The Role of Trunnion Tables in Multi-Axis Milling

Trunnion tables convert a three-axis machining center into a powerful multi-axis platform capable of machining multiple sides of a part in a single setup. A trunnion adds controlled rotation around a horizontal axis, creating access to faces, angles, and features that normally require repeated fixturing on a flat table. When paired with a CNC 4th axis a trunnion becomes the foundation for 3+1, 3+2, or full simultaneous five-axis milling. 

The advantage is simple. A trunnion places the part at the center of rotation, not the spindle. The machine no longer needs to reach deep along long tools or rely on multiple workholding setups. Instead, the trunnion brings each face of the part directly to the spindle in the most rigid and efficient orientation. The result is shorter cycle times, fewer setups, consistent datums, and smoother process control. 

Trunnion Tables for 3+1 Machining 

A trunnion table is a fixture platform mounted to a rotary axis that allows the workpiece to be rotated about a horizontal centerline. When added to a vertical machining center, it provides access to multiple faces of a part in a single setup, transforming a standard 3-axis machine into a practical 4-axis positional system. 

In 3+1, the trunnion table, generally, acts as a positioner rather than a continuously moving axis. The rotary motion is used to index the workpiece, which is then locked in place while machining occurs. This greatly increases productivity by eliminating multiple setups while maintaining the geometric accuracy of each operation. 

Because the rotary axis is held stationary during cutting, a braking system, hydraulic or pneumatic, may be used to supplement mechanical locking systems integral to the 4th axis, especially if the Trunnion Table is quite large or long, or if the cutting forces are quite heavy. Outboard supports and bearings prevent deflection and maintains alignment with the rotary centerline. 

Ideally, trunnion tables for 3+1 applications are machined from Class 35 cast iron, providing excellent vibration damping and thermal stability. Flatness and squareness are typically held within thousandths of an inch to ensure repeatable alignment between the trunnion faces and the machine’s linear axes. Tighter tolerances, while possible, are not typically required because most users will use a Tool Probe to “touch-off” a workpiece or fixture prior to the start of a machining cycle to eliminate scrap and crashes. With the simplicity of Touch Probes locating the workpiece in 3 dimensions and loading offsets to the Machine Tool Control, this additional step, even when not necessary, is often taken by the operator as a precaution.  

When combined with a quick-change system such as the Carr Lock® interface, trunnion tables also simplify fixture exchange—allowing operators to swap vises, subplates, or dedicated fixtures in less than a minute while preserving location accuracy. 

Trunnion Tables for Simultaneous 5-Axis Machining 

In simultaneous 5-axis machining, trunnion tables perform a much more demanding role. Instead of indexing and locking, both rotary axes move continuously in coordination with the linear axes, maintaining precise tool orientation throughout the cutting process. 

This continuous motion requires an order of magnitude more structural rigidity and positional accuracy than 3+1 work. As a result, trunnion tables for true 5-axis machining are typically built directly into the machine tool’s structure and integrated into the casting or mounted on precision-ground reference planes. These integrated systems minimize deflection, thermal drift, and stack-up error between the rotary axes and the spindle. 

The bearings, drives, and feedback systems in these assemblies are also far more sophisticated. Instead of external brakes, torque motors or harmonic drives provide direct, backlash-free motion control. This integration allows simultaneous movement at high feed rates while maintaining micron-level accuracy. 

While many standalone trunnion attachments enable excellent positional machining, only a fully integrated trunnion assembly provides the rigidity and dynamic stability required for continuous 5-axis toolpaths. 

How Trunnion Tables Improve Part Profitability and Spindle Utilization 

(For A/B testing: How Trunnion Tables Reduce Setups, Scrap, and Process Errors/Why Trunnion Tables Are the Fastest Path to High-Performance Multi-Axis Milling) 

A trunnion is more than a fixture platform. When installed on a machining center it fundamentally changes how the spindle can reach a part. Trunnion tables support multi-axis machining in several key ways. 

1. Improved Access to Multiple Faces: The trunnion rotates the part, not the tool. This reduces tool stickout, improves rigidity, and allows the spindle to reach deep features from better angles. Features that once required multiple setups can be reached in one cycle. 
2. Higher Accuracy Through Fewer Setups: Every time a part is refixtured, the risk of error increases. A trunnion keeps the datums aligned from face to face, improving feature-to-feature accuracy. This is especially important for aerospace, medical, and tooling components where accumulated error is unacceptable. 
3. Increased Throughput and Repeatability: Trunnion tables eliminate manual part repositioning. The machine indexes the part automatically and begins cutting the next face. Setup time drops sharply, and cycle time improves through better tool orientation and shorter tools. Shops often see significant reductions in WIP and fixture count. 
4. Better Use of Cutting Tools: With the part tilted or rotated into an optimal approach angle, shorter and stiffer cutters can be used. This improves surface finish, increases feed rates, and eliminates chatter in pockets or tall features. 
5. Expandable Work Envelopes: A flat VMC table limits how far the spindle can reach without long tooling. A trunnion rotates the workpiece into the spindle’s most favorable zone and reduces wasted travel. Even smaller machines can tackle larger and more complex parts when a trunnion is added. 
6. Simplified Fixture Strategies: A trunnion converts multi-sided machining into a structured, single-setup process. When paired with quick change systems like Carr Lock, fixtures can be swapped in seconds without losing location. This is ideal for production, families of parts, or flexible job shop environments. 
7. Bridge to Full Five-Axis Capability: A trunnion is the practical path for shops that are not ready to invest in a full five-axis machining center. It allows 80 to 90 percent of typical five-axis work to be done on a standard VMC at a much lower cost. When equipped with a high-performance 4th axis and modern control, a trunnion enables both positional and simultaneous multi-axis toolpaths.