Broaching Process

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Broaching Process

What is Broaching?
Broaching is a machining process that pushes or pulls a cutting tool (called a broach) over or through the surface being machined. Broaches are referred to as multiple-point linear-travel cutting tools and are used to produce flat, circular, and even extremely intricate profiles, as seen from a section perpendicular to the tool travel.

A broach is a series of single-point tools arranged successively in the axial direction along a tool body or holder. Each sequential tooth varies in size and shape in a manner that allows each tooth to cut a chip of the proper thickness.

The shape and spacing of each broach tooth is determined by the length of the part being broached, the amount of material being removed by each tooth, and tonnage restrictions of the broaching machine. The chip space between each tooth is designed to sufficiently accommodate the volume of chips generated.

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.

Types of Broaches
The many different types of broaches available today can be grouped into four major categories:

Internal Broaches

Internal broaches are either pushed or pulled through the part. Strength considerations usually limit the tool design. Internal broaches can generally be identified by two categories:

Round Broaches
Round broaches are designed to finish a hole that was originally formed by casting, drilling, punching, or forging. The round broach is then pushed or pulled through the hole to bring the hole to the desired dimension.

Spline or form broaches
Spline or form broaches are essentially the same as round broaches. The difference is that they have splines or some other type of form ground parallel to the axis of the tool. These splines can be angular, involute, straight-sided, cam, lobed, square, or hexagonal.

External Form Broaches

External form broaches are usually carried on a guided ram and mounted in a broach holding fixture. Strength is generally not a major consideration since the cutting force is transferred to the ram at many places along the length of the broach(es). Form broaches are typically identified in the following categories:

Flat surface broaches
Flat surface broaches are used to remove material from the external surface of a part. The broach passes “over” the work piece to produce the desired surface. In some instances, the work piece itself passes over stationary tools (i.e. chain broaching). Flat surface broaches can be constructed from either solid high-speed steel or inserted carbide.

Form surface broaches
Form surface broaches are used to remove material from an external surface as well. However, rather than producing a flat surface, they generate many types of contours and shapes. Applications for form surface broaching are myriad, and include:

serrated forms
angular spline and gear forms
“fir tree” forms
compressor slots
keyways

Blind-Spline Broaches

A whole new era in broaching dawned when General Broach pioneered the blind-spline broaching process. Unlike conventional broaching, blind-spline broaching allows for the machining of blind surfaces—surfaces that are impeded by a flange or protrusion that does not allow a cutting tool to pass completely through or over it.

Blind-spline tools are designed to cut either internal or external forms. Blind-spline tools are mounted on a rotary dial type or linear machine and the tools are kept in sets. Blind spline tools are single-tooth cutting tools, with each successive tool removing a predetermined amount of material until the desired profile is achieved.

The advantages of using blind-spline broaching over more conventional machining processes are:

quick and easy size
high degree of accuracy
extremely high production
extremely high production rates (over 10 times faster than shaping or hobbing)

External "Pot" Broaches

While this type of broaching is also external, it is a distinct category from external form broaching because of its unique process. This type of broaching is used when an external form is required. The form can be splines (straight sided, involute, angular), cam, or lobular.

In a pot broaching set up, the part is usually pushed up through an internal ‘pot’; a fixture that houses a series of form inserts. Some pots also include a sequence of shave rings that are mounted at the rear of the broach inserts and are used to enhance the form, spacing, and finish of the resultant part.