14 inch mirror-fine grinding problem

[dark star]

I have been making progress grinding a 14 inch mirror blank. Everything has gone more or less OK until now. However, when I reached 1000 grit- 50 emery, the last grit before polishing I picked up some small scratches. Thinking it might be due to the coin tool I am using I went back to 320 grit. But I have now got through 600 grit (125 emery) without any problem. When I got to 1000 grit for the second time I have got some fine scratches again.

The tool seems to be sticking the mirror, but only with the 1000 grit. I first smoothed the grit in to the mirror with my finger, mixing it with water. I then tried mixing it with water in a bottle, but I am still getting scratches.

The first question is do I need to use 1000 grit? Or could I go straight from 600 grit to polishing?  If not, how do I prevent the sticking which I presume is causing the scratches? I will probably go back to 320 grit again, but there is not much point until I have understood what is going wrong, otherwise the same thing will happen again!

I will post some pictures tomorrow, as I am rather tired after two hours of grinding!

David

[dark star]

One suggestion I have read is to add a few drops of washing up liquid when mixing the grit with water, this helps to break up the grit, has anyone tried this?

David

[dark star]

Having read Glasspushers 12 inch mirror grinding posts it seems to be OK to finish fine grinding with 600 grit (125 emery). I hope this is correct, as I cannot face risking going back to 320 grit and then getting scratches again with 1000 grit!

David

[faulksy]

why dont you pm crashtest, as he is doing a 22" grind at the moment, sure he wont mind helping you out

http://stargazerslounge.com/blog/1346/entry-1642-10mm/

[JRM]

Hello David,

  first off, gonig back to 320 is going to far back, you should only have to go back one grit size unless its a massive scratch, but

these sound like sleeks, second the drop or to of dish soap really does help, you can try that as well.

you can also stop grinding at previous grit size before the 1000 unless you are making a huge leap between grit sizes, it will only mean a longer

polishing time.

My advice... go back the one grit size, grind out the sleeks and do an extra 5-10 wets to insure the sphere is good all the way out to the edge.

that was my problem when I did 10", it was not a good enough sphere, this is also why I did extra wets with every grit size when grinding the

20" expecially the 12M before I went to 5 micron.

after you are done those extra wets you can either try the 1000 again or go staight to polishing, I would try it one more time myself, but its up to

you to decide, it wont make any difference other then polishing time, but do keep us all posted..

my two cents

Rick M

[Astrobits]

I had the same problem when I was using a steel button tool. Going finer than 12 micron always scratched. I simply stopped at 12micron and polished from there.

One of the anomalies in this activity is the variation in grades of abrasive. Different sources use different codes for the grit and If possible it is sensible to use the micron grit size as that is consistent. Here's a chart from Saint Gobain:

grit-conversion.pdf

As you can see the finer grits particularly can be different depending on the reference standard used. So one source of 600 grit could be only 9 micron while from another source it could be 20 micron.

Nigel

[Jesterowl]

wish i had the time, patience and skill to do something like this. 

[Glasspusher]

I have just picked up on this thread as I have been on holiday. The issue of fine scratches seems to be a regular topic here. I have experienced them myself which is why I recommend finishing on 600 grit Aluminium Oxide. Aluminium oxide particles are less aggressive than Silicon Carbide which is a harder substance, hence it is ideal for finishing with. I have found that trying to use finer and finer abrasives just results in scratches. It may take a little longer to polish out from Aluminium Oxide 600 but it does produce a scratch free polish. I recommend using a good quality white Aluminium Oxide from a reputable source. I second NIgel's comments, if you search the internet for grit comparison charts you will be confused because there often seems to be little agreement.

John

[Astrobits]

I forgot to mention that I also finish on 12micron Aluminium Oxide. As John says it is less aggressive and gives a finer finish.

One thing that the most grit suppliers fail to give is the spread of sizes in a particular grade. This Newport Glass Works chart does give the max and min sizes for each grade of grit from an unnamed supplier who specifically stated that the data is only approximate!!:

http://www.newportglass.com/grit.htm

Their 600 grit has a maximum grit size of 35 microns and a minimum of 9 microns with an average of 16 microns. 1000 grit has a maximum size of 23 microns and a minimum of 2 microns with an average of 7 microns. It is the largest bits of grit that will cause the scratches and going from 600 to 1000 does not give the improvement that you might expect.

Nigel

[crashtestdummy]

why dont you pm crashtest, as he is doing a 22" grind at the moment, sure he wont mind helping you out

http://stargazerslounge.com/blog/1346/entry-1642-10mm/

I only wish I was anywhere near that stage with my mirror

[mapstar]

I only wish I was anywhere near that stage with my mirror

Me too fella the tile tool is well set just struggling for time and not even looked for the mosaic tiles to cover it yet!!!