Thread: How do you know how sharp something is

The thumbnail test is often used to test sharpness of a kitchen knife. The story is that one leans the knife only with its own weight on the thumbnail and holding the handle lightly rock it back to a very acute angle. If the knife holds in position, its sharp.

Maybe it was my mum who told me that may be I read it, maybe my brain has turned to mush!

I thought that scalpels and razors are all finished "honed" with a laser.

Glen

Ok folks, here it is: Mick's Immutable Universal Law of Sharpness.

"The sharpness of a tool is in direct proportion to its propensity to jump off the bench or out of your hands and fall cutting edge first onto a concrete floor or alternatively, if you manage to keep a really tight hold on it, to immediately find an embedded piece of steel in the workpiece."

Simply put, if you sharpen a tool and manage to use it for more than ten mins without one of the above happening, you haven't sharpened it nearly enough.

Mick

Mick

I like that.

I would add, "sharp blades fall edge first".

We'll have to think of some more.

Regards from Perth

Derek

How do you know how sharp something is?????

I have just found out the hard way that I wouldn’t really recommend.

I had just sharpened my smaller skew chisel and everything was looking good. Upon arriving back at the lathe I noticed that the tool rest was slightly too high for what I wanted to do. In a reflex action I placed the skew between my legs, point facing up to adjust the tool rest. Well you may be now thinking the worst but I quickly moved my left hand down for some reason and caught the very sharp edge of the skew just above the first knuckle on my little finger. As predicted with the sharpness of the skew the hair cut smoothly and so did the skin. As the claret started to flow and I headed to the house to see the nurse the pain was dulled by the knowledge that I still had a super sharp skew waiting for me after I had been patched up.

The moral to the story …… don’t slice your finger as a method of checking the sharpness of any tool.

So are we proposing that to test the sharpness of a blade we place it on the edge of a bench. if it falls of and lands point down on the cement, statistacly it was probably sharp.

could we then quantify our results and see how long it takes for blades of a given sharpness to fall off a known bench onto a known floor of cement.

is it gravity.

could we simplify the arrangment. if sharp blades fall more readily, then gravity must act more upon them.

SO
on a sensitive scale measure your tool, if it weighs more after sharpening than before it is sharp.

Soundman

Your equation is scientifically unsound if it does not include provision for blood.

Derek

There exists a certain underlying principle of engineering. It's known by various different names: Sod's Law, Murphy's Law and, by the better-educated (so I've heard because I don't qualify) as:
The Innate Hostility of Inanimate Objects.

This principle manifests itself at its mildest when the falling slice of toast manages to flip itself in mid-air to land buttered side down.

The trick is to find a way to make the principle work for you. When I was a smoker (all of four weeks ago now: cough, wheeze - Jeez it's hard!) and back in the days when we were allowed to smoke in airport terminals, I could, three out of four times, get a call to board the delayed flight by ordering another beer and lighting a cigarette.

So - how do we get the Innate Hostility Principle to effectively gauge sharpness without sacrificing any claret? I don't pretend to know the answer but if we all work on it we'll get there (and it'll distract me while I'm gasping for a smoke!)

(Cough, wheeze!)

Col

OUCH!!!

sharp enough

A edge, when drawn lightly across a thumb nail should glide smoothly. If it has a slight drag it isn't razor sharp, it will however shave hair and cut meat beautifully.

The theory behind this is that if it drags it still has a wire edge on it, even if it is almost invisible to the naked eye, if it glides it has lost all of the wire edge and is honed to the ultimate final edge.

When this degree of sharpness is achieved it is ready for use as a woodcarving tool.

Most good carvers can attain this sharpness in less than a minute on regularly used tools and keep the fine edge for many months and with nothing more than a few seconds maintenance a day will have the edge become even keener as carving with the tool progresses.

Cheers - Neil