A question arises as to how to get the edges even on powered sharpening machines, by centering the dressed shape of the sharpening wheel on the blade. Some people, like the person who tried to show me how to use such a machine (badly), iterate the process - get it close, sharpen, get it closer, resharpen, etc. That wastes a lot of steel - and therefore enormously reduces the lifetime of the blade.
You can virtually eliminate such iteration by taking measurements. Let me suggest the opposite approach, and take many measurements. This will waste little or no metal, other than what is needed to make the blade sharp. Many people would say this is overkill, far more measurements and time than a well-practiced skate tech would need. But for a novice skate sharpener, the "magic of measurement" can compensate for a lack of experience.
1. Check that you really need to sharpen. 1/2 to 2/3 of the time, you can save metal by just straightening a bent over edge. You can feel a bent over edge by running your finger across the blade sides and over the bottom. (Do not run your finger along a blade edge or burr - it is easy to cut yourself that way.) Ideally you can straighten the edge with a "steel", such as is sometimes used on kitchen knives. I use a very fine grain (e.g. 5000 or greater grit number) flat sharpening stone. I run it gently along the sides. If you want to avoid roughing up the sides (a good idea), you can use tape almost up to the bottom (hollow) of the blades. You may wish to tilt the stone a tiny bit inwards, towards the hollow, especially if you apply tape, but keep the tilt consistent.
2. Dress the wheel to the desired ROH (Radius of Hollow).
3. If your sharpening machine's blade holder forces the blade straight while you sharpen, skip to step 6.
4. Place a straight edge across one side of the blade. The shortest dimension on the straight edge is usually the straightest, so place that against the blade. Do they touch perfectly, all the way, or is the blade warped? Now do it on the other side of the blade. In some cases, the blade sides aren't straight and parallel, so it will appear to be warped. In that case, in the best possible solution is the one that creates the same deviation from straightness on both sides.
5. If the blade is warped, straighten it. Many sharpening machines have a small tool (maybe on the side) that can push against the center of the flat part of the blade, to straighten it. If the machine doesn't have such a tool, use a vice. The vice won't quite make it straight - you may need to overcompensate a little, by putting a few thicknesses worth of paper against the side the blade is warped away from. Go back to step 4 and repeat, until the blade has no warp. Note: de=warping a very warped blade can break it. So one skate tech I have known returned new blades that had more than about 1 mm of warp. This probably annoyed the blade makers – he had to pay shipping costs, and they sent the returned blades to a less picky skate tech. But if it has significantly more than 1 mm, you very likely will break it, and you will have lost the costs of the blades.
6. Use a square to mark two straight pencil lines at approximately right angles to the bottom of the blade, one near the back, one near the front of the length to be sharpened. You will eventually take measurements along these lines to test centering.
7. For the next steps I assume there are two places to adjust the centering of the dressed wheel on the hollow surface of the blade, one near each end of the blade. Assuming the blade is mounted horizontally (turned on its side), this is done by raising and lowering the blade holder, or the table, at each end. If these is only one point of adjustment, the best you can do may be to compromise between centering at the two ends, or perhaps to use the center of the length.
8. With the machine turned OFF: Mount the skates (or blade) on the machine's blade holder, centered as well as you can on the blade holder by eye. If there is no holder, but just a table on which to rest the blade, likewise adjust the table height to be as well centered as you can.
9. With the machine still off: lightly touch the wheel to the blade at one end on the outermost pencil line. Adjust a
divider or
Inside Calipers to the distance between one side of the blade and that side of the wheel, along that line. Check that the distance between the other side of the blade, and the other side of the wheel is the same. If not, adjust the height of the blade. Repeat until the centering is perfect. You could get an even more consistent match by using a depth gauge, such as is included
here (though Harbor Freight tools are not always of high quality), or the depth probe present on many precision calipers.
10. Do the same thing at the other end of the blade. Repeat step 9 and 10 until both are perfect.
11. Things might still not be perfect, because some machines don't quite dress symmetrically on the wheel. So, with the machine still off, use the wheel to scratch a very short mark across those pencil lines at each end, moving the blade (or blade holder) by hand to make the scratch.
12. Now repeat what you just did in step 9 and 10 with the sides of the wheel, but use the sides of the scratch. Repeat this step until both until the scratches at both ends are perfectly centered on the blade.
13. Use a broad magic marker to create a dark layer across the whole hollow of the blade. (Some people use pencil - but I'm not clear how to make a uniformly thick layer of graphite with a pencil.)
13. With a cloth, wipe a very thin layer of light machine oil on the sides of the blade and along the hollow, so the cut will be clean. Not everyone does this - some people say it gums up the wheel, which then needs to be cleaned of oil. But I believe clean edges glide longer and faster.
14. Turn the machine on. Very lightly draw the length of the blade across the rotating wheel. (If there is no eye guard, wear safety goggles! Also, tie back long hair, and do not wear baggy sleeves, that can get caught in the machine.) You want to remove just barely enough metal so that the magic marker mark disappears. If you aren’t adjusting the rocker curvature or sweet spot, try to take the same amount of metal off the whole length. Do NOT round off the back of the blade, and do NOT touch the toe pick.
15. Take the skate or blade out of the machine. Using a square across the blade, check that the inside and outside edges are level. If not: the previous skate tech must have sharpened the blade very far off center; make another pair of pencil lines 1/4 - 1/3" inwards from the first ones, and go back to step 7. The repetition will waste metal. (It is actually a good idea to use pairs of straight edges, and line them up by eye. The two straight edges should be perfectly parallel. Some straight edges designed for the purpose are magnetized, so they stick to the blade)
15. At this point, depending on how coarse the wheel is, there may be a lip or burr, oriented outwards to the sides, along each edge. You can feel it by very gently running your finger across the blade sides, just like the bent over edges in step 1. (Again, do not run your finger along the blade edge or burr.) You can do one of two things: "Deburr" it by removing the burr, or straighten the burr, and polish it flat, to make an ultra-sharp foil edge. Most skate techs think they deburr it, but actually do something in between. True foil edges are very fragile – taking even one step off ice bends them over or breaks them. But they make the blade super-sharp even if the ROH is large, which I believe means they can be used to extend the lifetime of the blade. But people who don’t take good care of their edges should not use foil edges.
16. In either case, do the same thing with the steel or flat stone that I mentioned in step 1. If you tilt the stone slightly inwards, it tends to deburr rather than create a foil edge.