Straight sharpening vs. Side sharpening in Theory
I would like to preface this article with the statement as a reminder that herein all of the opinions expressed by me are really just that, opinions. I do not
possess a high powered microscope nor am I a trained or qualified metallurgist. My visions and ideas about valleys and peaks are my own invention to
describe what I feel would represent the ideally sharpened blade. The likelihood that any blade could actual be sharpened with delineated valleys and
peaks is just a wish and should be considered just conjecture on my part. Also the concepts of the peaks or ledges breaking off in certain sequences or
events is just guesswork on my part because I have not actually observed this, as of yet, at the microscopic level.

The successful process of sharpening a tool blade should result in the removal by reduction of metal from said blade in a systematic manner with the result
of a cutting edge comprised of freshly exposed tool steel. With a low powered microscope the freshly exposed steel may appear to have either; a random
scratch pattern on the blade if while sharpened it was rotated in a circular stroke while in contact with the abrasive stone, or a linear pattern if the blade was
stroked back and forth in a systematic manner. Any abrasive by their very nature of abrading will leave a scratch. Furthermore you might notice that each
scratch is shaped like a valley and that it has an associated ridge on either side of the valley. This peak/valley effect is highlighted if the surface of the metal
is illuminated with a raking light directed from a 90degree angle to the scratch pattern. The width and depth of the valleys and ridges should represent a
relationship between the size of the grit particle and the compression force factor upon the blade transmitted to the girt withheld in sharpening media.

The actual scratch pattern at the true cutting edge of the cutting blade, where it falls off into space is the business end. This is the primary concern to the tool
user. The portion of the tool that is behind that leading edge can be important to the tool user, but will not be discussed here. When the tool is put to the task
of cutting wood, this leading edge suffers all of the stresses of the cutting action firsthand. This is the exact point where the deterioration of the tool steel
begins at the atomic level and is affected by various outside agencies and internal factors like; oxidation, metal fatigue as in stress fractures, exposed weak
relationships between alloys, hardness & tempering issues, the hardness of the wood and its fibrous natures, foreign in-bedded non-cellulose particles
and so forth.

Compounding the metallurgical issues are the sharpening procedures, or lack there of, that the tool user follows in preparing the blade for his/her own use.
I speculate that the users sharpening techniques at the bench can play an important role in the durability of the leading edge of the blade. The system of
sharpening and the abrasive used are all part of the recipe in regards to the tools maintenance and performance.

I suggest that under ideal conditions, the scratches at the leading edge of the tool might appear as miniature combinations of peaks and valley all neatly
lined up next to each other if it were sharpened with straight on strokes perpendicular to the blades edge. The peaks will protrude beyond and represent the
actual leading edges of the tool while the valleys will appear to be receded behind and between the peaks. The edge will have a toothed or comb like pattern
were the peaks represent the teeth of the comb and the valleys represent the spaces between the teeth.

I further suggest that in angle sharpening, under those same ideal conditions, the peaks and valleys will be present but will appear to have and off setting
angle that leans to one side and that this angle's degree/off perpendicular would be directly related to the angle degree that the blade was presented to and
pushed into and along the sharpening media.

If the blade was sharpened in a circular or random manner the scratches would appear to be irregular and would enter and exit the blades edge in a
haphazard way with overlapping "teeth", those pointed angular protrusions of the comb like description.

Conversely if the blade was side sharped where the blade edge is pushed along in a motion parallel to the stone, the leading edge of the tool will look like a
series of ledges or steps running parallel with the blades edge. No peaks or valleys will appear along the cutting edge of the tool but would by necessity
appear on the side edge of the tool bevel and therefore out of the cutting area proper.

In Practice
The way I see it in my mind, in using the blade, if it were sharpened with a straight on sharpening stroke and if it is being pushed and moving into the wood
in a straight on direction, this forward force of the tool as it projects into the wood will transform the forward energy into a cutting action, and in doing so
because of the angular symmetrical peaks and valleys of the shaped steel, a of portion of the forward energy will begin to mimic and act as lateral forces,
thus creating all kinds of extraordinary stresses to the steel during the engagement between of the steel edge and the wood fibers.

