What is broaching machining? How is it different from turning and milling?

In the field of precision machining, how can a gearbox part with a complex spline hole be turned from a blank into a finished product with IT7 precision in 45 seconds? When the turning tool needs to move repeatedly and the milling cutter needs to change tools frequently, a "multi-tooth linkage" process is changing the status quo with the advantage of "one-time molding" - this is broaching machining. As the "efficiency leader" of precision machining, what is the essential difference between it and the turning and milling we are familiar with? This article will analyze the differences from principle to application to core differences.

1. Definition and principle of broaching machining

(1) Definition

Broaching machining is a core process that relies on multi-tooth broaches to achieve efficient precision machining. Its core lies in the relative movement of the broach and the workpiece, using the increasing cutting depth of each tooth to complete the composite cutting process of roughing, semi-finishing and finishing in one go.

The broach structure follows the "functional zoning" design: the front rough cutting teeth are responsible for the main material removal, the middle semi-finishing teeth are responsible for the modification of the profile, and the rear fine cutting teeth and correction teeth achieve the ultimate control of dimensional accuracy (IT6-IT7) and surface quality (Ra0.4-0.8μm).

(2) Movement characteristics

Main movement: Driven by the hydraulic/mechanical system of the broaching machine, the broach moves linearly or rotationally along the axis of the workpiece. The movement accuracy directly determines the straightness or coaxiality of the machined surface.

Feed movement: No additional feed mechanism is required. It is automatically realized by relying on the height difference (tooth lift) of the adjacent teeth of the broach itself. The cutting depth of each tooth is 0.02-0.2mm, ensuring that the cutting force is uniform and controllable.

(3) Process advantages

Efficiency revolution: Single stroke completes three-level processing, saving 60%-80% time compared to traditional step-by-step processing, especially suitable for mass production.

Precision guarantee: The micro-cutting of the correction teeth (0.005-0.01mm) eliminates the previous processing error, and cooperates with the high-rigidity guide rail of the broaching machine to achieve micron-level precision control.

Surface quality: The continuous cutting trajectories of multiple teeth overlap, effectively suppressing vibration, and the surface roughness can reach mirror level (Ra≤0.8μm).

2. Classification of broaching processes and their applicable processing objects

Broaching machining is an efficient cutting process that uses a multi-tooth broach to complete rough, medium and fine processing in one stroke. According to the broaching object, movement mode and cutting strategy, the broaching process can be divided into the following categories, each of which has its applicable part object.

(1) According to the processing object

1) Internal broaching:

It is used to process through holes of various cross-sectional shapes, including round holes, square holes, polygonal holes, spline holes, keyway holes, internal gears, etc. Before broaching, the lead-in hole needs to be drilled or reamed in advance. The hole diameter is usually 8-125 mm, and the depth does not exceed 5 times the diameter; in special cases, it can reach 400 mm in diameter and 10 m in depth.

2) External broaching:

Used for machining exposed surfaces such as planes, forming surfaces, grooves, tongue grooves, blade tenons, external gears, etc.; commonly used in mass production of large planes (such as machine tool bed guides) and complex contours (such as the outer surface of automobile cylinder bodies).

(2) According to the movement mode

1) Linear broaching:

The broach moves in a straight line in one direction, cutting through coarse, medium and fine teeth in sequence, suitable for various internal and external contours and axially symmetrical surfaces such as keyways.

2) Rotary broaching:

The broach or workpiece rotates simultaneously or alternately during the cutting process, which can process polygonal cross-sections (such as multi-faceted holes), spline contours, etc., and can effectively eliminate linear lines and improve surface quality.

Rotary broaching

(3) According to cutting strategy

1) Forming broaching:

The entire cross-section is formed at one time, the tool is a single forming tooth, with high precision (IT6-IT7), suitable for through holes with small apertures and high precision requirements.

2) Progressive broaching:

The rough, medium and fine teeth are arranged in sections, which is suitable for large and medium batch processing of complex cross-sections (such as splines and special shaped grooves), and the tool manufacturing is relatively simple.

3) Wheel cutting broaching:

Rough broaching adopts short-blade wheel cutting and dividing, and each block cuts a small amount of excess; then fine teeth or forming teeth are used for fine processing, taking into account efficiency and surface quality, and it is often used for fast processing of large apertures or deep holes.

4) Layered broaching:

Such as gradual type and same profile type, complex curved surfaces or step surfaces are processed by layer-by-layer cutting, which can effectively control cutting force and workpiece deformation.

3. Process flow of broaching machining

(1) Workpiece preparation and clamping

Blank inspection: Check the surface of the workpiece blank and the size of the pre-processed hole (introduction hole) to ensure that it meets the reference requirements before broaching.

Cleaning and decontamination: Degrease and remove chips from the workpiece to prevent chips and oil from affecting the positioning of the fixture and processing accuracy.

Workpiece positioning and clamping: Clamp the workpiece on the broaching table or special fixture, adjust the reference surface to be coaxial with the broaching tool guide sleeve, tighten the clamping bolt to ensure that the positioning is firm and there is no looseness.

(2) Broach selection and installation

Broach type selection: Select internal broach or external broach according to the processing object, and further determine the type of forming broach, progressive broach or wheel cutting broach.

Guide sleeve and broach holder assembly: Install the broach on the broach holder, adjust the guide sleeve to be concentric with the workpiece hole to prevent the broach from shifting during the broaching machining.

Center alignment: Use the center hole or indicator to center the broach and workpiece to ensure that the runout is ≤0.002 inches (≈0.05 mm) to ensure processing accuracy.

