How to reduce deformation of precision parts during CNC machining?
- How to reduce deformation of precision parts during CNC machining?
- Why should we pay attention to deformation during the machining process of precision parts?
- Common situations and causes of workpiece deformation during CNC machining
- Physical changes in the CNC machining of precision parts, resulting in deformation.
- Causes of deformation during machining of precision parts with different characteristics
- Technological measures to reduce machining deformation of precision parts
- More operational techniques to reduce machining deformation of precision parts
- Elimold: The right manufacturing partner for every CNC project
- Summarize
In CNC machining, machining deformation is a common and serious problem in the manufacturing process. It not only affects product accuracy but also leads to longer machining cycles and reduced production efficiency. Therefore, effectively solving the machining deformation problem is a pressing issue that needs to be addressed in the production of high-precision custom parts.
However, in the process of forming and machining mechanical parts, the most commonly used machining process is cutting. During cutting, factors such as cutting heat, frictional heat between machine tool parts, internal stress of the workpiece, and clamping force can cause deformation, leading to decreased precision and even scrap. Therefore, it is necessary to understand the causes of workpiece deformation and take preventative measures during machining. This article will delve into the common causes of deformation in CNC machining and their corresponding solutions.
Why should we pay attention to deformation during the machining process of precision parts?
The performance of precision parts is not only related to a company’s profits but also to safety. Products or equipment assembled from precision parts can bring economic benefits to the company while effectively reducing the probability of safety accidents. Therefore, avoiding deformation of parts during machining is particularly important. Machinists in companies manufacturing precision parts should consider various factors and take corresponding measures during processing to prevent deformation, ensuring that the finished parts can be used normally. To achieve this goal, it is necessary to analyze the causes of deformation during part machining and find reliable solutions to the deformation problem, thereby laying a solid foundation for the realization of modern enterprise strategic goals.
1,000,000 different projects for global clients over many years , the various deformations that occur during the manufacturing process of precision parts have diverse mechanisms, which dictates a wide variety of solutions and prevention methods. The appropriateness of the measures taken to address these deformations in the machining of precision mechanical parts directly determines the quality of the part’s machining and also greatly reflects the level of a company’s mechanical engineering capabilities. To improve the machining quality of parts, it is essential to deeply understand the causes of part deformation and take effective measures to avoid and reduce it.
Common situations and causes of workpiece deformation during CNC machining
Workpiece deformation during machining is a relatively difficult problem to solve. First, the causes of the deformation must be analyzed before any countermeasures can be taken. Below are some common causes summarized by our team based on years of experience.
The material and structure of a workpiece can affect its deformation.
The magnitude of deformation is directly proportional to the complexity of the shape, the aspect ratio, and the wall thickness, as well as the rigidity and stability of the material. Therefore, when designing parts, it is essential to minimize the impact of these factors on workpiece deformation.
Deformation caused during workpiece clamping
When clamping a workpiece, first select the correct clamping point, and then select an appropriate clamping force based on the location of the clamping point. Therefore, try to make the clamping point and the support point coincide, so that the clamping force acts on the support. The clamping point should be as close as possible to the machined surface, and a position that is unlikely to cause clamping deformation under force should be selected.
Deformation caused during workpiece processing
During the cutting process, the workpiece undergoes elastic deformation in the direction of the cutting force, a phenomenon commonly known as tool deflection. This reduces the resistance caused by friction between the tool and the workpiece, and also improves the heat dissipation capacity of the tool during cutting, thereby reducing residual internal stress on the workpiece.
Stress deformation after processing
After processing, the part itself has internal stress. The distribution of these internal stresses is in a relatively balanced state, and the shape of the part is relatively stable. However, after removing some material and heat treatment, the internal stress changes. At this time, the workpiece needs to reach force balance again, so the shape changes.
Deformation after processing (including after heat treatment)
Parts with a large length-to-diameter ratio will bend after heat treatment or machining and after being left for a period of time; that is, their straightness will be found to be greater than before upon inspection. Thin sheet-like parts with a particularly small length-to-diameter ratio will exhibit a “hat-shaped” bend after heat treatment or machining and after being left for a period of time, meaning that one end will bulge out from the center relative to the surrounding edges, resembling a straw hat; their flatness will be found to be greater than before upon inspection. Cast iron parts will also show a larger flatness after being left for a period of time after machining. Fork-type parts will warp after being left for a period of time after heat treatment or machining; that is, their perpendicularity will be found to be greater than before upon inspection. These phenomena mainly occur because there are internal stresses within the parts. The distribution of these internal stresses should be in a relatively balanced state, so the shape of the part is relatively stable. However, after machining, material removal, or heat treatment, the internal stresses change and need to be redistributed to a new equilibrium state, thus causing the shape of the part to change.
