1. measuring set-up (5579)
2. sharpening "triangle" (5577)
3. setting pivot post height, 66.3 mm = 14° (5580)
4. 9 mm thick riser block, 6° (5583)
I. Measuring set-up
The first Edge Pro tip I learned from this forum was to record the distance from the front edge of the blade table back to the knife guide, so as to always use the same setting when sharpening a given knife. That is an easy measurement to make, but what about sharpening angle? The colored marks on the pivot shaft indicate 2° increments, and are spaced almost 9 mm apart, hardly enough for your average obsessive! In addition, ever since I began buying and using Japanese knives (some of which are asymmetrically bevelled), I have wanted to be able to set and sharpen much lower angles than the lowest indicated setting of 13°. In order to make the necessary measurements and calculations, I made several temporary modifications:
(see photo 1: 5579)
1. Knife table. The black Edge Pro knife table is slightly chamfered on its edges, making it difficult to take measurements along the 30 cm (12") Staedtler rule. So I held an engineer's rule flat against the end of the table, and taped a white 4x6 card to the bed, carefully aligning the edge of the card to precisely coincide with the end of the bed as though it had never been chamfered. Both the sharp edge of the card, and its whiteness, made taking subsequent measurements much easier.
2. Tape blank. Since all water stones wear, I standardized my measurements by using a new tape blank supplied by Ben Dale as the hypotenuse of the triangle I was creating and measuring. To facilitate supporting the 30 cm ruler, I attached a few spare riser blocks of 3 mm plexiglas to the undersize of the blank with 3M double-sided tape. This ensured that the line I was measuring was effectively the same one as the surface of a stone or polishing tape.
3. Ruler. Especially when calculations are involved, I find that I prefer to work in the metric system, and so purchased a precision 150 mm metric rule from Mitutoyo, model 182-111, to use with the Edge Pro. For quick work, millimeters are pleasantly precise, and for very careful work (with either close-up glasses or a loupe), I can read and/or estimate within half millimeter markings fairly easily. On the other hand, I find 1/32" intervals a bit coarse, and 1/64" intervals very hard to read. The Mitutoyo satin chrome metric rigid rule can be found here for about $15 USD:
Rigid satin chrome rules like the Mitutoyo (or Starrett equivalent) are ideal for the sorts of measurements I was making, as they are both wide enough and thick enough not to rock, even when being stood on end. Once again, a strip of 3M double-sided tape made it easy to find the correct position for the rule, and then lock it into place each time.
II. Measurements and calculations
To begin, I wanted to check the accuracy of the marked angles on the Edge Pro, and arbitrarily selected 13° as a starting point. I made sure that the 15 cm metric rule was perpendicular to the bed, and used the double-sided tape to stick it to the square, black plastic block at the top of the pivot shaft (above the colored angle markings). This placement automatically formed a right angle with the blade table.
(see photo 2: 5577)
Next I needed to take two measurements:
1. Opposite. The height on the pivot post where it intercepted the line formed by the polish tape blank.
2. Hypotenuse. The distance from the front edge of the knife table (where a blade is sharpened), to the edge of the metric rule on the pivot post.
The sharp corner of the 30 cm metric rule made it easy to read out the height of the triangle on the 15 cm rule, and the white card made it easy to read out the length of the line thus formed. Using simple trigonometry (sin = opposite/hypotenuse), I then calculated the sharpening angle formed by these lines. In this case, my height (opposite) was 50.5 mm, and my hypotenuse was 224.5 mm, for a sin of .2249. Using the calculator's inverse sin (arc-sin) function, we find that this is exactly 13.0°, so Ben Dale's marking is very precise (as we would expect!). Here is an image of the Edge Pro set to 14°, with a post height of 66.3 mm. Again, I am taking my measurement from the top of the block in which the pivot post is seated, to the top of the block at the top of the pivot post.
