Broaching (metalwork)

Broaching is a process in metalworking that removes material in a consistent, patterned way by pushing or pulling a toothed cutting tool over or through the surface being worked. A variety of metals can be broached, but some of the most common ones are brass, aluminum, titanium, stainless steel, alloy steel, 12L14, and cast or malleable iron. Broaching can be traced back to the early 1850s, and has been used for crafting medical implants, hand tools like wrenches and pliers, shaft slots, appliances, farming equipment, and serrated knife blades. It has also been used in fields such as plumbing and automotive design.

Background

In broaching, a craftsperson uses a toothed tool to remove material in a consistent way, often in a pattern. This creates holes in the metal being worked, which can come in a variety of shapes including circles, squares, hexagons, rectangles, and keyhole shapes. The toothed tool is called a broach. It is similar to a saw except the teeth are usually larger and increase in size from one side to another. There are generally several different types of teeth on one broach—some begin the cut (roughing teeth), others start finishing the metal (semifinishing teeth), and others complete the finishing (finishing teeth). The amount of rise per tooth determines the amount of material that the tooth will remove each time the tool passes through the metal. There are a variety of styles of broaches, but they are all typically made from alloy steel or high-speed steel. High-speed steel broaches are coated with titanium nitride (TiN), which is wear resistant and reduces friction.

Broaching can be traced back to the early 1850s with early applications in cutting keyways in pulleys and gears. After World War I, broaching was important to the rifling—or arrangement of spiral grooves—on the inside of gun barrels. The broaching process made some advances with the introduction of grinding in the 1920s and 1930s. This enabled broaching costs to become more competitive with other machining processes.

There are two main methods of broaching: surface and internal. Surface broaching is more straightforward—either the broaching tool remains stationary and the workpiece (metal) is moved against it, or vice versa. Internal broaching is a bit more complicated and requires a starting hold or opening in the workpiece in order to insert the tool. The tool or workpiece is then pushed or pulled to force the tool through the starter hole. Most broaches utilize a pulling force; those that use forward pressure are called push knives. There are a variety of types of movement that can be used in broaching. These include linear or circular motion of the broach or workpiece, or a combination of both at the same time. Normally, the workpiece is fixed and the broach makes a linear cut across it.

There are several types of broaching machines, including vertical external broaching, horizontal broaching, continuous broaching, gear broaching, internal thread broaching, and multi-tool multi-station broaching. However, the two main types of broaching machines are vertical and horizontal. Vertical machines are more popular than horizontal styles since they take up less space. Most broaching machines run on hydraulic (water) power, but some machines use mechanical parts that are required for more complex broaching. Mechanical broachers generally have more power, so they can cut through harder metals in less time.

Some of the most common materials used in broaching include brass, aluminum, titanium, stainless steel, alloy steel, 12L14, and cast or malleable iron. The best materials to use in a broaching machine are softer, but hard metals like steel, one of the most commonly used materials in machining, can also be broached. Non-metal materials such as plastic may also be broached.

Impact

Broaching is used in a wide variety of items. These include medical implants, hand tools, shaft slots, appliances, plumbing fixtures, automobile parts, farming equipment, and cutting teeth for pocketknife saw blades. However, the primary use of broaching is to produce high-volume parts that require cuts that are accurate, repetitive, and complex.

Broaching machines offer many advantages. First, they have high production efficiency and a low processing cycle. This can reduce the cost of a single item in mass production. Broaching also has a wide range of applications. It can process various shapes of internal holes and can complete high-precision machining of difficult-to-cut materials like turbine disks, which are used in generators and aircraft engines. Additionally, broaching can achieve high precision with low surface roughness. Another advantage is that the process of broaching is simple and skill requirements for workers are low. Broaching can also be automated, which reduces labor and costs, minimizes defects, and optimizes production. Broaches also have a long service life.

However, there are also limitations to broaching machines. For instance, the initial cost for custom broaches is high and not suitable for low-volume production. That means it is not cost-effective in single-piece and small-batch production. The design, repair, and sharpening of broaches are also expensive endeavors. In addition, broaching suffers from application limitations, as blind holes, deep holes, stepped holes, and obstructed outer surfaces cannot be broached. Finally, each broach can only be used for a specific size and shape; a separate one is required whenever the shape and/or size of the workpiece changes.

Going into the future, the broaching industry could face challenges due to a slowing metalworking sector, according to Gardner Intelligence, an industry company that does research and analysis. The company expects slowing trends to continue: in 2022, Gardner said the 2023 market for machine tools would be down 19 percent in the United States.

Bibliography

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