Distortion of parts following machining can be a significant problem. A component leaves the machine within specification, but after a while it turns out that its geometry has changed and it no longer falls within tolerance. Thus, warping following CNC machining can undermine even the best-designed production process. This phenomenon is particularly critical in mass production, where process repeatability and stability are key. So how can internal stresses be prevented?
In this article, you will learn:
Internal stresses are hidden forces acting within a material which are not visible to the naked eye, but which have a huge impact on the geometric stability of workpieces. They can arise during the production of semi-finished products, heat treatment, rolling, forging, and also during the cutting process itself.
As Krzysztof Piwowarczyk from RADMOT explains:
“When material is removed, the balance of forces is disrupted, resulting in deformation of the component. Consequently, managing internal stresses becomes one of the key elements – but also one of the challenges – of the mass production of high-precision components.”
In industrial practice, two basic types of stress are distinguished: inherent and residual. Inherent stresses arise in the material as early as the semi-finished product stage – for example, during rolling or casting. They are, as it were, ‘embedded’ in the material’s structure and may only become apparent during machining.
Residual stresses, on the other hand, arise as a result of technological processes such as machining or heat treatment. A characteristic feature of these is that they remain in the material even after the external forces have ceased. As a result, a workpiece may change its geometry long after the production process has ended.
During machining, the material is subjected to very high mechanical forces and high temperatures. The machined layer undergoes localised heating, plastic deformation and rapid cooling. These changes give rise to new stresses, which superimpose on the stresses already present in the material.
In the production of components requiring high precision – for example, precision CNC machining with an IT6/IT7 tolerance – controlling these phenomena is crucial for maintaining dimensional stability.
Part warping is usually the result of a combination of several factors. In day-to-day production, it is most often caused by the method of material removal, the heat generated and the quality of the raw material.
Removing a large amount of material from one side leads to an imbalance in stresses. The material ‘releases’ the accumulated energy and deforms to restore equilibrium.
The more asymmetrical the cutting process, the greater the risk of:
The machining process generates a significant amount of heat, which causes local expansion of the material. Once machining is complete and the workpiece has cooled, uneven contraction occurs, leading to deformation.
The quality of the semi-finished product is of paramount importance. Materials that have been rolled or cast often contain non-uniform stresses, which only become apparent during machining.
It is important to be aware that some materials are significantly more susceptible to stress relaxation than others. They require particularly strict control throughout the entire production process:
The risk of part deformation can be significantly reduced as early as the design stage. Appropriate design decisions help to reduce stresses and increase the geometric stability of the part.
Symmetry in the design helps to distribute stresses evenly throughout the material. As a result, when removing the machining allowance, internal forces cancel each other out rather than causing deformation.
Designing symmetrical cross-sections:
Too large an allowance increases the risk of stress relaxation, whilst too small an allowance makes it difficult to achieve the required surface quality. Optimal allowance selection strikes a balance between stability and production efficiency.
Selecting the appropriate technological strategy is crucial for minimising workpiece deformation. In mass production, machining process planning often determines the success of the entire project.
In industrial practice, a two-stage machining process is often used: roughing and finishing. After the first stage, the material is left to rest for a specified period to allow for partial stress relaxation. As a result, the final machining takes place on a more stable workpiece, which significantly increases the likelihood of maintaining tolerances.
Removing material evenly from both sides of the workpiece helps to maintain stress balance. This is one of the most effective methods for minimising deformation in mass production.
The order of operations is of paramount importance. The largest volumes of material are removed first, followed by finishing operations. This strategy minimises the risk of deformation of the final geometry.
In many cases, it is necessary to employ additional technological processes to reduce stresses prior to final machining.
Annealing involves heating the material to a suitable temperature and allowing it to cool slowly. This process reduces residual stresses and improves the dimensional stability of the workpieces.
Modern stress-relief methods also include vibration and cryogenic treatment. These allow stresses to be effectively reduced without the need for prolonged heat treatment.
Please contact us by email or telephone to find out more about our services!
How can you predict whether a part will warp after machining?
The greatest risk applies to slender and thin-walled components. It is advisable to carry out a technology and material analysis as early as the design stage.
Can warping be corrected after machining?
In many cases, re-machining or grinding is possible. However, the best strategy is to prevent deformation in the first place.
What tolerances should be allowed for warping?
This depends on the material and the geometry of the component. In precision manufacturing, it is essential to plan the process to allow for stress relaxation.
How long should the material be left to rest after rough machining?
The time depends on the material and the size of the workpiece. In industrial practice, this can range from a few hours to several days.
Does hardening always cause warping?
Not always, but it significantly increases the risk of residual stresses. That is why stress-relief annealing is often carried out before final machining.