Sharpening the Rod, Disk, and Bullet Cutters

The immediate problem is that the tools that hold the cutters will not fit the jigs widely used for sharpening.  Even if the jig will accommodate the handle, the swan neck confuses the issue of setting up the jig and makes the whole attempt a bit awkward. And so, sharpening these cutters is a bit of a problem, but not a problem that can’t be solved.  

The solution described in this article entails installing the cutter on a shop-made holder when it is necessary to take a cutter to the grinder.  The holder is very light compared to the regular handles which I find rather cumbersome in a sharpening situation, and it is straight.  

A support is needed for the holder so that the bevel angle can be established.  The tool rest found on most grinders will serve this purpose.  All you have to do is figure out the angle at which to set the rest.  It’s not hard to do, and absolute accuracy is not required.  

The truth is, in practice, grinding the disk and bullet cutters is seldom required.  Simply honing with a small diamond hone is usually all it takes to restore the cutting edge.  It is only after extensive use, and honing, that a trip to the grinder is appropriate.  

The disk and bullet cutters are basically scrapers and are sharpened like scrapers.  Background information is given in the articles on scrapers on this site, especially Part 2. Check the articles index.

Honing the Disk and Bullet Cutters

There are two steps.  First, hone the top face of the cutter to remove any burr that may be present at the cutting edge. Second, hone the bevel to produce a very fine burr.   

This assumes the bevel angle on the cutter is in the range of 75 – 80º and that the bevel is flat – not convex or rounded over at the cutting edge.  If such is not the case, the bevel should be ground prior to honing.  Grinding is described farther down in this article.

To hone the top face, remove the cutter from the handle and place it face down on a 600-grit hone.  Swirl it around in a circle or figure eight a few times, and it will be done.  That’s all it takes! The burr will be gone.  

Drag a thumb or finger across the top of the cutter and off the cutting edge.  You should not feel a burr.  The edge will be keen, but there will be no burr.  

To hone the bevel, mount the cutter either on a holder or on its regular handle if there is no conflict between the handle and the hone.  Place the holder (or handle) on a flat surface so the cutter overhangs the edge.

The top face of the cutter should be up.

Place the hone flat against the bevel, holding it lightly.  Look carefully at the cutting edge which should be in contact with the hone. Make a swirling motion with the hone while advancing around the curved cutting edge.  Again, it doesn’t take much. About 20 seconds of swirling is all that’s required to hone the bevel of a bullet cutter, for example.  

Again, drag a thumb or finger across the cutting edge.  My guess is that you will feel a burr.  It will be a very fine burr, nothing like the ragged, jagged, prickly burr you get from a grinder.  But, a very fine burr is exactly what we’re looking for.  

Examine the bevel in reflected light.  Can you see where the hone made contact?  If so, does the honed area reach up the bevel to the cutting edge?  If it doesn’t, and if you cannot feel a burr, perhaps you should try the honing again with the hone tilted slightly toward the cutter.

Holders for the Cutters

At the grinder, a very light touch should be used when you apply the cutter to the wheel.  You can ruin a cutter in short order by applying too much pressure, taking off too much metal in a great shower of sparks, and spoiling the curve of the cutting edge.  

I have better control and get far better results when I remove the cutter from its (heavy and cumbersome) handle and install it on a wooden holder.  Only a few minutes are required to make a holder and no special woodworking equipment is required.  A description of the holders I’ve made is given in the following.  

Holders for disk and bullet cutters are simple to make. Basically, the cutter is screwed onto the end of a rectangular strip of wood.  Rectangular, so the holder will sit flat on the tool rest of the grinder.

The following diagram and photo shows holders for sharpening freehand.  The handles are about 13” long, which is a bit longer than what you need when using the tool rest of the grinder.  The disk cutter is mounted upside down on the holder to avoid having the holder make contact with the grinding wheel.  

Making the holder for the rod cutter is a bit more involved. Here’s one way to do it that doesn’t require anything special in the way of tools.  

(1)  Begin with 3/4” square blank 10 to 12” long.  Drill a 1/4” diameter hole centered on one end to a depth of about 3/4”.  

(2)  Split the piece lengthwise with a bandsaw, starting the cut at the end with the hole.  Locate the start of the cut carefully so that just slightly less than a complete half of a circle remains in the piece that will become the bottom part. Take the width of the kerf into account. The cut will not be centered on the hole.

