The 8.75" f/6.7 Mirror Grind

[Chris]

I have to say it's a bit like ground hog day trying to remove the pits with 180 grit! Since I last posted I've put in about 6 hours of grinding in over 4 sessions, and burned through a further 0.6kg of 180 grit. I can still see pits! 

I've also managed to make the mistake in assuming that the satiga depth can't possibly be getting much deeper if I haven't even managed to remove shallow pits. Well I measured the sagita today and got a bit of a shock, the target depth was around 2 to 2.1mm and now it's 2.6mm!  This equates to f/5.4 rather than the target of f/6.7! I used mainly short strokes as well?

Well I'm out of 180 grit... again, and I'm not ordering any more. For morale purposes I'm finally moving onto 220 grit  I'll work with the tool on top for a bit to try and bring the sagita back close to 2mm again.

[Peter Drew]

Are you sure they weren't grade 180 pits?  

[Chris]

29 minutes ago, Peter Drew said:

Are you sure they weren't grade 180 pits?  

I'm finding it difficult to ignore the possibility I must admit  They look like the pits pictured on the previous page, and they just won't go away despite painfully grinding down through 0.6mm of glass with 180 grit! I'm pretty sure the pits aren't 0.6mm deep, so, erm? yeah lol 

[Alan White]

Chris,

Great to see back at the grind, was wondering where you were with it.
Hat off to you for sticking with it, I would have had enough by now, I far too impatient.

Have you a plan where the mirror will finally rest once completed and coated?

[Chris]

1 hour ago, Alan White said:

Chris,

Great to see back at the grind, was wondering where you were with it.
Hat off to you for sticking with it, I would have had enough by now, I far too impatient.

Have you a plan where the mirror will finally rest once completed and coated?

Hi Alan, thanks, I've not forgot about it all  Progress has slowed in recent months I must admit. I'm still settling into a new contract at work with more hours per week, and I've been training to run an half marathon for cancer research UK which is now all complete, run, and money raised, so hopefully I'll get more time on my astro projects now    

I've been meaning to update the thread for a while, but didn't really want to do so until I'd made some actual progress lol. I've still not made much progress with only averaging a couple of hours each month, and being stuck at the same stage of course. Even so, I thought it worth mentioning how easy it is to overshoot your sagita depth when focused on grinding out pits!....I'm sure making mistakes is all part of the process 

The plan is to build it into a Dob with zenith at a comfortable standing height hence aiming for f/6.7 @ 1500mm focal length. Having said this, if it ends up an f/4.8 with me veering off course as I've just done, I might end up with something a bit different.

[Alan White]

22 hours ago, Lockie said:

Having said this, if it ends up an f/4.8 with me veering off course as I've just done, I might end up with something a bit different.

Chris, I hope you don't,
but if you grind too far and get a hole in the middle,
you can always turn it into a Mak or paint it White and pretend it's a big Polo 

[Astrobits]

Assuming that the pit you identified, as per my previous reply, disappeared then you can assume that any further pits are from the abrasive you are using and it is therefore pointless to continue with that grade grit. In your current situation I would continue with the 220 grit with the tool on top to lengthen the ROC. Keep the strokes very short ( much less than 1/3D ) or you will end up with a difficult shape to sort out. As before, select the largest pit you can find ( near the centre of the mirror with tool-on-top working ) and when that one is gone check the curve and correct if necessary. Then move on to the next grit.

Note that the quoted grit sizes are average mesh sizes ( usually holes per inch ) that allow the grit through. There is a range of particle sizes within a batch. For example, 180 grit has a maximum grit particle of 114 microns, an average of 76 microns and the finest 25 microns. 220 grit has max 102, average 63 and finest 20 microns. It is those biggest particles that give the bigger pits. Thus the biggest pits from 220 grit will be bigger than the average pits from 180grit. These are figures from NEWPORT GLASS who supply mirror grinding materials. There is no guarantee that grits from other suppliers will conform to these specifications. There is also the possibility that whoever packages the grits from bulk doesn't clean up effectively when going from a coarse grit to a finer grit contaminating the finer grade.

Nigel

[Chris]

On 12/10/2017 at 17:28, Astrobits said:

Assuming that the pit you identified, as per my previous reply, disappeared then you can assume that any further pits are from the abrasive you are using and it is therefore pointless to continue with that grade grit. In your current situation I would continue with the 220 grit with the tool on top to lengthen the ROC. Keep the strokes very short ( much less than 1/3D ) or you will end up with a difficult shape to sort out. As before, select the largest pit you can find ( near the centre of the mirror with tool-on-top working ) and when that one is gone check the curve and correct if necessary. Then move on to the next grit.

