Spinning Wheel 40 by Bob Keller
Winner 2008 OPLC Faceters Hobnob Most Beautiful Stone Competition
Synthetic Division

Spinning Wheel 40
Designed by Bob Keller 12.06.07
An exhibition design for a large glass stone.
Angles for R.I. = 1.45080 + 40 girdles = 120 facets
40-fold radial symmetry80 index
L/W = 1.000 P/W = 0.466 C/W = 0.113Vol./W^3 = 0.168
GemRay Brightness at 0 degrees tilt for RI = 1.45 - COS Brightness: 94.0% ISO Brightness: 94.4%

Meet at culet
Match P
Meet at apex

Download PDF format printer friendly cutting instructions: [ Download Icon Spinning Wheel 40 Printer Icon ]

Download the GemCad format design file: [ Download Icon spinning_wheel_40.gem ]

Molded Glass SRB During the 2007 Tucson Gem and Mineral Show I acquired several 50mm molded glass SRBs with an eye towards recutting them into large gemstones for exhibition and general showing off, one of which is shown at right. These molded glass stones were made in China with a rather dumbfounding lack of attention to the quality of the masters from which the molds were made. The proportions are horrid and the "meets" are appalling. The tables are humongous and the pavilion mains are below the critical angle for the material, so you can look right through them. However, the glass was available in some attractive colors and was surprisingly clear of bubbles, so they put a smile on my face as inexpensive, preformed rough.

A little over a month before the 2008 Tucson Show I realized the clock was ticking and time was becoming scarce to cut a stone to enter the Most Beautiful Stone Competition, which is held in conjunction with the annual Old Pueblo Lapidary Club Faceters Hobnob during the Show. I had not previously entered this competition and aspired to cut a stone to enter in the synthetic division. I figured a large enough glass might have sufficient bling to compete successfully against the CZs which have historically dominated this event. Dick Fairless won it in 2007 with a 15mm CZ short Portuguese, and Art Kavan and Dieter Irmischer have racked up a number of wins in preceding years with some impressive CZs.

So out came the glass SRBs. My first inklings regarding a suitable design ran along the lines of an extended Portuguese or an elaborated multi-facet mixed cut. However these designs are deep proportioned and my SRB preform was shallow. After a little rough gauging I decided cutting a deep stone would necessitate more reduction in diameter than I wanted to give up. When it comes to gathering photons there is no substitute for aperture, as true for gemstones as for cameras and telescopes. After some protracted deliberation I settled on a pink one, primarily for the bodaciousness of the color.

GemCad Ray Trace I ultimately elected to go for maximum aperture and let the swarf settle where it may. Shown at left is a GemCad generated ray trace diagram of Spinning Wheel 40. This cut was experimental and is interesting to me from a design point of view. As the ray trace illustrates, what goes straight in, comes straight back out. In terms of GemRay COS (94.0%) and ISO (94.4%) brightness, this is by far the brightest stone I have cut. But in terms of GemRayX, a version of the GemRay raytracer that incorporates "head shadow" effect into its model, this is also the blackest (conventional) stone I have cut, with GemRayX calculating COS (3.3%) and ISO (3.6%) brightness.

The general idea of this design is not new and dates back at least 30 years to the publication of Quincy Howell's "Spinning Wheel" in The Faceter's Gem Cuts in 1977. Quincy Howell's Spinning Wheel employed 16-fold radial symmetry with stacked crown and pavilion facets. Due to the simplicity of the cut I suspect there were probably even earlier "Spinning Wheels".

In this variation for large stones I increased the radial symmetry to 40-fold, hence the name Spinning Wheel 40. Of course the choice of 40-fold symmetry was predicated on the use of an 80 index. I also elected to unstack the crown and pavilion facets. They are staggered by 1/80th of a revolution, so that when viewed directly on axis, the lines between crown facets bisect the faces of the pavilion facets. (The pavilion and girdle facets are cut on the even indexes, the crown facets are cut on the odds.) As the viewing angle changes, this staggered arrangement enhances the characteristic "chasing" or "spinning wheel" effect that is the namesake of the design.

