Sharpening narrow cutting irons often tilt to surprisingly odd angles no matter how experienced you are at sharpening. It takes more than just countering the tilt to get the ends even close to never mind dead-on square. What’s really needed is a guide for the abrasive to give rigidity to the task. I designed this guide because of the frustration of diving from one ditch into the other. Freehand sharpening is often good for a single sharpening but repeated sharpening often follows the biases of our stronger or weightier dominant side and we end up with the inevitable skewed end. On deeper grooves that might work fine, but on inlays or shallow recessing the bottom of a groove mostly needs to be parallel to the top face of the wood otherwise our inlay or inset piece ends up tilted.
I should point out that on these cutters I tend to go with either a single bevel of 30º only or the two-bevel type and not my usual macro-camber as on all other planes. On narrow irons, it is more difficult to keep the cutting iron level across and the bevels tend to get rounded.
To make my guide, use any scrap of wood squared up and trued to somewhere around 6″ long by 1″ by 3/4″; these sizes can be bigger, depending on the cutter sizes.
Set a protractor or sliding bevel to 25º and strike the knifewall with a sharp knife.
Set the protractor or sliding bevel to 30º, measure a 1/4″ at the narrow end of the channel . . .
. . . .and strike the second knifewall.
Square vertical walls down onto the adjacent face using the angled lines to align the knife to.
Set a marking gauge to a depth of say 1/2″. This will take care of most of your plough plane cutters. If you want deeper use thicker wood stock if you want to but there is no real need to as the cutter can extend beyond the sides of the guide.
It’s important that the cuts you make along the lines are square to the face as these faces will govern the squareness of the sharpened edge.
With the sides sawn, chisel from both sides to try to remove the waste as intact as possible. You will use this piece to establish the retainer wedge shortly.
You can simply chisel the bottom of the recess parallel to the outside face as the bottom is not critical. It’s the walls that are more important. I own a narrow router plane.
Lay the angled waste along a straight edge and establish a cut line. making the line and the wedge longer will enable you to plane one of the faces if you are slightly off or out of square in your sawing.
Pare down the knifewalls if you need to correct any discrepancies too.
Slide the cutter in place and then the wedge. Tap it tight to establish a cut line and mark the cut lines as shown and cut to length.
To Use the Guide
I have diamond hones of different levels of fineness, coarse, medium and superfine, but abrasive paper glued to paddles works just as well. The advantage of diamonds is that you can push and pull to abrade. With abrasive paper, you can really go only in one direction in the abrading. It’s also good to remember that they make diamond nail files, they make large and small, and these work well for the coarser abrading.
Use a hammer to tighten the wedge against the cutting iron from the top side as shown.
For the coarse abrading place the cutter against the 25º wall and wedge from the other side. The lowest point on the bevel should be about flush with the face of the guide. Once the bevel is established . . .
. . . remove the wedge and wedge the cutter against the 30º degree wall using the same wedge.
This time the wedging of the cutter should be very near flush with the face of the guide as the bevel should be as small as you want. The bevel will get wider with subsequent honings and then you will reestablish the primary bevel. You need only a narrow band of honed edge for the cutter to work great with this initialising for the cutter.