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Skate Sharpening . . .   Techie stuff . . . . .
ERRORS THAT PRODUCE or RESULT with UNEVEN EDGES
Please Read And Understand "Checking for Even Edges" Before Reading This Section. 
Producing even edges on skates is actually very easy. Knowing how your skates ended up with uneven edges is not as important as knowing how to check for uneven edges. If you want a deeper understanding of how some operators end up with uneven edges then read on. There are two common sharpening errors. Both cause or result with uneven edges.
ERROR #1 "If I grind down on the middle of the skate blade I will always produce even edges."  This seems to make common sense, however this statement is false!  When skating aggressively, particularly in hockey, the inside edge is worn more than the outside.  Furthermore, the inside edge is worn mostly under the ball of the foot and not much at all near the tail of the skate blade.  Many skates needing to be sharpened will not just have dull edges; they will also have unevenly worn edges.  The outside edge, which is less worn, is therefore "sticking out" further than the inside edge and it will contact the sharpening wheel first.
Look at
Fig H. - The illustrations are exaggerated for effect.  Here a skate with a high top edge is coming into the grinding wheel.  Note that the initial touch or first-pass will only show grinding marks at the top of this skate. If the operator ignores this and continues to grind - the high material will be taken off and the grind will drop into the center producing even edges: see Fig I.   Go back to Fig H. - Again if you lightly touched the tip and tail of this skate to the wheel or took one light pass, you will see that you are only grinding at the top of this skate.  What does this tell you? The answer is NOTHING! This is NOT an indication of where you will end up. This is only showing the wear pattern on the skate blade. A poorly trained operator who saw that only the top of the skate blade was being ground in Fig H will feel the need to raise the height of the holder to correspond to Fig J or change the angle of approach to Fig K.
Notice that while
Fig J and Fig K will both produce an initial grind that is in the center of the blade, both positions will result in the blade ending up the same way it came in . . . With a HIGH Top Edge!  Making matters worse is that this type of operator will usually not check their finished work since they believe they "ground to the center".
These are "Method Operators"; they pay attention to their method but do not check for results.
TO SUM UP: (1) Grinding into the middle of an imperfect skate will repeat the error and (2) virtually every skate sharpened will be imperfect.  GOOD SKATE SHARPENING OPERATORS are results driven.  They check and verify that every skate has even edges.  After the skate has been ground exposing all new edges, the blade is checked.
No skate should get back to the owner, much less make it onto the ice, unless it meets accuracy specifications.
ERROR #2 "Once I get my skate holder to produce a perfect skate I should leave it set that way".  This statement is false!
The reason this statement is false is that skate blades vary in thickness much more than one might expect.  "Standard" hockey blade thickness is .110 inch +/- ?  If there is such a thing as standard anymore!  Variations in thickness occur between different brands.
Even if you stay with one brand, manufacturing tolerances can and do vary.  To understand how blade thickness variations effect skate sharpening accuracy, see
Figs L, M and N.   Fig L shows different skate blade thickness.  Skate #1 is obviously thicker than skate #2.   This could represent a figure skate versus a hockey skate; a hockey goalie skate versus regular hockey skate.   Fig M shows the skate holder height set to produce even edges on skate blade #2.   Note that skate blade #1 has a high top edge at this setting.   Fig N shows the skate holder set to produce even edges on skate #1.   Note that blade #2 has a high bottom edge at this setting.
CONCLUSION Operators committing Error #1 are working under a false assumption.  They are adjusting their holders too soon and too often and end up "chasing their tail" and cannot guarantee the results.  Operators committing Error #2 are the opposite and are NOT checking OR adjusting anything.
                                 Creating even and square edges is easy if the skate-sharpening operator is
results driven
BLADE THICKNESS
BLADE HOLDER HEIGHT
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The information above was originally presented by Wissota Mfg, at www.wissota.com