Testing for sharp

Best test for kitchen knives...

John in NM

I'd say is cutting a tomato.

Or bread - anything that people often do with a serrated knife to make it easier.

Guess or test

John K Jordan

I use a sharpness tester and a friend does also. It uses a calibrated monofilament and measures the force it takes to push the blade through it. My pocket and kitchen knives both measure very close to a single-edge razor blade after sharpening with a 3-belt Realsharp machine.

If you are interested enough to want objectivity rather than subjectivity, you might look into the SharpCheck: https://www.edgeonup.com/index.html I think I have the PT50B. This picture is of an earlier model.

(I have no connection with the company)

This device is apparently popular with knife makers and discussed on their forums. If you live close, stop by and try mine.

JKJ

Re: Testing for sharp

JL

From my B-I-L (who is an executive chef).

Some knives should be "razor sharp", others shouldn't be. It depends on what they are cutting.

He tests for sharpness cutting an onion or an apple or a whole piece of citrus.

If it will cut evenly without drag - it is sharp.

Re: Guess or test

Yonak

I can't even imagine how this $179 item could be significantly better and JL's BIL's method.

Re: Guess or test

John McGaw

Amazing. I guess that if a property exists then somebody will want/need to test it and somebody else will figure out a way. I'd certainly not have thought of such a device (or that it would be relatively cheap). I'd be afraid and probably ashamed to test most of the cutting edges in my shop or kitchen.

Re: Testing for sharp

Bill Tindall, E.Tn.

It would be the rare kitchen knife that could be sharpened like a razor blade. Most have crappy steel and they are not hardened to sufficient hardness to sharpen to a high degree.

A hand lens is good for observing what is going on. Many knives are factory ground at a steep angle. If you hand sharpen at an optimum angle it will take a lot of effort to establish the new bevel. You may think you are done but not have reached the edge with the new bevel.

Re: Testing for sharp

Dave Bair

The History Channel's "Forged in Fire" tests blade sharpness by slashing things like hanging pig carcasses, bamboo bundles and hemp ropes. They never use them to shave, however. I like the "sticky-sharp" method: the blade is sharp if it feels sticky when drawn like a card scraper across my thumb.

Chefs and Woodowrkers

Bill Tindall, E.Tn.

Chefs are the cause of knives woodworkers loath. We routinely sharpen Rc 60 and up woodworking tools with high tech abrasives, establishing a high degree of refinement, sharper than a razor blade. When we try to sharpen a chefs knife (quality Japanese knives excepted) we can not get this level of edge refinement. In addition to the inferior steel, compared to our woodworking tools, most chefs knives are heat treated so that the chef can sharpen with a sharpening steel. For it to work the steel must be significantly softer than a plane or chisel blade. This softer steel can not take or hold and edge like our chisels. However, as brother in law said, sharp enough to cut the food is all that is required. Woodworkers want more!

Re: My view

Larry Clinton At Frankfort, (Central) Indiana

I learned, right or wrong, many years ago that cutting edges were totally different for food processing and wood cutting. First, Food processing knives edges are to be left serrated and are used in a "slicing" motion to cut through the item being cut. You do not expect to slice steak or tomatoes by pressing down on the item, but by basically a sawing action. While most wood cutting edges are used to separate material by pressing directly into the wood. IE: plane blades, chisels etc.

Therefore food knives are best left with a microscopic serrated edge and the purpose of the "steel" is to re-straighten the serrations to the plane of the knife as they get bent over.

As Bill has stated food processing knives are as a rule, much softer steel which actually works better for the application! Usually they are in the 50RC to 60RC, if the knife is used to chop or hit bone they benefit from being on the lower end as the 60RC steel edge may chip if subjected to impact. Axes, hatchets, machetes etc. are even often softer!

Wood working cutting tools which to be tapered to the finest edge (that will hold up) need to be very hard to allow the pressure to force its way through wood, somewhere in the 60c to 64C Rockwell range, with some of the newer alloys being strong enough to be even higher.

My personal method to test sharpness of wood working tools is to first examine under high magnification and lighting the edge to see if any flat can be seen, and that the area near the edge is mirror bright. Then I do test with paper but only with a direct force into the paper "no slicing", a good sharp chisel or plane blade will slice the paper without bending it, I also sometimes shave the hair off my arm.

Re: Guess or test

John K Jordan

You are right, a subjective test should be good enough for most people. Another useful test (besides the paper test) is the fingenail test, good for lathe, bandsaw, and chisels too.

There are several things going on with the "cutting without drag", one is the thickness of the blade, another is the subjective feel of the drag.

What the instrument gives is a number, rather than a feeling. A scientist would understand.

An objective test is especially good when evaluating different sharpening methods.

The SharpChick is basically an inexpensive type of the very expensive industrial tester such as this one. http://www.catra.org/pages/products/kniveslevel1/st.htm I don't know the price but I see it's available for rent.

An the statement about the ideal sharpness for different applications is right on. Note this comment from the CATRA people: " the results from this system are not directly comparable as the cutting performance of any specific blade or knife varies dependant on the material being cut. For example a razor blade is great for shaving , but is useless at cutting down trees! "

JKJ