What is Broaching?- Process, Working, And Types

Broaching is a machining technique where a toothed tool methodically removes material to achieve precise results. This process comes in two main forms: linear broaching and rotary broaching, each tailored to deliver distinct surface finishes.

While both approaches have their specific applications, broaching as a whole tends to be reserved for scenarios where precision is essential and large production volumes are involved.

What is a Broaching?

Broaching is a machining technique that relies on a sharp, hardened tool fitted with a series of progressively taller teeth to systematically remove material from a workpiece.

What sets broaching apart is its ability to produce consistent and accurate cuts in a single, continuous motion. Each tooth on the broach is designed to take away a specific amount of material, so with every pass, the tool shapes the workpiece with remarkable precision.

In practice, broaching can be performed in a few different ways: either the broaching tool moves across a stationary workpiece, the workpiece itself moves while the tool remains fixed, or, in some setups, both parts might rotate in relation to each other. Depending on the application, broaching might be carried out internally—passing the tool through the center of a part—or externally, working along the surface.

The broach itself comes in two main varieties: linear and rotary. Linear broaching is the approach most people encounter, where the broach travels in a straight path along or through the material, often using a dedicated broaching machine (sometimes simply called a broach).

Rotary broaching, on the other hand, involves spinning the broach as it’s pressed into the workpiece, which allows for cutting axisymmetric shapes and is typically done on lathes or screw machines.

A notable advantage of broaching, whether linear or rotary, is that the entire cut is usually completed in a single pass. This efficiency is particularly valuable when working with complex or irregular shapes that require tight tolerances.

As a result, broaching is often the method of choice for producing features such as circular and non-circular holes, splines, keyways, and flat surfaces—especially in high-volume manufacturing settings.

Typical materials processed with broaching include small to medium-sized castings, forgings, parts made on screw machines, and stampings. Although the initial investment in broaches can be significant, the process tends to be cost-effective for large production runs due to its speed and repeatability.

A broach might look a bit like a saw at first glance, but its teeth increase in height along its length, allowing for a staged cutting action. The tool is usually divided into three functional sections: one for roughing, a second for semi-finishing, and a final set of teeth dedicated to finishing the surface.

Uniquely, broaching integrates the feed motion directly into the design of the tool itself—the rise per tooth (RPT), or feed per tooth, governs how much material is cut by each tooth and thus the thickness of each chip removed.

Since all the cutting features are built into the broach, the process doesn’t demand complex machine movements or highly skilled operators. In essence, a broach operates as a series of single-point cutters, each tooth taking its turn to shape the material, rather like running a shaper tool over the same area multiple times, but with each pass already arranged in sequence.

What is the Broaching Process?

The broaching process differs notably based on whether one is performing surface or internal broaching. Surface broaching tends to be more straightforward, as it essentially involves the interaction of one surface with another.

Typically, this means either the broaching tool remains fixed while the workpiece moves against it, or, conversely, the workpiece stays stationary while the broach passes over it.

Internal broaching, on the other hand, requires a bit more preparation. To begin, the workpiece is securely positioned in a work holder, which also serves as the mounting point for the broaching machine itself. The next step involves using the machine’s elevator to lower the broach into the workpiece.

Once in place, the machine’s puller mechanism engages with the broach pilot. When the elevator releases its hold on the follower, the puller then draws the broach completely through the workpiece. After this, the finished workpiece can be removed, and the broach is returned to the elevator, ready for the next cycle.

What are the Uses of Broaching?

Broaching finds its way into an impressive range of everyday and industrial tools—whether it’s hand tools, household appliances, plumbing fixtures, components in automotive design, farming machinery, or even military hardware. If you look closely, traces of broaching are all around us.

That said, the process is most commonly valued for its role in manufacturing large batches of parts that call for precision, consistency, and the ability to create intricate shapes. While broaching is versatile enough for many applications, it tends to work best with materials that fall within a hardness range of 26 to 28 Rockwell C.

Still, it’s worth noting that many manufacturers have successfully extended that threshold, achieving reliable results with materials as hard as 32 Rockwell C.

How to Care for Your Broaches?

When a new broach is put into service, it can typically achieve at least 8,000 cuts. With consistent maintenance and timely sharpening, however, its useful lifespan can extend dramatically—sometimes reaching 60,000 cuts or even more.

Given that broaches represent a significant investment—often costing $2,000 or more—many organizations find that replacing them after just 8,000 uses is neither practical nor cost-effective, especially for larger-scale operations.

To address this, it is often advisable to have broaches sharpened and reconditioned after around 3,000 cuts. This practice not only preserves the tool’s effectiveness but also helps manage production expenses over time.

If you find that more force is needed during cutting than usual, this can be a clear sign that the broach requires sharpening. Regular attention to these cues can help maintain both efficiency and budget.

Types of Broaching Machine

Here are just a few types of broaching machines to consider:

• Horizontal broaching
• Vertical broaching
• Internal broaching
• External broaching
• Rotary broaching
• Linear broaching

#1. Horizontal Broaching Machine.

Horizontal broaching machines operate using a pull-type approach to broach various workpieces, making them suitable for both internal and external broaching tasks. These machines are typically constructed with a bed, a broach pilot, and a drive mechanism.

