5 axis workholding system: How to achieve precise positioning and stable support for complex curved surface machining?

In the field of high-end manufacturing, from complex curved blades in aerospace to precision molds for automotive panels, five-axis CNC machining has become a "weapon" for conquering complex geometric parts machining with its unique advantages of multi-degree-of-freedom linkage. However, the efficient implementation of this technology is inseparable from a key premise - accurate and reliable workpiece clamping. As a "bridge" connecting machine tools and workpieces, the 5 axis workholding system must not only ensure the stable positioning of the workpiece in high-speed rotation and multi-axis linkage, but also break through the spatial limitations of traditional clamping and provide a processing path without dead ends for the tool. The following will introduce in detail:

1. What is 5 axis workholding?

5 axis workholding refers to the technology of fixing the workpiece stably on the machine table through special fixtures, tooling or clamping devices on a five-axis CNC machining center (with three linear axes X, Y, and Z and two rotary axes A, B or C), ensuring that the workpiece maintains precise positioning and posture during the five-axis linkage machining process, while allowing the machine tool tool to approach the workpiece surface at any angle and direction to achieve complex surface machining.

Six-claw pressure plate hydraulic fixture; shell lathe fixture

Its core requirements are:

The fixture structure needs to adapt to the motion range of the five-axis machine tool (such as the swing angle and stroke of the rotary axis) to avoid interference with the tool and machine tool parts;

The positioning accuracy needs to meet the high requirements of multi-axis machining (usually the error is controlled within ±0.01mm);

The clamping stiffness needs to resist cutting force, centrifugal force and vibration to ensure machining stability.

2. The role of the 5 axis workholding system

(1) Precise positioning and posture control

Five-axis machining often requires multi-angle and multi-directional machining of the workpiece (such as the twisted curved surface of aerospace blades). The clamping device needs to accurately align the workpiece coordinate system (WCS) with the machine tool coordinate system to ensure that the tool path runs according to the designed trajectory.

Example: Through high-precision positioning pins, hydraulic chucks or vacuum suction cups, the reference surface of the workpiece is fixed so that the rotation center of the rotation axis (such as the A axis) coincides with the design origin of the workpiece.

Five-axis machining of aerospace blade surfaces

(2) Expanding the processing range and accessibility

Traditional three-axis machining is limited by the axial direction of the tool and is difficult to process deep cavities, undercuts or hidden areas; five-axis clamping allows the workpiece to swing with the rotation axis (such as the B axis ±90° swing), so that the tool cuts in at the best angle to avoid interference between the tool holder and the workpiece.

Example: When machining a mold cavity, by tilting the workpiece, a shorter tool can be used to complete deep cavity machining, reducing the tool cantilever length and improving rigidity.

(3) Reduce the number of clamping times and improve efficiency

One clamping can complete the processing of multiple surfaces (such as top surface, side surface, and inclined surface) of the workpiece, avoiding the accumulation of positioning errors caused by traditional multi-process clamping.

Example: When processing complex box-type parts, traditional clamping is required 3-4 times. Five-axis machining can complete all curved surfaces and hole system processing with one clamping, and the efficiency is improved by more than 50%.

(4) Ensure machining stability and accuracy

During five-axis machining, the workpiece rotates at high speed with the rotating axis (such as 360° rotation of the C axis). The clamping device needs to provide sufficient clamping force (such as constant pressure of hydraulic clamps) to prevent centrifugal force from causing displacement or vibration of the workpiece.

For thin-walled parts (such as aluminum alloy aviation structural parts), flexible clamps or vacuum adsorption are required to reduce clamping deformation (such as deformation controlled within 0.02mm).

(5) Adapt to automated and intelligent processing

In conjunction with the machine tool's automatic tool change (ATC) and workpiece detection system (such as a trigger probe), the clamping device can achieve closed-loop control of the machining process and automatically compensate for positioning errors.

Example: Use the machine tool probe to measure the workpiece position before machining, correct the clamping deviation in real time, and improve machining consistency.