Types of heat deformation
(1) Thermal deformation of the cutting tool
Cutting heat heats the cutting edge and tool body, causing the tool head to deform and elongate, thus changing the workpiece dimensions. The elongation of the tool head is related to the depth of the tool head, the cross-sectional size, the thickness of the cutting edge, and the sharpness of the cutting edge. The greater the depth of the tool head, the greater the elongation. The cross-sectional area of the tool shank is inversely proportional to the elongation. The thicker the cutting edge, the smaller the elongation.
(2) Thermal deformation of machine tools
The heat generated by cutting and friction between machine tool parts can cause some machine tool components to heat up and deform. For example, the deformation of the lathe spindle box can increase the spindle center height and cause horizontal displacement.
(3) Thermal deformation of the workpiece
Cutting heat causes the workpiece to heat up, and the temperature rises. There are two types of workpiece heating: uniform heating and non-uniform heating. Uniform heating will change the size of the workpiece, but the shape will remain unchanged. In non-uniform heating, not only will the size of the workpiece change, but the shape will also change.
Physical changes in the CNC machining of precision parts, resulting in deformation.
The machining of precision parts is accomplished by cutting metal or plastic materials with a cutting tool; the cutting process itself is an extrusion process. This extrusion process generates a large amount of heat, causing the material to expand and its grain structure to change. Upon cooling, the material shrinks, and the grain structure changes again, resulting in residual stress within the part’s material. If these stresses are not eliminated, deformation and dimensional changes will occur. The solution is to divide the material machining process into roughing and finishing. After roughing, the part undergoes heat treatment to fully eliminate cutting and residual stresses before finishing.
Causes of deformation during machining of precision parts with different characteristics
Precision parts with different design features may deform during machining for various reasons. Below, we summarize common machining (clamping) methods for machining parts with different design features and the causes of deformation.
Machining of the outer diameter of parts with a large length-to-diameter ratio
The process employs a three-jaw clamping method at one end, which conforms to the cantilever beam model. Therefore, assuming a constant cutting force, the force on the part pointing towards the other side of the tool remains constant, while the non-clamped side experiences the greatest deformation, resulting in minimal material removal. The amount of material removed follows an inverted trapezoidal distribution from the clamped side to the free side. In the deformation model, the dashed line represents the tool tip trajectory. After turning, the part forms an inverted cone shape with a diameter increasing towards the clamping end.
End face machining of thin sheet parts with particularly small aspect ratio
The process involves using an electromagnetic chuck to clamp the part from the bottom, followed by grinding the top. Because the part has poor flatness before machining, it deforms during clamping to improve its flatness. This clamping deformation (clamping deformation) prevents the cutting tool from removing the bulging material that should be removed in the free state. Therefore, regardless of how it is flipped, the flatness is acceptable before the clamping force is removed. However, in the free state, the part’s elasticity returns to its previous state, making it difficult to achieve the required flatness.
The part (or part of the structure) contains one or more of the features of cantilever and sheet.
The design of some parts has both cantilever and thin-plate characteristics. Therefore, if the positioning and clamping are not done properly, the part may experience tool deflection due to insufficient rigidity, or clamping deformation. After milling, the flatness required by the drawing will not be achieved.
Technological measures to reduce machining deformation of precision parts
CNC machining part deformation is a common and troublesome problem. It not only affects product precision and quality but can also lead to material waste and reduced production efficiency. Many operators have struggled with this, but don’t worry, with the right methods, it can be effectively addressed. Today, Elimold’s CNC machining engineering team shares several practical tips to help you easily solve part deformation problems and improve machining results.
Reduce internal stress of billet
Natural or artificial aging treatments and vibration treatments can partially eliminate internal stress in billets. Pretreatment is also an effective process. For large billets, due to the large machining allowance, the deformation after machining is also large. If the excess parts of the billet are pre-machined and the machining allowance of each part is reduced, not only can the machining deformation in subsequent processes be reduced, but the internal stress can also be released after a period of time.
