Diamond Parting Tool Sharpening

Andrew Hilton

This is more of a "hint" rather than a full-fledged article but it takes a bit of explaining so I thought I'd put it in this format.

I had been asked a few times about how to sharpen a diamond shaped parting tool. More specifically, how do you sharpen one by using the method of laying it on its side rather than simply holding it up to the wheel like...

This method is the quickest and easiest but it does cause a hollow-grind to be formed on the tool because of the curvature of the wheel.  This may or may not be an issue for you depending on what you like and how you work with the tool.  Update 5/20/07: For example, I use my parting tool as a small skew a lot.  Even a slight hollow-grind messes that up for me. It really does come down to what you like and how you work.  Mainly, though, I sharpen these things on their sides for the next reason.  It makes a big big difference in deep parting cuts.  It can also be a problem getting an edge that is straight across where the outside tips on each side are in line with the thickest part of the tool (in the case of diamond shaped tools at least).

The method of laying a parting tool on its side (shown below) is often taught but a difficulty arises when you use a diamond shaped tool.  The ridge that runs the length of the sides of the tool makes the tool "tippy" as it's positioned with this ridge directly on the toolrest. This makes getting a good single bevel at exactly the place you want it difficult.  It's simple to fix....

You can simply cut a grove in your toolrest with a file that matches the angle you want to have on your parting tool.  A triangular file does best here as it will produce an angle that supports the ridge best without tipping.  It would be even better if you could get the groove cut in such a way as it matched the angle profile on the side of your tool but something is better than nothing.  Note that in the pictures above the grove doesn't run from the edge to the edge of the toolrest.  This is just done in the pictures to make the groove easier to see.  You should have the groove go all the way from edge to edge so the parting tool can slide easily up to the wheel.

This groove, made deep and wide enough, will keep the tool from tipping as you grind the tool because the edges of the groove support a wider area of the tool giving it support.  It also keeps the bevels a consistent length and angle.

If you can't, or won't, cut a groove in your grinder toolrest, you can make a little wooden sled that rides on top of your toolrest that has the proper groove cut into it that will do the job just as well.  You could even go to the trouble to make a jig where the sled captures and holds the parting tool in one position while the sled itself moves over the toolrest and into position bringing the parting tool up to the grinding wheel.