Sharpening Handbook

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.

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.

General Guidelines
Angles			Comments
α	β_Top	β_Bottom	Comments
45°	22.5°	22.5°	*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. 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 : it makes raising the burr faster and easier, and makes sharpening easier as the tool is being pushed into tool rest, not being pulled away from it. (My experience when trying this from the other position was that the tool was being constantly pulled away from the tool rest and holding it there was difficult.) This is my preferred approach.
α	β_Top	β_Bottom	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. 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 α, but the ranges are typically in the 50° - 75° range.

Notes & Comments

Information regarding Grindstones

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.

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.)

More Information

Books & Papers

Some information presented is from the Tormek handbook, "Water Cooled Sharpening of Edge Tools".

Videos & Presentations

Tormek Sharpening Classes: Part 5 - Woodturning Tools (YouTube video). Want to learn how you get your woodturning tools razor-sharp? In this week's sharpening class we take a closer look at the tools for the woodturner; Gouges, skews, parting tools, scrapers, cutters and more. Sebastien and Wolfgang will show you the methods for sharpening all these turning tools and how you achieve repeatable edges every time. Stay tuned and hit us with your questions during the stream.

Tormek Sharpening Classes: Part 23 - Reshaping woodturning tools on a bench grinder (YouTube video). In this episode of the Tormek sharpening classes we explain how you can quickly and with full control, re-profile your turning tools on your bench grinder. The BGM-100 Bench Grinder Mounting Kit allows you to use the same jig settings as on your water-cooled Tormek sharpening system. It is therefore easy to do the rough shaping on the bench grinder and then continue to sharpen and maintain the edge of your Tormek machine.

How to sharpen scrapers and cutters - Tormek SVD-110 Tool Rest - with Nick Agar (YouTube video). In this episode, Nick Agar shows how to sharpen a woodturning scraper creating a large burr and a small burr depending on the material you're working in. He uses the Tormek SVD-110 Tool Rest and the DF-250 Diamond Wheel Fine. In addition, Nick demonstrates how to sharpen small hollowing cutters and ordinary cutters using SVD-110 and SVD-186.

Tormek Tool Rest SVD-110 (YouTube video)

Tormek Turning Tool Setter TTS-100 (YouTube video)

Tormek AngleMaster WM-200 (YouTube video)