re-examined the mirror surface for pits - looks better but still not sure it's good enough, so might buy some more 180 grit. I'll examine the mirror again this time with an upturned 25mm Plossl, but my gut feeling is to stick with the 180 for one more session (see pics of mirror surface through microscope @ 20x).

 

[Astrobits]

What I do is to find the largest pit that I can and mark the edge of the mirror radially from the centre through that pit. I then measure and remember the distance in from the edge. After one wet re-check for that pit, ignoring the rest of the mirror. If it has gone then that grit is finished. If not, carry on.

Nigel

[Chris]

Thanks Nigel, that sounds logical, I'll use your method once I've received more 180 grit 

Next stop will be 400 grit! I think I favour larger steps between grit sizes, with more grinding on each, my thinking being I'm less likely to pick up scratches with fewer grit sizes?

Already, It's surprisingly satisfying feeling the ground surface get smoother and smoother 

 

[niallk]

Fascinating thread! Great work - will be following with interest 

[mapstar]

Its looking really nice Chris well done up to now.

Effectively you are now just repairing the surface whilst maintaining the curve.  

One of the most satisfying things I noticed as I went through the different grades was when you looked across the face of the mirror the reflective surface could be seen. As the grit size decreased the angle you could see the reflection from increased.

Keep up the good work fella 

[Chris]

Thanks for the encouragement Damian and Niall, I'm looking forward to seeing reflections of any kind, that will be great to see. I need to read a bit more of your grind and build threads, Damian, now they're encouraging!  

[Alan White]

Still watching this with admiration Chris,
you have far more patience than myself.

[Chris]

1 hour ago, Alan White said:

Still watching this with admiration Chris,
you have far more patience than myself.

Thanks Alan, I'm just taking my time and enjoying the process  

[Chris]

These 80/100 grit pits are certainly persistent! I've just completed roughly another 2 hours of grinding, burning through two more 100g packets of 180 grit. The pits are less numerous now at least so it's going in the right direction, but again I'll need to order some more 180 grit.

I'm not certain I'll manage to grind every single pit out, but I'll give it a go!  

[faulksy]

nice one chris

[Chris]

Hmm? plan of action needed, I've just looked on Fleabay and can only see 1 x 100g bag of 180 grit left! I can see quite a bit of 220 grit available though. I'm pondering grabbing the last bag of 180 grit, plus buying say 4x100g bags of 220 grit to try and get through these pits, even if it's at a slightly slower pace. 

I'm starting to think slotting a 220 grit between the 180 and 400 grits might be a good idea anyway! 

[Chris]

18 minutes ago, faulksy said:

nice one chris

Cheers Mike  

[Astrobits]

Yes, pits can seem to be very persistent, but they do, eventually, disappear.

I like to think of the grinding action in this way:

As a large particle of grit rolls over the glass it strikes the glass with one of it's points something like hitting a big lump of concrete with a pickaxe. This causes a crack down into the glass. At a later time another particle creates a crack that intercepts the first crack and a lump of glass is freed and a pit is formed. This second crack does not necessarily meet the first crack at the bottom, the first crack going deeper.  At the end of using a coarser grit the surface is covered with pits and there are sub-surface cracks left over. On starting a finer grit these create finer/smaller cracks, some of which meet the existing cracks creating a somewhat larger pit at the location of the deeper cracks. Again, the smaller cracks do not meet the remnant cracks at the bottom so the pits tend to be repeated at a particular location until they do, at last remove the traces of the coarser cracks.  This action is repeated with each grade of grit.

Have fun

 

Nigel

[Chris]

Thanks Nigel, well I've ordered the last bag of 180 grit, and I'll order several bags of 220 when I get paid tomorrow. 

Yes, all part of the fun, and it keeps me out of trouble  

[Peter Drew]

After a while it dawns on you how relatively cheap commercial mirrors are. Nothing like the interest and satisfaction of making your own though. 

[Chris]

2 hours ago, Peter Drew said:

After a while it dawns on you how relatively cheap commercial mirrors are. Nothing like the interest and satisfaction of making your own though. 

Indeed on both counts, Peter  Materials aside, imagine working out your hourly rate spread over the time it takes to grind, polish and figure a mirror! Eeek! 

I might work that out for a laugh....or cry  No seriously though, the attraction of making my own optics massively out weighs the cost, and I'm still enjoying learning and doing all this......it might be a while before I get to your Littrow blanks though, Peter, but I've starting reading this at least:

https://www.firstlightoptics.com/latest-books-software/making-a-refractor-telescope-how-to-design-grind-polish-test-correct-and-mount-a-doublet-lens-book.html

 

[LightBucket]

19 hours ago, Lockie said:

Indeed on both counts, Peter  Materials aside, imagine working out your hourly rate spread over the time it takes to grind, polish and figure a mirror! Eeek! 

I might work that out for a laugh....or cry  No seriously though, the attraction of making my own optics massively out weighs the cost, and I'm still enjoying learning and doing all this......it might be a while before I get to your Littrow blanks though, Peter, but I've starting reading this at least:

https://www.firstlightoptics.com/latest-books-software/making-a-refractor-telescope-how-to-design-grind-polish-test-correct-and-mount-a-doublet-lens-book.html

 

Is this of any intereste for you project....as I see you need some more abrasives....

http://www.astrobuysell.com/uk/propview.php?view=127575

[Chris]

1 minute ago, LightBucket said:

Is this of any intereste for you project....as I see you need some more abrasives....

http://www.astrobuysell.com/uk/propview.php?view=127575

Thanks for the thought, it's mainly just 180 grit I could do with a bit more of so I can see off these 80/100 grit pits. I wouldn't mind some of their 220 grit but it's only 1.99 a 100g bag on ebay so probably works out at a similar price to the ad. Luckily I've been well looked after with grtis, polish, and pitch, with just a few little gaps to fill 

Cheers though, those blanks look well priced if anyone wants to do a 10 or 12"?