Note that the quoted grit sizes are average mesh sizes ( usually holes per inch ) that allow the grit through. There is a range of particle sizes within a batch. For example, 180 grit has a maximum grit particle of 114 microns, an average of 76 microns and the finest 25 microns. 220 grit has max 102, average 63 and finest 20 microns. It is those biggest particles that give the bigger pits. Thus the biggest pits from 220 grit will be bigger than the average pits from 180grit. These are figures from NEWPORT GLASS who supply mirror grinding materials. There is no guarantee that grits from other suppliers will conform to these specifications. There is also the possibility that whoever packages the grits from bulk doesn't clean up effectively when going from a coarse grit to a finer grit contaminating the finer grade.

Nigel

Oops! Sorry Nigel I must have missed your post until I was ready to post again!  

I moved onto the 220 grit and used up the 200g of that I had with an hour or so of grinding, plus I used the tool on top as you say. The only problem is I continued with 1/3D centre through centre, not very short strokes as you suggest. Thankfully, I've just measured the sagita which still looks to be 2.5mm to 2.6mm. Logically, I can't have changed the curve shape a significant amount if the depth has only changed by about 0.05mm, so lucky I ran out of 220 grit hey!  

Regarding the pits, I wrongly assumed that I needed to remove any signs of them. I wasn't taking on board that a particular grade of grit would create it's own pits (this now explains why the pits seemed to move around!).

I've checked the pits after grinding today and although I didn't monitor a particular pit in the centre they are less numerous and shallow compared to the 180 grit. As I'll be building a scope to suit the mirror and not the other way around, it's tempting to go easy on myself and not worry too much about the f/ratio as long as it doesn't get any faster than it's current f/5.4. At my current slow rate of work it would takes months and months to get back to f/6.7 anyway. 

With acceptance of the above, it's tempting to move onto the 400 grit now and see how it affects the pits with the proviso that I can go back to 220 grit if it looks like I'm not making any headway.

What do you think about this approach, Nigel?      

[Chris]

Just a bullet point update, sorry about the slow progress: 

• Very carefully removed any traces of 180 grit including scraping out the wax from the grooves of the tool, thoroughly cleaning, then re-waxing.
• Switching to tool on top to prevent the sagitta getting any deeper I moved onto 220 grit and performed about 15 or so wets.
• Used upturned 25mm Plossl eyepiece to check how the pits were doing? Verdict, they look less numerous and more shallow than the pits from the 180 grit.

Next - need to decide if I move to 400 grit (which I have a ton of), or order more 220 grit?  

[Astrobits]

As you've discovered, each grade of abrasive leaves it's own variety of pits.

The reason for using shorter strokes is to prevent flattening the edge of the mirror at the expense of leaving the centre too deep (with tool-on-top). Once you have generated the curve with the 80 grit then all you are wanting to do is to remove an even layer of glass over the whole surface thus removing the pits. Whatever length of stroke you use the piece on top always gets more work at the centre and the bottom piece, the edge. The longer the stroke the more pronounced this effect, thus keeping the strokes short is beneficial. I would avoid continuous COC and move the tool about 1/2-1 inch left and right of the centre as in a W stroke.

As far as the grits to use now I would say, yes, go 400 and as I said before follow the biggest pit you can find until it has disappeared then that grit is finished.

I am a bit concerned that I can see no cushioning between the mirror and the wood. That can ( very possibly ) lead to astigmatism as the wood will not be supporting the mirror evenly over the whole area. Try to get some carpet between them and rotate everything frequently.

Nigel

[Chris]

2 hours ago, Astrobits said:

The reason for using shorter strokes is to prevent flattening the edge of the mirror at the expense of leaving the centre too deep (with tool-on-top). Once you have generated the curve with the 80 grit then all you are wanting to do is to remove an even layer of glass over the whole surface thus removing the pits. Whatever length of stroke you use the piece on top always gets more work at the centre and the bottom piece, the edge. The longer the stroke the more pronounced this effect, thus keeping the strokes short is beneficial

Thanks Nigel, I'm really pleased I have your guidance, it sounds like I had the wrong idea about how switching to tool on top worked. I understood that what ever was on the bottom got the edge worked more than the centre, but I totally thought by switching to tool on top I would just literally be reversing my mistake of overstepping the sagita depth. i.e I thought the curve would remain correct but would just become more shallow. From what you have said it is a bit more complicated than just that, and I run the danger of creating an uneven curve by keeping the tool on top, even using short strokes is only limiting the effect then.

