Stropping/Lapping
	Finest abrasives.
	Microbevels front and back.
	Use a jig.
	Copyright (c) 2002-12, Brent Beach

Contents

• Overview Stropping/lapping is different from abrasion.
• Context Testing applies to plane irons.
• Test Procedure Compounds, surface used.
• The Results Looking at stropped bevels.
• Effect on Edge Durability
• Other Compounds - Tormek, pure Chrome Oxide.
• Other Surfaces - Leather? MDF?
• Discussion
• Conclusions
• Further testing

Overview

I decided to look at the surfaces left by stropping to get an idea of why stropping was helping. To my surprise, in all cases stropping left a worse edge than I was getting after the 0.5 micron 3M abrasive sheet.

What do I mean by a worse edge? As discussed earlier, the bevels at the edge of a sharp tool have the expected geometry and steel condition. You really have to read about sharp if you are to understand this discussion of stropping.

In these tests, you will see that the condition of the steel at the edge is much worse after stropping. As well, stropping with a soft medium can change the shape of the tool at the edge, moving you away from the desired geometry.

The problem then is to reconcile the clearly observed dulling action of stropping with the reports of superior perceived sharpness. At the end of this section, I suggest a number of situations in which stropping may help.

Stropping

Stropping also usually has no mechanism for clearing filings and broken grit particles from the work area. The cutting oil used with sheet abrasives floats the filings off the abrasive and out of the work area. This significantly improves the quality of the steel - or rather, fails to degrade the quality of the steel through subsurface fracturing.

Final Grit

Plane irons do not need tooth in the same way that knives do - in fact, the larger the scratches at the edge the faster the iron wears. This page is actually concerned with stropping of plane irons after a final honing step using a 0.5 micron abrasive. This abrasive is off the USA grit scale and finer than 15,000 grit in the Japanese scale.

This page shows that if you want the smoothest possible edge and if you hone down to 0.5 microns, then stropping with any of the stropping compounds listed here is a mistake. If you stop honing at a rougher grit, then stropping may help. This result applies even if you use the Lee Valley Chrome Oxide honing compound advertised as half micron.

Stropping Compounds

One fact to keep in mind with any stropping compound: it is not the finest grit in the compound that determines the edge quality, it is the coarsest. The fact that the Lee Valley green stick contains some 0.5 micron Chrome Oxide is not important since the stick also contains a lot of 5 micron Aluminum oxide grits. I wonder why Lee Valley calls this 0.5 micron. They could also take a 1200 grit Aluminum Oxide sheet, throw on a little 0.5 micron Chrome Oxide powder, add a little green dye, and call that a 0.5 micron abrasive. Now, why don't they do that as well?

Update Jan 2010: The advertising for the Lee Valley green stick has changed since last I looked. They now say: "The average size of scratch pattern it leaves behind is 0.5 microns". Do not be confused. Since only the tip of a grit scratches, a 0.5 micron abrasive could not produce 0.5 micron scratches. There are people who sell Chrome Oxide paste and loose grits that are actually 0.5 micron graded.

The Test Procedure

In these tests I used a piece of smooth hard maple as the strop. In all cases, the stropping was done with the iron in the jig to ensure the blade was stropped at the same angle as the final microbevel. To set a baseline for how the maple alone scratched the honed bevel, I began with no stropping compound on the maple.

In each test the stropping compound was applied to smooth hard maple, then the iron was stropped using only a pulling motion at the same angle as was used for the 0.5 micron abrasive. In all cases the image on the left is before stropping, that on the right after. The stropping compounds included the green crayon from Lee Valley and 3 stropping crayons from Delta.

The Results

Bare Wood Strop To start, an iron stropped on bare wood. The is an old laminated Stanley iron. The area below the blue line is the 15 micron area. Between the yellow and blue is the 5 micron area. Above the yellow, the 0.5 micron bevel. Bare wood scratched up the iron a little.
Green Crayon The next iron is a different old Stanley laminated iron (both are V logo irons). The stropping compound was Lee Valley green. Again, the three different areas of the iron are shown on the left. The image on the right has both of the fine bevels well scratched. This buffing compound is advertised as "primarily Chromium Oxide admixed with other fine abrasives (0.5 micron size)" according to the catalog. Checking around the net, I have read that a particular manufacturer produces 6 different green rouges, with chromium oxide content from 5 to 90%.
Green Crayon Thinking I have blundered, perhaps not having started with a sharpened iron (I had just taken 6 irons out of the pile sharpened a couple of days ago), I took a fresh image of the Smoothcut iron, before and after the green crayon. In this case, I have marked the area on the right image where part of the 5 micron bevel has not been scratched. See the Lee Valley green crayon discussion for more on the actual grit content.
White Crayon Next, a Hock iron using a white crayon. Again, the picture on the left the iron before stropping, the one on the right after. The white crayon usually contains aluminum oxide as the abrasive.
Tripoli Then a second Hock with Tripoli. Tripoli is a naturally occurring mineral which is a crystalline silica. I am not sure if or how they grade it. I suspect considerable variability. [It turns out that tripoli is the "silicious coats of perished diatoms." A diatom is a small plankton, usually unicellular, with a cell wall made of silica (hydrated silicon dioxide -- SiO2). Silica is the abrasive usually found in toothpaste.]
Red Rouge Finally, a Lee Valley A2 blade on red rouge. Red rouge typically uses Ferric Oxide as the abrasive. Again, I have been unable to find any grading information for red rouge. I suspect considerable variability in grit size within the bar and between bars.