Pavilion in ProgressCrown in Progress

Several views of the stone in progress are shown above. With a single course of facets on both pavilion and crown, it doesn't get much simpler than this in terms of design. However cutting this one wasn't the short walk in the park I had hoped for and it proved to be significantly more challenging than I anticipated. My laps (electroplated solid steel Crystalites) are showing their age and I had some frustrating issues with scratching and chipping at the fine cutting and prepolish stages. I am able to move smaller stones about to work them in remaining "sweet spots" on my laps, but the facets on the pavilion and crown of this stone are considerably larger than those sweet spots. So no matter where I positioned them I had some rather nasty scratching zones cutting across the facet faces in one place or another.

Push really came to shove when I got to prepolish on the pavilion. I have been very attached to the Raytech Industries 1200 NuBond Lap for prepolishing quartz and glass as it is truly a magic bullet on these materials. However my current 1200 NuBond has also seen its share of stones and the remaining sweet spots on it were also too small for this one. None of the scrubbing and sanding restoration techniques I tried on my worn NuBond alleviated the scratching. Time was flying so I decided the most expeditious solution would be to simply purchase a new one. At that point I learned that Raytech's production on the NuBonds had been stalled for months and they were currently unobtanium, with Raytech prognosticating forthcoming production on a near event horizon still several months distant.

I had formerly prepolished quartz and glass using a 3000 Crystalite solid steel. The first 3000 Crystalite I purchased was a wonderful lap that I eventually wore out on large quartz. My second 3000 along with another 1200 acquired during the same time frame were both scratchy dogs that left me without doubt that Crystalite was suffering some serious quality control issues regarding the grading and/or contamination of the borts they were electrobonding to these laps. It was those doggy Crystalites that first motivated me to try the NuBond for prepolishing. Now I couldn't get another NuBond due to Raytech's production malaise and I didn't want to roll the dice again on new Crystalites.

At this juncture I was frustrated to the point of seriously considering abandoning efforts to finish the stone in time for the 2008 Hobnob. However, after enduring my rant several members of the USFG Faceters List suggested an alternative, and I ordered one of the new 2007 formula BATT Laps from Jon Rolfe that I charged with 3000 grit LapidaryPro High Grade Friable Diamond Powder obtained from Rob Kulakofsky. Thanks to Jon's and Rob's efficient service and timely shipping, I was able to get a serviceable prepolish lap up and spinning in time to finish this stone for the 2008 Hobnob.

I'm still on the learning curve with using the BATT Lap to prepolish glass and quartz. After a bit of scratching during charging and break-in, I was able to achieve a serviceable, uniform prepolish on glass with the BATT and 3000 diamond. To date I prefer WD-40 as an extender fluid for this combo over several of the more viscous oils I've also used. Big stones shed significant amounts of swarf even at the 3000 stage and the more viscous extenders begin gumming up quickly. I initially started using WD-40 to help thin swarf loaded oil on the lap but quickly gravitated to using it straight.

I don't much care for the swarfy, oily, yucky black mess associated with this approach and I have yet to achieve as fine a prepolish with the BATT and 3000 bort as I'd been achieving with the 1200 NuBond. However, I probably won't go back to my cherished clean, water lubricated NuBonds for prepolishing even after Raytech gets their manufacturing act back together on them, as I'm now close to pitching my second (or is it third?) 1200 NuBond. At a manufacturer's price of $130 and discount street price around $117 for an 8" 1200 NuBond now (about double what I paid for my first one) I've decided they have become prohibitively expensive relative to their durability.

My grinding and prepolish sequence on large quartz and glass is typically 60, 100, 260, 600, 1200, then 1200 NuBond (or 3000 diamond) followed by cerium oxide for polish. I started with 100 on my Spinning Wheel 40 due to the molded glass SRB providing such a close preform for recutting. Based on my preliminary results using the BATT Lap, I may experiment with extending the sequence to 8000 on a BATT as the final prepolish on fine cut quartz. After 10 years or so of cutting large quartz, rechargeable is now a +++ in my book and I'm finally determined to switch over to bort charged tin and copper laps as I can afford them to replace even my coarsest plated steel cutting laps.