Improve tool cutting ability
The material and geometry of cutting tools have a significant impact on cutting forces and cutting heat. Correct tool selection is crucial for minimizing machining deformation of parts. Appropriate selection of tool geometry parameters is essential.
- Rake angle: While maintaining the strength of the cutting edge, a larger rake angle can, on the one hand, produce a sharper cutting edge, and on the other hand, reduce cutting deformation and facilitate smooth chip removal, thereby reducing cutting force and cutting temperature. Tools with negative rake angles should be avoided.
- Clearance angle: The size of the clearance angle directly affects the wear of the flank face and the quality of the machined surface. Cutting thickness is an important factor in selecting the clearance angle. During rough milling, due to the large feed rate, heavy cutting load, and high heat value, the tool needs good heat dissipation performance; therefore, the clearance angle should be small. During finish milling, the cutting edge should be kept sharp to reduce friction between the flank face and the machined surface and to reduce elastic deformation. Therefore, the back angle should be larger.
- Helix angle: To make the milling process smoother and reduce milling force, the helix angle should be as large as possible.
- Main deflection angle: Appropriately reducing the main deflection angle can improve heat dissipation and reduce the average temperature of the processing area.
Improved tool structure
Reduce the number of milling cutter teeth and increase chip removal space. Because precision parts have high plasticity and undergo significant cutting deformation during machining, a larger chip removal space is required. Therefore, the bottom radius of the chip removal groove is larger, and the number of milling cutter teeth is smaller.
Sharpen the cutting edges. The surface roughness of the cutting edge should be less than Ra=0.4μm. Before using a new tool, the front and back surfaces of the cutting edges should be lightly polished several times with a fine oilstone to remove burrs and fine serrations left during sharpening. This not only reduces cutting heat but also minimizes cutting deformation.
Strictly control tool wear standards. After tool wear occurs, the workpiece surface roughness increases, the cutting temperature rises, and workpiece deformation increases. Therefore, in addition to selecting tool materials with good wear resistance, the tool wear should not exceed 0.2 mm; otherwise, chip agglomeration is likely to occur. During cutting, the workpiece temperature should generally not exceed 100℃ to prevent deformation.
Improved workpiece clamping method
For workpieces with poor rigidity and thin-walled design , the following clamping methods can be used to reduce deformation ;
Firstly, for thin-walled bushing parts, if a three-jaw self-centering chuck or spring chuck is used for radial clamping, the workpiece will inevitably deform once released after machining. In this case, an axial end-face clamping method with better rigidity should be used. Align the workpiece with the inner hole of the part, insert the self-made threaded shaft into the inner hole, clamp the end face with a cover plate, and then support it with a nut. When machining the outer diameter, clamping deformation can be avoided, thus obtaining satisfactory machining accuracy.
In addition, when machining thin-walled workpieces, it is best to use a vacuum chuck to obtain a uniformly distributed clamping force and to perform machining with minimal cutting. This can effectively prevent workpiece deformation. Furthermore, a filling method can be used. To improve the machining rigidity of thin-walled workpieces, a medium can be filled inside the workpiece to reduce deformation during clamping and cutting. For example, a urea melt containing 3%–6% potassium nitrate can be poured into the workpiece. After machining, the workpiece can be immersed in water or alcohol to dissolve and remove the filler.
Arrange the work process reasonably
During high-speed cutting, due to the large machining allowance and intermittent cutting, vibrations frequently occur during milling, affecting machining accuracy and surface roughness. Therefore, CNC high-speed cutting processes are typically divided into roughing , semi-finishing , and finishing. For parts with high precision requirements, two finishing operations are sometimes necessary. After roughing, the part can cool naturally, eliminating internal stresses generated during roughing and reducing deformation. The machining allowance after roughing should be greater than the deformation amount, generally 1–2 mm. During finishing, the machined surface of the part should have a uniform machining allowance of 0.2–0.5 mm to keep the tool stable during machining, thereby greatly reducing cutting deformation, obtaining good surface finish, and ensuring product accuracy.
