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Cutting Perfect Circles

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Author Topic: Cutting Perfect Circles  (Read 523 times)
Bruce
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« on: December 16, 2015, 08:16:13 pm »

I did some searching on here to see if this topic had been previously discussed, but I didn't find what I was looking for.

I was working at cutting a round cab for a mount that I had, and noticed that although I was pretty close to having the cab perfectly circular,  there were a couple of spots that were just a tiny bit off.  It wasn't noticeable when looking at the cab until I held it to the mount.  It would probably be fine and most people wouldn't even notice it...but I do.

Does anyone have any tricks/techniques that help to achieve perfectly rounded cabs?
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lithicbeads
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« Reply #1 on: December 16, 2015, 09:57:37 pm »

 Most commercial cabs including circles are cut on a semi automatic machine that rotates the dopped cab into  a grinding wheel incrementally until it matches the model attached to the machine. Some folks put the dopped cab in a nattery powered drill gun and while handholding it run the edge slowly into the wheel. After the girdle is done you can slowly rotate the stone towards the top and crown it as well. Incresing the speed as the girdle preform becomes quite round will help make it better if you hold the drill carefully. Minute hardness differences in the stone lead to the small asynetries you are referring to in many cases but the way our hands rotate can also make a perfect circle very difficult when hand cutting. Obviously no stone circle is perfect but I hope you get some to the point they do not bother you. I have a very similar problem  when looking at certain shapes.
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bobby1
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« Reply #2 on: December 16, 2015, 10:31:30 pm »

I wrote an article that was in last November's issue of Rock and Gem in my monthly column that addresses this specific situation. I use a circle template as most people do but when I draw the circle from the template I use a black ball point pen so the line is narrow and well defined. When I'm grinding to this line I go slowly and grind up to this line but never beyond it. I always ensure that I'm grinding at a 90 degree angle to this girdle. If you grind past the line you don't have a reference to know how far beyond you have gone. I use the same pen lying on its side to draw a line halfway between the top and the bottom of the preform. As I grind the first pass I'm always looking at that line and again I never grind below it. As I make the first grind depth into the dome I ensure I have ground it the same all around. I continue grinding around the preform keeping it even all the way around. I do this to keep the symmetry balanced. Sanding and polishing after the grinding activity proceeds as usual.
Bob
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Bruce
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« Reply #3 on: December 16, 2015, 10:49:34 pm »

Most commercial cabs including circles are cut on a semi automatic machine that rotates the dopped cab into  a grinding wheel incrementally until it matches the model attached to the machine. Some folks put the dopped cab in a nattery powered drill gun and while handholding it run the edge slowly into the wheel. After the girdle is done you can slowly rotate the stone towards the top and crown it as well. Incresing the speed as the girdle preform becomes quite round will help make it better if you hold the drill carefully. Minute hardness differences in the stone lead to the small asynetries you are referring to in many cases but the way our hands rotate can also make a perfect circle very difficult when hand cutting. Obviously no stone circle is perfect but I hope you get some to the point they do not bother you. I have a very similar problem  when looking at certain shapes.

I have tried the drill technique with fair success.  I do think that the minute hardness differences is what is causing most of what I'm describing. I'm thinking of attempting to build a mount to hold the drill at 90 degrees that will allow me to slowly move it into the grinding wheel, and then by releasing the lock allowing it to swivel to crown it.

I wrote an article that was in last November's issue of Rock and Gem in my monthly column that addresses this specific situation. I use a circle template as most people do but when I draw the circle from the template I use a black ball point pen so the line is narrow and well defined. When I'm grinding to this line I go slowly and grind up to this line but never beyond it. I always ensure that I'm grinding at a 90 degree angle to this girdle. If you grind past the line you don't have a reference to know how far beyond you have gone. I use the same pen lying on its side to draw a line halfway between the top and the bottom of the preform. As I grind the first pass I'm always looking at that line and again I never grind below it. As I make the first grind depth into the dome I ensure I have ground it the same all around. I continue grinding around the preform keeping it even all the way around. I do this to keep the symmetry balanced. Sanding and polishing after the grinding activity proceeds as usual.
Bob

