Broaching: Definition, Process, Working, and Types

What is a Broaching?

Broaching is a machining process where a sharp, hardened, toothed tool removes material from a workpiece in a consistent, continuous, and accurate way. The process of broaching uses a tool with raised teeth of differing sizes that precisely cut away a specified amount of material with every pass over the workpiece.

In broaching, the multi-cutting tool moves while the workpiece remains static, or the workpiece moves while the tool is static. In some cases, there may be relative rotation between the tool and workpiece. The broaching process can be internal through the center of the workpiece or external across the surface of the workpiece.

Broaching is a machining process that uses a toothed tool, called a broach, to remove material. There are two main types of broaching: linear and rotary. In linear broaching, which is the more common process, the broach is run linearly against a surface of the workpiece to effect the cut.

Linear broaches are used in a broaching machine, which is also sometimes shortened to broach. In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axisymmetric shape.

A rotary broach is used in a lathe or screw machine. In both processes, the cut is performed in one pass of the broach, which makes it very efficient.

Broaching is used when precision machining is required, especially for odd shapes. Commonly machined surfaces include circular and non-circular holes, splines, keyways, and flat surfaces.

Typical workpieces include small to medium-sized castings, forgings, screw machine parts, and stampings. Even though broaches can be expensive, broaching is usually favored over other processes when used for high-quantity production runs.

Broaches are shaped similar to a saw, except the height of the teeth increases over the length of the tool. Moreover, the broach contains three distinct sections: one for roughing, another for semi-finishing, and the final one for finishing.

Broaching is an unusual machining process because it has the feed built into the tool. The profile of the machined surface is always the inverse of the profile of the broach. The rise per tooth (RPT), also known as the step or feed per tooth, determines the amount of material removed and the size of the chip.

The broach can be moved relative to the workpiece or vice versa. Because all of the features are built into the broach, no complex motion or skilled labor is required to use it. A broach is effectively a collection of single-point cutting tools arrayed in sequence, cutting one after the other; its cut is analogous to multiple passes of a shaper.

What is the Broaching

What is the Broaching Process?

The broaching process varies depending on whether surface or internal broaching is being used. Surface broaching is the most straightforward operation because one surface acts on the other.

For example, either the broaching tool remains stationary, and the workpiece is moved against it, or the workpiece is stationary, and the broach is moved against it.

Internal broaching is not so simple. For the use of an internal broach, the workpiece must be secured in place with a work holder, which also acts as the mount for the broaching machine. Then, using the elevator on the machine, the broach is lowered into the workpiece.

Then, the machine’s puller grabs ahold of the broach pilot, and then when the elevator releases the follower, the puller pulls the broach entirely through the piece. Finally, the workpiece is removed, and the broach reengages with the elevator.

What are the Uses of Broaching?

There are a plethora of uses for broaches. In fact, you have likely seen types of broaching in hand tools, appliances, plumbing, automotive design, farming equipment, and a variety of other military and industrial applications.

However, the primary use of broaching is in the production of high-volume parts that require accurate, repetitive, and complex cuts. While the broaching process can be used for a variety of applications, the best materials to use are those with a hardness rating of between 26 and 28 Rockwell C.

However, many production companies have seen success by using materials up to a hardness rating of 32 Rockwell C.

How to Care for Your Broaches?

When you receive a new broach, it is likely good for at least 8,000 cuts. However, if the broach is cared for and regularly sharpened, you can increase that original estimate to nearly 60,000 or more cuts.

Broaches are expensive parts, with some costing roughly $2,000 or more, and while the estimated 8,000 cuts might be OK for smaller operations, not every company can afford to replace these tools so frequently.

Therefore, after cutting 3,000 parts, it might be worth sending your broaches out to be sharpened and reconditioned to prolong the broaches usefulness and reduce overall production costs. If you have noticed that there is an increase in force required to cut your parts, then your broach may need some sharpening.

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 use the pull-type method for broaching the workpiece and are used for interior and exterior broaching. A horizontal machine has a bed, a broach pilot, and a drive mechanism.

The main broaches on a horizontal machine are keyways, splines, slots, round holes, and interior shapes. Though horizontal machines take up floor space, they are perfect for long broaches and heavy workpieces.

2. Vertical Broaching Machine

A vertical broaching machine pushes or pulls the broaching tool, with push broaching being the most common type. The design and configuration of vertical machines make it possible for them to perform multiple operations since it is easy to pass a workpiece from one machine to another.

The three types of vertical machines are push down, pull up, and pull down. The pull-up design is the most popular. Vertical broaching machines can be powered by hydraulics or an electro-mechanical drive; hydraulic is the most popular and cost-effective.

3. Surface Broaching Machine

A surface broaching machine removes access material from the surface of a workpiece. The cutting tool passes over the workpiece to produce flat or contoured surfaces. The unique feature of a surface broaching machine is that the cutting tool is attached to a ram that is forced along past the workpiece.

The ram can move horizontally or vertically depending on the design of the machine. It is possible for a surface broaching machine to have two rams; this is described as duplex broaching.

4. Continuous Broaching Machine

In continuous broaching, the workpiece is constantly moving while the broach is held stationary. The movement can be straight, horizontal, or circular.

With horizontal continuous broaching, workpieces are loaded on a fixture that is mounted on a chain. Continuous broaching is used for cutting similar parts at the same time.

5. Rotary Broaching Machine

Rotary broaching is unlike the other forms of broaching and is known as wobble broaching. It is a precision method for producing internal polygonal shapes internally or externally. In rotary broaching, the cutting tool is placed at a 1o angle to the centerline of the workpiece; this causes the broach to cut with a chiseling or scalloping effect.

Rotary broaching can be completed with a lathe or mill using a horizontal or vertical spindle machine where the spindle is allowed to spin freely. The shapes that can be created using rotary broaching include hexagons, squares, serrations, keyways, involute, spline, spur gear, numbers, and letters.

6. Turn Broaching Machine

Turn broaching machines are used for circular, linear, and spiral cutting where surfaces need a high finish. The tool has multiple inserts to remove material by rotating the crankshaft between centers. The components of a turn broaching machine are capable of finishing or roughing. The type of machine determines the tool.

Roughing inserts and their segments depend on the requirements of the stock removal rate. The finishing segments have adjustable cartridges, which are ideal for producing tight tolerances.

7. CNC Broaching

Computer Numerical Control (CNC) machines can be used for the broaching process due to recent technological developments. Parts can be broached using the rotary or linear processes in a single step. Broaching companies have designed tools, tool bodies, and holders that can be included in the CNC process.

CNC broaching has indexable insert broaching tools and tool holders. The special tool holders make it possible to have several specialized inserts. CNC broaches include rotary or wobble, punch, keyway, splines, polygons, and serrations to name a few.

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.