Do you really need 1000 grit??

[Sfarndell]

I finally decided to finish my old 18" mirror. It had accumulated a few scratches over the last 4 years so I decided to go back to fine grinding but i forgot how frustrating it can be.

I've twice picked up 2-3cm long (deep) scratches at 1000 grit about 10-15 minutes before fully ground out (+-45 mins into the grit size, +-3 wets) while lifting/replacing the mirror for a new 'wet'. After going back so often to 800 grit I'm questioning if it is really worth persevering with the 1000, or should I just polish out from here?  How much time would I really save? My lap is a 70% subdiameter lap. I know these aren't big scratches, but I'd rather invest a little more time now for the longer-lasting personal satisfaction of a job well done.

Thoughts/advice/experience??

[JRM]

   1000 grit equates to about 7 micron, I know of many a folks who have finished fine grinding at 9 micron, and a few who have polished after 12 micron,

 the finer you grind the shorter the polishing takes so they say...800 is like 12 micron and yes you can polish from there, as long as the mirror

surface is  uniform ( which it should be at that point) and no sparkles can be found when testing for pits, (none)  because once your done with 800 or 12 micron

there should be no more pits left to grind out,  if any pits are remaining they will not polish out.. 

it a matter of personal preference.

The scratches you are getting, are you sure its not a sleek, reason I ask is any time I got a scratch I had to go back to 320 grit to get it out, if you are removing it by going back to 800 it sounds more like a sleek, maybe try grinding more with the 1000 if you get it again

my two cents, hope it helps.

Rick

[Astrobits]

I don't think 1000 grit is necessary over 800 grit.

The quoted grit size is only the average size of the grit. There is a range of grit sizes that will be present in every batch as the table from Newport Glass shows:

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

As can be seen here, the nominal 800 grit is an average of 12 microns with the maximum particle size of 30 microns. Going to 1000 grit with a nominal 7 microns average it will have a maximum particle size of 23 microns.  I don't think that that is a sufficient improvement over the 800 to be worth it.

The above table is from one particular supplier and other suppliers will be different.

Personally I use Aloxite rather than Silicon Carbide for the finer grinding stages as it breaks down more easily and gives a finer finish. The last wet of grinding I extend by adding only water as most of the large particles will be much smaller than a new charge of abrasive would introduce with the consequent possibility of scratches.

Nigel