What is Spline Broaching?

Spline broaching is a machining process used to create a series of grooves or teeth, known as splines, on the inside of a bore or along the outside of a shaft. These splines form a drive connection between two mating parts, allowing torque to be transmitted while maintaining alignment. The process is essential in applications like gears, couplings, drive shafts, and other components that rely on rotational power transfer.

Splines can be straight-sided, involute, or custom-shaped depending on the design. Each type provides a specific balance of load capacity, fit, and ease of manufacturing. In modern machining, spline broaching has evolved from traditional broach machines to advanced CNC processes that deliver higher accuracy, flexibility, and repeatability.

The CNC spline broaching process

In traditional setups, spline broaching required long, multi-tooth tools and dedicated hydraulic machines that pushed the tool through a pre-drilled hole. Each tooth removed a small amount of material until the final spline form was achieved. This method worked well for mass production but required expensive tooling, specialized equipment, and long lead times for custom designs.

CNC spline broaching changes that approach entirely. Instead of using a multi-tooth broach, CNC Broach Tools use a single carbide insert programmed for multiple passes. Each programmed pass removes a controlled amount of material, simulating the effect of traditional broaching while taking advantage of the precision and control of CNC motion.

During a spline broaching cycle, the machine moves the broach tool in and out of the bore, cutting along each spline tooth position. Using commands such as G68 for coordinate rotation or index moves through the C-axis, the machine precisely indexes the spindle to align each spline tooth. The process continues until all grooves are completed.

This modern method allows machinists to broach internal splines directly on standard CNC mills and lathes without needing a separate broaching machine. The result is a flexible, cost-effective way to produce both standard and custom spline profiles.

Internal and external spline broaching

While spline broaching most often refers to cutting internal splines inside a bore, the same principles apply to external forms as well. Internal splines are common in gears, hubs, and couplings, while external splines appear on shafts and drive components.

• Internal spline broaching on a mill uses the spindle to control feed and depth of cut with high precision.
  
• External spline cutting can be achieved with appropriate fixturing and programming to maintain concentricity between the shaft and tool.
  

CNC spline broaching excels at internal forms because the tool can operate within the bore using rigid setups, short tool lengths, and controlled chip evacuation.

Carbide broaching inserts for spline machining

CNC Broach Tools relies on carbide inserts engineered specifically for broaching operations within CNC machines. Carbide offers superior hardness and wear resistance compared to high-speed steel, allowing the insert to maintain a sharp edge over long production runs and in difficult materials such as stainless steel, titanium, and Inconel.

Because CNC Broach Tools use a single insert rather than a multi-tooth broach, the cutting action is distributed across multiple passes. This reduces cutting forces, improves surface finish, and helps maintain consistent geometry. It also allows machinists to fine-tune depth of cut, feed rate, and number of passes depending on the material and desired finish.

With proper programming and rigidity, carbide broaching inserts deliver precise spline profiles that meet tight tolerance requirements. The combination of carbide durability and CNC repeatability gives shops a reliable, long-term solution for spline production.

Advantages of CNC spline broaching

The transition from dedicated broaching machines to CNC-based broaching provides several important advantages:

• No secondary operations – Machining splines directly on CNC mills or lathes eliminates the need to move parts to another machine.
  
• Reduced tooling cost – A single insert can cut multiple spline types through programming adjustments rather than new tooling.
  
• High flexibility – Machinists can quickly modify profiles or depths without ordering new broach tools.
  
• Tight tolerances – CNC motion control ensures consistent, repeatable spline geometry.
  
• Faster lead times – Parts can be machined complete in one setup, improving productivity and reducing outsourcing delays.
  

For manufacturers producing small to medium batches or custom parts, these advantages can dramatically lower costs while improving quality control.

Applications and materials

Spline broaching plays a vital role in power transmission and mechanical design. Common applications include:

• Automotive drive shafts and gear hubs
  
• Aerospace actuators and couplings
  
• Industrial pumps and gearboxes
  
• Robotics and automation assemblies
  

These components often require durable materials and precision fits, making carbide-insert CNC broaching an ideal solution. CNC Broach Tools’ process supports steels, aluminum alloys, superalloys, and other materials commonly found in demanding environments.

Programming and setup considerations

Success in CNC spline broaching depends on careful setup and accurate programming. Key considerations include:

• Rigidity – Use hydraulic holders or split sleeves to prevent deflection.
  
• Alignment – Dial in the tool centerline within 0.0005 in to maintain straight, consistent cuts.
  
• Relief – For blind holes, ensure a relief groove or cross-hole for tool exit.
  
• Depth of cut – Program small incremental passes, typically 0.0008 – 0.0015 in per pass, for clean results.
  

These practices help prevent insert chipping, deflection, and tapering, ensuring precise spline geometry across production runs.

Why spline broaching matters

Splines are critical in engineering because they transfer torque evenly across multiple contact points, reducing stress and wear on parts. A properly broached spline ensures that components engage securely and operate smoothly over their service life.

CNC spline broaching brings this capability into the modern manufacturing workflow, allowing machinists to produce complex internal forms with speed, accuracy, and confidence. By combining advanced carbide tooling, optimized programming, and proven setup methods, CNC Broach Tools makes spline broaching accessible to any shop running standard CNC equipment.

In short, spline broaching forms the grooves that make splined parts function, and CNC Broach Tools provides the proven technology to perform it efficiently, accurately, and in-house.