In practice, horizontal broaching machines are commonly used to produce keyways, splines, slots, round holes, and a range of internal shapes. Although they do require a significant amount of floor space, their design is particularly well-suited for handling longer broaches and accommodating heavier workpieces.

#2. Vertical Broaching Machine.

In a vertical broaching machine, the broaching tool is either pushed or pulled through the workpiece, with push broaching being the method most often encountered in practice. Thanks to their design and layout, these machines are well-suited for performing a series of operations in succession, making it straightforward to transfer a workpiece from one station to the next.

Vertical broaching machines generally fall into three categories: push down, pull up, and pull down. Among these, the pull-up configuration tends to be the most widely used. In terms of operation, these machines may rely on either hydraulic systems or electro-mechanical drives, though hydraulic drives are typically favored for their cost-effectiveness and reliability.

#3. Surface Broaching Machine.

A surface broaching machine is designed to remove excess material from the surface of a workpiece. In operation, the cutting tool moves across the workpiece, shaping either flat or contoured surfaces as required. What sets this type of machine apart is the way the cutting tool is mounted—secured to a ram that moves forcefully past the workpiece.

Depending on the specific design, the ram may travel horizontally or vertically. Some surface broaching machines are equipped with two rams, a configuration known as duplex broaching.

#4. Continuous Broaching Machine.

In the process of continuous broaching, the workpiece moves continuously, while the broach itself remains fixed in place. This movement can take different forms—it might be straight, horizontal, or even circular, depending on the requirements of the operation.

For horizontal continuous broaching in particular, workpieces are typically positioned on a fixture attached to a moving chain. This setup allows for the simultaneous machining of multiple identical parts, making it a practical choice for high-volume production runs.

#5. Rotary Broaching Machine.

Rotary broaching, sometimes referred to as wobble broaching, stands apart from other broaching techniques. This method is particularly valued for its ability to produce precise internal and external polygonal shapes.

The process involves positioning the cutting tool at a slight 1-degree angle relative to the centerline of the workpiece. As a result, the broach carves material away through a unique chiseling or scalloping motion, rather than simply pushing straight through.

What makes rotary broaching quite versatile is its compatibility with both lathes and milling machines, whether they have horizontal or vertical spindles. In these setups, the spindle is free to rotate, allowing the tool to interact with the workpiece efficiently.

The range of shapes achievable with rotary broaching is impressive, covering everything from hexagons and squares to serrations, keyways, involute profiles, splines, spur gears, and even custom shapes like numbers and letters.

#6. Turn Broaching Machine.

Turn broaching machines serve a crucial role in producing circular, linear, and spiral cuts, especially in applications where a high-quality surface finish is essential. These machines operate using a tool fitted with multiple inserts, which gradually remove material as the crankshaft is rotated between centers.

Depending on the specific needs of a project, a turn broaching machine can be equipped for either finishing or roughing tasks; the design and capabilities of the machine ultimately dictate the type of tool used.

When it comes to roughing, the choice of inserts and their segment configuration is tailored to meet the desired rate of material removal. For finishing operations, segments are fitted with adjustable cartridges, which allow for fine-tuning and are particularly useful when precise tolerances must be achieved.

#7. CNC Broaching.

Recent advances in technology have made it possible to perform broaching operations using Computer Numerical Control (CNC) machines. Today, both rotary and linear broaching can be accomplished in a single step, streamlining the manufacturing process.

Manufacturers specializing in broaching have responded to these developments by designing compatible tools, tool bodies, and holders that integrate seamlessly with CNC systems.

One notable feature of CNC broaching is the use of indexable insert tools and specialized tool holders. These holders are engineered to accommodate a range of inserts tailored for specific applications.

Depending on the requirements, CNC broaching can be used for a variety of forms, including rotary (or wobble) broaching, as well as punch, keyway, spline, polygon, and serration broaching, among others.

Advantages of Broaching

• Very high production rate higher than milling, planing, boring etc.
• High dimensional and form accuracy and surface finish of the product.
• Roughing and finishing in a single stroke of the same cutter.
• Needs only one motion of cutting, so design, construction, operation and control are simpler.
• Extremely suitable and economical for mass production.
• Expertise not needed.
• Remarkable finished face.
• Short cycle time with high accuracy.
• Little skill is required to perform a broaching operation.
• Broaching can be used for either internal or external surface finishing.
• A tolerance of +-0.0075 mm and a surface finish of about 0.8 microns can be obtained in this process.
• Cutting fluid may be readily applied where it is most effective because a broach tends to draw the fluid into the cut.

Disadvantages of Broaching

• Only through holes and surfaces can be machined.
• Usable only for light cuts.
• Cutting speed cannot be high.
• Defects or damages in the broach severely affect product quality.
• Design, manufacture and restoration of the broaches are difficult and expensive.
• Separate broach has to be procured and used whenever size, shape and geometry of the job changes.
• Economic only when the production volume is large.
• A very large workpiece cannot be broached.
• High tool cost.
• Broaching cannot be used for the removal of a large amount of stock.
• Parts to be broached must be capable of being rigidity supported and must be able to withstand the forces that set up during cutting.

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