Understanding how CNC machining processes affect internal stress and dimensional stability is essential for producing reliable and accurate parts. Each stage of machining — from rough cutting to finishing — can influence not only the tensile stress within the material but also cause stress release that results in part deformation and unstable dimensions. This section explores how CNC operations impact internal stress and why managing this stress is critical to achieving dimensional precision.
How CNC Processes Induce and Release Internal Stress
During CNC machining, materials experience both induced stress (from cutting forces, tool engagement, thermal gradients) and stress release (from removing constraints on pre-existing residual stress in raw materials).
How Residual Stress Gets Released
Most metal stock — especially extrusions, rolled plate, or castings — contains residual stress due to prior processing (e.g., rolling, casting, heat treatment). These internal stresses are balanced internally until machining begins.
When material is removed unevenly (e.g., one side more than another), these internal stresses are no longer balanced. The result is stress redistribution, often causing part warpage, bowing, twisting, or dimensional shift even after the part is unclamped.
Example: A symmetrical aluminum plate may remain flat until one face is face-milled. Once machined on one side, internal compressive stress on that side is relieved, and the plate may warp concavely toward the cut face.
Stress Release vs. Induced Stress
- Induced stress comes from mechanical cutting and thermal effects during machining.
- Stress release occurs when machining removes material that previously held residual stress in balance.
Both can affect dimensional accuracy — but stress release is more unpredictable and cannot be corrected merely by optimizing cutting parameters.
Why Stress Release Leads to Dimensional Instability
Stress release results in non-uniform deformation because internal stress is not evenly distributed. Unlike thermal expansion or tool deflection, this deformation:
- Occurs after machining, sometimes during inspection or assembly.
- Cannot be corrected by adjusting CNC paths or speeds alone.
- Compromises tolerances, especially flatness, parallelism, and hole positions.
This is especially critical in parts requiring:
- Tight position tolerance between features
- High flatness or straightness
- Assembly-fit critical components
Without stress-relieving measures, parts may pass inspection right after machining but later shift in dimension, leading to assembly or performance failure.
Best Practices to Manage Stress and Prevent Deformation
To prevent stress-related deformation and improve dimensional stability, consider the following strategies:
| Strategy | Description |
| Rough and Semi-Finish Before Final Machining | Allow the part to release stress gradually. Leave stock and finish in later passes. |
| Stress-Relief Heat Treatment | Especially for aluminum, titanium, and tool steels, use annealing or aging to reduce internal stress before precision machining. |
| Symmetrical Machining | Remove material evenly from both sides to keep stress balanced. |
| Use Low-Stress Raw Material | Choose stress-relieved or pre-machined stock when available (e.g., MIC-6 cast aluminum, ground flat stock). |
| Allow Rest Periods Between Machining Steps | In critical applications, let parts “rest” after semi-finish to allow stress relaxation before final machining. |
| Fixture Support Carefully | Avoid over-constraining parts in ways that mask or artificially suppress warping during machining. |
Summary of CNC Machining’s Influence on Stress and Dimensions
| Cause of Dimensional Instability | Mechanism | Prevention |
| Residual Stress in Raw Material | Stress redistribution after cutting | Stress-relief treatment or balanced machining |
| Aggressive Machining Parameters | Induced mechanical and thermal stress | Optimize tool speed, feed, coolant |
| Unbalanced Material Removal | Asymmetric stress release | Symmetrical machining and planning |
Understanding that stress release is not simply a byproduct, but a root cause of dimensional deviation, is essential for high-precision CNC manufacturing.