I've been using an extra fine point sharpie for line work.  The last piece I was working I stopped with a consistent millimeter or so left before I got to my line.  I intended on sanding with 400 to work the last bit to the line, and everything looked good until I placed it to the mount to check the size.  There were two spots that were (at least to me) a little off. My lines were still just visible, but it was just enough to be noticeable in the mount...as stated above...I think the subtle differences in the stone got it a little off.  It's not much, but I'm always looking for ways to make it better.
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finegemdesigns
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« Reply #4 on: December 17, 2015, 01:54:53 am »

http://www.gravescompany.com/Preformer-Attachment.html

Circles work with this tool on both horizontal and vertical wheel mountings. For ovals though you need to use vertical setup only. Horizontal mounting over a flat lap makes slightly unsymmetrical ovals due to uneven rotation angle to gemstone.
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Kent
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« Reply #5 on: December 17, 2015, 07:40:13 am »

As a woodworker I use templates at times for accuracy. With that same thought I'd be tempted to try using a guide under the stone. Could be a washer, or fender washer already cut out or you could cut a template to a specific size yourself using a hole saw. That would be the free form method of forming the outer edge. You could also develop a jig that holds your dop stick horizontally to your grinder and work it in a manner similar to a lathe.  Could be something as simple as a wood knee clamped to your bench with a hole in it for the dop stick.
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« Reply #6 on: December 17, 2015, 04:08:03 pm »

Hi Bruce

Don't feel bad.Cutting a calibrated stone is hard to do. I have specialized in doing that for over 35 years and I still have not produced a perfect calibrated stone. I do love seeing how close I can come and I still chase that. I just entered a 30 mm round in the killer cab contest. The high spot is 30.16mm and the low is 29.89mm  bricks

When I was first beginning I tried to use templates and could never make it work. I just use my eye and a set of calipers. Most of the form is about free form cabs and I have no problem with that but I would like some day to set up a section for people that do calibrated stoned. It is a different world.

Don't give up it takes practice .

Bless
Shawn



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wyrock
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« Reply #7 on: December 18, 2015, 07:05:22 am »

They now make hole saws in just about every size imaginable. If you can find one with the correct inside diameter you get an almost perfectly round plug. I can not remember the name of the dealer but there is one out there that also lists the inside diameter of their hole saws. Very simple to use in a drill press and a pan of water with a piece of wood or a brick to back up the slab.
Jim
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« Reply #8 on: December 18, 2015, 07:14:01 am »

I can do a perfect circle on my faceting machine with the quill set to 90 degrees and in free wheel mode (dop is not locked).  I'm guessing you could do the same thing with some type of jig and a flat lap as long as you could adjust the device vertically up or down.  On the faceting machine it works a treat.
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finegemdesigns
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« Reply #9 on: December 18, 2015, 12:12:34 pm »

A faceting machine works well also. But if you don't have a faceting machine then the Graves is a good choice. For example:





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Trails
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« Reply #10 on: December 18, 2015, 12:46:35 pm »

I use my dop sticks for gauging my base. I actually have wooden dowels from 1/8th of an inch all the way up to 3/4 of an inch. I make gauges for ear's from time to time, so getting a near perfect diameter at the set gauge is key for trying to match material to the hole in someone's ear. I just glue my rock cutout to dowel of the size I need, and grind down till I'm a few even millimeters from the stick. Usually by the time I'm done through 400/600 sanding I'm down to dowel size.

I run off my grinding benches, both have clearance so I can get the dop stick at a 90 degree angle and I just watch myself with magnification. Thats been the best way I've figured how to do nearly perfect circles. I've been waiting for someone with inch or bigger gauges to come forward to give me a challenge, but same time I fear it'd be too much weight for the cartilage unless I took some rock out from the center..
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Tay


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