(3) Process parameter setting and test pulling

Cutting speed and pulling speed: According to the material hardness and broach tooth type, set a reasonable main cutting speed (generally 0.5–2 m/min) and pulling speed, taking into account both efficiency and tool life.

Pulling force and tensioning: Adjust the hydraulic or mechanical pulling force to ensure that the tool teeth maintain stable cutting when entering and exiting, avoiding breakage of the broach or scratching of the workpiece.

Lubrication and cooling: Select the appropriate cutting fluid and spray it evenly on the contact area between the broach and the workpiece to reduce chip adhesion and thermal effects and improve surface quality.

Test pulling test: Perform a test pull to measure the finished product aperture, surface roughness and geometric dimensions to verify the accuracy of the parameters; if necessary, fine-tune the speed, pulling force or feed.

(4) Formal broaching machining

Start the broaching machine: After confirming that the broach and the workpiece are aligned, start the broaching machine main mechanism so that the broach passes through the workpiece at the set speed in one stroke to complete the three levels of rough, medium and fine cutting.

Cutting process monitoring: Observe the broaching sound and pulling force fluctuations in real time, and continue only when the cutting is smooth; if abnormal vibration or broach jam occurs, stop the machine immediately for inspection.

Multiple strokes: For deep holes or high-precision holes, multiple strokes may be required. After each stroke, clean the chips and apply cutting fluid, and then perform broaching again until the standard is met.

(5) Post-processing and quality inspection

Deburring and cleaning: After broaching, use a deburring brush or a deburring machine to clean the burrs on the hole, and clean and dry the workpiece.

Dimension and surface inspection: Use three-coordinate, plug gauge or roundness tester to detect the hole diameter, roundness and surface roughness (Ra0.4-0.8 μm), and record the data for archiving.

Tool maintenance: Check the wear of the broach teeth, and regrind or replace the tooth block if necessary to ensure the processing accuracy of the next batch.

4. Applicable parts and industry applications for broaching machining

Broaching machining is widely used in the manufacture of various parts due to its high efficiency and high precision of completing rough, medium and fine cutting in one stroke.

The multi-tooth structure and built-in feed of the broaching tool make it particularly suitable for mass production of parts with complex cross-sections and strict tolerance requirements.

(1) Parts that broaching machining is good at processing

1) Gears and spline shafts

It is used to process internal and external splines and gear profiles to ensure module consistency and meshing accuracy.

Commonly used in high-reliability occasions such as automotive transmissions and aerospace transmission systems.

2) Keyways and through slots

Process standard keyways, T-slots and special-shaped through slots, one-time forming without secondary milling.

Applied to guide grooves of machine tool guide rails, hydraulic valve bodies and automation equipment.

3) Precision hole processing

Broaching and finishing of holes after drilling and reaming can reach IT7~IT8 level accuracy.

Widely used for positioning holes and oil holes of pump bodies, cylinder bodies and hydraulic components.

4) Planes and grooves

Surface broaching can process large-area planes, grooves and steps, and the surface roughness can reach Ra0.4μm.

Mainly used for functional planes such as machine tool bed guide surfaces and hydraulic block oil grooves.

5) Gun bores and cylinder holes

Rotary broaching technology can be used for finishing of gun barrel spiral rifling and high-pressure cylinder body inner holes.

Applied in the fields of military, oil drilling and high-pressure cylinder manufacturing.

(2) Main industry applications

Automobile manufacturing: mass production of transmission shafts, crankshaft splines, steering system components, etc.

Aerospace: turbine components, landing gear pins, heavy-duty spline couplings, etc.

Hydraulic and petroleum equipment: hydraulic valve bodies, oil pump cylinders, drilling casing inner holes, etc.

Machine tool manufacturing: bed guide rails, slide slots, precision positioning holes, etc.

Medical equipment: implantable joints, guide sleeves, micro gears and other high-precision small batch parts.

5. Comparison of broaching with turning and milling

Comparison Dimension	Broaching	Turning	Milling
Primary Motion	Linear motion of the broach	Workpiece rotation	Cutter rotation
Tool Structure	Multi-tooth continuous tool	Single-edge tool	Multi-edge rotating tool
Machining Efficiency	Highest (one-pass forming)	Medium	Relatively high (multi-edge cutting)
Precision	IT6-IT7	IT7-IT8	IT7-IT8
Surface Roughness	Ra 0.4–0.04μm	Ra 0.8–1.6μm	Ra 1.6–3.2μm
Applicable Objects	Through holes, formed surfaces	Rotational parts (shafts, discs)	Planes, grooves, gears
Cost	High tool cost, suitable for mass production	Low	Medium

The above comparison shows that broaching machining has obvious advantages in the rapid mass production of specific parts, but single-piece production or special-shaped complex surfaces rely more on turning and milling.

Selection suggestions and comprehensive application

Batch and precision priority: When the annual output is large and the aperture or spline precision requirement is ≥IT8, broaching process is given priority;

Flexibility and complex surfaces: For single pieces, small batches or special-shaped complex surfaces, turning and milling are more flexible;

Cost and cycle balance: The initial investment of broaching tools is high, but the cost per piece is low; turning and milling are suitable for small batches of multiple varieties, and the tool investment and programming investment are controllable.

6. Summary:

Broaching machining, as an efficient and high-precision cutting process, occupies an important position in the machinery manufacturing, automotive, aerospace and medical industries. By reasonably selecting the type and technology of broaching, improving the process flow, and combining it with turning, milling and other processes, it is possible to achieve the best balance between efficiency, precision and cost, providing a solid guarantee for parts manufacturing and assembly.