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Cutting a really good girdle

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pete
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« on: December 23, 2011, 04:20:44 am »

Cutting an even continous line around an oval at a consistent angle to establish the girdle line I find very difficult. For some reason the long axis always cuts closer to the base of the cab than the short axis, even when I try to compensate by moving the cab slightly further from the wheel when approaching the long axis.
Getting the back bevel to meet the girdle in one or two passes is also difficult on ovals.
Girdles and bevels on round stones are much simpler because there's no short and long axis and the stone can be easily presented to the wheel at a consistent angle and turned continuously.
This is not a matter of using a rest for the stone as to do a proper girdle a line must be marked around the stone and the rest simply gets in the way.
I always spend a lot of time at the 600 stage fine tuning the girdle line to try and perfect it. Perhaps this is the way it has to be.
I know there's some complex mathematics involved in doing something like this but that sort of thing is beyond me to comprehend. The best I can do is to rely on hands, eyes and simple devices!
My initial search of this site didn't find much discussion on this technique but if there is, please point me to it
Cheers
Pete
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Allen
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« Reply #1 on: December 23, 2011, 06:11:21 am »

Pete there has been discussion over time.

This one may help  http://gemstone.smfforfree4.com/index.php/topic,4216.0.html

I find various blocks of wood as rests give the correct angles. Then its just a gentle touch.
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rickn
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« Reply #2 on: December 23, 2011, 09:41:25 am »

The way in the clip is the way I do it. I then take the girdle up to a 600 polish. Then I mark the girdle line at the hight I want it and never grind below the line as I am doming the cab. After the dome is finished and pre polished I polish the girdle thru the remaining wheels. After the 600 wheel no more stone is really removed and I get a good crisp even line.

RickN
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pete
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« Reply #3 on: December 23, 2011, 03:57:49 pm »

I checked that video bofore posting. The video describes cutting a cab type outline on a large flat stone using a rest which is no doubt handy for some but  rests just get in the way if the stone is small and dopped.
If your'e making a calibrated oval with close tolerances it's no good to dome right to the bottom of the stone then flip it over and create a back-bevel to give a girdle. Calibration goes out the door. Same thing happens if the back bevel is cut then the girdle line marked. The girdle line should be marked on the preform while the sides remain at 90 degrees, only then can you start to get a very accurate result.
For most stones it doesn't matter if the outline, girdle, curvature, flatness, polish etc is not a top job because they still look OK but to the judges trained eye using a loupe those stones are put aside. I'm aiming for more stones on the top shelf.
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hulagrub
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« Reply #4 on: December 23, 2011, 05:08:15 pm »

Pete, I mark the girdle also, but with agates and jaspers, my girdle is done and over at the 220 wheel. At the 300 for softer stones, and maybe the 600 for really soft stone. Guess I try to get my girdle as close as possible on the coarsest wheel and then just fine tune it on the next wheel. I use both a sharpie or a girdle marker to make an even line. Practice is the biggest thing to me.
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bgast1
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« Reply #5 on: December 24, 2011, 01:23:15 am »

I'm gonna get in trouble here, I just know it.  lol  hide,  I don't cut a girdle on my cabs. I read about how to make a cab the old school way and that's the way I do it. If I had tons of sales and someone wanted my cabs to be exactly the way they want, I would change the way I do things. But for now, all of my customers have been happy with what they buy.
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pete
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« Reply #6 on: December 24, 2011, 02:59:55 am »

I'm gonna get in trouble here, I just know it.  lol  hide,  I don't cut a girdle on my cabs. I read about how to make a cab the old school way and that's the way I do it. If I had tons of sales and someone wanted my cabs to be exactly the way they want, I would change the way I do things. But for now, all of my customers have been happy with what they buy.

 omg Heretic!!
Don't worry Bob. The only rule I know of is the comp rules. Jewellers also love girdles coz it makes their job easier but unfortunately makes ours harder!
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hulagrub
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« Reply #7 on: December 24, 2011, 10:09:03 am »

This is why we need to cut even girdles. I posted this pendant, awhile back. Notice at the upper right, how the bezel lays over, flatter than the rest of the pendant, due to an uneven girdle.

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Taogem
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« Reply #8 on: December 24, 2011, 11:20:52 am »

How small a calibrated ovals are you trying to do Pete..
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« Reply #9 on: December 24, 2011, 01:48:31 pm »

This is why we need to cut even girdles. I posted this pendant, awhile back. Notice at the upper right, how the bezel lays over, flatter than the rest of the pendant, due to an uneven girdle.



Dave, I am by no means a jeweler yet. I only cut cabs and place them in extremely cheap findings. In fact most of the time they are glued in. I do want to start learning how to make my own jewelry but that is yet in the future.

My question would be, what is the difference with an old school cab like I cut. About a 10 degree bevel all the way down, then the bottom bevel as well. It seems to me that would work just fine. (?)
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« Reply #10 on: December 24, 2011, 02:09:10 pm »

The difference is the final product..

If the cab is rounded (old school) all the way down as compared to a even girdle for one, is being able to keep the bezel tape wrinkle free when we burnish it tight to the cab.



Same with step type bezels..

Everything just looks so much more neat..

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« Reply #11 on: December 26, 2011, 12:53:27 am »

George, my cabs are not rounded all the way over, most of the time but not always. In fact, I am not sure if someone was paying me a compliment or a criticism once when she said that my cabs have such nice straight edges.
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pete
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« Reply #12 on: December 26, 2011, 02:32:06 am »

How small a calibrated ovals are you trying to do Pete..

3mm rounds were about the smallest I've cut and many in the sub 6mm range. Recently I recut about 30 facetted rubies that were 1mm x 2mm (accent stones)! They were native cut and not exactly calibrated but were bloody small to work on!

The reason I brought this subject up is because I like cutting for the  lapidary comps in Australia and there's nobody in my area that does the same.  To restate the obvious, stones at comp level need to be highly accurate with very high lustre: it's a challenge I love but is very time consuming.
I know it's all in the hand and eye techniques to get the best girdles, especially on ovals. But you can't simply place them on a rest and hope for the best.
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« Reply #13 on: December 26, 2011, 09:28:24 am »

I'm glad you brought this up. I would actually like to be sure that I am cutting cabs that someone can use and would likely come back for more. Or even like you do, win a cab contest should I ever enter one.
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« Reply #14 on: December 26, 2011, 09:46:55 am »

Pete, you are so right about the use of the rest, no guarantee there!
Bob, the girdles I cut, are usually 1.5 mm, with me keeping in mind that Charlotte may want to wrap the cab. If I know that it will be put in a bezel, I go for a wider girdle, but I always aim for an even girdle. The secondary effect of an even girdle (for me) has been so much better cabs, which is what we all strive for.
We teach cabachon classes at our club, and when someone strives for that even girdle, 90% of the time, their cabs look wonderful. When they get lazy, the resulting cab, is somewhat less satisfactory.
Charlotte and I, and one younger couple are the only ones regularly cutting cabs, although we have many curious people watching our works.
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