3.5 axis workholding fixture structure classification

Type	Representative Fixtures	Application Scenarios	Technical Features
General Fixtures	Precision Vises, Indexing Heads	Medium-small sized regular workpieces (e.g., flanges, gear boxes)	Repeat positioning accuracy: ±0.01mm; Adjustable clamping force (500–2000N)
Special Fixtures	Blade tenon positioning fixtures, mold core fixtures	Complex shaped workpieces (aerospace blades, automotive panel molds)	Customized positioning surfaces (accuracy: ±0.005mm); Integrated cooling and chip removal channels
Flexible Fixtures	Vacuum chucks, electromagnetic fixtures	Thin-walled workpieces, deformable materials (aluminum, plastic)	Non-contact clamping; Deformation < 0.01mm; Response time < 2 seconds
Automated Fixtures	Robot quick-change fixtures, gantry-type fixtures	Mass production, smart factory scenarios	Supports M2M communication; Automatic workpiece type recognition (e.g., RFID tags)

4.5 axis workholding's impact on CNC machining

(1) Positive impact: Improve machining capability and quality

1) Complex surface machining capability

Support high-precision free-form surface machining (such as automotive cover molds, aerospace impellers), and achieve geometric shapes that traditional three-axis machine tools cannot complete (such as negative angle surfaces and cross surfaces).

Aerospace blade surface processing

2) Improved precision and surface quality

Reduce the number of clamping times and avoid repeated positioning errors (the traditional multi-process clamping error is about ±0.05mm, and the five-axis one-time clamping error is ≤±0.01mm);

The tool can be fed along the normal direction of the surface to reduce the fluctuation of cutting force, and the surface roughness Ra can be improved from 3.2μm to below 0.8μm.

3) Efficiency optimization

Shorten processing time: complete multi-faceted processing through one clamping, eliminating auxiliary time such as clamping and tool setting, and improve the processing efficiency of typical parts by 30%-70%;

Reduce the number of tools used: Five-axis linkage can achieve "milling instead of grinding" through the tool swing angle, replacing some precision grinding processes.

(2) Challenges and precautions

1) High complexity of fixture design

Interference must be avoided: the fixture structure must avoid the motion range of the rotating axis (for example, when the A-axis swings, the fixture height cannot exceed the machine tool swing angle limit). During design, motion simulation must be performed through CAD/CAM software to simulate the collision between the tool and the fixture to avoid;

Balance between light weight and rigidity: When rotating at high speed (such as C-axis speed above 100rpm), the excessive weight of the fixture will increase the machine tool load, and lightweight and high-strength materials such as carbon fiber and aluminum alloy must be used.

2) High requirements for programming and process planning

The impact of the clamping position on the tool path must be considered: for example, when machining the bottom of the workpiece, the fixture support point cannot block the machining area;

Tool axial control: Five-axis programming requires setting a safe distance for the clamping device (such as the minimum distance between the tool handle and the fixture ≥ 20mm) to avoid collision during swing angles.

3) Risk of workpiece deformation

Uneven distribution of clamping force: When clamping thin-walled parts, excessive local pressure will cause the workpiece to bend (for example, when the thickness of the aluminum alloy web is less than 2mm, multi-point uniform support is required);

The influence of thermal deformation: The heat generated by long-term high-speed processing may cause the fixture to expand, and thermally stable materials or cooling systems need to be selected.

4) Cost and maintenance requirements

High cost of special fixtures: Customized fixtures need to be designed for special-shaped workpieces, and the cost of a single fixture can reach tens of thousands of yuan;

Five-axis machining fixture, multi-workpiece dovetail fixture

Regular precision calibration: The positioning surface of the clamping device needs to be calibrated regularly with a three-coordinate measuring machine (CMM) to ensure repeated positioning accuracy.

5. Typical application scenarios and clamping device selection for 5 axis workholding

Application Fields	Workpiece Types	Clamping Devices	Technical Key Points
Aerospace	Titanium alloy blades, aluminum alloy frames	Vacuum chucks, flexible support fixtures	Low clamping force, anti-deformation, automatic centering with probe
Automotive Molds	Injection molds, automotive panel molds	Hydraulic chucks, precision vises	High rigidity, quick clamping, stable positioning for multi-angle machining
Precision Parts Machining	Medical devices, optical lenses	Electromagnetic fixtures, pneumatic dividing heads	Micron-level positioning accuracy, suitable for high-speed rotational machining of small-sized workpieces

6. Summary

5 axis workholding is the "rigid foundation" of five-axis machining, and its technical level directly determines the processing feasibility and precision limit of complex parts. From basic positioning reference design to high-end intelligent fixtures, it is necessary to comprehensively consider the workpiece material, structural characteristics, processing technology and machine tool performance, and achieve the leap from "machining capability" to "efficient and precise machining" through the systematic design of "precise positioning - rigid clamping - interference avoidance - automated integration".