Granted, the wood fibers will in most likely be irregular in their structure, but at least in the case of the steel edge we are beginning with a known
symmetrical structure. The peaks and valleys are three dimensional, and the ridge of the peak radiates back as a triangle to the base of the peak where the
valley intersects at its lowest point. This creates a geometrically regular shape based on the action of the abrasive with a triangular profile that acts to evenly
deflect and distribute the forces from the forward movement of the tools engagement of the wood back into the ever thicker body of the blade behind the
actual leading edge. As the tips of the protruding forward peaks are engaging with the wood, they are stressed back and forth over and over again in
downward and sideways movements and by doing so will begin to be broken off as the result of the various forces impacts them including metal fatigue,
abrasive properties within the wood, heat transformation of the steel generated by friction, etc. The peaks will generally be broken off singly or in closely knit
groupings but in time all the peaks will break as the tool is used resulting in dullness and the need to resharpen.

Angle sharpening strokes created by various tool-to-stone angle degree ratios will result in similar valley/peak profiles as in the straight sharpening stroke
but the peaks will be pointed askew to the tools cutting edge as the sharpener designed. In the case of angle sharpening I speculate that as the blades
leading edge in the forward movement engages the wood, the peaks will be stressed up and down and back and forth (as described above in straight
sharpening), but the stresses will noticeably be more towards and favoring the direction of the askew sharpened angle. This favored angle pattern should in
effect be acting as a multiplier factor of those stress forces. I conjecture that this happens because in a mechanical sense the peaks are acting as wedges
and they are converting the tools straight on forward push force to a side force that is in relation to the angle of the wedge. This converted side force exerted
on the peaks after straight on sharpening allows the peaks to flex from side to side and up and down. By contrast the converted side force present and
affecting the peaks after angle sharpening tends to favor one side of the wedge/peak because of the exposed and leading long side of the triangle shaped
wedge. Thus the converted side force begins to act as a
dominating side force to over affect the one short side of the triangle peaks. This dominating side
force places the tips of the peaks at an even greater risk of premature stress fractures, fatigue and breakage. This I believe is in part due to the inability of the
peak to reverse the flex pattern or to in other words to flex back to the "at rest" position during the cutting action, and to then flex further over to the opposite
direction and thereby equaling out the internal stresses of the peak. If this could be shown, it would point to a possibility of premature metal fatigue as the
result of repeated bending in only one direction.

I am guessing that in the case of a
side sharpened blade where the leading edge resembles not valleys and peaks but instead a series of steps or ledges,
the dulling of the blade will take effect in a different manner. As the blade is engaging the wood fibers there will be less side to side flexing of the blades
micro edge because there are not the triangle shaped peaks present to encourage this, but instead because of this ledge profile, most of the flexing will be
in the down reference. During the flexing in the down manner the same stresses will be placed upon the blade to maintain its integrity as before but
because the engagement of the wood fibers at the leading edge. The blades leading edge is challenged to be both be flexible enough to bend and to return
to the at rest position but at the same time rigid enough to sever the cellular fibers of the wood. In doing so when one area of the ledge fails a fissure will
begin to develop at the base of the ledge this stress fracture will lead and run down the length of the ledge in a millisecond. The leading edge that is made
up of first ledge may or may not break off as one long piece, but it will all snap off at some point and once this first ledge has broken off the next ledge is at
risk. This next ledge has a thicker profile and tougher but the blade will act duller. This may be a good working edge for some tasks like chisel work or
general planing but would likely be challenged in producing  ultra thin shavings over a long duration of bench time.

Finally in the case of the
random or circular sharpening stroke the edge will become dull in a random or unpredictable sequence. Some parts of the
leading edge that look like steps will break off in wider pieces and some that look like peak of various angles will break of in small groups of all sizes.
Circular sharpening strokes are used by some knife sharpeners but not usually used on broad tool blades because the desired flatness of the blade bevel
is difficult to maintain in a circular motion and thus this sharpening stroke is not broadly taught or recommended for tool users.

For sharpening Japanese kanna blades, one expert sharpener recommends that in the sequence of the course to medium grit stones an angle type
sharpening stroke works well  by helping to establish the blade bevel flatness. He begins with strokes through the medium fine stones with the blade
towards a more straight on stroke. The final finishing stones he recommends that those strokes be straight on and that the final polish be consistent with
straight on strokes to the stone. This is the free hand sharpening method I also use for chisels and plane blades.