Introduction of Broaching

Broaching Principles

Broaching is a machining process that pushes or pulls a cutting tool (called a broach) over or through the surface being machined. Its high-production, metal-removal process is sometimes required to make one-of-a-kind parts. The concept of broaching as a legitimate machining process can be traced back to the early 1850s. Early broaching applications were cutting keyways in pulleys and gears. After World War 1, broaching contributed to the rifling of gun barrels.

Advances in broaching machines and form grinding during the 1920s and 30s enabled tolerances to be tightened and broaching costs to become competitive with other machining processes. Today, almost every conceivable type of form and material can be broached. It represents a machining operation that, while known for many years, is still in its infancy. New uses for broaching are being devised every day.

Figure 1. Cutting action of a broaching tool.

Broaching is similar to planing, turning, milling, and other metal cutting operations in that each tooth removes a small amount of material (Figure 1 ).

Figure 2. Typical push keyway broaching tools and a shim.

The broaching tool has a series of teeth so arranged that they cut metal when the broach is given a linear movement as indicated in figure 1. The broach cuts away the material since its teeth are progressively increasing in height.

Properly used, broaching can greatly increase productivity, hold tight tolerance, and produce precision finishes. Tooling is the heart of broaching. The broach tool's construction is unique for it combines rough, semi-finish, and finish teeth in one tool (Figure 3).

Figure 3. Parts of a broaching tool.

There are two types of broaching procedures: internal broaching and external broaching. For exterior broaching, the broach tool may be pulled or pushed across a workpiece surface, or the surface may move across the tool. Internal broaching requires a starting hole or opening in the workpiece so the tool can be inserted. The tool, or workpiece, is then pushed or pulled to force the tool through the starter hole.

Almost any irregular cross-section can be broached as long as all surfaces of the section remain parallel to the irection of broach travel (Figure 4). Helical cuts can also be produced by twisting the broach tool as it passes the workpiece surface.

Figure 4. Different types of broaches.

Hints for successfully broaching a keyway

Set-up

Maintain a rigid set-up at all times. The workpiece must be solidly fixed or nested perfectly square with the baseplate and ram face. Check to make sure that all square and parallel surfaces on the face of the ram and the baseplate remain true.

Alignment

Proper alignment of the broach, workpiece, and ram is the most important factor in all broaching operations. Misalignment can cause drifting, deflection, and even breakage.
Alignment Tips--If a keyway broach drifts and cuts a taper, try the following:

1. Reverse workpiece or turn broach so teeth face toward the back of the press.
2. Let the bushing protrude above the workpiece to give more support to the back of the broach thereby helping to keep it aligned. If a collared bushing is used, place it upside down under the workpiece.
3. Make sure the broach is centered under the ram at the beginning of the cut. If the broach moves out of alignment after starting to cut, back off the pressure on the ram and align the broach itself. Repeat during successive cuts to ensure perfectly straight cuts.

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What is a Broaching Machine?

The term broaching may have been derived from an ancient roman word braccus, which meant an object having projecting teeth. The operation started in the year 1850 when broaching tools were called “drifts”. These drifts were being hammered in blacksmith shop through the work or pushed through with an arbor press.

Broaching is a method of removing metal by pushing or pulling a cutting tool called a broach. The tool may be pulled or pushed through the surface to be finished. Surface is finished by broaching flat or contoured and may be either internal or external. Broaching is generally limited to the removal of about 6 mm of stock or less. Different types of broaches are used for different types of work.

Broaching machines are probably the simplest of all machine tools. These consist of a work holding fixture, a broaching tool, a drive mechanism and a suitable supporting frame. Although the component is a few, several variations in design are possible. There are two principal types of machines, horizontal and vertical type. In addition to the standard types there are special and continuously operating machines. Both horizontal and vertical types have one or more rams depending on production requirement. Dual ram models are arranged so that when one ram is on cutting stroke, the other is on return stroke, and return stroke is performed quickly to gain time. This time can be used to unload and load the machine.

Gear Hobbing

Hobbing is the most frequently used method of making gears. A gear hob is a cylindrical cutting tool with rings of teeth that are the involuted shape of the desired gear tooth. The gear hob is positioned at a right angle to the gear blank being cut. In this manner the gear blank rotates with the hob feeding across the cutting plane. Each successive tooth on the hob cuts a little deeper until it has generated the precise gear tooth profile desired.