(see photo 3: 5580)
In order to calculate in advance the amount that the pivot post needed to be raised or lowered to change it by one degree, I measured the distance between the most extreme positions marked, 52 mm, between the 13° and 25° marks, and divided 52 mm by the 12° difference to get 4.33 mm/degree of change. I proceeded to lower the post by roughly that amount, take measurements and calculate to verify that these values would hold for all the angles between 20° and 7° (with the pivot post all the way down, I measured 7.16° as the lowest position the stock Edge Pro Pro can assume).
III. Angle settings
These are the calculated, and verified, pivot post heights for each angle, in millimeters:
7° 36.0 mm
After cutting a riser block out of 9 mm thick plexiglas (48 x 74 x 9 mm), I was able to determine additional post heights for 5° and 6°, which is as low as I need to go for any of my current knives.
5° 35.0 (with 9 mm thick riser block on blade table)
6° 39.5 (with 9 mm thick riser block on blade table)
(see photo 4: 5583)
Note on marked angles. As has been discussed before, while the "marked" or calculated angles listed here are quite accurate, they don't take into account the fact that almost all blades taper in thickness even before the primary and secondary bevels are reached. If a blade, in cross-section, was a rectangular block sharpened only on one edge, then the marked angles would truly describe the actual included cutting edge angle. However, as the bodies of most blades are also wedge-shaped, the actual included angle will be several degrees more acute than the values listed above. I wonder, for example, just what the actual angle of my Takeda bunkabocho is, since it tapers radically, in addition to being sharpened at the 9° angle setting. I thought it was sharp when I received it, but it's now the one knife that my fingers won't slide on when doing the Murray Carter three-finger test!
IV. Adjustments and stone/blank thickness
I have still not used the Edge Pro enough to feel truly expert with a whole variety of knives, but after doing these calculations and studying its construction carefully, I came to a few additional conclusions that may be useful to other users.
1. Fine angle adjustments. Using the marker technique to color the bevel, we all know that small adjustments in raising or lowering the pivot post can make the difference between polishing the bevel or adding a micro-bevel to the cutting edge. I now know that changing the post height by 4.3 mm means changing the sharpening angle by 1°, which means that a 1 mm change equals about 1/4°, a very fine increment indeed.
2. Stone/blank thickness. Using a dial caliper, I measured the thickness of my current set of Edge Pro stones and polish tape blanks, and was surprised at what I discovered. I lap my stones frequently on DMT Dia-Sharp plates when I am sharpening, and so they're quite flat (I also check them periodically against an accurate engineer's square). The thickness, however, varies between stones by as much as 1.5 millimeters, with the thickest at 7.3 mm. The polish blanks are substantially thinner, as low as 5.8 mm, and ranging up to 6.4 mm. Seeing how different the tape blanks are one from the other, I am going to drop them off at a machine shop next week and have them all milled to the same thickness so that once I'm polishing I won't have to make any further angle adjustments unless it's for a specific effect (the difference in the thickness of the PSA tapes themselves ought to be negligible). That will mean lapping them again, but the consistency will be worth it.
As for my stones, I now know that as they stand — and they don't change thickness very fast — I'll need to adjust my pivot post height up and down with each stone change so that I'm hitting the bevel at an equivalent angle at each grit. The rule here is that as stone thickness decreases, the effective sharpening angle increases, which is why micro-bevels are formed by raising the pivot post and stone arm slightly (raising the post increases the angle).
This leads to a simple rule for stone thickness: if you are moving from a thicker to a thinner stone, lower the pivot post by the amount of difference between the stones. Example: my MF (medium fine, 220 grit) stone is 6.9 mm thick. My EF (extra fine, 320 grit) stone is 6.4 mm thick. When I'm ready to change between these stones, I need to lower the pivot post by .5 mm to compensate.
I still have some questions about changing guide clip bolts, guide clip lock bar clearance and raising the guide clip when using thick riser blocks on the "Pro" Edge Pro, but I'll save those for another post.