Full Chisel Blog

July 22, 2009

Variable Grit Abrasive

I really can’t call this a variable grit sandpaper because it is not sand and it is not paper, but it is similar.  It can be used as a burnisher in its polished condition and it can be altered into other grits by abrading it with equivalent abrasives.

I selected a piece of quartersawn white oak (Quercus alba) and cut it to a size that was comfortable in my hand,  2 1/8″ by 4 1/2″ by 1″ thick.  The wood will be stable even if I use it wet or dry. 

variable grit1

I cut the piece of sting ray skin (shagreen) to the size of the oak block with a wax pencils width larger than the block.  I then used large shears to cut out the material.  This stuff is tough, the little tubercules of mineral deposits are very hard.

Variable grit2

I allowed the glue to dry, I used liquid hide glue and just kept working it flat and smooth over about a 30 minute period every several minutes to make sure it made good adhesion.  I put a bit of glycerin in the hide glue to keep it flexible as there may be some small movement.  I will also treat the edges with a mixture of distilled water and alum to make the edges waterproof.  For a complete discussion of making Hide Glue waterproof see Hide Glue – Historical & Practical Applications.

After about 20 minutes work with sharp shears and mostly a coarse file I was able to shape the abrasive block (sanding block) so that the sting ray skin was even with the edges of the block and smoothed to a burnish finish.

Variable grit3

I can use sandpaper to make this abrasive block any grit I want and the grit will correspond roughly to the grit of paper I use.  Because these mineral deposits are so hard they will hold their edge for quite some time.  This is an old material and technique that I find fascinating.

Stephen

December 11, 2009

Woodworking Magazine

Filed under: Of Interest,Uncategorized — Stephen Shepherd @ 9:59 am

The current issue of Woodworking Magazine, winter 2009 is out, I now have a subscription and overall it is a good magazine and I actually read the entire thing, unusual for me as many magazines talk of stuff of which I have no interest.

On page 4 in the Shortcut section is my little contribution on Variable Grit Abrasive.

Happy reading.

Stephen

May 12, 2008

Grinding tools

Filed under: Sharpening,Uncategorized — Stephen Shepherd @ 9:09 pm

Fortunately grinding is not done that often so it is not that much of a grind.  When one grinds tools, one takes those fine expensive laminated steel blades, be it planes or chisels and puts them to a spinning abrasive wheel to remove the metal that is preventing the tool from being sharp.  All the time trying to prevent the tool from overheating, spoiling the temper of the tool.

I have a 30 inch diameter Indiana limestone grindstone, hand cranked.  And I have found it impossible to turn that wheel and grind a tool and heat it up enough to prevent me from having my fingers near the end.  The tools do warm up, but they don’t get hot and certainly don’t get hot enough to change color.

It also helps that I keep my fingers near the grinding so I am forewarned if any undo heat is being developed.  Do I have problems turning a crank with one hand and holding the tool freehand in the other against the face of the grindstone, well I have learned how and it isn’t much of a problem.

Another advantage of slow speed grinding is that things happen a little slower than with high speed grinders, however one must pay close attention, but at least with my grindstone, it doesn’t take much time at all to get a tool ready for flat stones.

Now unlike flat stone work, I do on occasion use a water bath to lubricate the stone.  I have a dripping water can, it is a mess but keeps the stones clean and of course the tool cool.  The water isn’t necessary and I don’t use it all the time unless it is a great deal of grinding that needs to be done.

I had a 15 inch diameter modern stone, that I mounted on a hand crank and it worked well, but sold it when I was offered more money than it was worth ( I like that when it happens).  I never used a lubricant on that stone and it worked fine, never glazed and never burnt a single tool.

Stones can glaze and need to be dressed, the easiest way to dress a limestone grind stone is to saturate it with water then use a piece of hard steel as a scraper and smooth off the surface.  A flat piece of sandstone or other abrasive stone can also be used to dress an old stone.

