Why do machine tools need stress relief？

Within the machine tool industry, there's a saying: "Machine tools must release stress, and the higher the precision of the machine tool, the more important stress release becomes." While this statement may seem like mere experiential advice, it actually has a very solid foundation in engineering and materials science.

As modern manufacturing demands ever-increasing machining precision, machine tool machining accuracy has progressed from millimeter and micrometer levels to sub-micrometer and even nanometer levels. In this context of such precision, any minute structural deformation can be amplified, directly affecting the overall geometric accuracy, stability, and long-term reliability of the machine. Residual stress is one of the "invisible killers" affecting machine tool precision.

This article will systematically analyze the nature of stress, why machine tools must release stress, the time required for stress release, and common and effective methods.

1.What is Stress?

From a mechanical perspective, stress refers to the interaction force generated within an object under the influence of external factors. When an object deforms due to external factors (such as external forces, temperature changes, humidity changes, phase transitions, etc.), internal forces are generated between the various parts of the object to resist these external factors and attempt to restore the object to its original state. These internal forces are called stress.

Stress does not necessarily originate from external loads. Even without external forces, stress may exist within a material; this type of stress is called residual stress.

In machine tool manufacturing, the most important concern is mechanical residual stress, especially concentrated in large structural components such as the bed, column, and beam. These stresses mainly originate from temperature gradient changes and microstructural transformations during casting, welding, heat treatment, and rough machining, including thermal stress, structural stress, and localized plastic deformation stress generated during machining. Due to the large size and complex structure of the components, these stresses often persist for a long time and are latent.

2.Why Machine Tools Must Release Stress?

A. Chronic Structural Deformation Caused by Residual Stress

Many people mistakenly believe that once a part is machined and passes inspection, the structure will not change. However, residual stress is actually a form of "latent energy." Under conditions of varying ambient temperature, thermal cycling from long-term machine tool operation, long-term self-weight, and changes in constraints during subsequent finishing or assembly, stress gradually releases and transforms into deformation. This deformation is often slow, irreversible, and difficult to detect, yet it accumulates continuously.

B. High Sensitivity of High-Precision Machine Tools to Stress

On ordinary machine tools, micron-level structural changes may not be noticeable; however, on high-precision machine tools, even minute stress release can cause errors in guideway straightness, spindle coaxiality, geometric accuracy drift, and decreased repeatability. Therefore, the higher the precision of the machine tool, the more stringent the requirements for structural stability, and the higher the requirements for stress release.

3.How Long Does Stress Release Take in Machine Tools?

A. Key Factors Determining the Stress Release Cycle

There is no single answer to how long stress release takes. Material type (e.g., gray cast iron, ductile iron, or steel), component dimensions and wall thickness, structural complexity, casting and heat treatment processes, and the stress release method used all significantly affect stress release behavior.

B. Understanding the Time Scope in Engineering Practice

Based on professional literature and engineering experience, the stress release cycle for machine tool castings under natural aging conditions typically ranges from several months to several years. Early practices of burying castings underground or submerging them in water, while utilizing environmental changes to promote stress release, have been largely phased out by modern manufacturing systems due to their extremely long cycles and poor controllability.

4.When Does Stress Release Generally Occur?

A. Stress Release Nodes in the Manufacturing Process

It is particularly important to emphasize that stress release for machine tool components is usually completed before final assembly. Common processes include casting completion, rough machining, one or more stress release cycles, semi-finishing, necessary secondary stress release, and finishing. This phased approach helps reduce the risk of deformation during subsequent processing and use.

B. Stabilization Process After Assembly

After the machine tool is assembled and deployed, manufacturers usually require it to be left to stand for approximately 10 to 20 days to further stabilize the structure under its own weight and environmental conditions before final accuracy testing. Although this process is short, it is crucial for maintaining accuracy.

5.Common Stress Relief Methods in Modern Machine Tools

A. Mainstream Methods of Artificially Accelerated Stress Relief

Natural settling is the most traditional method, offering stable results but with extremely long cycles and high costs. In modern manufacturing, thermal aging is more widely used. This involves heating, holding, and slow cooling to induce minute plastic deformation within the material, effectively releasing stress. Vibration aging utilizes the principle of resonance to redistribute internal stress, offering advantages such as low energy consumption and short cycles, but requiring precise parameter matching.

B. Surface Stress Control and Auxiliary Methods

Shot peening and rolling primarily improve the overall stress state by introducing residual compressive stress on the workpiece surface, often used for local structures or as auxiliary methods. In addition, methods such as thermal shock aging, acoustic aging, and explosive aging are mostly applied in special scenarios or research fields.

6.Material Selection is Also Part of Stress Control

A. The Influence of Metallic Materials on Stress Problems

Traditional metal machine bed materials inevitably generate residual stress during casting and machining, thus requiring subsequent processes for release and control. Appropriate material selection and structural design help reduce stress levels at the source.

B. Application of Low-Stress Materials in High-End Machine Tools

In the field of high-end machine tools, natural granite and mineral castings are increasingly being used in machine beds and key structural components due to their extremely low internal stress, good thermal stability, and excellent damping performance, thereby reducing stress problems at the material level.