I don't think there has been much discussion about preforming before, and was wondering what different people do. How much preforming do you do? I have seen some people preform to the point the stone is almost finished cut, others just cut a flat surface dop and start cutting? It would be interesting to see how much variation in approach there is.
I started working a stone yesterday, and will post some pictures of the rough, after its on the dop, and the finished cut.
none...just start with the limiting facet area and cut till you have enough to get rid of the shallow spot or flaws and go from there.If you use wax you can adjust the stone for a better yield too..I hate grinding my finger tips and chasing stones across the room.
Here is the above stone finished. Finished up at 5.945 ct. so 29.01% yield. That's a bit low for me, my average is 33.4% but this stone was deep and I knew a lot was going to get cut away.
Joined: Fri Jun 25, 2010 10:06 pm Posts: 2267 Location: Chapel Hill, NC / Toronto, ON
Gene, I think I do something similar to you. I'll knock off any rough bits or inclusions that I want out, and I'll grind a flat (or prepolished viewing window), and maybe clean up the rough to have more of the approximate shape, but then I'll just cut. None of this "preform the rough to have a pavilion and crown already outlined" - I think that ends up with a higher loss than otherwise.
Joined: Tue Apr 05, 2016 7:58 pm Posts: 1424 Location: San Marcos, CA
Gene, Not much more can be said but flat out wonderful. Wish you would start a thread and share your photography skillset with us.
I typically get the same range of yields on the average. It is hard to get a higher yield when cutting brilliant style gem designs.
I tend to preform more than needed I think. I believe it allows me more time to judge the stone off the dop. Typically I find the area for the table and let the ruff dictate the shape, remove bad areas with preforming girdle and pavilion areas to a rough shape. I cut mostly for commercial work with a Jang, and specialty work with a Poly-Metric Scintillator.
Although I had training in preforming with an elderly German miner, who mined in Tanzania during the 70's before government takeover. He preformed all his rough before sending for cutting to control loss. He did not facet but boy can he preform smaller stone ruff. Mostly grossular garnets, tanzanite, some spinel and sapphire from other surrounding areas.
Joined: Wed Aug 12, 2015 7:11 am Posts: 93 Location: Gold Coast Australia
I pretty much do the same as yourself Gene. Sometimes I polish a quick window just to make sure the internals are ok. Fantastic gem you cut there also!
Joined: Sat Dec 12, 2015 2:44 am Posts: 35 Location: Sedona, Arizona
It varies for me depending on the type of stone and size. If it's a softer stone or smaller I'll grind a flat surfaces for the dop and jump right in. But if it's a larger stone like 20cts (finished) or more and a harder stone I will definitely take the time to perform a little more as it seems to make the whole process faster.
And Alex was right, now I have to go and collect my socks from across the room! Nice work!
Precision Gem, WOW, that stone will knock your socks off. Can you share the design? I would love to try that one. Absolutely stunning!
The design is really a Jeff Grahm called Nigerian Tourmaline. It's a pretty basic square cushion, but I used concave facets on the pavilion. I find this design cut with traditional flat facets to have a lot of extinction in tourmaline.
Joined: Mon Aug 20, 2012 11:32 pm Posts: 1747 Location: Florida, United States
Precision Gem wrote:
Alex Edwards wrote:
Precision Gem, WOW, that stone will knock your socks off. Can you share the design? I would love to try that one. Absolutely stunning!
The design is really a Jeff Grahm called Nigerian Tourmaline. It's a pretty basic square cushion, but I used concave facets on the pavilion. I find this design cut with traditional flat facets to have a lot of extinction in tourmaline.
Joined: Fri Mar 18, 2016 12:45 am Posts: 178 Location: Vancouver, B.C. Canada
Precision Gem wrote:
Alex Edwards wrote:
Precision Gem, WOW, that stone will knock your socks off. Can you share the design? I would love to try that one. Absolutely stunning!
The design is really a Jeff Grahm called Nigerian Tourmaline. It's a pretty basic square cushion, but I used concave facets on the pavilion. I find this design cut with traditional flat facets to have a lot of extinction in tourmaline.
Very nice!
As for preforming I try to just put the rough straight on the dop and have at it. I really like using wax because I am often off center by a bit and can nudge it to where I want it.
I use wax too, for the same reasons mentioned, and it's always simple and quick to remove the stone from the dop. For preforming, I'll use depending on the size of the rough, anywhere from a 100 ripple lap to a 360 lap. Once on the dop, I'll spin or very roughly cut a tier or two with the 360 before doing all the finial facet placements with a 1200 sintered lap. I don't use 3000 diamond, but prepolish with 8000. A heavy charge of 8000 will certainly move facets, yet gives a bit better finish than 3000. Finial polish is with either oxides or diamond.
I think 600 to preform is way too slow, and using 3000 for facet cutting would take way too long unless you preformed to the extent where the stone was pretty much final cut. I know many cutters overseas do a preform that look almost like a finished stone.
Joined: Mon Aug 20, 2012 11:32 pm Posts: 1747 Location: Florida, United States
Precision Gem wrote:
I think 600 to preform is way too slow, and using 3000 for facet cutting would take way too long unless you preformed to the extent where the stone was pretty much final cut. I know many cutters overseas do a preform that look almost like a finished stone.
The cutting speed would depend on the concentration of diamond in a sintered lap. Low concentration = low stock removal I use a hard metal bonded 34 micron (close to 600 mesh) sintered diamond lap with 100% concentration. Preforming goes very quickly, even on corundum.
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