ARTICLES & REVIEWS
Loose Diamond Sharpening
by Bill Tindall and Phil Smith
Most correctly, this would be Bill's and Phil's status of loose (where the diamond is not bonded to a substrate) diamond sharpening. The objective of this post is to convey what we do and where opportunities are for improvement with the hope that others will add to this experience. Eventually we will arrive at what is best. It should not be presumed that we have found what is best yet.
We sharpen chisels, lathe tools, and plane irons. Our planing is primarily domestic hardwoods, and planing operations would be judged typical of persons combining hand tools and power tools. We aren't avoiding sandpaper for final surface preparation yet we strive for a planed surface that requires only light sanding. Perhaps a few persons planing foreign woods, competing in planing competitions, or looking for a final finish off the plane, could have more demanding requirements that we fail to recognize.
Advantages of loose diamonds
Diamonds are the hardest material known and they are also very resistant to crushing. As a result a diamond abrasive cuts aggressively for a long time before the particles get dull or fracture to smaller less useful sizes. Diamond penetrates and cuts even the most abrasion resistant steels vastly better than aluminum oxide, the abrasive most commonly used for sharpening. While silicon carbide is hard enough to adequately penetrate very hard steels, it useful life is limited by its ease of crushing to uselessly finer particles.
Diamond sharpening is not messy. It involves small amounts of lubricant as opposed to the water issue with water stones. Other than occasional recharging, using diamonds is maintenance free.
Loose diamond, in the form of paste or powder, is very cheap. Twenty dollars worth of diamond will last the typical woodworker several years. We have flattened the backs of dozens of chisels and plane irons and have yet to use up a 5 gram tube of paste.
We find it relatively easy to hand-hone on a diamond charged cast iron plate as compared to other abrasive formats. The steel glides smoothly across the plate and it is, therefore, easy to know the bevel is oriented properly on the abrasive. Because diamond cuts fast, it typically takes a dozen strokes on a diamond charged plate to accomplish final honing. It is easier to maintain proper tool presentation for 10 strokes than 100 as might be the case on a slower cutting abrasive. We hone with a pull stroke with the tool angled to the plate. Angling the tool helps keep the bevel oriented on the plate because the bevel is effectively widened by this approach. Because angling is preferred, the plate does not have to be as wide as the widest tool one needs to sharpen.
Issues in using diamonds
The challenge is how to best take advantage of these desirable diamond properties for typical woodworking sharpening. Factors to be considered and optimized are:
Every new tool needs more or less grinding to flatten the back, and sometimes a lot more. We have found that even surface ground tools require more than simple polishing. Typically, 30 to 60 micron diamond is required to initially flatten a back in a reasonable amount of time. For some tools with particularly unflat or pitted backs, power grinding prior to diamond grinding is the only practical recourse. We have found a Makita horizontal wet grinder to be ideal for this task, while a very carefully used belt sander with 60 or coarser grit, preferably zirconia (blue), can also be used. Care must be taken to not grind near belt edges or severe lack of flatness can result if the belt edges have a curl.
The typical woodworker sharpens when the tool stops cutting effectively. (Others may sharpen based on surface appearance or some other factor. We have no experience in these matters and will not further address them) Compared to final edge refinement (honing) a lot of metal must be removed to create the wire edge that indicates all the old "dead" edge has been removed. We want this step to be accomplished quickly. Either coarse diamond grit (30 or 60), coarse abrasive sheets (100 or even 80 grit zirconia) or coarse bench stones must be used, or power grinding is required. Sharpening that produces a hollow grind at this stage is beneficial for the edge refinement step, see below.
Edge Refinement (honing)
Studies by Steve Elliot indicate that it is desirable to finish the sharpening process with a diamond grit at least as small as 1 micron. This size grit will not accomplish the initial sharpening in a short amount of time to produce a burr as discussed above. Therefore sharpening must be at least a two step, two grit, process.
The substrate must be hard enough so that the diamonds don't uselessly bury into it, but soft enough so that the diamonds embed enough to become immobile during use, yet stick up to cut. One wants the diamonds to be fixed in the substrate and the tool to move across them during sharpening. Of course the substrate must be flat. For centuries cast iron has been the substrate of choice for charging with diamonds. The problem is that no one yet sells appropriate cast iron plates for sharpening. A group (club) can buy cast iron bars and have them flattened (surface ground) at a machine shop. This economy of scale can yield 2 x 12" plates for $25. A trip to the junk yard may also yield suitable cast iron pieces that a machine shop can turn into usable plates.
Acrylic (Plexiglas) is one of the hardest plastics available. It is cheap and flat, and available from MSC and other machine shop supply houses. We have found that coarse diamonds (eg 60 micron) embed better in it than in cast iron. We recommend this substrate for coarse grinding. Fine diamond (eg. 1 micron) does ok on it but cuts slower than on cast iron. Steel sticks to acrylic more than the self-lubricated surface of cast iron, which makes control in free hand sharpening more difficult, but not unreasonable.
For grinding concave curved tools (eg flutes and coves) 60 micron diamond charged onto an aluminum cylinder/rod that is then spun in a drill press or lathe works well. The diamond charged cylinder surface nestles into the flute or concave bevel and grinds away, quickly compared to alternatives. We have not explored charging aluminum plates with diamond for plane iron sharpening.
