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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 1:00 pm 
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Ever try to use a dull machete? It doesn't cut, it crushes.


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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 1:05 pm 
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Arya,

I see what you are saying, but do not agree. You compare a new sintered lap with a worn plated lap. Your assumption is that the plated looses (lots of)diamonds, Which I do not agree with.

Also, a new lap probably has diamonds sticking up higher than the majority, hence deeper scratches. So there are some diamonds that do what your premise says. This is the only way that I can see your premise working, but unfortunately that would only work with a NEW lap.

From what I read and hear here, most new laps do have some unevenness of diamonds. Jon has mentioned several times that he does dress the sintered laps before he sends them out. And when he did not do this with, IIRC, the 8 inch laps, there were problems and he said he would go back to dressing.

A premise could be considered that the sintered will cause MORE subsurface problems as those few higher diamonds experience the effect you mention re the worn lap.

Also, am I correct in thinking that the sintered lap has bronze holding onto the diamonds and the plated has a harder metal, nickel. Would not the softer metal release more diamonds as Al Balmer worries about? And loose diamonds do generally cause deeper scratches, do they not.

As I look at dressing my sintered lap, the thought comes that when I dress it I am removing some bronze plus some diamonds. Would not some diamonds end up with LESS metal holding them in and/or a higher profile, leading again to your premise that a few high diamonds leads to sub surface fractures?

Lastly, your premise assumes that a significant percentage of the diamonds on the worn lap are missing. What amount needs to be missing to substantially increase the energy used by the remaining diamonds? 20%? 30%?


Hence, and from my own experience, I agree with Precision Gem that while Worn Lap 1 is possible, Worn Lap 2 is much much more likely. I have cut strontium titanite, calcite, cerussite and other fragile stones and found that a worn, smooth lap is necessary and preferable.

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 2:31 pm 
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Arya, you theories go completely against my experience. So from you theory, a manually charged lap would make deeper scratches when pre polishing with say 3000 or 8000 diamond AFTER you have polished a few stones with it. I find that to just opposite, and that an older charge of 8000 is for all practical purposes a good commercial final polish. This is like saying the worn sand paper will leave bigger scratches on wood then new sandpaper. This just isn't true.

My 12 year old 360 lap left a better finish than the new 1200 Sintered from Jon did. I have often gone from the 360 to 3000 pre polish. Believers in subsurface damage would never dream of even preforming with a 360!

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 5:18 pm 
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If the diamonds are worn with planar tops the subsurface damage is not the consequence. The diamonds are wearing the material away. In cutting (Metal, etc) we are pushing a wedge through a material. In a brittle material we are producing a trail of fractures. The real peril in a dull tool is excessive Force needed to accomplish material removal. My secret to cutting spodumene and enjoying it is to start with a new plated lap just for that job The force per unit area is high, but the unit area from the sharp points is miniscule. The damage comes from trying to push a dull tool through the work , and this is true of any activity and material. The thing spodumene cannot survive is heat and pressure when cutting it. Here is a case where the excessive force needed for material removal outweighs the sharp point damage. A complication is that often "Well worn" laps, especially 1200 plateds, start throwing loose diamonds, and begin rumbling betwenn the stone and the lap creating the free body abrasion producing a distinctive trail of impact fractures.

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 5:37 pm 
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My experience so far has been that a well-worn lap is slower but generally produces a smoother surface. And while I don't really like using it all that much (contamination, black gunk etc), I find that the difference between a loose #3000 grit and a fixed grit plate of the same mesh size to be dramatic in the condition of the pre-polished surface. The fixed is always frosted while the loose is often so bright that it's not far short of polished.

That said, I find a #3000 plated lap does a good job for most things and they usually polish nicely and rapidly from there. Except sapphires which tend to orange peel a fair bit from the finer fixed grits.

But I'm only still a rookie :)


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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 6:51 pm 
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Hmmm...Bob and Gene, you've given me a lot to think about. I like your thoughts from theory and what you're saying has a lot of merit.

Time for empiric testing :)


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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 7:05 pm 
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Jon,

From your post and Precision Gems diagrams, isn't there an assumption that the diamonds are oriented point end up? Is this what actually happens; some procedure lines all the diamonds point up?

I would have thought random orientation, which would explain , perhaps, why friable diamond bort is preferred by some?

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 7:39 pm 
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Jon, if I'm understanding this right...does that mean that the main problem with dull laps isn't necessarily diamond distribution, but instead the extra force that people may or may not apply on the stone to try and get the stone to cut faster?


