Recent Comments

Sharpening Tools, Part 3

The Gritty Details

Here is a table of the relative grit sizes of the different standards used.  CAMI is the American Standard, and for a long time is was the main standard.  The European standard, FEPA, is more precise, and so FEPA graded sandpaper tends to work faster than CAMI.  JIS is used for Japanese water stones, and Mesh is a measure based on the screen used to sort grit.  An inch is 25400 microns across so a micron is pretty small.  A pair of lines a half micron apart is right at the limit of what can be seen by an optical microscope.

I personally like to use microns for most calulation and estimates.  Microns are a consistent scale and when things are measured in microns, they are usually much more precise.  I also like to use grit graded with a range.  Without a range of size, the grades listed can be pretty far from precise and have a wide range of grits mixed together.  Using mixed grit sizes slows down your work.  If you want to sharpen quickly mixed grits are usually something to avoid.

Usually when sharpening, you are best off, just touching up the blade with the finest grit surface you have available.  Only when initially sharpening, removing nicks from blades, or correcting dubbed edges, do you need to go through a range of grits.

To grind down a surface using an abrasive, the best, fastest method is to start out with the largest grit that cuts a line that is not as deep as the amount of material you wan to remove.  You then grind down to nearly the depth you want and then switch to a grit size that is half as big as the size you just used.  You keep reducing half the size until you reach your final goal.

The reason why you want to switch grit sizes, is that it can speed up your work considerably.  Here are six blocks in order, each is half the size of the last one.

The big one is roughly 32 times as tall as the smallest one.  If I need to grind with ten passes at a particular grit, to get the surface ground to it’s maximum fineness with that grit, then I would have to do a total of 60 passes to work through the grits and reach the final size.  If however I start with the smallest grit I will have to make 360 passes.  Using multiple grits is faster.   There are more reasons than just speed to use multiple grits.  As an example I will assume we are using sandpaper.  if my abrasive sheets are good for 20 passes, then I can either half use six sheets, or use up eighteen sheets.   Time and economics matter.  If you only learn one thing from these discussions, this is not a bad one to learn.  In this example you will be able to work six times as fast.  In real life, large grits are faster by more than just scale, so in real life, you will be faster still.

Another thing that can speed you up considerably is to not go too far with each level of sharpening.  A lot of sources will tell you to grind one way with an abrasive, and then grind a different way with another abrasive until the first ones marks are all gone.

In the illustration above, the right hand side is a cross section of what you get when you grind the previous grits scratch entirely away.   On the right hand side is what happens when you grind down to the same level, or just a hair lower with each grit.  You end up grinding half as much from the blade.  This makes the job twice as fast and uses half the materials and lets your blade last twice as many years before it is ground too far down to use.

There are a lot of different methods of sharpening.  Most of them can produce a sharp edge.  A lot of folk have switched sharpening methods, thinking that they can get a sharper edge by changing systems.  Maybe they can, but most experts on sharpening can sharpen pretty well on a sidewalk slab.  Some of them can then finish up to an edge you can shave with, with the top of their boot.  I am not making a wild claim here.  My aim is for you to be able to easily become one of those experts.  Using these primitive methods is far from ideal or as fast, but it can be done.

An ideal cross section of most sharpening systems, be it sandpaper, oil stone, whetstone, or grindstone, might look something like this.

The tips will fracture soon enough.

While the stone is usually much harder than the steel, pressure and wear will quickly make the surface more like  this.

A lot of abrasives will fracture into sharp shards that will still grind off metal.  This will be slower, but with the flattening of the teeth, and the fine grit produced, the abrasive left will leave a much finer finish.  So you can, if you want to, keep on sharpening forever, while your stone gets more worn down, clogged up, glazed, loaded, and finer grained.  High pressure on the stone will cause this to happen sooner.  A stone that is clogged up, will still sharpen, but it is very slow to do it.  They make tools for cleaning up these surfaces, big gum rubber erasers for sand paper, diamond points or spinning steel stars for grind stones, resurfacing stones and blocks for oil and whetstones.  Top quality stones often will wear down on their own, exposing new surfaces.  A gentle touch on these stones is a requirement for best results.

Another method is to use loose grit on a flat steel surface.

Loose grit gives you all of the advantages of the finest stones, with a much lower cost.  It gives you the loose grit surface that a fine whetstone will produce.  This allows the grit to roll and use all edges.  One of the methods that is considered an inexpensive way to get great results is the Scarysharp method.  It consists of using sandpaper on glass or stone plates.   In the short run, it is quite inexpensive, but it ends up being pretty comparable in cost to stones and sharpening machines as you keep replacing sandpaper.

Loose grit is much less expensive.   They use it to make sandpaper.  After preparing paper, and glue to bond the grit, and packaging it, the cost of sandpaper is going to be higher than loose grit.  With sandpaper you only get to use one tip of the grit to sharpen with.  When it dulls, you don’t get the fine pointed polishing action of loose grit, except to a very minor degree.

In my own experience, loose grit is not as fast as the first few strokes made with good quality sandpaper.  Norton 3x sandpaper will, for a while, beat loose grit of the same grade.   A good stone freshly surfaced, will perhaps also beat loose grit for a short while.  Then the difference becomes apparent.  The loose grit keeps on grinding and fracturing and polishing.  There is a reason experts use loose grit on a  slab to flatten the bottoms of planes and other tool surfaces, and polish the faces of stones.  It is a faster, more precise, and less expensive  method.

In the seventh installment, I will detail an even faster and less expensive variation on this method, using thousands of diamonds to sharpen.

Bob

Sharpening Tools, Part 1, Part2, Part 3, Part 4, Part 5, Next part is Part 4

4 comments to Sharpening Tools, Part 3

Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>