Abrasion Rate vs “Grit”

It is a common assumption that coarse hones (lower grit rating, larger grit dimension) remove metal faster than fine hones.  However….

In the Swarf! article, it was observed that coarser hones remove larger pieces of metal.   At the same time, we should expect that a finer hone will contain a higher density of abrasive particles.  The overall rate of metal removal will depend on both the size and number of metal swarf particles removed over a given distance traversed by the blade on the surface of the hone.

5and10

In this conceptual drawing, the red circles represent 5 micron particles and the blue circles represent 10 micron particles. There can be four times as many 5 micron particles as 10 micron particles in the same area.

Obviously there are other factors to consider, the pressure applied, the shape of the grit particles (and how they wear with use) and the mechanical properties of the steel, to name a few.  The obvious approach is to simply measure the rate of metal removal on a variety of hones to see what happens…

In this experiment, a small piece of hardened carbon steel was used as a workpiece to compare the abrasion rate for a variety of hones.  The sample was cut from the end of one of the vintage straight razors used in other experiments for this blog.  This is not intended to be a systematic study with precise control of the pressure and stroke, but rather to simply to determine ballpark numbers for the metal removal rate for a few hones over a range of rated grits.

carbon-steel

It should be expected that the rate of material removal will increase with the downward pressure applied.  In this experiment, the workpiece was “scrubbed” back-and-forth by hand with approximately 500g equivalent force applied.   The force and surface area were chosen to be in the typical range used for honing a straight razor.  The angle of the workpiece was varied relative to the direction of motion every ten strokes to minimize abrading parallel to existing scratches.  At varying intervals, the workpiece was weighed with a 1 milligram precision scale to determine the change in mass due to metal removal.

The pressure was chosen to be typical of that used with straight razors, however it is certainly less than would be applied to remove substantial amounts of steel when free-hand sharpening a very dull knife.  Since pressure is inversely proportional to surface area, it should be expected that a similar pressure to that used here will be achieved when flattening the back of a broad chisel or plane blade.

swarfhone

Each of the waterstones (Sigma, Shapton, Chosera, King) were lapped with an Atoma 400 (or 140 in the case of the 220 and 320) diamond plate to produce a fresh surface, and each stone was lapped again after every 500 strokes of the workpiece.  All of the diamond plates have all been used sufficiently to be broken-in, but not nearly worn-out.

The results for the Shapton Glass 16k hone are shown in the plot below. The average rate of removal was determined by fitting the data to a straight line.  Steel is removed at a rate of 2.7 micro-grams per stroke, or 2.7 milligrams per thousand strokes.  Based on the dimensions of the workpiece, each milligram decrease in mass corresponds to 2 microns of steel abraded.  One micron of steel is removed every 200 strokes.

16k-plot

The measurements were performed on a variety of hones and grits, and the results are tabulated below.  The rate is given in micrograms per stroke (1 microgram corresponds to 2 nanometers) and in microns/ 100 strokes.

hone

For the waterstones, there is essentially no measurable difference (within experimental uncertainty) for the abrasion rate of the Shapton 320, Chosera 1k,and the Shapton 2k and 4k hones.  These four hones removed metal at the same rate.  The 8k and 16k Shaptons removed metal at about 1/3 the rate of the coarser hones.  The removal ratio between the King 1k and 6k hones is much higher than observed with the various grits of Shapton hones, however the King stones are rated for their actual grit size rather than their “effective” grit size.

Only two of the waterstones tested “became muddy” during use, the Sigma 220 and the the King 1k.  These two hones were also among the fastest metal removers.

The results shown in the Swarf! article demonstrated that the dimensions of the removed metal particles do increase with the rated grit of the hone.   However, the rate of metal removal will depend on both the size AND quantity of the metal debris.  The results here corroborate the idea that the finer hones remove smaller particles, but that they can remove proportionally more of those particles, for an overall similar removal rate.

The Atoma diamond plates produced the same removal rate with 140 and 1200 grit with the 400 grit performing at half that speed.  Having fewer contact points than the DMT plates produces a higher pressure at each diamond for a given force.

The four DMT plates were the same ones used in the Diamond Plate Progression where we observed the seemingly anomalous inverse relationship between grit and keenness.  The measured removal rate displays the same inverse relationship with the “finest” hones being fastest and the “coarsest” hones being slowest.  Astonishingly, the DMT Coarse (325) removed steel at the same rate as the Shapton 16k glass stone.  Among the fastest hones were the DMT EEF (8k) and DMT EF (1200), and this is almost certainly due to the presence of oversized diamonds in those hones.  The slow removal rate for the Coarse and Fine plates indicates that there is not sufficient pressure for the full diamond particles to abrade the surface, rather that abrasion is dominated by small asperities on the surface of those larger diamonds.

