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.

The key with a Negative Rake Scraper is that the 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
Angle Comments Recommended
Tormek Jig
α = 45°

βTop = 22.5°

βBottom = 22.5°
Side View of the tool

Tom Wirsing recommends 45°, and has an impressive background to make such a claim. The platters he makes are very well done.

Every time you resharpen, re-grind both sides of this scraper. This 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 :

  1. it makes raising the burr faster and easier, and
  2. makes sharpening easier as the tool is being pushed into the 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.


SVD-110
α = 65°

βTop = 15°

βBottom = 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

Videos & Presentations

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About this site
Remember : The goal of sharpening is to produce sharp tools, and these tools can injure you if mishandled. Safety measures should be followed to protect yourself and those in your shop. Be sure to read and follow all instructions from the manufacturer, and and utilize proper safety equipment. Never consume alcohol or anything that could impair your judgement before sharpening tools, or using sharp tools. Comments can be sent via eMail to me at SharpeningHandbook@Gmail.com.