[jyl]

There is no inherent advantage to sharpening with a machine (i.e. grinding wheel or belt) or by a commercial service (which will almost certainly use a machine, for economic reasons), versus hand sharpening (using stones). Except that hand sharpening takes time and practice.

Sharpening basically requires the edge to be beveled and then polished smooth. The bevel is typically at an angle between 30 degrees to 15 degrees and two-sided; in some cases a more or less acute angle; in some cases on only one side with the other at 0 degrees; in some cases a double bevel. The polish is usually as fine as your stones or strops go and your patience lasts; in some cases you want to leave more roughness ("tooth").

So, for your standard $60 Henckel knife of relatively soft stainless steel for your typical hard-on-knives housewife , you might do 30 degrees two-sided, and just use your coarse stone, this will be a thick, rough edge that will stand up to some abuse and last as long as possible given that she won't ever bother to maintain the edge. Your sushi chef with the $500 Japanese blades with very hard carbon steel edges might do 15 degrees one-sided with secondary bevel, polished to the equivalent of 8000 grit with a succession of waterstones that might cost as much as his knives, and he will touch up his edges every day.

I do my knives at about 15 degrees, single-bevel, two-sided, and I polish down to an Arkansas stone. That seems to work for the quality of my knives (decent-to-good, but no better) and available stones.

Your run-of-the-mill knife sharpening service won't hold the bevel angle any better than a skilled hand sharpener can, and won't polish the edge near as finely as a hand sharpener with the right stones can. They will probably use the same angle etc for every knife. And if they are too rough with the grinding, they could conceivably damage the metal.

I know this sounds geeky, it isn't - really serious knife sharpening geeks would call me a total slap-dash amateur.