Effect on Edge Durability

Any one of these stropping compounds does a good job of scratching up the edge. On a knife, or a carving tool used with a slicing action, this may be important. On a plane blade, how does stropping affect durability?

Here is the Smoothcut mentioned above, put through the usual test. The last time the Smoothcut did quite a bit better than this - getting a wear bevel of only 7 pixels (0.0005") compared to 9 pixels this time (0.007").

Using a stropping compound has reduced the Smoothcut to the performance of the Parplus blade.

This result is one small confirmation of the legion of tests done by Metallographers on the effects of various grit sizes on the crystal structure of steel. The larger grits fracture the crystal structure, weakening the metal at the edge. The metal wears away more quickly. The tool dulls faster.

Even if you manage to strop in a way that does not change the shape of the bevels at the edge, you are weakening the steel as compared to the edge you get using my jig and sequence of abrasives.

Other Compounds

There are stropping compounds that contain only finer grits. Tormek makes a stropping compound that contains Aluminum Oxide grits from 1 to 3 microns. This is also intended to be used on a powered strop, although the Tormek strop spins a lot more slowly than most powered strops, turning at 90 rpm. Tools for working wood and Hand America make bars and pastes that contain only Chrome Oxide grits in the 0.5 micron range.

Because these purer compounds contain only very fine abrasives they can only be effective when used with micro bevels. They remove metal so slowly that if they were used on the entire bevel it would take a very long time to remove the scratches left by the previous grit.

John D. Verhoeven used pure Chrome Oxide abrasive on a powered wheel and got good results. He used micro bevels. You could perhaps spend enough time on this type of powered strop to make a difference even if honing the full bevel. I am thinking of doing some testing on this.

Other surfaces

Leather

It turns out that leather by itself, with no stropping compound, has no significant ability to remove metal. Testing of clean leather as a strop, with subsequent examination of the stropped tool using a Scanning Electron Microscope (SEM) by John D. Verhoeven showed the abrasive grooves on faces and the bur size along the edge were not significantly modified. In the picture, the left is the edge bevel before stropping on a clean leather strop, the right image is after.

Stropping using leather with a stropping compound can be very effective, even with a stropping compound that contains only 0.5 micron grits (see previous section).

With light pressure on a powered leather hone the tool will only slightly dent the surface. The faster the leather moves past the tool the more likely the tool is to ride on the leather rather than sink in. Hand stropping on leather, even hard leather, is almost certain to have the tool sink into the leather and dub the edge.

After creating a flat bevel on a hard abrasive surface, it makes little sense to take that bevel to a soft surface that really can only make the bevel less flat.

As well, any leather surface will become contaminated with filings. As the leather loads up with filings it is no longer a neutral base for your abrasive.

Medium Density Fibreboard - MDF

However, think about how fast MDF dulls power tools - much faster than ordinary wood dulls power tools. MDF is all by itself a powerful abrasive. What would a bare piece of MDF do to your tools? I am going to test this soon, I hope.

MDF, like all surfaces used for stropping, ends up with particles of abrasive and metal filings embedded in its surface. As well, before these get pushed into the surface, they roll around between the tool and actually wear down the MDF. Your surface will soon go out of flat. Can you tell when this has happened?

Finally, metallographers - who make a science of polishing metal - always flush the abrasive surface continually during use to ensure that filings and broken grit are moved away from the working area. If you use baby oil on 3M abrasives, the cutting fluid lifts the grits and filings. The blade sweeps them out of the working area. You cannot do anything similar with MDF because the oil would soak into the MDF causing it to expand.

Steel plates

Once the diamond grits are embedded into the steel, you can use baby oil with this surface - you gain the ability to keep the stropping surface clean while stropping.

A well polished stropping plate made of steel like this and used with a cutting fluid like baby oil could be comparable to the 3M Chrome Oxide abrasive sheet. More testing opportunities!

