Thread: How do you test sharpness?

What we dont' appreciate is that the 2000+ year old way we sharpen edges is on a microscopic scale like using dump trucks to sort smarties. We use a more or less randomly arranged lump of (fine) stuff, get it as flat as we can and then throw that at a microscopic edge and hope it becomes sharp. Because this actually works in most cases we don't seem to need any significant improvement but with the ability to manipulate indivdiual atoms around the corner there is almost certainly another way. In theory the outer edge of a blade can be no less than a single line of single atom or crystals of the blade material. If there was a real practical demand I think it could be done now (for a price) but would you pay say $10000 to sharpen your plane. OK you would have an ultra razor sharp blade but it might only stay that sharp for 2 or three swipes and 20 - 30 swipes later you would be at the same degree of sharpness that you could achieve at home?

A materials engineer will then say - change the blade material. Unfortunately there is no known material that holds together on an individual atomic scale under even under the sorts of moderate forces we apply during wood working. Many materials like carbon fibre do hold together very well under tension and compression, but shear forces are another kettle of fish, and most materials will not wear shear forces especially on the atomic scale. A single line of carbon atoms on the tip of a ultra solid diamond blade would suffer some damage on the first swipe. It would still be much sharper than your sharpest plane but at what cost? And at what point will it reach the best sharpness you can generate in your shed, and then what will it cost to sharpen, never mind the cost of the blade to begin with - and DONT YOU DARE DROP THE BLADE OR PLANE.

In 1994 I visited Japan and bought what was then "a revolutionary new kitchen knife" (ceramic) and a high quality layered carbon steel kitchen knife. The steel knife was superb but the ceramic knife was just awesome (I used the soft tomato test). Within months of about equal use I noticed the ceramic knife had many tiny chips out of the edge of the blade, it still cut the soft tomato fine (probably because of the nicks in the blade), but after about a year I could clearly sharpen the the carbon knife sharper than the ceramic. A year after that the cermic knife shattered after falling on the floor. The steel knife is still alive and the most favoured of our kitchen knives.

Sharpness is a law of diminishing returns. What's the point of a super-ultra-razor sharpness that lasts for 2-3 swipes - there has to be a balance between the nature of materials involved, the practical longevity of use and $$$. Whether we are at that point - I don't know.

OK - here endeth the science lesson of the day.
Cheers

I think when we are ready to go that far, putting a sharp edge on something to then go and cut something else with it will be a thing of the past. If you can manipulate atoms to that degree, then why not just turn the evil heat ray on a bit of wood and have it manipulate it into a coffee table?

OK, that would take all the fun out of it, but you get my point

silentC: happy to have holes shot in my theory. I guess it depends on a few other factors, such as the relative sizes of the lint fibres and the scale of the edge roughness. A worn edge might also have some rounding involved as well as faceted breakages, depending on the metallurgy. A freshly honed edge might also have some 'bending' of the edge induced by the honing method, called a 'wire edge' I think. I've seen photomicrographs of wire edges in Lee's book, but I've never seen any of worn edges, so I don't know if 'breakage' or 'rounding' is the dominant effect of wear. It might even depend on the chemistry of the wood.

I wonder if we've descended into Derek's realm of the "obsessional and silly"? At the risk of doing so, I'll keep going...

I believe that industrial sharpness standards are tested by the amount of force required to drive the item under test a certain distance into a standard medium (e.g. http://www.catra.org/pages/products/kniveslevel1/st.htm). In the home shed this is hard to do because we don't have accurate force gauges (though measured weights could do the job), and -- more importantly -- measuring the small distances involved is difficult to do accurately. I propose a home-shed sharpness test as follows.

The test is designed for chisel or plane blades (i.e. ideally linear edges, rather than points). A piece of polystyrene foam acts as a standard substrate medium. The aim will be to push the edge under test into the foam using a set of standard weights, totalling an amount W (in kg). The length of the edge, L, (in metres) corresponds to the width of the chisel or plane blade. A dial micrometer or other method of measuring small distances is used to measure the penetration, d, into the foam. I'd suggest that d be kept small (say 1 mm), otherwise the test will depend on the smoothness of the actual bevel, rather than the properties near the edge. A measure of sharpness, S, will be given by the ratio

S = dL/(Wg),

where g is the acceleration due to gravity (9.8 m/s^2), and converts the weight in kg into a force (in Newtons, N). The units of S are m^2/Newton, or inverse-pressure, 1/Pascal, which seems intuitive. I'm not aware of a name for this particular unit, so I propose naming it the Zato, after the famous blind swordsman, Zatoichi.

Assume that 1 mm is the smallest penetration depth that can be accurately measureable (say to two or three digits accuracy) in the shed. For a standard penetration depth of 1 mm, the sharpness will be

S(1mm) = L/Wg.