More operational techniques to reduce machining deformation of precision parts
While many factors can cause deformation of mechanical parts, different factors require different solutions. On one hand, operators should practice and operate the parts frequently. On the other hand, appropriate machines and materials should be selected and used in conjunction. Besides the reasons mentioned above, practical operating methods are also crucial. Below, we summarize several operating techniques that can reduce deformation during the machining of precision parts.
1. For parts with large machining allowances, symmetrical machining should be used to achieve better heat dissipation and avoid heat concentration during machining. For example, if a 90mm thick sheet needs to be machined to 60mm, one side can be milled first, and then the other side can be machined in one go to the final size, achieving a flatness of 5mm. If symmetrical machining is performed using repeated feed, each side needs to be machined twice to achieve the final size, which can ensure a flatness of 0.3mm.
2. If a sheet metal part has multiple cavities, it is not advisable to use a mold-by-mold machining method, as this can easily lead to uneven stress and deformation of the part. Multi-layer machining should be used, machining all cavities simultaneously on each layer before moving to the next layer. This ensures even stress distribution and reduces deformation.
3. Reduce cutting force and heat by changing cutting parameters. Of the three cutting parameters, cutting force is most affected by back-cutting parameters. If the machining allowance is too large, the single cutting force will not only cause part deformation but also affect the rigidity of the machine tool spindle and reduce tool life. Reducing the back-cutting amount will significantly reduce production efficiency. High-speed milling in CNC machining can overcome this problem. At the same time, by appropriately increasing the feed rate and machine tool speed, the cutting force can be reduced, thereby ensuring machining efficiency.
4. Pay attention to the feed sequence. Roughing focuses on improving machining efficiency and maximizing the amount of material removed per unit time. It typically uses top milling. That is, it removes excess material from the surface of the blank as quickly as possible, essentially forming the geometric contour required for finishing. Finishing focuses on high precision and high quality. It should use milling. Because the cutting thickness of the cutting teeth gradually decreases from its maximum value to zero during milling, the degree of work hardening is greatly reduced, and the degree of part deformation is also reduced.
5. Thin-walled workpieces inevitably deform during clamping during machining, especially during finish machining. To minimize deformation, loosen the clamping blocks before finishing to the final dimensions, allowing the workpiece to return to its original state. Then, gently press down to just clamp the workpiece (based entirely on feel), achieving the desired machining effect. In short, the clamping force should ideally be applied to the supporting surface, and its direction should align with the workpiece’s rigidity direction. The clamping force should be as small as possible while ensuring the workpiece remains stable.
6. When machining parts with cavities, never insert the end mill directly into the part like a drill bit. Doing so will result in insufficient chip space and poor chip removal, leading to overheating, expansion, chipping, and breakage. It is recommended to first drill with a drill bit of equal or larger size than the end mill, and then use the end mill for milling. Alternatively, you can use CAM software to generate a helical cutting program.
Elimold: The right manufacturing partner for every CNC project
Meeting the demands of CNC manufacturing, especially for small and medium-sized enterprises (SME), can be resource-intensive, requiring expertise in parts machining and CNC certification. To address these challenges, collaborating with specialized CNC machining suppliers like Elimold has become a common industrial practice.
Elimold is an ideal manufacturing partner, providing top-tier CNC machining services. Operating in China, a global manufacturing hub, the company boasts an outstanding track record. Their team of experts is capable of achieving tolerances to 0.01 millimeters, encompassing a range of services such as CNC turning, CNC milling, plasma cutting, and laser cutting.
Furthermore, Elimold is not only ISO 9001 certified but also boasts state-of-the-art quality control processes, ensuring that every project meets the highest customer satisfaction standards. This combination of expertise and quality assurance makes them a reliable choice for meeting a wide range of CNC machining requirements. Start your CNC project today! With cutting-edge technology and professional craftsmanship, we bring your innovative designs to life.
Summarize
In CNC machining, machining deformation has a significant impact on production efficiency and product quality. Therefore, we can effectively solve deformation problems during machining by optimizing cutting parameters, selecting appropriate tools, rationally arranging the machining sequence, increasing coolant usage, improving workpiece clamping methods, and enhancing equipment rigidity, thereby improving machining accuracy and production efficiency. With continuous technological advancements, more efficient and precise solutions will emerge in the future, helping manufacturing companies gain an advantage in the fierce market competition. If you need services from companies that can manufacture precision parts for your project, please contact us.