What is rotary broaching? Which one has a wider application range, it or linear broaching?

In the field of mechanical processing, broaching is an efficient cutting process, which is mainly divided into two forms: linear broaching and rotary broaching.

Linear broaching completes three levels of rough, medium and fine cutting in one stroke along the linear guide by the broaching tool, which is suitable for various symmetrical surfaces such as internal and external holes, keyways and planes;

Rotary broaching uses the "swing" principle of a small inclination angle to process non-circular holes, splines and polygonal contours on ordinary lathes or milling machines without the need for special broaching machines.

This article will compare the concept principle, process characteristics, application scope and industry cases in four aspects, deeply analyze the advantages and disadvantages of the two and the applicable scenarios, and demonstrate which method is more widely used in modern manufacturing.

1. Overview of broaching process

Broaching is a cutting method that uses a multi-tooth broaching tool to complete rough, medium and fine processing in one stroke, and is widely used in the processing of high-precision and large-volume parts.

Tool structure: The broach consists of coarse teeth (roughing), medium teeth (semi-finishing), fine teeth (finishing) and correction teeth. The tooth height increases gradually and has a preset step distance. No machine tool feed motion control is required, and the feed is built into the tool.

Process advantages: Multiple levels of cutting can be completed in one stroke, with high productivity; forming and correction teeth are used together, with an accuracy of IT6-IT7 and a surface roughness of Ra0.4-0.8μm; the tool can be reground many times, suitable for mass production.

2. Straight broaching

(1) Definition and principle

Straight broaching is the most traditional and most commonly used broaching form. The broach moves in a straight line in one direction on a special broaching machine. The broach is passed through the workpiece through a tool puller or a push rod to achieve cutting.

Movement mode: The main movement is provided by the broaching machine, and the broach passes through the workpiece along the guide rail at a constant speed; the feed movement is automatically completed by the height difference of the broach teeth, and the depth increases by one step distance for each tooth passed.

Processing parts: Mainly used for processing inner holes (such as round holes, spline holes, keyways), outer contours, planes and through grooves and other axially symmetrical surfaces.

(2) Typical applications

Keyways and splines: spline holes in automobile gearboxes, keyways of mechanical transmission parts, etc., to ensure contour accuracy and meshing consistency.

Hole finishing: Finishing of drilled or reamed holes, with a hole diameter of 8–125 mm and an accuracy of up to IT6~IT7; suitable for positioning hole processing such as hydraulic valve bodies and cylinder bodies.

Plane broaching: High-finish processing of large-area planes such as machine tool bed guide surfaces and hydraulic component oil tank surfaces.

3. Rotary broaching

(1) Definition and principle

Rotary broaching, also known as eccentric broaching, is a process on ordinary lathes, milling machines or CNC lathes that introduces an inclination angle of about 1° between the tool and the workpiece axis, so that the broaching tool produces "eccentric" cutting as the spindle rotates, completing one-time forming.

Principle of deflection: The broach is installed in a special tool holder, and its axis is offset from the workpiece axis by a small angle. As the drive part (workpiece or tool holder) rotates, the broach cuts into the sheet metal in an elliptical trajectory, cutting out non-circular cross-sections tooth by tooth.

Rotary broach holders and shanks

Feed control: The ideal cutting feed rate (IPR) ≈ tool diameter × sin (deflection angle), which is actually slightly lower to avoid complicated cutting and tool blocking. After cutting, spiral patterns are generated due to deflection. If necessary, the process can be reversed or cut in sections to eliminate the spiral.

(2) Typical applications

Polygonal holes and splines: square holes, hexagonal holes, star holes and other fastener positioning holes, as well as fast processing of small internal and external splines.

Blind holes and deep holes: Blind holes that cannot be processed by ordinary linear broaching. Rotary broaching only needs to leave enough chip space at the end of the workpiece to complete the cutting of internal special-shaped holes.

External contours: Process polygonal or special special-shaped contours on the outer surface of the turning plate and cam plate to avoid multiple tool changes and complex programming.

4. Comparison of application scope

Item	Linear Broaching	Rotary Broaching
Machine Requirements	Dedicated Broaching Machine(Horizontal/Vertical)	General Lathe/Milling Machine/CNC Machine
Part Types	Symmetrical Surfaces:Internal/External Holes, Keyways, Planes	Non-circular Holes, Polygons, Blind Holes
Batch Size	Medium to Large Batches(Tooling Cost Allocation)	Small to Medium Batches,Even Single Piece
Machining Precision	High (IT7~IT8)	Medium-High (IT8~IT10)
Surface Quality	Excellent (Ra0.4~0.8μm)	Good (Slight Spiral Marks,Post-Processing Possible)
Cost Investment	High (Custom Broach,Broaching Machine Investment)	Low (Using Existing Machine Tools& Tool Holders)

As can be seen from the above table, linear broaching has significant efficiency and precision advantages in large-scale symmetrical surface processing; while rotary broaching relies on ordinary machine tool environment, is more flexible, and is suitable for special-shaped holes and small-batch processing.

5. Industry applications and typical cases of linear broaching and rotary broaching

(1) Automobile manufacturing:

The spline holes in the transmission are mostly broached by linear broaching to ensure consistent modulus and high precision;

Square holes and hexagonal holes of fasteners are often completed with one-cut rotary broaching in combination with lathes.

(2) Aerospace:

Large-scale blade positioning holes and contour surface broaching mostly use linear broaching;

Complex special-shaped holes (such as fast loading and unloading pin holes) can be quickly realized on CNC lathes using rotary broaching.

(3) Machine tool manufacturing:

Flat linear broaching is often used for bed guide rails and slide groove surfaces;

Small batches of spline shafts can use rotary broaching on CNC lathes to reduce tool change cycles.

(4) Medical devices:

High-precision micro-holes (catheter insertion holes, implant positioning holes) are processed by rotary broaching on a lathe in one clamping process.

6. Which one has a wider range of applications?

In general, linear broaching and rotary broaching have their own focuses, but from the perspective of machine tool popularity and process flexibility, rotary broaching has a wider range of applications:

Low machine tool threshold: No special broaching machine is required, only the tool holder needs to be modified or the yaw tool can be used on most turning and milling centers, reducing equipment investment and factory layout restrictions.

Modified Hexagonal Broaches

Strong batch adaptability: It can handle processing from single pieces, small batches to medium batches of multi-shaped surfaces, without the need to share the cost of broaching tools in large batches.

Various processing objects: It can handle asymmetric, special-shaped, blind holes and outer contours, etc., expanding the role of broaching technology in modern flexible manufacturing.

However, for large batches, symmetrical cross-sections and ultra-high precision parts, linear broaching is still irreplaceable, and its efficiency and precision advantages are obvious.

7. Conclusion

Linear broaching and rotary broaching have their own advantages:

Linear broaching - special machine tools and broaches, high precision, high efficiency, suitable for large-scale symmetrical surface processing;

Rotary broaching - low machine tool threshold, high flexibility, can be implemented on conventional turning and milling centers, suitable for special-shaped holes and small and medium-sized batch processing.

On the whole, with the growth of flexible manufacturing and small batch diversification needs, rotary broaching is used in more working conditions, but linear broaching still maintains its core position in the field of high precision and large batches. The combination of the two can achieve the optimal balance of quality, efficiency and cost in modern manufacturing.