Discussion

First, my testing shows that

Second, many people find that stropping with these compounds improves the usefulness of their blades. There are many anecdotal reports of much better tool experience after stropping. Many of these reports are related to carving chisels, but some apply to bench chisels and plane blades. Proponents of stropping are absolutely convinced of the benefits of stropping.

How then to reconcile my results with the popular experience with stropping? There are two slightly different cases: 1 - restoring a slightly dull tool, and 2 - as the last step in sharpening.

Restoring a dull tool

The blue line represents the ideal resharpened result - a new lower bevel parallel to the original. This result is not achievable with a strop that uses a very fine abrasive - fine abrasives remove metal too slowly. If you were to use a coarse stropping compound (the Lee Valley compound) you would be compromising the condition of the steel - resulting in faster subsequent wear. If you strop freehand or use a soft stropping material like leather, you cannot achieve this result.

The two pinkish lines represent other possible (more likely) new profiles. In the first, slightly flatter case, you have raised the back of the blade slightly (are stropping at a slightly larger angle) so may slightly reduce the size of the bulge in the wear bevel while only slightly increasing the included angle (from the original). You could increase the stropping angle even more and create a flat facet that reaches the edge, but you have increased the included angle and reduced the clearance angle. Even though the bulge in the lower wear bevel is gone, this blade may now not have enough clearance to cut into the wood.

With a worn edge, you have two options. Repeated stropping which produces a slightly different worn edge, or the three step honing which produces a sharp edge.

People who argue that they just spend a few seconds bring the edge back are dodging reality. One strop proponent reported giving his bench chisel a quick touch up before each tenon. A well sharpened tool should do dozens of tenons without any attention. This person is spending a lot more time sharpening and worse is always using a dull tool.

Last step in sharpening

But say you do notice improved sharpness and edge durability? The only possibility is that you did not start with the ideal edge.

Perhaps grinding has produced a thick edge - your final abrasive was coarse. The thickness of the metal at the edge is a function of the depth of the scratches you are producing. Put another way, the thickness of the edge is a function of the heights of the scratch walls. Stropping, even with a coarse stropping compound like the Lee Valley green crayon may reduce the heights of those valley walls, making the edge thinner.

A second possibility is related to freehand honing. People who freehand do not get flat bevels, they get faceted bevels. A couple of careful pulling passes on as strop could remove some of those facets, making the surface of the tool near the edge more regular and hence more usable.

The conclusion? If stropping improves sharpness then your honing failed to produce a good edge.

Resolution of this conundrum will require testing in the field. Have microscope, will travel.

Conclusions

1. If you are using 3M microfinishing abrasives you should not strop using any of these compounds on a maple board.
2. If you are using some other abrasive or blade holding method, and if stropping is improving the sharpness of your blades, then your previous honing step is producing a bad profile. You should investigate other honing techniques - in particular, the techniques discussed in these pages.

Qualifications

Some possibly better (than the test) combinations include:

1. It may be that some other manufacturer's Chromium Oxide compound does not contain the range of grits found in the bar I bought. Lee Valley's current green crayons may not even be the same. In fact, you can buy a Chrome Oxide paste that is 99.9% chrome oxide.
2. It is possible that the same compounds on some other surface will produce different results. I have argued above that leather and MDF are not suitable. Metallographers use diamonds on linen. I have never hear of anyone hand stropping on linen, but it could work. Diamonds on steel might also work.
3. It is possible that the same compounds on powered strops (either discs, felt wheels, or leather belts) might work better. I have read, but not verified, that high speed grinding makes the grit appear smaller -- for example, the scratches from a 60 grit wheel might look like those from a 180 grit bench stone. I plan to test this option.
4. If you use other sheet abrasives, even 3M wet-and-dry abrasives sold in lots of stores, often auto stores, your bevels may be so bad (the front and back bevels near the edge have deeper scratches than those produced by the honing compounds) that these compounds can help.
5. If you don't use back bevels and you are getting a fine wire edge that is not being removed during honing, stropping on a bare leather surface may help.
6. If you don't use back bevels, then stropping on the front and back of the blade may improve the quality of the back face - smooth the back wear bevel a bit. This would only apply if you stropped using a jig on a hard surface.
7. Verhoeven's testing was limited to a electron microscope examination of the surface of the tool. He did not actually compare the resulting edge in use. We know from Metallography that the crystal structure of metal can be damaged below the surface even if the surface looks smooth. Verhoeven is a metallographer. I am not sure why he was satisfied with looking at superficial edge smoothness when he must be aware of the issues with sharpening methods using large grits at the edge.

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