Limestone grindstones should not soak in water, as a cooling trough, as it will soften the stone producing an eccentric grind stone.  The sides of the stone if somewhat dressed can also be used for grinding and the variable speed of hand cranked makes that process easy.

Hand cranked small geared grindstones are also very helpful as they can be cranked slowly and are much safer than those powered by water wheels or steam engines.  It is difficult, but I am sure not impossible to injure yourself using a hand cranked grinder, but somehow I think the injuries will not be as severe.

I do not know what grit my grindstone would be.  And I don’t think I need to know what it is, it just works, then I go to my coarse flat stone, then the medium then the fine.  Do I know what the exact grit is?  No.  Do I care, well maybe, well not really, it doesn’t matter.  The grind stone is rough, the coarse flat stone is not as rough as the grind stone.  The medium stone is not as rough as the coarse stone and not as fine as the fine or hard stone.  And the fine stone is finer that all of the others.  Then there is the tripoli impregnated leather strop for final finish, which I am sure is finer than the stones.

It is like the grit of sandpaper, numbers like 220 and 600 are applied today to indicate grit, in the nineteenth century (yes, we have sandpaper, although it is expensive and has to be imported from the States), 0, 1, 1 1/2, 2, etc indicate what?

Grind slow then move from coarse to fine on the flat stones and heating up during sharpening should never be a problem.  Adding high speed to grinding really didn’t improve anything, but it did create a lot of tool burning and ruined temper and ruined tempers.  Sometimes faster isn’t better, I think I may have said that before.

Stephen

May 6, 2008

Shagreen. ends my chagrin

Filed under: Of Interest,Uncategorized — Stephen Shepherd @ 8:10 pm

Well at long last my chagrin has ended with the purchase of a fine piece of shagreen.  For those of you unfarmiliar with shagreen, it is the skin of a sting ray.  Also some shark skins are called shagreen, but it usually refers to ray skins.  Now shark skin, especially dogfish skin makes excellent sandpaper, but it only sands in one direction.  Shagreen is different.

Shagreen, sting ray skin

Traditionally sting ray skins were dyed green, hence the name, and was the traditional handle covering of swords in both the orient and in the West.  What is unusual about this skin are all of those little dots, they are mineral tubercules that are very hard.  When the skin is prepared the sharp edges are scraped, sanded and ground down to make them smooth.

That is why they look like shiney spots, they are an off white color and the leather is dyed black and the tubercules stay white.  And they are also very hard.  The reason they are used for sword handle grips is that you can hold on to the sword even if it is covered in blood or sweat or both.

Shagreen is also used to cover eating utensil handles (George Washington had a set), easy to hold in greasy fingers.  Medical instruments would have handles covered with shagreen to help with the grip when things get bloody.

Aside from its colorful uses, shagreen is an excellent traditional replacement for sandpaper.  (Sandpaper is early but quite expensive and not as durable.)  I will be saving the center section where the big tubercules are and some other areas of different grit.  I wonder how to classify the grit, spine, snoot, tail, edges.  When the tubercules end the top of the ray ends and the underside is smooth and not made into leather.

The spacing of the tubercules (I like that word for some reason) varies over the surface of the skin, but the grit is determined by the user.  Shagreen is the only variable grit sandpaper, now there is a novel thought.  But the leather comes prepared and the mineral nodes are smooth.  In order to make them more abrasive, I use a file to roughen up the surface, the finer the file the finer the grit.  It is basically scratching the tubercules to get them to have sharp cutting edges which last until they mechanically wear off.

Left smooth the shagreen can be used to burnish woodwork, including turnings, similar to ‘boning’, the process of burnishing by using a piece of smooth animal bone or hardwood stick to polish the work.  Shagreen is a useful material and I will make a tool handle or two, I need a piece for a friction match case to ignite the ‘lucifers’, and a goodly supply of a fine flexible abrasive.

Still searching for dogfish skin, I have had a bit of sharkskin and I did like the way it worked and it lasted for quite a while.  I will be putting this stuff to the test and will talk of it later.

Stephen

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