Thread: fast, cheap way to lap chinese 12k

I bought a chinese 12k that had really big saw marks left on it. I had a bunch of loose abrasives left over from a telescope mirror grinding project from a few years ago. I bought a granite floor tile (cost $4) and started with a slurry of 220 SiC grit, then 500 SiC, then 9 u Aluminum Oxide. The slurry cuts *really* fast (much, much faster than wet or dry sandpaper) and the AlOx left it really smooth. I have some 5u lying around but I'm not sure it would be that much of an improvement - the stone is really flat and smooth right now. One thing that is funny is that when you get to the fine abrasive, the stone really sticks to the tile!

I looked online and there is a place called gotgrits.com that sells these abrasives. A 1/4 lb of SiC 500 mesh is about 2 bucks, same of Al Oxide (I think) was about 1.75.

One other thing that seemed to work well - I started on the underside (unpolished side ) of the floor tile whose textured surface helps gather grit (it wears down with the 12k) then finished on the polished top side.

Anyway, just thought I would pass on the idea.

- Ian

You built your own telescope mirror, cool. Not a "simple" job.

Jeff

Very good ideas here,thanks for the info.

Originally Posted by ruday555

Very good ideas here,thanks for the info.

I forgot to mention that I figured I would do the final smoothing with the slurry stone that came with it (after the 9 u Al Oxide). That worked out really well - both have a kind of glossy look to them. I would love to hear the opinion of some of the honing experts here.

I figure the whole process, from really rough to dead smooth took about 15-20 min. I didn't see any problems with embedded grit at all.

Too soon to tell about its effectiveness as a hone - at first try, it seemed to work well (but I am definitely a newbie at this).

One question to anyone with a geology background: what kind of rock, exactly, is a chinese 12K? It looks like a silicaceous shale (has the shale smell). Just curious (actually, very curious). It is a very interesting rock.

thats awesome.. i wonder how some SiC 220 grit would work on an arkansas? probably better than my worn dmt325

Originally Posted by ezpz

thats awesome.. i wonder how some SiC 220 grit would work on an arkansas? probably better than my worn dmt325

Lapidary grit works well on hard stones - I always use it on novaculites - as well as any other hard hone. It is far faster than a DMT and cheaper - novaculites can inflict severe damage to the diamond coating on DMTs. The subject has been covered in depth quite some years ago, but here is the oldest reference I can find to it - the older postings must have got lost somehow.

I used to finish off with wet and dry paper, but now I use a fairly worn DMT 600 grit which is kept well flushed with water - that shows up any irregularities, such as may have been caused by the support for the lapidary grit (eg stout sheet glass) being abraded. The support will dish after a while - which is how you form telescope mirrors and simple lenses - the stuff on top of the support will become convex as the support becomes concave. It's an age-old process.

Even large old glass sheets for prestige applications were flattened using this technique in the old days - the large sheet was supported flat on a table, and small 'rubbers' were worked over it in one direction and then another and then another and so on with a fine grit paste as the lubricant - eventually all the high areas were flattened - the same sort of principle as using a small hone rubber to flatten the hone.

It seems that if you go far enough back in time, someone somewhere had already thought of whatever it is that we have just come up with - or 'rediscovered' to be accurate. It seems that we have forgotten a lot...

Regards,
Neil

Originally Posted by invention13

... I looked online and there is a place called gotgrits.com that sells these abrasives. A 1/4 lb of SiC 500 mesh is about 2 bucks, same of Al Oxide (I think) was about 1.75.

Just so there's no confusion, that should be gotgrit, not gotgrits.

gotgrit.com

Sorry, going from memory.
The guy who runs this is really nice - he called me personally to tell me that the 500 SiC I ordered would be a day or two late.
If anyone read this - don't order more than 1/4 lb of anything - it goes a long, long way. I probably used < 1/2 teaspoon to lap the hone I did.