Naked Science Forum
General Science => General Science => Topic started by: decepticon on 06/09/2009 22:28:38
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hi all,
just wondering recently whether sharpness of razor blades, knives and drill bit edges for example, have a value of sharpness?
could this really be defined?
regards
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I don't think so. When I have seen sharpens reported it is as the thickness of molecules at the edge of the blade. An obsidian blade is one of the sharpest, only one molecule thick on its edge.
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An interesting question. I will have a "stab" at answering though I am not sure how sharp the answer will be. I don't know of a measure of sharpness as such.
I guess that sharpness can mean slightly different things in different applications. In everyday language sharpness may have different specific meanings. For the purposes of cutting a certain degree of serration (deliberate of incidental) may be of advantage. The blade may even feel sharper as a result (it will cut more easily) even though the blade as a whole may be poor. It might cut rather unevenly but may do so with some ease. This is useful with a weapon (for example) and would probably be an advantage whereas a scalpel has to cut very smoothly.
It could be considered that sharpness is the the acuteness of the edge. But this would not apply to drill bits or opposed slicing blades (like scissors) where the angle is only a little less than a right angle but where the edge is considered sharper if consistant along its length. It is hard to cut your finger even on a sharp drill bit for example.
In general, I would guess that sharpness of an edge is really to what degree the angle (of the two sides that converge to form an edge) is maintained all the way down to the edge itself. I would guess this could be measured as the average radius produced at the edge. This would then be a measurable value and useful in describing a blade or point for a specific application. I have used microprobe needles which I have sharpened myself to have a point whose radius is a few microns for example.
Perhaps someone else can confirm that the radius of the edge or point is a good measure though I have never seen Gillette use this in their adverts :-)
Ah, someone has posted whilst I was typing, but I'll post anyway.
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The sharpness of a blade, (how effective it was at cutting), would be in part due to the hardness of the material it was made from.
there are scales of hardness ... http://en.wikipedia.org/wiki/Indentation_hardness
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Would it be measured on the Sweeney Todd scale?
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I think JimBob has it right.
There's no 'sharpness' scale that I'm aware of.
The 'cutting ability of something is more to do with how 'thick' it is. Thus, if obsidian blades have been made that are only one molecule thick on the cutting edge, then the only 'sharper' thing you could possibly have is something comprised of smaller molecules.
A few Sci-Fi stories have featured single molecule filaments i.e. a long length of filament consisting of a single molecule. Rather than being 'sharp' they'll cut through anything in the same way that a cheese-wire will cut through cheese: not because the cheese-wire has 'sharp' edges; it is round, but because it's very thin.
Sharpness is something that really just applies to angles i.e. the angle between the two surfaces of a blade, but it is the thickness of the cutting edge, not the angle between the two surfaces that dictates how well it cuts. Scissors are a good example. If you look at a pair of scissors you wouldn't describe them as being 'sharp' because the two surfaces of the cutting edge are at nearly 90 deg to each other.
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You're not really cutting anything when you're dealing with gases and fluids because the molecules aren't bound together in the same way they are in a solid, so I'm not sure that the stuff about wings leading edge radii is relevant here.
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I suppose it would be necessary to include the angle between the cutting planes as well as some measure of the radius. For example, machine tool cutting bits can have "negative rake" and the angle might be 90° or more, but they still get blunt with use.
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I can't find the book on my shelves but I have (had) a book on basic knife sharpening that did discuss sharpness in graded terms.
They were very general though, nothing quantitative. And I don't know if they were widely accepted terms or unique to that book.
Razor sharp will cut hair with ease, utility sharp will cut paper but not shave, and so on.
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thanks to everyone who has taken time to answer my query
as i expected there isn't a definable value to sharpness but glad i asked and some thought provoking answers
cheers
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ISO 8442-5:2004 specifies the sharpness and edge retention of knives which are produced for professional and domestic use in the preparation of food of all kinds, specifically those knives intended for hand use.
Powered blade instruments of any kind are excluded.
Generally these types of knife are manufactured with blades of either plain edge design or with edges incorporating particular features to enhance or optimize aspects of cutting ability.
The following two types of knife blade are suitable for the cutting test.
Type A edges: cutting edges which can be resharpened by the user and edges with a pitch greater than 1mm;
Type B edges: cutting edges which are not intended to be resharpened on a steel.
Whilst these knives are predominantly manufactured with blades made from various grades of heat treated steels, the testing of knives of any construction or blade material is not precluded providing that the test criteria are met.
The principle of the testing is to reproduce a cutting action, by forward and reverse strokes, against a pack of synthetic test medium under controlled parameters.