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Negative Rake Scraper |
The cutting edge on a Negative Rake Scraper (also known as an NRS) is formed by one step on the grinder. The grinding action forms the hook.
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A craftsman knows in advance what the finished result will be, while the artist knows only what it will be when he has finished it. W. H. Auden |
The key with a Negative Rake Scraper is that the α angle must be less than 90°. If not, it won't work. For all practical purposes though, 70° is about as high as you can go for a functioning tool.
There seem to be two schools of thought on the shape of the cutting edge for this tool, and both options are shown below.
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General Guidelines |
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| α | βTop | βBottom |
Comments |
| 45° | 22½° | 22½° |
Side view of the tool Tom Wirsing recommends a β of 45°, and has an impressive background to make such a claim. The platters he makes are very well done. Every time you resharpen, you can choose to re-grind both sides of this scraper, or not. If you do re-grind both sides, the action will clean off the residue left on the top edge by the scraping process (especially when scraping oily woods). Making the top and bottom angles the same makes this significantly easier as you can sharpen the top, flip the tool, and then sharpen the bottom. 
click on the gray block to see more On the Tormek, sharpen with the Tormek SVD-110 Tool Rest attached to the support bar in the vertical position (with the grinding direction being towards the edge of the tool). This does two things:
This is my preferred approach. |
| α | βTop | βBottom |
Comments |
| 65° | 15° | 50° |
Side view of the tool Marty Kiminsky noted that 25° top/40° bottom is generally recommended for this tool, and that is also his personal preference. See also, "Woodturning" magazine, issue # 352. Grind the top first, and make it about 3mm (3/16 inches) wide. Then, you resharpen by only re-grinding the bottom. Once the top gets down to 1mm or so, you will need to re-grind it again.
click on the gray block to see more An advantage to this approach is that you only need to sharpen one side at a time, so it is faster to resharpen (by a few seconds or so). This can add up as scrapers must be resharpened very often. Various people recommend different angles for β, but the ranges are typically in the 50° - 75° range. |
If grinding this on a high speed grinder or sander, be sure to manage the temperature so that the temper is not lost due to overheating.
| High Carbon Steel |
Temperature control is critical for high carbon steel. Tempering will be lost in the steel if heated until it is blue (~550°F / 300°C). Cool by dipping as needed in water. Dr. Larrin Thomas noted in his book, Knife Engineering: Steel, Heat Treating, and Geometry (2025), The lack of sparks or color changes is no guarantee that the edge was not overheated. Therefore, sparks are not necessarily a sign that heat has or has not been generated in the steel. Color changes can occur from overheating of steel in grinding, but during grinding, the surface is being removed, and the affected volume can be relatively small. Therefore a color change is not always observed. Microhardness measurements of edges sharpened with a belt sander show a drop in hardness at edge. This results from “over-tempering” the edge by heating it above its original tempering temperature. |
| High Speed Steel |
Cool by dipping as needed in water. |
| Steel with Carbide Insert |
DO NOT cool by dipping in water. This will cause the carbide to crack due to shocks.
It is also recommended to use a high grit wheel for sharpening carbide as lower grits can cause cracks in the edge leading to it being far more crack-ridden and brittle. (Lower grits can be used for shaping the edge -- see also, Grindstones and Other Sharpening Media - Quick Selection Guide.) |
