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Hi, I have an 18 inch rotary (not a vib) flat lap that has a steel plate. I wanted to polish some thunder eggs and was hoping to get some input as to how to go about doing it! I've found lots of info online about how to use vib laps, but there doesn't seem to be much out there for rotary laps. So far, I've started grinding some eggs with 80 grit silicon carbide. For this stage, it is pretty easy to figure out when they are "done". When you don't see any more saw chatter marks or scratches, your done! So far it seems that 30 minutes to 2 hours is about right depending on how bad the saw marks are on the halves.
The questions I have is what about for the other grits? I've put some through 200 grit, and they look smoother than after 80 grit, but how to you know they are "done"? Is there a rule of thumb for grinding X hours at each grit? I've been going about an hour or so. Or is it just a matter of grinding until you think they are as smooth as they are going to get?
I also put some through 400 grit and they really don't look much different than after the 200 grit. So I'm not sure if I doing OK, or rushing things. I suppose I can just try taking it to the end and see what they look like, but I thought I'd ask here to hopefully save me from having to go back and regrind a bunch of thunder eggs.
And while I'm asking questions... The grits that I have available and plan to use are 80, 200, 400, 600, 1200 and then polish. Does that seem reasonable? I am also going to have to figure out how to do the polishing. I assume I will need a felt pad or something to hold the polishing compound. I don't think just putting the polish on the steel plate like for the courser grits would work too well.
My 16 inch unit has a separate plate with wool carpet and cerium for the final polish, but rather than change the heavy plate I use my 15 inch jiggle pan. You might try comparing similar rocks with and without the 1200 step. A local dealer and friend who has 2 48 inch units only runs coarse on his grinder plate, then sands them by hand at something like 280 and 600 on a bull wheel, then straight to the other 48 inch with the carpet and polish. His finished pieces are very nice.
Last Edit: Jun 20, 2015 15:54:27 GMT -5 by Rockoonz
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Post by jakesrocks on Jun 20, 2015 16:25:35 GMT -5
Herb,what make is your flay lap ? The maker may have a polish pad made of heavy felt which is bonded to a disc of galvanized metal.
For flat lapping, you really shouldn't have to go beyond 600 grit. The grit itself will break down and become finer.
There is no set rule for length of time in each grit. You just keep grinding until you cant see the scratches left by the previous grit. One thing. Rotary flat laps are much quicker than the jiggle pans are.
Interesting that you don't need to go beyond 600 grit! I wouldn't have guessed that you could get a good shine going straight from 600 to polish!
I just remeasured the metal plate, I was mistaken, it is a 16 inch lap not an 18 inch. I don't know the brand. It looks like it is home made, or maybe a Frankenstein home made from various commercial parts.
A few months ago, I did a little bit of searching for felt pads, but I don't think I found anything large enough. The wool carpet idea sounds like a possibility. I did try just using a regular piece of thick soft felt cloth, held down with some spray adhesive but that didn't seem to work too well. I ended up with lots of soggy fur balls and clumps of felt! I need to find something stiffer that can stand up to the abrading.
Post by jakesrocks on Jun 20, 2015 18:53:19 GMT -5
LOL. In the old days of lapidary it was all but impossible to find grit or sanding belts above 600 grit. When grinding cabs us old timers would use a worn out 600 grit belt as a pre-polish belt. Any grit still contained in the belt had been broken down to a finer grit size. We'd go straight from that belt to a polish wheel with cerium oxide. Worked great.
Once your piece is lapped to your final grit, you can go straight to a polish disc or wheel instead of trying to polish on the flat lap.
Remind me some day and I'll tell you how to convert your flat lap into a single head sphere machine.
LOL. In the old days of lapidary it was all but impossible to find grit or sanding belts above 600 grit. When grinding cabs us old timers would use a worn out 600 grit belt as a pre-polish belt. Any grit still contained in the belt had been broken down to a finer grit size. We'd go straight from that belt to a polish wheel with cerium oxide. Worked great.
I always wondered when reading old issues of Rock and Gem or other lapidary magazines why they always bothered with using worn out belts instead of just using a finer grit belt to begin with. Now I know!
this is very interesting how do you keep the slabs or eggs stationary while the lap is turning I can see it would be very easy one at a time just holding them but if you have a whole tray of them how do you keep them from just spinning with the lap? It would be hard to do that with varying heights of the eggs, if you could, you should be able to grind them down in a matter of minutes like 5 min. If they were slabs of the same thickness you could glue them to a piece of ply wood or plastic the size of your lap and a dowel attached to the centre so you could hold on to it in other words a mega dop stick if you used plastic maybe some carpet adhesive spray (PSA) would work, still don't know how you would do eggs for sure. I think you should be able to go through 100,220,400 and 600 in half an hr or less depending on the saw marks not including rinsing the grits off as you proceed.
Minerken wanted to know how I keep the thundereggs on the lap. Here is a couple of pictures that show the jig I built.
I basically took a scrap piece of plywood and cut out a circle leaving about a 1 inch strip of wood in the center so I had 2 semicircles. I made the diameter of the circle about 1/4 inch smaller than that of the lap plate so the eggs would not hang over the edge of the lap. I then cut out a chunk of the 1 inch wood strip to accommodate the washer and nut in the center of the lap. Finally, I added some more strips of wood to be able to span the distance over the washer and nut for strength. To hold the jig to the lap, I use a couple of spring clamps. I also have a few thin strips of wood underneath the jig to raise it up enough so the lap plate doesn't scrape the wood.
Here is an overall shot:
Here is a close up shot showing the gap around the washer and nut.
And finally, here is a shot of what it looks like in action:
The lap turns counter clockwise. So you can see that the eggs just bunch up against either side of the 1 inch wood strip. Once or twice during the grind at a particular grit, I stop the lap and turn the eggs 90 degrees or so and sometimes move them from the outside of the lap to the inside etc just to help assure everyone is evenly ground.
The only things you have to be careful about is not letting the plate get too dry. If it does, the eggs stick and the whole thing seizes up and the lap stops spinning. Luckily the drive belt off the motor is loose enough that the motor doesn't burn out!
The other thing you have to watch out for is if you have eggs that have thin edges. They can get jammed up either on the 1 inch wood strip or on the outer edge of the wooden cutout.
With this setup, I can process 10 - 20 eggs at a time, depending on their size. I am getting my brother, the computer engineer, to build be an automated grit feeder so I don't have to babysit the lap by constantly feeding it more grit. Hopefully I will have that in a few weeks. Right now, I'm finding that the rough 80 grit stage takes from 30 to 2 hours for the eggs with really bad saw chatter marks. And the other grits I am running for 1 hour each. I think I might be too stingy with adding grit, especially at the finer grits. It seems you have to add a lot of grit to keep the grinding going.
Yes, right now the grit feeder is my first finger and thumb! Quite time consuming. Hopefully my brother won't get distracted with other projects and will finish building the automated feeder for me soon.
On my 30 inch flat lap, I use 180 grit until all saw marks are gone. Then I use a bull wheel with 100 grit belt, then 220 grit belt and then a 400 grit belt. From there it goes on the 24 inch polisher using cerium oxide.
I don't have a sweep to move the rocks around on the flat lap so I move them around by hand a lot. Kind of a pain on the larger pieces.
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Apr 23, 2024 10:17:01 GMT -5
Welcome to the Rock Tumbling Hobby Forum where we share a love of rocks and a sense of community as enduring as the stones we polish.
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