Should I switch back to MOT and just accept that my mirror will become slightly faster still, but be more assured about my curve, or would you continue TOT bringing my sagita back up a touch, and like you say, just limit the damage to the curve by keeping strokes short?? 

3 hours ago, Astrobits said:

As far as the grits to use now I would say, yes, go 400 and as I said before follow the biggest pit you can find until it has disappeared then that grit is finished.

Thanks, will do, whether MOT or TOT, I will move onto 400 grit and monitor a large pit until it goes (my bad for not monitoring a particular pit with the 220 grit, I found a window of time to get a grind in and totally rushed things making several mistakes it seems  

3 hours ago, Astrobits said:

I am a bit concerned that I can see no cushioning between the mirror and the wood. That can ( very possibly ) lead to astigmatism as the wood will not be supporting the mirror evenly over the whole area. Try to get some carpet between them and rotate everything frequently.

 My bad, again, I remembered about cutting a fresh circle of carpet towards the end of my 220 grit. I'll have that in place for the 400 grit for sure. I have been continuously rotating the tool, mirror, and myself at least   

3 hours ago, Astrobits said:

I would avoid continuous COC and move the tool about 1/2-1 inch left and right of the centre as in a W stroke.

I did wonder when I should start doing the W stroke? Does this just further increase the randomness or does it serve another purpose? I'll do the W stroke with my 400 grit then, just need your feedback on the above question regarding keeping the tool on top or not?

I'm getting sloppy and forgetting things I've read and been told which is leading to mistakes. Hopefully if I can build up a bit more momentum again I can get back on top of things  

 

[Astrobits]

It doesn't really matter, MOT or TOT for smoothness of curve. My preference would be to keep TOT, the longer the ROC the easier it will be to figure.  The top piece gets more action in the centre and the bottom piece the edge. Therefore with MOT the ROC will get shorter while with TOT the ROC will get longer. The longer the stroke the more severe this action will be, probably leading to non-spherical surfaces one way or another.  At 400 grit this action will be small but noticeable. The reason for the small W stroke is to smooth out any tendency to produce zones caused by some regularity in the tile tool ( even if you can't see any obvious regularity ).

Nigel

[Chris]

14 minutes ago, Astrobits said:

It doesn't really matter, MOT or TOT for smoothness of curve. My preference would be to keep TOT, the longer the ROC the easier it will be to figure.  The top piece gets more action in the centre and the bottom piece the edge. Therefore with MOT the ROC will get shorter while with TOT the ROC will get longer. The longer the stroke the more severe this action will be, probably leading to non-spherical surfaces one way or another.  At 400 grit this action will be small but noticeable.

I'll go with that, TOT with short W strokes it is. I've scraped and re-waxed my tool, and cut a fresh circle of carpet for the mirror, so I'm all set.

 

18 minutes ago, Astrobits said:

The reason for the small W stroke is to smooth out any tendency to produce zones caused by some regularity in the tile tool ( even if you can't see any obvious regularity ).

Thanks for that, I did wonder.

Cheers for the help as always  

 

[mapstar]

This has moved on since I last popped by which has been quite some time due to other non astro related projects.

Glad to see you progressing with it Chris and it's nice to be moving on with the 400grit.

Keep on pushing so to say 

[Chris]

1 hour ago, mapstar said:

This has moved on since I last popped by which has been quite some time due to other non astro related projects.

Glad to see you progressing with it Chris and it's nice to be moving on with the 400grit.

Keep on pushing so to say 

Hey Damian, good to hear from you, sounds like you've been busy  Thanks, yeah I'd be a further along in all honesty if I hadn't spent ages trying to grind out pits created by the actual grit a was using to get rid of said pits lol...you live and learn though hey 

[Chris]

I had about 4 hours on this yesterday, half the time used to clean down, scrape and re-wax the tool, then I cleaned everything again to make sure no 220 grit was left hanging about. Also a bit of bevel maintenance so I don't chip the mirror whilst grinding. So, about 2 hours actual grinding with the 400 grit after all said and done.