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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 8:27 pm 
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It's generally true of any tool used for any process. Machinists know that dull tools consume more horsepower and generate much more heat. Woodworkers know that dull tools are dangerous because of the greater pressure required to cut..so people press hard and when the tool slips, injuries are more serious. They know a dull power saw blade burns the wood.
I have mentioned in the past other scenarios:
1) The dentist informs you that a "well-worn" drill will be used. You bolt from the chair and run away.
2) The surgeon informs you that because of cost controls, sterilized previously used scalpel blades will be used in your upcoming procedure. You run away.
...etc.
These are universal rules in craftsmanship and industrial practice. It is Good Manufacturing practice.
The Optics Industry, if it wants a fine finish, uses appropriate fine grit sizes. They never use grazing dull composites because they must KNOW how they obtained the predictable finish. Even an amateur telescope maker knows that.
Only in this field do people suggest dull tools. I suppose they get good results, somehow. We are producing one-of-a-kind items, not manufacturing, so that's probably the reason.

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 8:30 pm 
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The diamonds are randomly oriented to an extent, but using "Blocky" diamond it is not so chaotic. The friable diamond is more effective because, like flint and garnet (The latter being popular for waterjet cutting) they fracture to sharp aggressive edges as they crush down.

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 8:33 pm 
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I find that the difference between a loose #3000 grit and a fixed grit plate of the same mesh size to be dramatic in the condition of the pre-polished surface. The fixed is always frosted while the loose is often so bright that it's not far short of polished.

That is at least partly due to the fact that loose grits embed into the lap surface, so that a 3K lap may give the same finish as an 8K plated.


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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 8:49 pm 
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That is generally true of composites as well, because the depth of cut is limited. At no point is the full dimension of the diamond crystal exposed. If it were, the diamond would have fallen out of the bond. The only drawback to relying on this is that each particle will have a different depth of cut because the bonding depths and geometries and orientation are not identical. So a buried 3K network can give you an AVERAGE scratch size of 3µ, but does not guarantee the absence of 6µ scratches.

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Fri May 27, 2016 9:06 pm 
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The reason I prefer using a sintered or steel lap for cutting over charging a lap is that it cuts consistently from day to day. Sure over the years it slowly slows down, but day to day it's the same.
When I charge my pre polish lap, a fresh heavy charge of 8000 will cut facets like crazy, by the end of the stone, it is no longer cutting like crazy but just a bit. Tomorrow, it is not cutting only polishing. I mix and switch from 3000 to 8000 depending on the size of the stone and what type of stone it is, and how I feel that day.
I would never keep a worn out lap, just to save money. I sell stones, so this makes no sense for me. Often my stones are sold for several thousand dollars, and the rough cost thousands. So the price of the lap is kind of dwarfed in the big picture.
What is important to me is time, and therefore I use more aggressive fast cutting laps.

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Sat May 28, 2016 10:14 am 
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Folks,

Love this discussion !!!

There are two other considerations here as well.

For people who just like to facet, a worn lap gives them one big advantage -- it reduces the chances of over cutting. I tell my students that the 'trick' in faceting is to know when to "stop cutting". Many many times they will cut and cut and cut and the newer the lap, there is a much greater likelihood of over cutting. So we use worn 1200's after newer 600's because it cuts slower and gives the student more opportunity to 'stop' in time. Related to this is reminding them that there is a speed control on the Facetron's we use and it's there for a reason. There are times when you want slow and smooth cutting.

Second, Speed is also a factor. Run the lap faster and more energy is delivered and, to a point, faster cutting and greater possibility of sub surface damage. So pressure is not the only possible cause.

Related to 'stop cutting', think how many times you 'should' turn the machine off and rub in a facet, especially with the first tier of barion facets...

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 Post subject: Re: Faceting Strontium Titanate
PostPosted: Sat May 28, 2016 10:45 am 
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That's interesting that you bring in speed Bob. I didn't really think about it, but maybe that is an issue. I really never cut with my machines at full speed, normally I'm around 60%. I don't think it cuts any slower, and I guess subcounsouly I felt like it cuts faster a bit slower. At high speed it feels like the stone floats on top of the lap.
Before I bought a sintered 1200, I would cut most stones with a 600 Nu-Bond. Left a very nice finish that would not require pre polish. I would go right to a 60,000 or 100,000 diamond polishing. The problem with the Nu-Bonds is they would wear kind of quick, and for a period of time, the quality went down and the new laps were not very good, hence the switch to a 1200 sintered.

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