knifeopedia
Production of Bolster Knives
While bolsters offer numerous benefits, they also increase production costs. There are several methods for attaching a bolster to a knife, each with its own advantages and cost considerations:
Single piece hot drop forged
This is the traditional method for producing a bolster knife. A single piece of metal is heated until it becomes red-hot and malleable, then it’s shaped using a large drop-forging hammer - hence the name drop-forged. In this method, the blade, bolster, and tang are all crafted from a single piece of steel.
PROS
Blade made of one part. Hammer-forging improves material structure and reduces carbide size.
Traditional and straightforward process that can be applied to both artisanal production and mass production on industrial scale
CONS
More expensive, energy-intensive production method.
Limitations in handle shape and blade materials that can be forged.
Hot drop forging remains the most common method for producing forged bolster knives.
Single piece upset-forged
Upset forging is a more modern knife production technique, developed with the insight that, when using a flat strip of steel, the blade and tang already have the required shape, leaving only the bolster to be formed into a 3-dimensional structure.
In this process, the middle section of the steel strip is heated until soft. Pressure is then applied from both ends, causing the heated metal to bulge. This bulge, which forms the bolster, is then forged into shape with a hammer. Since the forging area is limited to just the bolster, the process is faster, uses less energy, and requires less force compared to traditional drop forging.
PROS
Blade made of one part. A more affordable and less energy-intensive forging method compared to traditional hot drop forging.
CONS
Material structure of the blade may be slightly less refined compared to drop-forging.
Only a limited range of steels is compatible with this process, and there are constraints on handle shapes and designs.
This technique is only feasible for industrial-scale production, requiring machinery investments and specialized expertise.
Upset-forging today is used by market leading manufacturers of German knives like ZWILLING or WÜSTHOF.
Welding
In single-piece forging, the blade, bolster, and tang are all made from the same material. However, using the same hard steel for the bolster and tang offers no functional advantage, as hardness isn't necessary for these parts. This led to the development of knives made by welding two or more different metals together. In this approach, the blade is crafted from hard knife steel, optimized for cutting, while the bolster and tang are made from softer, less corrosive metals.
For welded knives, the bolster and tang are often produced as a single unit using cast stainless steel, in a process called the "lost-wax" method. This method allows for a wide variety of shapes and designs
PROS
Allows easy combination of blades and handles without the need for expensive tooling.
Reduces production cost by limiting the use of expensive knife steel to the blade itself.
Enables the use of high-carbon steel for the blade, which may be difficult or impossible to forge using traditional methods.
No limitations on handle construction, designs, or materials.
CONS
The biggest disadvantage of welded knives is that the heat generated during welding can alter the material structure around the seam, increasing the risk of corrosion or even breakage. However, by employing a special production sequence and technology that is more expensive, this corrosion or breakage risk can be completely eliminated.
The vast majority of bolster knives with multi-layer blades are welded, as are most China made bolster knives.
Glued/riveted
In this construction, used exclusively for half bolster knives, the bolster is made up of either of 1 U-shaped part that is wrapped around the tang, or 2 parts that are added onto the tang. The bolster parts are secured to the tang using either rivets, glue, or a combination of both.
PROS
The primary advantage is that this method avoids welding, thereby eliminating the risk of corrosion or breakage in the welding area.
CONS
The transition between the blade and bolster is not entirely smooth, creating a dirt trap that is slightly harder to clean than in welded or fully integrated designs.
This construction is not commonly used but is applied (among others) in BOB KRAMER knives, both the original versions and the licensed production models by ZWILLING.