The term scraper refers to a whole class of tools that are purposely sharpened to a burr and used at a downhill slant. The word scrape is misleading because in reality a scraper is taking a highly positive shear cut. A turner’s scraper works exactly as a common cabinet scraper. The burr is what does the work because it is actually a micro cutting edge. The short length of the burr effectively limits the depth of cut per revolution of the work, making it a safe, predictable tool.
There are times when it is impossible to use a gouge, so a scraper is called for. Scrapers are the workaday implements that get the job done and often carry the day. A gouge may be the best way to cut wood because it leaves the best finish, but it requires a fair amount of skill and cannot reach blind areas. A scraper cuts wood handily in all situations and requires only a modicum of skill. You can also grind scrapers to complicated shapes, making it easy to duplicate details.
A scraper is quite different from other turning tools, and at first it can be a little unnerving to use. Up until now, you have been taught to shear cut, which requires keeping the bevel of the tool rubbing on the work surface. Suddenly, you must now point the tool downhill and drag the burred edge for it to cut properly. In fact, pointing it uphill can cause a nasty catch.
The trick of using a scraper effectively is to point it slightly downhill and touch it lightly to the work. Inside a bowl it is necessary to raise the rest a bit for the tool to point downhill but to cut on the centerline. You will have to play around with how far downhill you slant your scraper depending on how you created the burr. Burnished burrs need more slant than ground burrs, and no two people burnish exactly the same way. The trick is to find the right angle for your sharpening methods.
Simply pointing the tool slightly downhill is not always enough, though, especially when using a dome scraper on the inside of a bowl. If you start a scraping cut at the inside center, your concentration is naturally on the tip of the tool. As you approach the sidewall of the bowl, the side of the scraper starts to cut. If you do not pay attention, you may find the side of the tool cutting uphill instead of downhill and a catch may occur. The trick is either to swing the handle to the right, roll the tool on the rest, or both.
You should hold a scraper loosely in your fingers and place it ever so lightly against the work. If a scraper is pushed, it goes from a positive-angle cut by the burr to a negative-angle plow cut, with tear out aplenty. For heavy faceplate work, the heavier the scraper the better. For bowl work, I have a dome scraper made out of an old jackhammer chisel. It weighs about 5 lb. and never jumps around on the rest.
There are two ways to sharpen a scraper: grinding and burnishing. Grinding a scraper is simplicity itself. Set the grinding rest so that the bevel will be ground to about a 75 ° inclusive angle (most turners refer to this as 15° of clearance in the edge). Place the tool flat on the rest, and push it into the wheel. Keeping firm pressure on the tool, swing it to achieve the desired shape (see the bottom photo above). You do not need to grind the tool upside down to achieve the burr; it forms just as well by grinding right side up and you can see what is happening. A burr is the natural outcome any time a tool is ground.
Even though a ground burr works quite well, burnishing creates an even better burr. A burnish is no more than a round or oval bar of hard, polished steel. It is like a file without teeth. When burnishing a commercially available scraper, you must use a carbide burnisher because a standard cabinetmaker’s burnisher will only work if a tool is soft enough around 55 HRC. HSS turning tools (scrapers included) are in the range of 58 HRC to 62 HRC.
It is possible, though, to make a scraper from a softer piece of metal and burnish it successfully with a cabinetmaker’s burnisher. You can also draw the temper back on an old file to make it soft enough to burnish. To do this, grind the file to the desired shape and buff the area adjacent to the cutting edge to a good polish. Carefully and evenly heat the file with a propane torch until the polished area turns a deep blue color. Keep the torch moving all the time, and do not apply too much heat at the cutting edge. You should apply more heat behind the edge so that the heat flows out to the edge. When the tip area is bright blue, quench it in water. You have now drawn the temper back to about 55 HRC and can burnish with a standard cabinetmaker’s burnisher.
The burnishing process changes the crystalline structure of the steel at the cutting edge, increasing the hardness in this area. This is called work hardening. It is not possible to burnish without first removing this work hardened area by filing or grinding it away. Once the cutting edge has been filed or ground, use an oil stone to bring the bevel and the back (top surface) of the tool to a good finish. It need not be a polish, just a good finish. Next, rub the burnish along the bevel edge with considerable force. By slanting the burnish toward the back of the tool, you roll a burr. To get sufficient leverage, you may find it necessary to clamp the tool in a bench vise and place some body weight on the burnish. Waxing or oiling the burnish also helps because it reduces friction.
Burnishing a Scraper
Burnishing a turning scraper is much the same as burnishing a cabinet scraper. It is a three-step process: grinding away the old, work-hardened edge; bringing the ground edge to a hard, polished corner with oil stones; and burnishing a burr with a burnishing tool.
Although burnishing works well and produces a burr superior to grinding, it is not worth the trouble on small scrapers, especially if a scraper has a complicated shape. When doing heavy faceplate work with a large scraper, a burnished burr offers considerable advantages because it takes heavier shavings and holds it edge slightly longer.
Veritas makes a burnishing fixture that will roll a burr even on HSS scrapers (see the photo above). It employs a carbide cone fixed in an aluminum base, which screws to a work surface. A fulcrum pin, which can be located in two positions, allows you to lever the cutting edge of any tool against the carbide cone to roll the burr. The fixture works well and is inexpensive.