The 400 grit is much smoother than I expected, you can hardly hear it cutting. I used short W strokes rotating both the mirror and tool. Before I started I eyed up a little bunch of pits which formed a little triangle in the mirrors centre. After about 300g of 400 grit I could no longer see these. I could see the odd little pit here and there still, but think they might be caused mainly by the 400 grit from what I'm starting to understand. The surface in general looks much smoother under the magnified gaze of the upturned 25mm Plossl, so I feel like progress is being made    

I think I might be ready for the next grit size, so not really remembering what I actually have I've took a quick inventory:

- Alu Oxide 400

- ( 9 micron Alox (600)

- 1st smoothing 225

- 2st smoothing 175

- 3rd smoothing 125

- Pitch and rouge.

I'm not sure if I should use the Alu oxide 400 before the 9 micron Alox 600? Any help deciding would be appreciated. My gut feeling is to use the Alu oxide 400 first as despite being 400 grade like I've just used, I'm thinking it would be softer than the carbide so would leave a smoother finish. The smoothers are in numerical order, after which I think that's when I need to test the mirrors curve for sphericity.  

 

[Astrobits]

You will find that Al Oxide is much softer than SiC and gives a much finer surface. I would typically polish after 12 micron Alox so I would recommend that you go with the 400 and then the 600 and then polish. The finer you try to grind the greater the chance of scratches which often take longer to grind out than the time possibly saved with a finer surface. If memory serves, the numbers of the smoothing grits are the particle sizes with a decimal point missing, 125 being 12.5 micron etc. If that it correct the 600 Alox is finer than the three smoothing grits in your list, so they would be a retrograde step.

Nigel

[Chris]

 

Sounds good Nigel, I'll skip the smoothers in that case and see how things look after the 400 and 600 Al Oxide. 

I'd better start reading up on making a pitch lap and setting up test equipment  

 

[Chris]

Back on the grind tonight! After the usual tool scrape/re-seal, clean down, re-bevel, I burned through all the Al Oxide 400 I had, about 12 long wets worth. I was making my wets last several minutes each to try and get the most out of it. I'll check for pits tomorrow, but fingers crossed all will be well and I can move onto the final Al Oxide 600 before reaching the polishing stage!

I've noticed that when the mirror is wet it's pretty much see through like a window, and when dry it's frosted, but large text can be read through it. It's a bit easier to see where I am with this visualisation rather than by stating grit grade  

 

 

[Astrobits]

Looking good.

Nigel

[Alan White]

The VSG continues, looking good Chris.

[mapstar]

Keep at it fella. Can't wait to see it polished as you'r very close now 

[Chris]

Thanks guys  The project sometimes gets put on the shelf, but I vow for it not to collect dust on the shelf  

 

[Astrobits]

You can put the grinding stages on the shelf for as long as you like without problems. Unfortunately when you make up the pitch lap it will change shape if left off the mirror and leaving it on the mirror it might stick to the glass if it dries out. You will then have a lot of work/time to get it back to a good polishing condition. So plan to be able to polish out without significant breaks. If you need to break for a day or two then get a thick plastic bag and put the mirror in with pitch lap in contact and a good squirt of water to keep it wet. Leaving it too long with a soft pitch lap will see the channels close up and you will need to re-cut the channels before continuing.

Nigel

[Chris]

2 hours ago, Astrobits said:

You can put the grinding stages on the shelf for as long as you like without problems. Unfortunately when you make up the pitch lap it will change shape if left off the mirror and leaving it on the mirror it might stick to the glass if it dries out. You will then have a lot of work/time to get it back to a good polishing condition. So plan to be able to polish out without significant breaks. If you need to break for a day or two then get a thick plastic bag and put the mirror in with pitch lap in contact and a good squirt of water to keep it wet. Leaving it too long with a soft pitch lap will see the channels close up and you will need to re-cut the channels before continuing.

Nigel

Err, I hadn't fully considered this if honest. I knew about storing the pitch in contact with the mirror in a moist bag, but hadn't realised it was quite so time critical. 

I'll need to plan accordingly by the sounds of it, and maybe do the polishing stage during annual leave.....although all my annual leave is to cover child care which could be a problem, they're harder work than work sometimes! 

I'm assuming I'll need my test equipment ready at this stage as well, Ronchi or Facault. 

Any idea how the polishing stage compares to the grinding stage time wise will be much appreciated? Might help with planning this. 

I'm sure I'll be able to find more time for my grinding projects as my kids get older, I still have 3 children under 6 years old so I guess I'm lucky to have anytime for my own hobby at the moment.