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Post by hummingbirdstones2 on Feb 5, 2022 11:48:26 GMT -5
Yeah, to do a "perfectly" flat back you lock the wrists and fingers and just use elbows and shoulders - both arms moving like opposed pistons.
Maybe it's a little like "Wax on. Wax off." with the arms and hands in a different orientation. Moving the back of the stone only within that one plane.
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Post by holajonathan on Feb 5, 2022 21:46:19 GMT -5
Hmmmm - holajonathan - Were you trying to flatten those St Johns on your flat lap? After your very informative post on sintered wheels, I assumed you were grinding those pieces on a wheeled machine.
Flat surfaces are possible on wheels, once the muscle memory is there. I'm trying to figure out how I would explain it, if anyone here hasn't done it this way.
Easiest way (might make no sense at all) I can describe it - is to grind it any way you want until it's more or less flat. Then hold the cab firmly in the same orientation relative to the wheel as the cab back is moved in a circle over the same spot on the face of the wheel.
The angle of the cab relative to the wheel does not change. It's easiest when holding the cab with both hands to keep it in exactly the same orientation as it is moved around and around against the wheel. A dop stick can be left on to help hold it, but keeping even pressure on both "end" of the cab is necessary to make it work.
If anyone who's never done this understands ANY part of what I just said, then today is a very good day.
This method will definitely flatten a surface enough to move on to polishing on a flat lap or whatever.
I am familiar and at least proficient with the technique. A 90 degree plane (perpendicular to the point of contact) might be easiest because it's easy to lock into that position visually. But any plane works as long as you stay on that plane. Staying on that plane while moving the piece is the key; the angle of the plane doesn't actually matter. My description probably only makes sense if you already understand the technique. Harder to explain than to actually do, I'd say. I was actually trying to flatten out some rather ambitiously sizes pieces on the flat lap -- not exactly cab size pieces, more like gnarly 5 square inch pieces. You might say I was trying to use it like a planer to make some small slabs good for a couple of cabs each. I do have a little indoor trim saw (Rock Rascal), but with a 6" x 0.020" blade on it, I can only use it for cutting out preforms. There is absolutely no possibility of cutting little slabs or grinding with the blade. It's a thin, wobbly little MK303, good for one thing only (but quite good at that one thing.) I was going to save you this long and boring story before, since you pointed out (correctly) that my supposed approach didn't make much sense, I'll give you the novella version. Although I am excited to work the material, I resigned to waiting until a little snow melts and I can do it the way that I know works -- with bigger saw + stiff blade. I also need a new 80 grit lap disc. It's still full of diamonds, they are just very dull. |
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Post by hummingbirdstones on Feb 5, 2022 22:03:10 GMT -5
Hmmmm - holajonathan - Were you trying to flatten those St Johns on your flat lap? After your very informative post on sintered wheels, I assumed you were grinding those pieces on a wheeled machine.
Flat surfaces are possible on wheels, once the muscle memory is there. I'm trying to figure out how I would explain it, if anyone here hasn't done it this way.
Easiest way (might make no sense at all) I can describe it - is to grind it any way you want until it's more or less flat. Then hold the cab firmly in the same orientation relative to the wheel as the cab back is moved in a circle over the same spot on the face of the wheel.
The angle of the cab relative to the wheel does not change. It's easiest when holding the cab with both hands to keep it in exactly the same orientation as it is moved around and around against the wheel. A dop stick can be left on to help hold it, but keeping even pressure on both "end" of the cab is necessary to make it work.
If anyone who's never done this understands ANY part of what I just said, then today is a very good day.
This method will definitely flatten a surface enough to move on to polishing on a flat lap or whatever.
