Sharpening on Granite/Glass Plate FWW#165

The current FWW issue has an article about sharpening and lapping on a granite plate using silicon carbide powder. My question is if doing this method of sharpening on granite (or plate glass), does the abrasive significantly change the flatness of the substrate (granite or glass)? Like a sharpening stone, do you lose the flatness...the most important feature? If not, why?
- Al
ps: sorry for the re-post. I actually replied to another thread by accident.
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: The current FWW issue has an article about sharpening and lapping on a : granite plate using silicon carbide powder. My question is if doing this : method of sharpening on granite (or plate glass), does the abrasive : significantly change the flatness of the substrate (granite or glass)? Like : a sharpening stone, do you lose the flatness...the most important feature?
I use chromium polishing paste on glass - please see my web site - Sharpening Notes - Some Scientific Light On Sharpening Technique' .
The surface of the glass is somewhat degraded of course, but I haven't noticed significant wear. However carbide powder might be much coarser, hence some wear is inevitable.
When using fairly coarse carbide grains on glass for flattening plane soles, I've found that the scratches act rather like glass cutter scratches and make the glass rather more fragile than usual.
: If not, why?
Until the surface of the glass is fragmented by numerous scratches, the surface will not be attacked in the same way as an edge does to a stone. Ie the steel is not trying to tear the grains from the supporting matrix.
Well, that's a theory anyway!
Jeff G
-- Jeff Gorman, West Yorkshire, UK Email address is username@ISP username is amgron ISP is clara.co.uk Website www.username.clara.net
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Jeff, Thanks for your response.
Same to all the other posters to this thread.
Regards,
- Al

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I can say for sure that lapping a plane sole with silcon carbide on glass will cause the glass to become glazed and dished, and will put a measurable concavity on the plane sole. DAMHIKT.
Glass is generally said to have a Mohs hardness of about 6. (Mohs hardness is a measure of resistance to scratching. Harder minerals will scratch softer minerals). The most abundant minerals in granite are feldspar (Mohs hardness 6) and quartz (Mohs hardness 7). The slab shown on the FWW article appears to me to be "black granite" (gabbro), which generally doesn't have much quartz. So the granite or black granite plates are about the same hardness as glass or a little harder. IIRC, steel has a hardness of about 5, but I'm sure the tempered alloys used for plane irons and chisels are much harder. I haven't been able to find a hardness for SiC either, but the fact that its used as an abrasive means its very hard. Granite is usually polished with a grit consistng of tin oxide, which I bet is not nearly as hard as SiC.
The bottom line: the SiC abrasives and tools are probably harder than the plate; if so, they will scour and erode the plate with use. You could test this by using your plane irons and chisels to try scratch the granite plate, and vica versa.
-JBB

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This is a bit confusing to me. Diamond lapping compound is in common usage, and, as you know, it's as hard as there is. I thought the theory of lapping was to move the lappee with as little pressure as possible over the compound, not to abrade as if it were sandpaper. If you're cupping your piece, you're doing it wrong. In industry the lapping I've seen involves a moving substrate and stationary piece.
http://www.surfacefinishes.com/lapping.html
The use of inexpensive "float" glass as a substrate would provide both initial flatness and disposability.

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What actually happened there is that the lapPING plate is supposed to be softer than the lapPED plate. This applies to both flat and diameter lapping. Typically, I use brass rounds with diamond powder to lap out carbide bushings.

Again, what happens here is the abrasive embeds itself into the softer material...in this case, the plane sole...an abrades the harder material. I know that this may seem counter-intuitive, but if you think it thru and use your minds eye to see what is happening while you are lapping something, you'll see what I'm saying.
Long story short on this one...if you want to lap without using SiC paper, instead using powder, you'll need a cast iron lapping plate, which is more brittle than steel but also softer. It is porous, so it allows the abrasive to embed and cut the harder steel. The greater the difference in hardness between the two items the better...but NEVER use aluminum as a lapping plate. DAMHIK
Any questions, please, feel free to ask.
Mike
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...if you want to lap without using SiC paper, : instead using powder, you'll need a cast iron lapping plate, which is more : brittle than steel but also softer.
An old cast-iron plane should be able to act in this way!
Jeff G
-- Jeff Gorman, West Yorkshire, UK Email address is username@ISP username is amgron ISP is clara.co.uk Website www.username.clara.net
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A quick geology lesson:
If you need a surface that has a consistent hardness over the area then granite or any natural stone will not work. As mentioned in the previous article that talked about the Moh's hardness of various minerals, granite contains much more than quartz and feldspars. Granite is simply a common name that does not encompass the different varieties, of which "gabbro" is not included. "Black granite" simply has a higher percentage of plagioclase, pyroxenes and amphiboles.
The problem with granite in this context is that they typically contain very soft minerals called micas. Which therefore helps in the pitted appearance of many granites after it has been polished. So it is very likely that the powder may wear down the softer minerals in an uneven fashion. I am not sure that this would affect the techniques that you describe, though.
Hope this helps

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To go on and on:
"Granite" and "granitic" are often used, especially in laymen's parlance, to include the whole spectrum of plutonic igneous rocks, in which granite (senso stricto) and gabbro are included. A coarsly-crystalline rock consisting mostly (or entirely) of plagioclase feldspar (with varying amounts of the other minerals mentioned), is properly termed "gabbro" and often referred to as "black granite", and is commonly used for monuments, floors and walls of buildings, even the sills of the drive-through window at "In-and-Out" burger restaraunts.
-JBB

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