Card scrapers are indispensable tools. It won’t replace a plane and it isn’t a substitute for sandpaper, but they do compliment both. Planes, no matter how well tuned can cause tearout; a well sharpened and properly employed scraper can quickly remove that tearout. Sandpaper removes wood rather inefficiently and with somewhat limited control, yet scraped surfaces, at least in my experience require sanding. Scrapers do have a few limitations. They have to be used with care, since they lack a guiding surface, making it all too easy to create a depression or a washboard surface. Finally, scrapers work poorly if at all on softwoods. Actually there is one more issue with scrapers, they have a reputation of being difficult to sharpen.
Ask 100 woodworkers about sharpening a card scraper and you’re likely to get 95 different answers. In some areas there is considerable agreement, in others not so much. Other than the “ruler trick”, honing seems to have the most common ground. By contrast, when it comes to turning the burr, there is little consensus. Some advocate light pressure, others heavy. The number of stages in forming and turning the burr also vary . Even the recommended tool used to turn the burr diverge, ranging from valve stems to the backs of chisels, but all do agree that the burnisher must be hard and smooth. Finally, some,unnecessarily complicate the process, or try to uncomplicated it by relying on jigs and specialized tooling. In truth, getting a good burr requires no specialized tooling, can be accomplished in seconds and is simplicity itself. This is a good thing, because even a well turned burr has a relatively short working life.
Purchasing a quality scraper will go a longs ways towards success. A scraper made with poor steel, won’t be able to take or hold a decent burr. An overly hard scraper will make it difficult to form a burr, and that burr will be prone to work-hardening, which leads to premature breakdown of the cutting edge. I have had excellent success with the thicker scraper sold by Lie-Nielsen. I have not found the thinner version to be very useful as a scraper, but it does work great as a blunt chisel, when making half blind dovetails.
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Sharpening begins with establishing a square cutting edge. In keeping with my philosophy of not relying on jigs for sharpening , the edge is cleaned up with a fine cut mill file that is housed in a recess, routed in a piece of scrap wood. Both edges of the scraper are passed over the file, while holding the scraper at 90 degrees to the file. It is essential that the edges be square, otherwise one side of each edge won’t cut effectively or at all. Two or three passes over the file, is all it should take to re-establish a good edge. Some time ago, I dispensed with honing the edge. Neither the cutting edge’s longevity or the resulting surface finish were negatively impacted by skipping the honing.
The faces of the scraper, do need to be honed, and for this I use an inexpensive, fine diamond hone. The diamond hone cuts quickly without the need for lubrication. When the scraper is first purchased the faces are typically not polished well enough for optimum cutting, so it pays to spend a few minutes with diamond hones of various grits to bring the last ½” of the cutting edges to a high polish. The only key point to honing, is to never allow the diamond hone to tip over the edge. Doing so will round over the edge, destroying the essential sharp arris.
Years of working in a machine shop, gave me access to an endless supply of broken carbide end mills, which are perfect for burnishing. The end mill is simply shoved into an appropriately sized file handle. I follow a three step burnishing process; drawing, flattening and rolling.
Drawing is accomplished by laying the scraper flat near the edge of the bench and with firm pressure on the burnisher, draw it down the face of each edge several times. At various times, I have tried holding the burnisher flat on the face of the scraper or letting it tip slightly off the horizontal. I can’t say for sure, but holding the burnisher flat on the face, results in a more predictable burr and one that seems to last longer. This would make sense, in that going off the horizontal would have essentially the same effect as allowing the hone to tip over the edge. If viewed under a microscope the edges of the scraper would have minute projections of steel drawn out, resembling a C or U in cross section.
The next step in forming the burr is the one where there is the most divergence of opinion . Many woodworkers, go directly to turning the burr, but I have found that the resulting burr lasts longer, is more consistent and can be turned at least one additional time(re-sharpened it as it were) if an additional step is taken; flattening the projections.. With the scraper on its edge, draw the burnisher down the edge. The weight of the burnisher will supply all the pressure necessary to flatten out the projections of metal formed in the previous step. At first you will feel a slight drag as the burnisher is applied to the edge. After a few strokes the burnisher will glide smoothly over the edge. At this point you can make one additional pass with moderate pressure on the burnisher to ensure the projections have been flattened out. The reason for the light touch is to gently flatten the projections, which lessens work-hardening the steel, and prevents galling the edge. The extreme hardness of the carbide burnisher, also works to prevent galling.
The final step is to turn or roll the burr. Position the scraper flat on the bench with its edge extending a short distance past the edge ofthe bench. Using firm pressure on the burnisher, with it held about 8 degrees off the vertical, make two passes down each cutting edge. One pass is made pushing the burnisher away from you and the other drawing it towards you. This double pass, ensures a uniformly turned burr, because it is difficult to start the burnisher at the end of the scraper. The 8 degrees at which the burnisher, is not to be taken as an absolute. Much more of an angle than that will form a burr that is too aggressive and will require the scraper to be leaned quite far forward to make it cut. Leaning the scraper like this makes itdifficult to control. Conversely a lesser angle on the burnisher ,leaves a burr that will require the scraper to be held almost vertically, making the flexing necessary for proper use difficult. The edge of a scraper, no matter how well formed and despite the quality of the steel, will break down rather quickly. Producing just dust, is a clear sign that the edge has broken down, but it is best to recondition the edge before it has become dull. To re-condition an edge start by repeating the drawing step, but in this case, applying very light pressure on the burnisher for the first few passes. As with flattening the projections, at first you will feel a slight drag as the burr is “turned” over and made flat. Once the dragging disappears, the pressure on the burnisher can be increase slightly. Heavy pressure and too many passes should be avoided to prevent work-hardening the edge. From here on the same steps are repeated as with a freshly filed scraper. Depending on the quality of the steel, how far the edge is allowed to degrade, and the care taken to avoid work-hardening, the edge can be re-conditioned up to two times before needing to resort to filing the edge.
You can watch a video of this process here