Soft or porous substrates, such as wood or leather, require a comparatively huge amount of diamond to charge because most of the diamond buries uselessly into the substrate. It is also difficult to float away the grinding gunge from these materials. Hard substrates, such as glass, defeat the purpose of this kind of grinding. If the diamond does not embed in the substrate, it will embed in the steel and wear away the harder material.
When grinding gem stones with diamond charged cast iron, the cast iron lasts essentially forever. In our case we know that some diamond sticks to the tool. These diamonds will abrade the case iron. Apparently this process is slow for we have not seen any significant plate wear in more than one year of use, even after flattening dozens of plane irons and chisels made from very abrasion resistant CPM 3V and M4. This usage is more than a life time of wear for most people just sharpening.
WD 40 is the lubricant of choice for cast iron and mineral spirits is suitable. Olive oil is the traditional lubricant, but we think it is too viscous. We are wary of using any oily lubricant on acrylic for fear that it may soften it. We use propylene glycol, but ethylene glycol, antifreeze, glycerin, or just water could be used. A lubricant on some other substrates may be problematic due to the substrate itself being problematic as discussed.
Charging the Plates
It takes a very small amount of paste or powder to charge a plate. A ¼" worm of paste or the tiniest mound of powder is all that is desirable. This diamond can be smeared about the plate with a thin piece of plastic grocery bag stretched over your finger and a little added lubricant (the bag is to minimize contamination). Overcharging is undesirable (see below). In most cases on cast iron one can hear the diamond cutting. Recharge when the cutting lessens. Relube when the plate gets sticky. Float the grinding gunge off the edge of the plate with added lubricant. From time to time, wipe the plate clean with a paper towel. Don't worry about wiping away diamonds as the ones that are doing the grinding are embedded.
Contamination by Carry Over
We find that 1 micron diamond is sufficient for final honing. But for back flattening or initial sharpening, something coarser must precede this grit. Some diamond particles stick in the scratches of the steel being sharpened. We have explored every imaginable way of removing them and failed. We think the only way to be rid of them is to grind them out (overcharging a plate will result in lots of these particles in the steel which is not desirable). For this reason it is undesirable to sharpen with a succession of diamond grit sizes because the coarser particles will contaminate the substrate used for the finer particles. Of course you can choose to just live with this problem and resurface your plates from time to time to remove contaminating grits.
If one chooses to ignore the carry-over issue and use a succession of diamond sizes to accomplish initial grinding as well as edge refinement, then it will be best to reduce the grit sizes in small steps, that is use several plates charged with a decreasing succession of grit sizes. Phil uses a 30, 15, 3, and 1 micron series.
The assumption is that a worn diamond particle from the first size that carries over to the next smaller size would nearly match the cutting action of this next smaller grit. So, while the carry-over issue is not solved it is minimized. Periodically, the coarsest, or next to the coarsest, plate used in this process will need to be resurfaced to remove its diamonds. This fresh plate will become the plate for the finest grit and the other plates move up one notch in grit size. Personally, I (BT) am not fond of this approach as it gives up some of the advantages of diamond sharpening while Phil and others use this approach.
We have explored two other ways to accommodate carry-over. When flattening backs with a loose coarse grit diamond this coarse grit grinding can be followed by brief grinding on any material where the abrasive is fixed, for example 3M film or a diamond bench stone. Presumably this intermediate abrasion will remove any stuck coarse diamond particles prior to continuing on to honing with finer particles, while not adding any new contaminating particles to the steel. This approach has not been confirmed by microscopic examination after each step, but we intend to do so in the future. Another approach is to only use one size of diamond for final honing (see below for this preferred procedure).
Preferred Procedure For Edges
Far and away the fastest approach is to grind a hollow bevel to produce the initial sharpening burr with a bench grinder, followed by honing with 1 micron diamond. The diamond cuts so fast that one can use even a 60 grit wheel on the grinder and the diamond will still quickly remove these scratches and polish the edge. In fact, the 1 micron diamond will completely remove the hollow grind in a short amount of time.
A challenge to using this approach is that it takes some practice to be able to grind a straight edge with the bench grinder. A light touch and a slightly crowned, very slow cool cutting wheel is helpful. The 1 micron diamond will not quickly straighten a crooked ground edge. There are two fixes for wavy edges off the bench grinder. After a few strokes with one micron diamond it will be clear where the diamond is cutting, and not. A trip back to the bench grinder can quickly straighten the edge using the 1 micron polish line as a guide. Or, the edge can be straightened on a coarse fixed abrasive, for example a coarse diamond bench stone or 30 micron 3M film abrasive, prior to moving to 1 micron diamond. On most days we can grind a sufficiently straight bevel such that about ten strokes on the 1 micron plate is all that is necessary to complete edge refinement. Most of this honing is to straighten the bevel as only five or fewer strokes are necessary to remove the grinder scratches.
A slow cutting wheel helps in controlling grinding to make the edge straight. A homemade grinder assembled from an arbor (available Lee Valley), a small motor, and a fine (100 or smaller grit wheel) would be cheap and ideal for this step, but we have used a 3600 rpm grinder, a 100 grit wheel, and a very light touch with success. BT slowed grinding to gain control by attaching a variable frequency drive to a 3-phase bench grinder. The wheel turns at around 500 rpm for this operation.
This is where we are at, at this particular point in time. We encourage others to explore better ways to take advantage of sharpening with diamonds and report their results.
. . . Bill Tindall and Phil Smith
© 2005 by Bill Tindall and Phil Smith. All rights reserved.
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