It is not easy to bring 40 facets to a decent meet point at a culet or apex. I've cut other designs with 24 and 32 mains converging at the culet and that previous experience helped on this one. My approach was to first meet four facets on 20, 40, 60 and 80 at the culet point. Then bring four more on 10, 30, 50 and 70 to meet with the preceding four at the culet point. Now eight facets are met at the culet, which is all that are required on an SRB, but on a Spinning Wheel 40 your culet is then only 20% complete... Next on 4, 14, 24, 34, 44, 54, 64, 74, then 6, 16, 26, 36, 46, 56, 66, 76, then 2, 12, 22, 32, 42, 52, 62, 72 and finally 8, 18, 28, 38, 48, 58, 68, and 78. Of course this exact sequence is not gospel, but the general approach and procedure is to first establish the meet and then follow on with symmetrical sets of radially equidistant facets in an alternating sequence that leaves uniform sets of unmet facets remaining between those that are met. If you have not mastered the paradigm and practice of "cut a little, look a lot" before cutting a Spinning Wheel 40, you will at least more fully appreciate it by the time you have finished one.

In addition to the task of meeting 40 facets at the apex, the crown on this stone provided some additional challenge due to the low angle (12.70°). My machine, an extensively modified Graves MK I, has enough vertical play in the head that it vibrates at an apparently resonant frequency when polishing facets at such low angles.

I think most faceters experience some squawking, particularly while polishing with oxides. Now imagine that occasional nails-on-chalkboard squawk ongoing as continuous "singing" and that's what my machine was doing the entire time I was polishing the crown of this stone. This effect was very irritating in more ways than one. Beyond the psychological trauma inflicted on our dogs, the relentless vibration actually loosened fasteners in my machine's quill and head while I was polishing. I first became aware of this problem when I had just finished polishing a crown facet and lifted the stone to disengage the index gear pawl to change the index setting. But before I disengaged the pawl, the stone turned about 10 degrees in the quill as I lifted it. The vibration had loosened the set screw that secures the dop shaft from rotating in the quill. Needless to say the issue with vibration loosening fasteners was messy, time consuming business until I learned to keep checking and re-tightening critical fasteners like the quill set screw that normally don't warrant a second thought.

All that squawking and vibrating going on while attempting to polish in 40 crown facets to a respectable apex was just not a good thing. I couldn't figure out a quick fix to arrest the vibration or even dampen it significantly, and with only a couple of days remaining before the 2008 Hobnob, I just pressed on and did my best to deal with it. Faceting is a hobby for me and something I do for recreation and the fun of it. My frustrations faceting this particular stone were greatly compounded by cutting it against a deadline that loomed due to my own procrastination. Had I simply started it a month or two earlier I could have resolved most of the issues at my leisure instead of feeling under pressure and I think there is a lesson learned here.

Spinning Wheel 40Bragging Rights

The Results

My finished Spinning Wheel 40 is about 46mm in diameter and weighs approximately 217 carats. I'm still pondering and deliberating over the perceptual results regarding GemRay verses GemRayX brightness and the ramifications of "head shadow" for lighting models in gemstone raytracers. This design is experimentally extreme in terms of ignoring head shadow and is not at all liked by GemRayX. But that did not prove an overwhelming liability in terms of the votes for Most Beautiful Stone among the faceters and guests attending the 2008 Hobnob.

All said and done, I think this stone demonstrates there is considerably more going on than brightness when it comes to the perception of beauty in gemstones. It is cut from garden variety glass, which is a weak material in terms of refractive index and dispersion compared to other popular synthetics, particularly so the CZs that have tended to dominate the synthetic division of the Hobnob's Most Beautiful Stone competitions. It throws some nice dispersed flashes here and there, but doesn't fountain rainbows like a CZ. It doesn't scintillate in the conventional sense, but it does chase or spin nicely as it is moved about slightly off axis, and I think the chasing effect is the primary attractor in terms of its visual interest. Last but not least, it is a big stone, and when you get right down to it, size really does matter...  ;)

On a closing note, I'll mention I'm planning to have some more fun designing and faceting large recuts from those molded glass SRBs. If you have an interest in these, check out the Cutting Rocks Web Site to see what is available.

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Bob Keller