Geode #2

Here’s another Joba geode. It’s heavy, probably no cavity, tough basalt under the mud ball, quartz veins, ~2” diameter, slightly egg-shaped/oblate. Using my juice bottle caps with SiC grit and super glue, 1.5” bottle caps. It’s slow going. Light grinding pressure because heavier pressure results in it spinning on the shorter axis, making it even more egg-shaped. Now most of the mud is gone, and it’s down to the basalt and quartz. Tough stuff.



These SiC cups seem ok, but they get dull quickly.  Refreshed them too with a sprinkling of fresh grit and glue, but again soon dull.  Needs diamonds.  Duh, Robbie.

In a previous post, Brian mentioned cups to hold diamond grinding discs - Thanks, Brian! It’s a bit of a diversion from the geode, but I think it can be an improvement over SiC bottle caps. Don’t have a lathe, but do have a drill, and a 2 1/8” hole saw gets a 2” core in 3/4” PVC board. Added a 1/2” countersink to help a ball find the center.



Before I had any rocks suitable for spheres, I used Home Depot 3/8” thick marble floor tile, cut into 3” octagons, laminated with super glue, then ground round. Still have a few of those around, so thought to use one to grind a cup shape. Mixed loose grit, gel hand soap, water to get a paste.



Worked ok for a while, but at ~1” diameter the cup acts more like a wiper, entraining very little grit into the cup. Tried a ball with 1” velcro dots stuck to it, and SiC 60-grit sanding discs. Seemed ok until the dots got ripped off by the cup. This should have worked better than it did.






A more direct approach is to grind the cup with a solid abrasive ball. Drizzled a little super glue on a floor tile ball and rolled it in loose grit, kinda like rolling a doughnut in candy sprinkles. Did only portions of the ball at a time, and let it dry a few hours. Had too much grit in the tray resulting in gobs of grit instead of a thinner uniform layer. Glued a lot of grit to my fingers.



Solid abrasive ball is lumpier than I like, but some of the high points come off with a coarse file. It’s still chaotic in the sphere machine - violently thrashing about. After awhile it settles down. SiC grinds PVC easily, and after a mere 2 hours of grinding and messing around, finally got 2” cups. These cups are now ground concentric with the motor shafts, so they are staying velcroed to the backing pads to preserve concentricity, since cup/velcro placement is imprecise. Its ugly, but it works. Woohoo!



30-grit 2” diamond discs won’t conform to the compound curve of the cup, and 2” peel&stick velcro discs won’t conform without wrinkling, so used the Dremel and a razor blade to cut the diamond discs and velcro into pie-shaped pieces. Finished radius is slightly larger than the geode. All this is a lot of messing around just to grind this geode. I guess it’s an experiment in the tooling.









I like the bottle caps with SiC grit because they put a lot of grit in the grind.  They get dull too quickly though.
Even these cup-mounted diamond pads suffer from only a little diamond doing any grinding.  Maybe I should have more patience and just let them run.

I wanted something more aggressive so got some diamond segments for replacements on core drills. These are made for boring a hole in a sidewalk or a concrete slab, like for a pipe, or bigger for geology core samples. Instead of welding segments to the end of an iron pipe, a simpler way is just glue the segments to a disc, so I used a hole saw to make discs from 3/4” PVC board. Used water bottle caps to center and align the diamond segments, then added a little super glue. Been grinding more than 40 hours and they haven’t failed yet. Still have the problem of only a little contact area between the segments and the sphere, but maybe the segments will wear-in and develop a chamfer with more surface area.

They are more aggressive than the plastic diamond pads so I now prefer them for roughing.







And there's a cavity!  Woohoo!



Both the quartz and basalt are seriously pitted. Quartz polishes, but basalt does not, so didn’t go any further than roughing.
Sphere measures 1.7”.



On a red flashlight with a mirror…




It’s not perfect, but it’ll do.

I’m a rookie but I like that I machined a bubble - a quartz bubble.

Hope you like it too.