Heat is the part nobody sees
Kitchen knives are hardened steel, and hardness is put there by heat treatment at the factory. Heat can also take it away. A grinding wheel spinning against a thin edge builds temperature at exactly the place with the least metal to absorb it — the last millimetre of the blade, which is the only part that does any cutting.
When that happens the steel loses temper. The knife still looks fine. It sharpens up on the day and then goes blunt in a week instead of a season, and it keeps doing that for the rest of its life, because the damage is in the steel and not on the edge. There is no fixing it short of grinding the softened metal away entirely.
A water stone cannot do this. It cuts slowly, with water carrying the swarf away, at hand speed. The blade never gets warm enough to matter.
How much steel each one takes
Sharpening is removal — you are not adding an edge, you are cutting metal away until two surfaces meet cleanly. The question is only how much.
A hand-stone progression takes a fraction of a millimetre: enough to reach fresh steel and shape the bevel, no more. A powered wheel, run for a few seconds by someone working through a queue, takes visibly more, and it takes it fastest at the tip and the heel where the pressure concentrates.
That is why an old knife that has lived on a grinder develops a scooped, narrowed profile — the belly worn back, the heel eaten in. Nothing was broken. It was just spent, a little at a time.
Edge life, in practice
A properly stoned edge on a decent kitchen knife holds for months of home use. The same knife off a fast wheel is often sharper for the first two days — a coarse wheel leaves a toothy edge that bites well — and noticeably worse by the second week, because that toothy edge is fragile and the steel behind it may already be soft.
This is the whole trade. Speed and price on one side; the life of the knife on the other.
When a grinder is genuinely the right call
Honesty is worth more than a sales line here. A powered wheel is the correct tool for reshaping a badly damaged blade — a snapped tip, a deep chip, a completely wrecked profile — where a great deal of metal has to come off before any refinement is worth doing. Done carefully, cool, with the right wheel, that is real work.
It is the wrong tool for routine sharpening of a kitchen knife you intend to keep. That is a stone's job.
What 956 Sharp does
Hand stones only. Every blade goes through a coarse stone to re-form the edge, then finer stones to polish the bevel, and is tested on paper and on a tomato before it is wrapped and returned. $15 per knife up to 8 inches, $17 over, 24-hour turnaround.
Chips and broken tips are quoted after we see the blade, because how much steel has to move decides whether it is a sharpening or a repair.