I am familiar and at least proficient with the technique. A 90 degree plane (perpendicular to the point of contact) might be easiest because it's easy to lock into that position visually. But any plane works as long as you stay on that plane. Staying on that plane while moving the piece is the key; the angle of the plane doesn't actually matter. My description probably only makes sense if you already understand the technique. Harder to explain than to actually do, I'd say. I was actually trying to flatten out some rather ambitiously sizes pieces on the flat lap -- not exactly cab size pieces, more like gnarly 5 square inch pieces. You might say I was trying to use it like a planer to make some small slabs good for a couple of cabs each. I do have a little indoor trim saw (Rock Rascal), but with a 6" x 0.020" blade on it, I can only use it for cutting out preforms. There is absolutely no possibility of cutting little slabs or grinding with the blade. It's a thin, wobbly little MK303, good for one thing only (but quite good at that one thing.) I was going to save you this long and boring story before, since you pointed out (correctly) that my supposed approach didn't make much sense, I'll give you the novella version. Although I am excited to work the material, I resigned to waiting until a little snow melts and I can do it the way that I know works -- with bigger saw + stiff blade. I also need a new 80 grit lap disc. It's still full of diamonds, they are just very dull. Kudos to you, sir, for being proficient with the technique. I can't do it. Honks me off, too. Lord knows I've tried and failed too many times to even think about it anymore.  If I'm working an end piece or something that's not already flat I give it to Vince and tell him to do it. LOL! |
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Post by holajonathan on Feb 5, 2022 22:10:06 GMT -5
Kudos to you, sir, for being proficient with the technique. I can't do it. Honks me off, too. Lord knows I've tried and failed too many times to even think about it anymore.  If I'm working an end piece or something that's not already flat I give it to Vince and tell him to do it. LOL! I do most of my cabbing standing up with the Genie around waist level on a table. So I am looking down at the cab / wheel when cabbing. This is bad for my neck, but good for a lot of things, including keeping a cab on a certain plane if using a wheel to flatten. I don't think I could do it if I were sitting in front of the machine, or if I were using a dop stick. |
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Post by hummingbirdstones2 on Feb 6, 2022 10:51:16 GMT -5
holajonathan - I knew by seeing your cabs that you'd know how to flatten a back. I'm easily confused, and flat lap references threw me.
Not a boring story at all, either, as your descriptions of techniques are way more to the point than my rambling babble.
I sit at the Genie, so the back of the cab hits the wheel at 4-5 o'clock. Lets me see the cab and contact point as you described.
Easy to understand why those pieces were especially difficult - that's a big chunk, and I know how gnarly the rough was.
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Post by holajonathan on Feb 6, 2022 16:02:27 GMT -5
holajonathan - I knew by seeing your cabs that you'd know how to flatten a back. I'm easily confused, and flat lap references threw me.
Not a boring story at all, either, as your descriptions of techniques are way more to the point than my rambling babble.
I sit at the Genie, so the back of the cab hits the wheel at 4-5 o'clock. Lets me see the cab and contact point as you described.
Easy to understand why those pieces were especially difficult - that's a big chunk, and I know how gnarly the rough was.
It was the same catch-22 you always get when working with hard rocks. I've got lots of ways to shape them fast -- everything from a cup wheel on an angle grinder, to a 30 grit sintered wheel. But such options create deep scratches, and worse yet, subsurface damage, so it's no timer saver in the long run. My dream grinding wheel would cut like a new 220 plated wheel -- fast, precise, and smooth, with no nasty scratches left behind. I have considered changing one of the pulleys on my 8" grinder to increase the arbor speed by about 50%, while also plumbing a fresh water feed to flood the wheel with water. (Right now I use an aquarium pump + spitters, like the Genie.) A faster speed with a lot of water to keep the wheel cool and clean should make any wheel cut a lot faster, especially my 8" sintered wheels. This seems like a gentler way to increase grind rate, as opposed to pressing harder or using a coarser grit wheel. If I dull the wheels quickly with the higher speed, I can dress them and bring them back to life. Dressing sintered wheels seems to remove so little of the metal matrix, it seems that I will be able to dress them hundreds of times before wearing the out. I have been abusing and repeatedly dressing my 8" sintered wheels for over a year, and I don't think I've worn through more than about 0.5mm of the matrix. Have you ever experimented with a high arbor speed for the first step of cabbing? My 8" grinder currently runs at something like 1200 RPM, which given the larger wheel diameter, ends up being the same speed at the point of contact as the Genie. |
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Post by hummingbirdstones2 on Feb 7, 2022 9:10:38 GMT -5
No - I've never tested higher RPMs on a diamond wheel. Never thought about it, really, but it might be okay with sintered wheels. Makes sense that sintered wheels would tolerate the higher SFM better than plated. Interesting!
I guess the ability of the diamonds to tolerate the higher speed would be the limiting factor. Sintered matrix should hold them in place.
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If I'm working an end piece or something that's not already flat I give it to Vince and tell him to do it. LOL!
If I'm working an end piece or something that's not already flat I give it to Vince and tell him to do it. LOL!