The sharpness, S(1mm), will increase as the weight required to achieve 1 mm penetration decreases. If a blade of 4 cm length requires 0.5 kg of weight to drive it 1 mm into the test block, then the sharpness S(1mm) will be 0.04/(0.5 x 9.8) = 8.2 milli-Zatos. For two edges of identical sharpness but one twice as long as the other, the penetration of the longer edge will be half the penetration of the shorter one. (This last point needs verification. If the linear relationship doesn't hold, an appropriate exponent will need to be inserted into the expression for sharpness to handle the effect of edge length.)

The diagram shows the test set-up. Some jigging and support structures will need to be added to actually carry out the test. If penetrations of 1 mm are not achievable with convenient shed-based equipment, we could agree to substitute an even softer material, as long as it is cheaply and widely available, and of consistent properties. Perhaps cheese (at a fixed temperature) or butter (ditto) could be used (an agreed brand name should ensure consistent quality).

happy to have holes shot in my theory

That wasn't my intention, just wanted to put it to the test. It wasn't a very scientific process though, so you might have something but it needs more analysis.

I wonder if we've descended into Derek's realm of the "obsessional and silly"?

I think we've been there for a while. These things are interesting from an intellectual point of view but whether they translate to any practical advantage remains to be seen.

I'm going to have to learn to calibrate my thumb as that seems the most popular option but my thumb is so work-hardened. I'll have to look at some other part of my anatomy which is more sensitive and able to discern between the different levels of sharpening .

BTW my thumb has numerous little cuts in it and I don't know if I can spare any more but I guess it's readily available unlike tissue paper etc. Sharpening is a hot topic as most people think their method is the best. I'm not interested in going any further than just knowing that the tool is sharp enough to carry out its intended work.

Originally Posted by silentC

I think when we are ready to go that far, putting a sharp edge on something to then go and cut something else with it will be a thing of the past. If you can manipulate atoms to that degree, then why not just turn the evil heat ray on a bit of wood and have it manipulate it into a coffee table?

OK, that would take all the fun out of it, but you get my point

Nar. Just use sketchup to define an object, then let loose the nanobots on the nearest pile of raw material and wait...

I hope I'm not around when a carpenter's competence is judged by how well they can program.

BTW my thumb has numerous little cuts in it and I don't know if I can spare any more but I guess it's readily available unlike tissue paper etc.

My wife looks at my hands in horror. They are always a mass of cuts and abrasions. I wonder what my patients think? The trouble is that I frequently brush a finger tip lightly on a blade corner. The blades are so sharp that I am not aware of having cut myself - and deeply at that - until I see the blood flow . I hate cuts on finger tips!

Regards from Perth

Derek

let loose the nanobots on the nearest pile of raw material

I read a book that used that concept. I suppose it's not a new idea. They had these machines and vats of base material. The machines used nano technology to build anything you wanted from the raw materials. I suppose when that time comes, we could have the machine build us a nice pile of white oak and what ever hand tools you wanted. When the blade goes blunt, scrap it and get the machine to build you a new one. Then we could concentrate on the fun part

Originally Posted by zenwood

An edge (line) is sharp when it is formed by the intersection of two exact planes at a narrow angle. This is the ideal. When the two planes are at a shallower angle, it will be easier to push through a piece of wood, but will be more liable to breakage.

To approach the ideal, the two surfaces must be abraded (honed) to approximate the ideal, perfect, planar surface. A rough-honing will result in the two surfaces having deep peaks and scratches, and the edge (formed by the intersection of these two surfaces) will have large indentations and protuberances. This is as good as you can get with, say, a 1200 waterstone. The edge will be 'sharp' but the large protuberances will quickly break off, resulting in large blunt areas (coloured red in the diagram). The edge will quickly become hard to push through wood, and will need resharpening.

Going to finer grades of honing, say with a, 8000 waterstone or a strop, the two surfaces will have a larger number of smaller peaks and scratches, and the edge will have smaller indentations and protuberances. The small protuberances will still break, but the result is a smaller set of blunt areas along the edge. The edge will remain easier to push through wood for a longer time.

If this theory is correct, a good test of sharpness would be to gently draw the edge sideways (parallel to the sharp edge) along a linty cloth (wool? tissue?). Mere rough-honed edges will tend to grab fibres and pull them out attached to the protuberances along the edge. Fine-honed (sharper) edges will tend not to grab fibres, cutting through them instead.

I'm with you Zen, this is what I said a couple of years ago and I haven't changed my mind.
http://woodworkforums.ubeaut.com.au/...rpening+chisel
I think there may be only a small difference in sharpness between 1200 and 6000, but a big difference in durability of the edge. While you have the chisel in the sharpening jig it takes about 10-20 seconds to finish the honing off on the 6000-8000 grit. Given the extra durability you get, I just don't know why you would not do it.
Cheers
Michael