The experiment was repeated for a few of the hones, but with an estimated 5 times the force applied.  At approximately 2.5 kg force equivalent, this is the limit of what I could apply with two fingers and still keep the workpiece moving  smoothly across the hones.

honepressure

At this higher pressure, we begin to see the coarse hones outpace the finer hones.  Even still, there is little advantage in the 320 and 1k grit stones over the 4k, when the greater apex damage caused by those coarse hones is considered.

There is a simple explanation for these results: coarse stones are only faster if sufficient pressure is used.   In the study and literature on the topic of grinding and wear (tribology) the terms “sliding, ploughing and cutting” are commonly used.  There is a critical pressure required for an abrasive to cut into the workpiece, otherwise it will simply slide without removing metal.  Efficient grinding occurs when the particles are cutting.   If the exposed abrasive becomes worn, the contact area increases and the effective pressure decreases (for a given applied force) and we will move from cutting to sliding.   In performing these measurements, I did observe an increased abrasion rate on the coarsest hones immediately after lapping, but this was typically short lived (less than 50 laps) and had minimal influence on the measurements.

As usual, we should be cautious in generalizing these results.   It is not my intention to “rate” various brands and types of hones.  However, this is not a difficult experiment for others to perform themselves.

Advertisements

11 responses to “Abrasion Rate vs “Grit”

  1. Thank you again. I hope you didn’t wear out your fingers rubbing that little piece of steel on those rocks. Weighing the differential is a delicate procedure that is waaaay beyond most of us.

    This information gives more support to your article on “too big of a jump”. Progressing from say a 1k stone to an 8000 DMT might make quick grinding, although the damage from rogue diamonds on the 8000 plate might make this a poor choice. However, progressing from a 1k bevel setter to an 8k finisher, like your Shapton, without any intermediary stone in the progression seems like it would take longer to achieve a finished edge than using the 4k in the progression.

    I have found that using a slurry on my synthetic stones makes honing much quicker. I have been experimenting with a 4000 Gouken Hayabusa and 8000 Gouken Fuji, mid priced, matched hones. I rub the two stones together to make a slurry on the 4000 which cuts quickly and the slurry keeps the stone surfaces clean. I progress to the 8000 hone with slurry which I slowly thin out to a clean surface (no slurry) for the last few laps, and edge trailing strokes. I am pleased with both the performance of the hones and the results on my bevel and edge. I think the slurry makes a slightly micro-convex edge. The burr is minimal, and requires only a few strokes on the loaded denim strop for removal.

    If the edge is chipped, I find that 800 SiC (corundum) paper, laid wet on glass, removes metal quicker than anything I have — cheap, flat and fast.

    Like

      • Thanks for the correction. You did not go on to say that SiC, or silicon carbide, is also known as carborundum which was spell checked to corundum — a different compound as you pointed out. Aluminum oxide “sandpaper” is often made to look black, like carborundum “sandpaper”, but the performance is different on steel. I much prefer carborundum paper for sharpening purposes because the grit is very sharp and it seems to be friable, breaking down to a finer grit during use. I like it for sharpening both razors and knives. I had a ¼” glass plate cut to 4 ½” x 11″, with polished edges, so that I can use half a sheet of 8 ½ x11 inch paper. I also use the paper dry with masking tape over the ends to hold the paper on the glass. A rubber eraser cleans out the swarf.

        Like

  2. Thank You for this valuable report!! When they say one have to get to know his stones…
    I have quite a little bunch on stones and i can say that there are finer grit ones that cut faster than the rougher; Now you have given sound numbers to this kind of observations!!!

    Like

  3. Very interesting conclusions. So if the goal was quickest metal removal to take out damage on larger blades what would the best choice be? Am I gathering that a coarse diamond stone would be a poor choice since you need to use high force which greatly decreases the life?

    Like

    • I have no use for coarse diamond stones. However, the DMT EF (1200) is probably my most used hone for fast removal in low-pressure situations; straight razors, chisels and plane blades. For water stones, I wouldn’t go below 1k.

      The simplest (and most common) way to increase pressure is to decrease the surface area by convexing the bevel. This allows you to access the higher removal rate of the coarser stones. This occurs naturally when you freehand sharpen.

      Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s