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How to Sharpen a Parting Tool

Thin parting tools.  Unless the cutting edge is in really bad shape, you may not need anything more than a hone to do the job. A hone is quite effective because of the small surface area involved.  

I find a credit-card-sized hone to be hard to control on the narrow surface of a thin parting tool.  It wobbles a bit much and tends to round the cutting edge.

A better method is to clamp the tool flat across the corner of a worktable and then use a much larger diamond “stone” held vertically.  After a few strokes, check the surface to see your progress.  If the cutting edge is skewed instead of being square, you are not holding the stone exactly vertical.  If it is rounded, you are letting the stone wobble during the stroke.

If you determine that grinding is required, set the tool rest so the bevel meets the wheel at the proper angle and lightly touch the tool to the wheel.  Metal will be removed quite rapidly; only a second or two of grinding may be needed.  

A belt sander may be used instead of a grinder and is my preference. You can easily switch to a higher-grit belt for a less aggressive approach.  Just be sure the point of the tool is presented to the belt “in trail” to avoid the possibility of getting a dig in.  (See the article on sharpening skews where the belt sander method is described in detail.)

The burr.  Grinding or honing the cutting edge almost always results in a burr that makes the tool more aggressive in the cut.  If it is too aggressive or tends to be grabby, you can remove the burr. A couple of swipes with a hone is all it takes, or, it may go away on its own after a bit of use.

If you find you have to tilt the point of the tool slightly downward to get it to take hold and cut, and if it is then overly aggressive, you may have a burr that has a significant backward curl. Removing the burr will probably improve the situation.  This effect might also indicate that a lighter touch should be used in the grinding process.  

Sharpening symmetrical-bevel parting tools.  This includes the rectangular and diamond parting tools as well as the much wider beading/parting tool.

The same techniques used to sharpen a skew can be applied to these tools, so let me refer you to the article on sharpening skews. The un-skewed cutting edge of the parting tools actually makes them much easer to sharpen.  In general, the method is to use either a grinder or belt sander and then finish with a hone.

As always, don’t rush to the grinder unless you have a situation where honing alone will not suffice.  Once a proper edge is established, honing is generally all that’s required.

With the skews, getting flat bevels is a priority in an effort to keep the cutting edge from gradually curving deeper into the wood.  For a skew in a planing cut, the cut occurs on a curved surface, but the direction of the cut is in a straight line, parallel to the axis of the piece.  

With parting tools, the cut actually follows a curved surface.  It may be that a concave bevel on a parting tool will make the cut progress more easily, but I will leave that for you to decide.  

On a diamond parting tool, be sure the cutting edge is located precisely at the widest point of the tool.  If it is otherwise, you will lose part of the benefit of the diamond cross section and the tool may be more prone to bind in the cut.

In honing a beading/parting tool, many turners recommend that you hone the sides of the tool as well as the bevels.  

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