Check to see that the cutter will rest in the resulting slot with the flat surface of the cutter slightly proud of the surface of the holder.  

(3) Use a mixture of epoxy and sawdust or coffee grounds to fill in the slot on the piece that will become the top of the holder.  After the epoxy cures, sand it flat.  (This method is suggested so the two pieces will fit back together reasonably well even if the cut line is not straight along the length of the piece.)

(4) Put the two pieces back together with the same orientation as they were originally.  

Use two wood screws located near the handle end.  

(5) Find a machine screw and a nut to use for the clamping screw, or use a wood screw captured in a knob as shown in the photo. Drill appropriate holes for the screw.  If you use a machine screw and a (wing) nut, installing or removing the cutter will be simpler if the head of the screw is captured.  

(6) Taper the end of the holder as you see fit to give additional clearance when you manipulate the holder at the grinder.

Grinding the Cutters

We need one other item, a template of sorts to help set the angle of the tool rest on the grinder.  It’s basically a strip of wood with the proper angle cut on the end.  The angle will be the same as the bevel angle you want to put on your cutters.  

So what angle?  I measured the angle on a brand new bullet cutter and found it to be 80º.  I think any angle in the range from 75 to 80º will work fine.  I split the difference in the following illustrations.

A paint stirrer makes good timber for the template, or cut a strip from the side of a board.  It should be at least 3/4” wide; the thickness is not all that important.  

Lay out the angle at one end of the template.  Use whatever angle-measuring device you have available or use the method illustrated in the diagram at right.

Make a saw cut along the line.  If you use a bandsaw, place a backing piece at least 1/2” thick under the template to get a cleaner cut. A small, fine-tooth hand saw or even a hacksaw will work as well.  

Place the template against the handle of your cutter holder and mark the location of the cutter.  This identifies the part of the template that should contact the grinding wheel when you are adjusting the tool rest.  

Do it.  Grind the cutter.  Set the tool rest at the proper angle using the template.  Attach the cutter to the holder, place the holder flat on the tool rest, and make a “test touch” of the cutter to the rotating grinding wheel. Check the cutter to see where the contact was made, and if it looks reasonable, proceed with the grinding.  

Let me repeat this one more time: use a very light touch.  The cutter is relatively thin so you will need much less pressure against the wheel than when grinding a bowl gouge, for example. You probably will get a few sparks, but try to keep them to a minimum.  

To do the grinding, simply pivot the holder from side to side, keeping the holder flat on the rest at all times.  Rotate the cutter on the holder as necessary.  Try not to grind too much in one spot which will create a flat on the cutter.  

After grinding, you should feel a rather prominent burr at the cutting edge.  This is the burr that you remove prior to honing, described earlier in this article.  So, do the honing operation and the cutter will be ready to use.  

A note on grinding wheels.  The wheel I use is 120-grit aluminum oxide, one of the wheels that come standard on a popular slow-speed grinder.  In my opinion, an 80-grit wheel is too course; it’s too aggressive for the small cutters and gives a rougher surface.  

For a minor touch-up of the bevel, I use a well-worn 120-grit WoodRiver diamond wheel.  The diamond wheels get a lot of bad press now that CBN (cubic boron nitride) wheels are gaining in popularity, but mine has served me well.  I’ve used it extensively over a period of three years or more, and it is still doing the job.  

Grinding the Rod-Shaped Cutter

One end of my rod-shaped cutter is ground like a round-nose scraper.  I sharpen it using the exact same procedure as described above for a bullet cutter.  

The other end I have ground to the shape of a V, which does well for me as a wood-removing, shaping tool.  I sharpen it freehand, using the holder shown in the photos above.  

If you’re not “practiced and experienced” in freehand sharpening, place the holder flat on the tool rest and grind the shape of the V, with the angle of the rest set as for grinding the bullet and disk cutters.  The resulting grind will need to be rounded on the underside, but you can do this freehand.  Round it to your liking. Just stay away from the cutting edge.  

Honing the V-shape is optional.  Whether I do or not depends on the nature of the wood I’m cutting. If it tends to be soft and stringy, I will hone the V in an effort to get it as sharp as possible.  

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