Zero Radius

Been having a discussion on sharpening in the Scary Sharp post recently, wanted to add more detail, and thought others might also be interested in the discussion.

So far, the discussion has been:

Reader

I’ve found that if you sharpen something too sharp (too fine an edge), the edge will not be durable enough to do much real work. Picture whittling with a straight razor vs a real whittling knife. I notice this particularly with my machete.

Stu’s Shed

Sorry, but have to contradict you on that – you cannot have too sharp an edge where it comes to durability, but you can have too fine a bevel angle.

What is happening is there is not enough material backing up the edge, so it is falling apart under load.

Think about a pane of glass. It is SHARP, yet the bevel angle is 90 degrees! Now glass isn’t the best example because it is such a brittle material in the first place.

Sharpness is how close to a zero radius corner you have on the junction between two surfaces. A zero radius is ideal (and impossible), so from there on you want the minimum radius possible.

Depending on the tool, its purpose, and the brittleness of the material you choose a bevel angle accordingly.

Reader

Zero radius equals flat.

Flat no cut, flat smash, said in a cavemans voice. 🙂

Wouldn’t this be more accurately described as having an angle (bevel) as close to 360 degrees as possible? With a back up bevel to support the finer bevel?

Glass cuts by being like a very fine saw blade composed of microscopic peaks and valley very much like the edge of a knapped flint arrowhead.

My (latest) reply:

A lot of my information has come from Ron Hock’s book on Sharpening, so to really get to understand the topic, that is a superb reference.

The Zero-radius corner reference comes from Ron’s own description (my imaging of his concept)

Radius Corner vs Zero Radius

A lot of information on sharpening can be gleaned from Ron’s own blog on Sharpening – The Sharpening Blog

With bevel angle, I’m not sure what having a bevel angle approaching 360 degrees would look like!  However, where it comes to having too fine a bevel angle:

Bevel Angle

Both these tools are sharp, even equally as sharp as they both have an identical tip radius.  The difference is the top one has been sharpened to too fine a bevel angle – the point is too long and thin and easily breaks away resulting in a non-functional blade.  The lower blade has sufficient material backing up the tip, so it can survive increased punishment.

Finally, glass may have microscopic peaks and valleys that aid its cutting action (although I still claim glass is also just simply sharp – minimal corner radius), but steel is no angel either (from Ron Hock again)

Steel Edge 500x mag

3000x mag

3 Responses

  1. Hi Stu
    I think that covers the subject in the broadest possible way, if they have any doubts now then their future in woodworking needs further thought.
    cheers
    Don

  2. Safety week? , this makes me sad, i wonder if any of us today were told about safety, or was it something taught in us, instilled, my father never taught me about the safety of a circular saw, i just looked at it, read the manual, and understood this was a very dangerous tool. i understood the principles of its operation, i have a hard time understanding, why this generation needs such babysitting.

  3. Great post. It seems like you have this one right. Pictures and Ron Hock’s book help complete your case!

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