How do you know how sharp something is

[Arron]

Hi, when you are sharpening something, how do you test just how sharp it is. I'm just asking in case there is an original solution I havent heard about.

I am keen to know what you do after you have removed all the hair from your left arm. I usually just carve into a nearby piece of scrap wood, but its a fairly poor test and not something I really want to do to a just-finished blade. Is there a better way ?

thanks
Arron

[Wood Borer]

You can shave hair with a chisel or a plane blade when they have slightly lost their edge particularly on a wide plane blade when you have been planing narrow edges.

I check my blades when the cutting is OK but not quite 100%. A close inspection of the blade in the right light at the right angle usually reveals a shiny white line on the very edge. A hone with a 6000 water stone usually brings it back to a good edge again.

- Wood Borer

[derekcohen]

Arron

Good question. Yes, after you have shaved all the hair from your arms you have only your legs to turn to, and I don't think I want to go there.

Wood Borer is absolutely spot on when he described what to do when you sharpen the blade, how you see the sharpness. But below is how you experience it.

The best test of a truly sharp blade, either a freshly sharpened plane blade or chisel blade, is to see if you can cut pine end grain. If you think that is easy, just try it! Soft wood tends to be crushed before it is cut, and only a sharp edge will cut the wood fibres before they bend.

For the same reason pine is one of the hardest woods to chop handcut dovetails into. The ends just crumble and crack. To cut clean edges in pine you have to slice the ends. And only a truly sharp edge will do this.

One alternative to pine is Western Red Cedar. Same story.

Regards from Perth

Derek

[soundman]

If you happen to cut yourself & don't even know you are cut till you see the blood, the tool is probably sharp.

It all depends on what you call sharp, sharp enough to your average builder would be useless to a fine cabinet maker.

[journeyman Mick]

Many years ago, as an apprentice I almost cut my thumb off with a chisel I had just sharpened. :( (I was doing the wrong thing and I knew it, it's the one and only time I've needed stitches). The doctor that stitched me up congratulated me on the cut, said he couldn't have cut it any cleaner with a scalpel.

Mick

[derekcohen]

Mmm ....

I wonder how scalpels are sharpened?

Regards from Perth

Derek

[journeyman Mick]

Nowadays they use disposable blade scalpels (or in a lot of cases, disposable scalpels) a bit like an exacto knife. Need you ask really, in this day & age isn't almost everything disposable?:(

Mick

[derekcohen]

Mick

Sadly so.

Still, I wonder how they are sharpened to such a high degree when manufactured. Anything to learn from this?

Derek

[journeyman Mick]

Hmm, never gave it a thought, guess it's the same process that gives us razor blades, craft knives etc. Hmm disposable tip chisels and planes? (Oh actually the planes have already been done, laminated bodies with disposable cartridge knives or even TCT knives, made in Switzerland or Austria from memory. ).

Mick

[Arron]

Good fish hooks these days are usually sold as 'chemically sharpened' rather then 'mechanically sharpened'. Always wondered how they do that.

Arron

[coastie]

I dont think you would find too many surgeons usung scalpels these days,mostly Laser, bloodless.
If it cuts hair on your arm with only the slightest pressur its sharp.keep fingers away!!

[Woodchuck]

Hi all,

I believe scalpels, lancets & most medical sharps are extremely sharp because they are electropolished/silicone coated. They are first cut to a sharp, low angle point then electropolished for smoothness. They are then given a silicone coating that virtually eliminates friction also. Scalpel blades I would think would be single use as to assure there sharpnes.

Cheers

Woodchuck

[Wood Borer]

Perhaps something to bear in mind if a scalpel is used on a piece of wood that later needs to be polished.

I will keep them away from my shed.

- Wood Borer

[burn]

When I was a boy, my 'master woodworking' neighbour said that after honing (spell?) a chisel or plane blade, you would lightly run it down your thumb nail to see how small a sliver you could peel off.

I suppose once you got down to skin, you'd use other fingers ... hmmm ... I can't remember how often I sharpened my tools as a boy ... by hey! ... that was nearly 30 years ago. Opps ... I just remembered, a few months back, I honed some chisels and used the same method ... just checked, the thumb's nail is smooth! I wonder how quickly thumb or finger nails grow? Luckily I've not had to sharpen lots of blades at the same time!


Burn

[Ben from Vic.]

If you want to read about some truly amazing blades, try here.
They are sharpened down to a few atoms (sort of).

http://www.popularmechanics.com/science/extreme_machines/2003/10/knives/

Ben.

[outback]

Sharpness (or lack of it), is somewhat subjective. I bet if you grab a hundred people and asked them to sharpen a blade to the stage they considered it sharp, there would be degrees of sharpness. For some a sharp blade would be "half" sharp for someone else, and maybe for the next guy, they would never allow a blade to become this blunt.

For me I use the old shave yer arm technique.

[Pete J]

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!

[Glen Bridger]

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

Glen

[journeyman Mick]

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

[derekcohen]

Mick

I like that.

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

We'll have to think of some more.

Regards from Perth

Derek

[Sir Stinkalot]

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.

[soundman]

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.

:D

[derekcohen]

Soundman

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

:D Derek:D

[Driver]

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

[Dustcollector]

OUCH!!!

sharp enough

[Neil]

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 :D