What is purpose of fullers on blades?

Timo Nieminen · 21st June 2016, 09:51 PM

Lightening/strengthening the blade, and/or ornament.

Ornament is obvious. Sometimes fullers are cut into blade to remove flaws.

Lightening and strengthening deserve a few more words. A blade is usually strong enough in the front-to-back direction, but side-to-side stiffness depends on tradeoff between weight and strength. Stiffness depends on the cube of the thickness, so a little bit more thickness can have a large effect on the stiffness. But thickness adds weight. So, enter fullers - a blade of the same thickness with fullers has a smaller cross-sectional area. There is less steel, so there is less weight. If the thickness of the blade is the same, the fullered blade is almost as stiff as the unfullered.

How to make a blade lighter with a fuller: take a blade, and cut a fuller into it. You remove steel, and the blade is lighter. You only reduce the stiffness by a small amount.

How to make a blade stiffer (i.e., stronger): when forging the blade, forge the fuller. You have the same amount of steel, and if you make the blade the same width, it will be thicker. The weight is the same, and the stiffness is greater.

The usual analogy is the I-beam - not as stiff as a solid beam, but a much higher stiffness-to-weight ratio.

Some people will claim that cutting a fuller into a blade makes it stronger. No, it makes it weaker. Forging a fuller can make a blade stronger.

The same kind of weight/stiffness benefits can be obtained from hollow-grinds, T-spines, raised ridges, Z-section blades, etc.

Some martial arts people like fullers because a fullered blade makes more noise when they swing. The noise is sensitive to blade alignment and speed, so it's useful audio feedback.

21st June 2016, 09:51 PM	#2
Timo Nieminen Member Join Date: Mar 2012 Posts: 422	Lightening/strengthening the blade, and/or ornament. Ornament is obvious. Sometimes fullers are cut into blade to remove flaws. Lightening and strengthening deserve a few more words. A blade is usually strong enough in the front-to-back direction, but side-to-side stiffness depends on tradeoff between weight and strength. Stiffness depends on the cube of the thickness, so a little bit more thickness can have a large effect on the stiffness. But thickness adds weight. So, enter fullers - a blade of the same thickness with fullers has a smaller cross-sectional area. There is less steel, so there is less weight. If the thickness of the blade is the same, the fullered blade is almost as stiff as the unfullered. How to make a blade lighter with a fuller: take a blade, and cut a fuller into it. You remove steel, and the blade is lighter. You only reduce the stiffness by a small amount. How to make a blade stiffer (i.e., stronger): when forging the blade, forge the fuller. You have the same amount of steel, and if you make the blade the same width, it will be thicker. The weight is the same, and the stiffness is greater. The usual analogy is the I-beam - not as stiff as a solid beam, but a much higher stiffness-to-weight ratio. Some people will claim that cutting a fuller into a blade makes it stronger. No, it makes it weaker. Forging a fuller can make a blade stronger. The same kind of weight/stiffness benefits can be obtained from hollow-grinds, T-spines, raised ridges, Z-section blades, etc. Some martial arts people like fullers because a fullered blade makes more noise when they swing. The noise is sensitive to blade alignment and speed, so it's useful audio feedback.