Turning The Burr
The cutting component of a card scraper is the burred edge of the scraper. The burr is a sharp hook of metal which is turned on the edge of the scraper by burnishing with a steel rod. A file or sharpening stone is used to joint the edge of the scraper before burnishing. Cabinet makers typically joint the edge square, or at a right angle to the face of the scraper, which allows a fine burr to be turned on both sides. Luthiers often use a beveled edge, which allows a more aggressive burr, but on one side only.
There are a number of variations in the process of turning the burr, the choice of which is down to personal preference. However the basic concept is that the burnishing rod is held against the edge of the scraper at a slight angle and drawn along the edge a number of times until the burr is created. One variation involves holding the rod flat against the side of the scraper for the first few passes to create a burr pointing away from the edge and this is then rolled back down by drawing the rod along perpendicular to the scraper.
With the burr properly turned, a scraper will produce miniature shavings resembling those from a plane. If only dust is produced, either the scraper is not sharp, or not being used correctly.
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—Henry David Thoreau (1817–1862)
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