Linear Broaching & Rotary Broaching: Process Guide

What is Broaching? How does a broach work? Broaching is a subtractive manufacturing process that produces features by removing material form a part with a toothed tool known as a broach. The process is divided into two main types: linear broaching and rotary broaching. Both variants are detailed in the article below.

This process is an effective and efficient technique as it produces a feature in a single pass.

Linear Broaching – basic principle

The tool is moved linearly against the work piece surface. The linear version of the process is by far the more common of the two. The principle of this process is shown in the diagram below.

Linear Broaching – round pull broaches

Round pull broaches have a 360° profile. See the diagram below for an example.

A round pull broach Image © 2018 EngineeringClicks

Geometry limitations of linear broaching

The image below shows an assortment of tools for creating various profiles. The nature of the process (fundamentally removing material along a single axis) means that the achievable geometry is simple: a 2D extruded shape.

An assortment of tools for creating different profiles

What is the difference between internal and external broaching?

Here are a few examples of the types of geometry that can be created using internal and external broaching process. The diagrams below show common internal and external forms, but a part could easily feature a combination of both if required (external and internal forms).

Common internal forms, Image © 2018 EngineeringClicks

Common external forms,  Image © 2018 EngineeringClicks

Rotary Broaching

In rotary broaching the rotating tool is pressed into the work piece, producing an axisymmetric shape. This process variant can be performed by using a milling machine or lathe. Blind shapes are easily possible using this method, and the tooling is relatively simple. The diagram below and video give further details:

Geometry limitations of rotary broaching

The image below shows examples of internal and external geometries that can be achieved.

Common external forms Image © 2018 EngineeringClicks

Typical Applications

• Gear Teeth
• Key seats
• Gun barrel rifling
• Ends of connecting rods
• Flat surfaces
• Helical spline and straight splines
• External, internal, regular and irregular profiles

Rifling profile of a gun barrel created by this process

Design Considerations

• Part geometry is limited.
• For internal forms, a hole must first be drilled in the work piece.
• A typical process has maximum stroke length of 25 mm and minimum stroke length of 3 mm.
• Sharp corners, blind holes and large surfaces are not recommended for this process.
• On any surface an average of 0.5 to 6 mm of material should be removed.

Materials

• All types of metals can be processed using this technique, including metals, ceramics, plastics, and any other solid material.

Process Variations

• There are three types of broaching machine for this process: rotary, vertical and horizontal. The machine required will depend on the part and the feature required.
• The process can be used for both roughing and finishing.
• There is a variety of tools available: single type, combination type, internal type and external type
• Surface broaching can be done with the help of index-able insert broaches, which can be coated with titanium nitride for longer tool life.

Economic Considerations

• Depending upon the part and feature, the process can have a moderate to high production rate up to few hundred pieces per hour.
• Production rate can be increased using automation.
• Lead times can range from a few days to several weeks, depending on the complexity of tooling.
• Poor material utilization since chips are created, but material may be recycled.
• Labor, equipment and finishing costs are low. Tooling costs can be high.
• Flexible process, as a single tool can be used for many products.

Quality Considerations

• Tearing can be a problem with soft workpiece materials.
• Power required, tool life, surface integrity, cutting force and surface finish all depends on the machinability of the work piece material.
• Excellent surface details can be produced.