Lapidaryforum.net
Rock Art => Cabochons, Intarsias, Cameos => Topic started by: R.U. Sirius on September 25, 2020, 04:36:06 PM
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As a long-time beginner (sic!) who struggles with occasional flat spots on cabs, I want to know if there is a right way to orient the dopped cab relative to the grinding wheel when starting to bevel the edge to form the dome. Looking at the youtube videos and my peers at the rock club, and thinking about the mechanics of grinding, I see several distinct possibilities:
- Keep the face (crown) of the cab vertical and facing to the side, and tilt sideways (in horizontal plane) to adjust the bevel angle. Grinding action is along the edge. I prefer this, as it allows me to clearly see and maintain the angle by looking straight down onto the cab as I rotate the stick to work around the stone.
- Start with the face oriented towards the wheel, and tilt in vertical plane to control the bevel angle. Grinding action is then perpendicular to the edge. I need to look from the side to judge the angle between the stone and the wheel.
- Keep the face vertical and facing the wheel, and move (not tilt) up or down the wheel to control the angle. Again the grinding action is across (perpendicular to) the edge I am beveling.
How do these (or other) approaches compare in terms of consistent and fine control of the bevel angle, and in terms of flat spots on the dome and the girdle? Does the answer change once I move to sanding on expando wheels?
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Wow great question. It is going to be different for every person I am sure. I am a person who hand holds stones and does not use dops (except for rare occasion). I actually change the way I am holding depending where I am in the beveling.
I start with edge up face to wheel and bottom edge tilted out towards me. I get pretty precise edge bevel heights because I am watching the width of the bevel as I go around the cab setting the first step. This first is to set the heaviest bevel of the dome of the cab closest to the edge.
After the 1st step I change the angle and move the stone lower on the wheel so as I look down from above I can see how much of the first step I remove and how close far the 2nd step travels towards the center.
After this 2nd step of the "pyramid" I turn the top of the stone facing up (as you can see by the pictures "up" still has the edge higher than the top of the stone so it isn't truly "face up") and to the left so I can watch how wide and how close the next step goes across the 2nd step and advances to the center.
Now that being said I never hold the stone parallel with the ground, it is always tilted towards the wheel. My descriptions are describing more of how much of the top of the cab I see during these steps. I took a couple pictures of one hand with the cell phone and one on the cab to give examples. You will see the black marker on the edge of the side bevel of the stone. After roughing my dome I marker the edge before putting the bevel on the side of the stone as it gives me a visual reference as I have it on the 220 wheel setting the angle of the side bevel.
I think the biggest problem that leads to flat spots is not doing enough rocking motion across the top of the stone and as you are going over the bevel steps, feeling the hips as the stone rolls. Again this is hand held so with a dop, a whole other beast. As I rock the stone from one edge across the top and to the other edge I slow down the speed of the rocking the further I am from the center and this helps smooth those flat spot hips. As I rock the stone it is back in the orientation of the step 1 where I can't see it but feel it in my fingers. I repeat this same procedure on the 220. Then I can change to step 2 holding and rolling left to right and right to left as I rock across the dome and this is also on the 220.
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I never rock , never have. I just turn concentric circles for a round cab for example with the stone never leaving the wheel or changing angle until the full stone has been cut at that angle. Also I have cut hundreds of thousands of stone by hand and never dopped. Many do not. One old time method doppers used was called running the stone. You start the grind at the top center of the stone and turn and pull the stone upward until you grind to the edge.Rotate a hair and repeat. No matter what style you use a key wheel totally ignored today is the 600 hard wheel. After going over the stone with the 600 hard and repeating the pattern you used for the coarser hard wheels you can easily see the mistakes you are making at the coarser grits. A worn 600 hard is the best for taking out small scratches as well without removing excessive amounts of your stone. As a production cutter I relied on it to make very fast fixes with minimal resanding after finding the scratch.
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I use a dop at 90 percent of the time. For me, concentric circles are part of it, but rocking the stone while slowly turning it is a big part of preventing flat spots. This is true for wheeled machines like shown above as well as flat laps. Also running it around in a twisting motion when working near the top can help as well. (Hope that last is clear.) You may need to experiment a bit to find what works for you. The above described 600 thing is so helpful, too.
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I personally see your questions as two fold. When you say "flat spots" I first think the top of the dome.
To address this is the area you are having flat spots in I would suggest practicing more and not cutting low dome cabochons. Practice cutting tradition high dome cabochons where you are able to properly roll over the girdle to the top of the dome. Once you practice cutting the top of the dome. you will be able to identify better what stage you are moving past too fast when cutting low dome cabs. By drying the stone in-between stages and properly inspecting it before moving to the next stage.
I see this a lot online when people are selling finished cabochons. Where there are still viable scratches right on top of the dome but the sides are perfect.
The second part comes to mind is flat spots all over the shape.
This often happens when cutting soft stones and requires more movement and not slowing down the movement of shaping. This slowing down or stopping causes flat cutting like faceting a stone. This also happens really only on the hard wheels. And why rubberized drums or sanding wheels are used in latter grit stages. However it can happen on all stages if the stone is soft. To combat this In my experience when cutting soft stones dopping has an important roll in it. As it allows your wrist to move more freely without trying to hold the stones with your hands. Thus not slowing down or stopping the cut. When you use your hands and let's say it a small soft stone like Turquoise. Getting the right held position for your hand will dictate how it is being cut to the wheel. Stopping and repositioning your hold will cause these flat spots to appear.
In the end it really is practice on what works for you as others point out.
For me I mostly cut by hand, but I tumble finish and recut girdles. Low or high dome cuts are never a problem. But I dopp soft stones mostly as well as small ones and don't tumble them. But if it is a small agate I still cut it by hand. On the latter sanding stages for certain soft stones I do use mostly concentric circles too and move fast all over the belt.