Since chisels and similar blades have a flat upper surface, the WM-200 angle tool works fine for that. The recommended grinding angles for those blades are between 20-30 dps (although there is only one side to be sharpened). The blade itself is angled at about 45° from the working surface, which leaves several degrees of relief behind the cutting edge. In my case the bevel is down.
Spoiled by sharpening by numbers, I looked if setting the angle by T-USB was feasible, which would allow for grinding and honing at a precise angle, just as it is customary todays for knives.
All associated programs ask for the jig diameter. They were obviously developed with knife sharpening in mind, so they silently assume a symmetrically round jig (at the spot where it rests on the USB at least). All past knife jigs fulfil this demand, and the diameter was chosen for simplicity and convenience.
The real value of interest however, is the distance from T-USB to the imaginary center plane that is constructed by the line through the center of the spine (of a knife) and the cutting edge (let´s call this the center plane for the lack of better words).
This is the parameter the program works with, because this distance is a crucial part in constructing the triangle that spits out the T-USB-to-stone value we are after; on a standard knife jig it comes out as 12 mm / 2 = 6 mm.
The graph here Calculations used for calculating SVM Knife Projection displays those relations quite nicely.
But note, this is a source of error: the older models of the knife jig used to have a 12 mm metal bar and thus a consistent diameter throughout. They had the potential of non-symmetric clamping, which would lead to uneven bevels. Its current successor was made symmetric, but has an inherently variable bar size due to its construction. Using it at the outmost rest will indeed yield 12 mm, but on the inner rest, for smaller knives, it will be more like 14 mm. This introduces an error probably overseen by many.
Fortunately sharpening of knives is not rocket science, a fraction of a degree up or down won´t hurt as long as the settings are consistent, repeatable and symmetric, but keep that source of error in mind particularly if you are the nit-picking type.
I used the
The first parameter to determine is jig diameter. As mentioned before, the internally used value is the distance from T-USB to the center plane. In our case, the latter coincides with the top of the blade (there is no second blade side and there is no upper half of the jig rod). Consequently the choice for naming this parameter "jig diameter" turns out to be an unhappy one, but that´s how it evolved. Let´s call this real parameter center plane displacement for the exercise.
For the SE-77 square edge jig center plane displacement comes out as
According to the above explanations, this is only half of what the jig diameter parameter expects despite the fact that it internally works with exactly that number, so you have to enter double of that value, i.e.
One more measurement we need is projection. Projection is defined as the distance between the point where the center plane meets the stone and a plane that is in a 90° angle to it that touches the USB bar at the stop screw (let´s call this stop screw plane, see picture). This is a bit tricky to measure. (Note that neither the jig axle's rest nor the jig stop's contact point are necessarily meeting T-USB. They are different points, but close. Refer to above mentioned graph). Thus directly measuring the distance from the USB's rear side to the cutting edge in parallel to the center plane with a vernier is not yielding the right result, since the SE-76's flanges protrude towards the operator and obstruct any direct access to the USB with the caliper's feeler fingers.
See fig. 2 "measuring stop screw plane to rear jig offset". Place a try square flush to the top of the jig. (Left or right of the stone the jig can be flipped forward or backward and stay put in a resting position). Its one leg is in parallel to the center plane. Measure the distance from the inner edge of the other leg to the USB with the vernier.
It turns out that this offset is exactly
I chose to grind at 27.5 dps, and the T-USB value produced by the program is
I sharpened with this value, and after sharpening I checked the edge against the angle gauge notches on the WM-200 (fig.4: "controlling the final angle"); it fits perfectly into (one half) of the 55° notch. So this is another proof that the number method works universally.
A small appliance akin to the ones existing for knives could be made to mount the planer blade at a certain projection every time, in which case one could work with the very same setting over and over until the wheel diameter has changed. Even thicker or thinner blades would not make a difference, since the jig opens towards the bottom.
I later sharpened a plain chisel with the same method. Without going into detail (since the basic procedure is well documented), the angle turns out correct.
Edit: I updated the jig dimension measurements above after finding a more exact method.