Production of Bolster Knives
The various advantages of having a bolster don’t come for free. Adding a bolster significantly increases production cost. There are different ways to attach a bolster to a knife:
Single piece hot drop forged
This is the traditional way of producing a bolster knife. A piece of metal is heated until it is glowing red and thus softened, so that it can be formed. It’s then forged into shape by a big, dropping forging hammer, thus the name drop-forged. The whole metal part of the knife (blade, bolster, tang) is made of one piece and from one type of steel.
Blade made of one part. Hammer-forging improves material structure and reduces carbide size.
Traditional and easy process, applicable from single piece production to mass production on industrial scale.
More expensive, energy-intensive production method.
Limitations in handle shape and blade materials that can be forged.
Hot drop forging is the most common method of producing forged bolster knives.
Single piece upset-forged
Upset forging is a more modern production technique based on the insight that, when using a strip of steel to produce a knife, the blade and tang are already flat, and it’s just the bolster that needs to be transformed into a 3-dimensional shape.
Thus a strip of metal is heated in the middle until the metal is soft. Then pressure is applied from both ends of the strip, so that the metal bulges where it is hot and soft. This bulge (bolster area) is forged into shape by a hammer. Since the forging area is much smaller, the process is quicker, less energy consuming and requires less force.
Blade made of one part. A less expensive and less energy-intensive way of forging knives.
Material structure of blade slightly inferior to drop-forging.
Very few steel compatible with this process. Limitations in handle shapes and designs.
Only feasible on industrial scale, requires machine investments and high technological know-how.
Upset-forging today is used by market leading manufacturers of German knives like ZWILLING or WÜSTHOF.
In single-piece forging of bolster knives, blade, bolster and tang are made of the same material. But the harder material used for the blade has no advantage from being used for bolster and handle, where the hardness is of no use. Thus the idea of producing a knife by combining 2 (or more) different metals and welding them together. In such a set-up, the blade would be made from hard knife steel, and the handle from a softer and less corrosive metal.
With welded knives, usually bolster and tang handle are combined in one part and welded to the blade. They are usually produced in cast stainless steel (so called “lostwax” method), which allows the production of all different kinds of shapes and designs.
Easy to combine blades and handles without tooling cost.
In case of expensive knife steel, production cost can be reduced by restricting material usage to the blade.
High carbon steel can be used for the blade that would be difficult or impossible to forge.
No limitations in handle construction, designs and materials.
The biggest disadvantage of welded knives is that the heat generated by welding transforms the material structure in the area of the welding seam, thus increasing the risk of corrosion or even breakage. However, by employing a special production sequence and technology that is more expensive, any 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.
This construction is only applied for half bolster knives. Here, the bolster consists either of 1 part (in U-shape) that is wrapped around the tang, or 2 parts that are added on the tang. The bolster part(s) is (are) fixed to the tang by either rivet or glue (or both).
The biggest advantage is that welding can be avoided, thus eliminating corrosion and breakage risk in the welding area.
A disadvantage is that the transition from blade to bolster is not completely smooth, creating a dirt trap that is slightly more difficult to clean.
This construction is not a common one, but applied (among others) for BOB KRAMER knives, both the originals as well as the license production by ZWILLING.