Monday, September 8, 2014

How a Serrated Blade Works

Serrated blades work on a couple of different principles. In general, you have a continuously variable cutting angle as you draw the knife in the cutting stroke.

(image courtesy of Sypderco via Amazon)

Other serration patterns exist too of course. I own a Victorinox 10" Bread Knife and New West Knifeworks Super Bread Knife. The Victorinox has had a best buy rating from Cook's Illustrated as it's a good performer for a very reasonable price. The thin blade is a little flexy for certain uses though. The Super Bread Knife is a very good knife for quite a bit of money and uses a less common gentle wave serration. I also like how the tip is un-serrated on the Super Bread Knife for a clean finish to the cut.

The Victorinox uses  a fairly common pointed scallop.

And both are chisel ground, meaning they're flat on the one side. Serrations are ground in with shaped wheels. As each wheel will wear somewhat differently, it's difficult to match up the pattern on both sides of the blade. A chisel grind is a reasonable solution.

For a given length of blade, a serrated blade has more total edge length than for a plain edge knife of the same length. There's more distance to a curvy route than a straight one. So for the same cutting stroke, a serrated blade can apply more cutting surface.

Serration patterns with points gain some sawing effects making serrations more efficient on fibrous materials. Further, those fine points concentrate the pressure on a small area increasing the apparent cutting power, but also increasing wear on the points.  The points tend to round over with time and use. 

Serrated blades seem to hold an edge longer. This is both true and false. The points of the serration are what contact the cutting board. The points take the abuse and are structurally more prone to wear and dulling.  Dave_Martell at Knifeforums has a very good photo of this as shown below.

However, the points are the least amount of edge on the blade and protect the curves from the more dulling aspects of cutting thereby preserving the overall apparent sharpness of the blade.

Performance differences 

Comparing my two serrated blades, they're fairly similar for cutting bread overall. The serration patterns behave a little differently. The pointy serrations start the cut in crusty bread faster. However, once into the bread, the super bread knife is a bit faster and creates less crumb on the board.

They were indistinguishable in cutting tomato. 

When making a long deep cut, as in cutting a cake horizontally for filling, the thinner more flexible blade of the Victorinox can wander towards the tip making for an uneven cut. Converserly, in a denser material, like squash, the thicker blade of the Super Bread Knife has more wander. 

There are some simple brackets to help you make the cake cut evenly though. 

Sharpening a serrated blade

The primary trick to sharpening a serrated blade is to find a sharpener with a radius equal to or less than that of the serration. Dave_Martell in the knifeforums link above demonstrates using Edge Pro sharpening films around dowels to do so. Spyderco's Sharpmaker has finely rounded corners on the triangle stones purposely for sharpening serrations, though it tends to round over the points a bit. Sharp round tips, but not as pointy anymore.  DMT makes some tapering diamond files for this purpose as well. Generally speaking, serrations are ground at 30 degrees on one side only. 

It is this fitment issue that precludes some serrations from being resharpened fairly simply and usually requires factory equipment to do so. This is the issue with Cutco style serrations for example. 

As the serrations get finer and finer, they become harder and harder to sharpen. Inexpensive knife sets often take this path to offer an initial sharpness and then just become food saws in cheap steel.

The wavy scallops on the Super Bread knife are easier to sharpen than the pointed scallops, at least on the Sharpmaker. It's not a big difference though.

Drawbacks to the serrated blade

  • Exaggerated glide to finish the cut. When the edge contacts the cutting board with just the points, the cut is incomplete. You have to move the points through the the rest of the cut to finish cutting. 
  • Most of the edge does not contact the cutting board
  • Requires special tools and techniques to sharpen
  • Marks the food when. cut Consider those steak knives that tear the steak more than cut it. Serrations are reasonable for a steak knife where they're used on hard ceramic plate that would dull a plain edge. But I don't like what they do to steak. You'll see the marks in other foods as well, even from a sharp blade. Waviness left behind and such. 
  • Wanders in the cut
  • Junk masked by serrations. Many inexpensive knives are serrated on poor steel because they'll still saw through pretty much anything
  • Impossible to sharpen some serration patterns without factory equipment. 

No comments:

Post a Comment