Why are serrated knives more effective than normal knives?

[Josh0768]

TL;DR

Why are serrated knives more effective than normal knives?

What physical processes does serration take advantage of that make cutting something with a serrated knife more effective than cutting something with an ordinary knife? What is the optimal shape of each tiny segment of a serrated knife? Would cutting effectiveness increase as you add more serrations (and if so, at what rate? Does it increase indefinitely?)

 

[BvU]

A serrated knife saws (tears) instead of just cutting.
Tooth size depends on the fibers to be cut. Bread knives are often coarser.
For sharpening check out handsaw sharpening (crosscut; no need for setting the teeth)

 

[berkeman]

Summary:: Why are serrated knives more effective than normal knives?

What physical processes does serration take advantage of that make cutting something with a serrated knife more effective than cutting something with an ordinary knife? What is the optimal shape of each tiny segment of a serrated knife? Would cutting effectiveness increase as you add more serrations (and if so, at what rate? Does it increase indefinitely?)

Can you say specifically which kind of serrated knife you are asking about, and what kind of materials you are cutting? There are differences between serrated bread knives, serrated meat knives and serrated utility knives that you would use to cut rope, etc.

Here is an article about serrated knives used in cooking:

https://www.cooksillustrated.com/equipment_reviews/1704-serrated-knives

And this is a typical utility-type knife with serrations for cutting ropes, seatbelts, etc:

 

[Baluncore]

Summary:: Why are serrated knives more effective than normal knives?
What physical processes does serration take advantage of that make cutting something with a serrated knife more effective than cutting something with an ordinary knife?

A serrated knife is not a saw. The fundamental difference between a saw and a knife is that the saw produces saw dust or crumbs. A knife cuts softer material, without waste.

When a serrated knife hits a hard surface, only the high spots are damaged, while the bulk of the cutting is being done by the protected hollow ground serrations between the high spots.

A serrated knife therefore cuts for much longer before it needs sharpening, but when it does need sharpening, it is significantly more difficult, so it is disposable.

A serrated knife does not cut better than a new straight edge knife, but the straight edge knife will need continuous maintenance over the same lifetime.

 

[sophiecentaur]

Summary:: Why are serrated knives more effective than normal knives?

What physical processes does serration take advantage of that make cutting something with a serrated knife more effective than cutting something with an ordinary knife?

Cutting involves penetrating a surface. That needs Pressure. The pressure under a flat ended rod is low so it doesn't penetrate easily but a pointed rod will penetrate. (More force needed for the same local pressure).

Straight edged and serrated knives tend to be used differently. Straight vertical slicing, without moving across the surface just separates the two sides of the cut. Serrations make the contact length (hence area) greater so the local pressure is less. A straight edge gives a cleaner and easier cut with a Normal Force alone. But a razor blade will not last long, compared with a hacksaw.

Things are different when we use a sawing action with a serrated edge. The tips of the serrations present a smaller total area and achieve a higher local longitudinal and vertical pressure. It works on small parts of the surface and can remove them (tear) whereas a straight edge can clog up and it presents no longitudinal pressure.
Any serrations can produce swarf (chips in US) which needs to be shifted and it can require the tips to be offset from the line of the blade (i.e. a saw). But when you are cutting (haha trying to cut) a soft tomato, the advantage of a serrated blade is that the local pressure is high but the total Normal Force is small enough not to squash the fruit. Starting to cut bread (or even slashing the top of risen dough) is better with serrations for the same reason.

Cutting wood and metal requires both straight edges (chisels, axes and lathe tools)) and serrations (saws and files). The choice can depend on the finish you want as well as the toughness or hardness of the material. Neither is 'better' - just more suitable for a particular task.