For Jonathan

rockjunquie

Fabulous examples. Thank you so much!

I was intuitively moving in this direction because I realized that doming cabs with flat sides, points, or geometric shapes required a different approach. You have confirmed that explicitly, and these photos will really help. The outdoor photos are especially good because the reflections show dome angles better than light box photos.

I am good at polishing cabs (tumbling experience helps a lot). Being able to make consistently good domes and even girdles is the missing piece.

That is some really nice bloodstone, by the way. Wow. And the indo pet wood is no slouch either.

rockjunquie - Thank you So much for posting those photos! I searched the forum endlessly when I started making cabs, looking for photos of cabs from the side angle...because I had the same questions about the dome that holajonathan has (had). And that Dino Gem Bone is one of my favorite pieces of ALL time!

stardiamond - Thanks for posting what you did...it is along the same lines as to what I'm about to say.

holajonathan - I watched your other thread with extreme curiosity as to what people were going to chime in about the subject. You know I haven't been doing this very long, which is why I hung back. However, this is what I've come to believe on the subject. It matters what kind of dome YOU want on the cab. It doesn't matter if it's a high dome, low dome, dome located in the geometric center of the odd shape, dome at the top, dome at the bottom, even dome along the axis, or sloping dome along the axis. What matters is there needs to be SOME kind of dome in order to create a curved surface to eliminate the scratches. Like Tela's examples, the points on the Bloodstone (almost faceted) are worked completely differently than those on the Dino Bone (more rounded and sloping leading into the corner). The Dino Bone and the Chrysocolla are the best examples of how I try to work these shapes.

I agree with stardiamond and jasoninsd that design features are the artist's prerogative, period.  My question, however, was a little different. 

There are two things that seem fundamental to a good cab:

(1) Flat spots on the top are generally undesirable, if for no other reason than it's hard to remove scratches and polish a flat top.  Most would also agree that cabs with flat spots do not reflect light in as pleasing of a way as a domed top, but I suppose that could be the artist's prerogative as well.  

(2) The girdle should be a consistent height.  This is both a practical requirement (related to setting or wrapping a cab), and most would also agree that an even girdle looks better.  

So while there is not a single "right way" to shape a dome, the dome shape should avoid flat spots and allow for a consistent girdle height in almost all cases. 

With an oval cab, a lens shaped dome satisfies those two requirements.  The dome height can vary (low dome vs high dome) and the exact profile or curvature of the lens could vary also for artistic reasons.  But generally speaking, oval cabs will all have domes that looks similar.  

My original question concerns cabs with straight lines, inside curves, long points, and geometric shapes, and how to avoiding flat spot while maintaining a consistent girdle height.   If I try to dome such cabs like an oval cab, I bump into limitations imposed by rules of geometry.  The shape of the cab's perimeter, combined with the necessity of maintaining a consistent girdle height and avoiding flat spots, does not allow for a perfect lens shaped dome on anything other than a round and (I believe) an oval cab.  This is not merely a question of artistic prerogative, but rather, a geometric limitation.  

(As a quick aside, a perfect lens shaped dome can be desirable for aesthetic reasons.   Light reflects evenly from the surface of a lens regardless of the angle.   In practical terms, if you tilt a cab with a perfect lens shaped dome underneath a light bulb, the size and shape of the light bulb's reflection will not change regardless of the angle.)  

As shown in her photos, rockjunquie addresses this issue by leaving rounded ridges (some more rounded than others) in the face of her angular shaped cabs.  This is one way to maintain a consistent girdle height while avoiding flat spots.  As stardiamond points out, the those ridges can be flattened out to a large extent (at least with the bulging triangle shape) by spinning the cab against the face of the wheel and while still maintaining an even girdle.  (The bulging triangle shape used by stardiamond  allows for a closer approximation of a lens shaped dome than a triangle with straight sides.)

jasoninsd observes that Tela rounded off the ridges in her dino bone cab to a great extent, whereas her burro creek cab goes to the opposite extreme, using prominent ridges as a design feature.  Both the dino bone cab and the burro creek cabs avoid flat spots and have a consistent girdle, although the domes are very different.  

But none of the photos posted by rockjunquie  shows a lens shaped dome.  They can't.  It is geometric impossibility.  If someone thinks they can dome a triangle cab with a perfect lens shaped dome, while avoiding flat spots and maintaining a consistent girdle, I would love to see it!  You can flatten and hide the ridges to a large extent with the bulging triangle pattern, but a perfect lens shaped dome is not impossible.    

Yes, you get to choose the dome you want on a cab, but if you are to avoid a flat spot on the top, and to have a consistent girdle height, your choice is not unrestrained and your options are not unlimited. 

stardiamond illustrates one end of the spectrum, where you round off the ridges to the greatest extent possible while maintaining an even girdle.  That approach creates an almost lens shaped dome, although I don't think it is possible for it to be truly lens shaped.

rockjunquie illustrated the opposite end of the spectrum with her burro creek cab (angular dome with little rounding), something of a mid-point with her bloodstone cab (clearly angular dome with some rounding), and her dino bone cab approaches the dome shaped used by stardiamond (heavily rounded dome angles approaching a lens shape dome).

So you can choose what you want, but those are your basic options.  Nailing down the mechanics and techniques to actually achieve each of those options is where my original question was pointing.  rockjunquie and stardiamond have provided valuable information on mechanics and technique.  Now I've just got to practice.  

IMO dome should be centered “center mass” not necessarily mathematically centered with a caliper.
Again just an opinion

Think Yul Bryner or Kojak, not Conan or Kaepernick.

I guess I misunderstood your original question...which basically made my comments more of a "Captain Obvious" type of statements more than anything else. And again, maybe I'm too new to this to try and give any worthwhile insight...but that isn't going to stop me from trying! LOL


Your originally stated issue...

"I have no problem putting a decent dome on a tear drop or oval, but when I make cabs with narrow points or with lots of flat sides, I get confused about how to shape the dome. I often end up grinding the dome too low (girdle too thin) as I approach the points, and leaving it too high along flat sides."


I guess for me, I go for the geometric center of the cab, regardless of shape. I don't grind the girdle too thin or too high. The very first grind I make on any cab (regardless of shape) is a 45(ish) grind that follows the girdle line all the way around the perimeter of the cab. I also draw the girdle line around every cab I work. I don't work from the dome down toward the girdle line (until the entire shaping is done - then I will to "smooth" out the grinding lines). Once that initial grind is done, the girdle line is set. The next grind is at an angle that removes half of the material from that initial grind to the geometric center (the "top" of the dome). The third grind "rounds" out that last bit of material that's left.

When it comes to shapes with a narrow point, the first and second grind is "elongated" in that section...it's more of a 20-30 degree in the narrow point section versus the 45 degree - depending on how pointed that section gets. To avoid the "ridge" that would be created following two straight edges on that initial grind, the movement of the cab is curved or rounded as I go around that corner. I think Tela tried to describe this move...if I was reading what she was writing correctly.

If you want an easy way to inspect your dome “topography” as you grind away, park a four-foot two-lamp strip above your cabbing bench and look at the refection of the lamps on the face of the cab as you look it over.



H/T . . . Bobby1

parfive

I already took that advice from you Mr. Parfive (from an old thread), and I've got (3) 4-foot LED tube lights above the cabochon creation counsel in my barn. One would do, but I like the extra light. Works as advertised!