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As a primary school student I took woodworking classes and recall the teacher sharpening the gouges and chisels that we used on the lathe. I use the word sharpening but that is, perhaps an assumption. He ground and polished the cutting edges of the tools.

Years later in the workshop of a university department, I asked the instrument maker how sharp the cutting tools on the metal lathe needed to be. He said that he thought that hardness, not sharpness, was really the issue with metal lathe cutters, and then simply assumed that I fully understood what I'd been told.

And then yesterday, using a pair of old hair clippers, I caught myself wondering whether they needed sharpening, which reminded me once more of the sharp vs. hard distinction which I never did investigate. Does it make sense to talk about wood-lathe tools as being sharp as distinct from being hard, and vice versa for metal lathe tools? I can think of all sorts of ways in which wood is not like metal (!) but what are the differences that lead to qualitatively (as opposed to quantitatively) different characteristics of the cutting tools? ... and do hair-clippers get sharpened, or does the hardness of the steel versus the softness of hair solve all problems?

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    There is an entire branch of science called tribology that is focused on wear, friction, and lubrication. The questions you're asking have simple common sense "solutions," but those are not really answers - just how we end up doing things practically. It is not intuitive at all why hard metal gets dull when cutting "soft" materials like wood or hair. Likewise, some extremely hard materials can only be "cut" by grinding, not by slicing or shearing. One good rule of thumb (at least on the soft side) is that the softer the material the sharper the blade must be... think tomatoes vs carrots.
    – aaron
    Commented Apr 8, 2019 at 14:15
  • Just a headsup if you want to dive into the world of sharpness and sharpening, the subject is deep and broad. And unfortunately it's full of half-understood principles and outright falsehoods promulgated as absolute truths by their respective believers (who, by definition, are at loggerheads with each other if their 'truths' are different). So, uh, prepare for the arguments! I'm referring to sharpening in the broadest sense of the word here, covering every possible permutation including some steps that tend to be separated into the description honing, which is an aspect of edge refinement.
    – Graphus
    Commented Apr 8, 2019 at 18:11
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    "do hair-clippers get sharpened, or does the hardness of the steel versus the softness of hair solve all problems?" All cutting tools blunt, even cutting stuff as soft as vegetables.... so yeah, clippers can do with being sharpened eventually. They are designed in such a way as to minimise the need for this by using a shearing action, the cutters being quite hard and the edge geometry being very stout (90° I think!) which provides masses of support for the cutting edge. By comparison the stoutest edge angle you'd encounter in a more conventional cutting tool might be in the range of 45-60°.
    – Graphus
    Commented Apr 8, 2019 at 18:15

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Hardness and sharpness are two distinct attributes that can be applied to these devices and/or materials.

The instrument maker is off-base if he considers that sharpness is not important or a factor to be considered when dealing with steel cutting tools. Certainly hardness is important. A soft steel cutter will possibly cut if suitably sharp, but will not last long. A hard steel cutter will not necessarily cut if it is not appropriately shaped and sharpened.

I use high speed steel cutting tools on a mini lathe and have superior performance when the tool is sharpened to the profile required for a particular task. That performance is improved if I use a diamond hone on the cutting edge. The resulting surface finish is quite a bit nicer with a honed tool than with a sharpened only tool.

Grinding is the method for sharpening steel. Typical grinding stones are coarse grit, even the fine grinding wheels are not particularly fine. The edges that are created by sharpening will have microscopic irregularities that transfer to the workpiece. A honed edge reduces these irregularities and the improved sharpness also reduces (slightly) the load on the machine.

A cutting tool such as hair clippers use a shearing action. A dull edge on the twin cutting edges will also increase the load on the machine, although it is likely powerful enough to overcome this.

Sharpening the shears will improve the speed of the cut, reduce the strands of snagged/stuck hair and at the microscopic level, improve the quality of cut on the hair. I would not be surprised to learn that one can incite split-ends by using poorly sharpened shears.

The steel in hair cutting shears are most-certainly hardened, and also sharpened. Un-hardened steel will not hold an edge for very long. The action of steel in shears, movement against each other, will also dull steel, hardened or otherwise. The difference between the two is how much time is involved.

I've sharpened ordinary scissors, also called shears, and the improvement in performance is striking.

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    Oh, yeah, paper cut: really sharp! not hard at all.
    – fred_dot_u
    Commented Apr 8, 2019 at 17:54
  • Pardon the pun, but aren't we really splitting hairs saying that grinding and honing are truly different tasks, outside the granularity of the material. For instance, if I "grind" on an 80 grit wheel, then "hone" using a 300 or higher grit plate, the action and result is still the same except for it being finer with higher grits. If I have a 1000 grit wheel, is that grinding or honing?
    – user3158591
    Commented Apr 14, 2019 at 1:47

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