Many times, customers will directly ask us how much machining accuracy we can achieve, but without information such as drawings, materials, and post-processing, it is difficult for engineers to give a prepared evaluation. There are many factors that affect the accuracy of CNC machining.
Firstly, let’s start from the perspective of the parts themselves. What factors can affect accuracy?
1. Component structure design
There are many workpieces that usually require multiple processing techniques to be completed. For example, for a complex workpiece, the overall frame is first machined using a three-axis equipment, then the curved surface is machined using a five axis CNC, and then the right angle is cleaned using an electric spark. Finally, burrs are manually processed, and each step of the process flow will affect the accuracy of the workpiece. If there are no drawings, the tolerance range cannot be evaluated.
2. Material selection
If you choose different materials for the same workpiece, the tolerance will also vary greatly. For example, the tolerance of aluminum alloy can reach ± 0.01mm, while ABS production can only reach ± 0.1mm. The hardness, toughness, uniformity and other characteristics of the workpiece material will affect the cutting force and deformation during the machining process, thereby affecting the machining accuracy.
3. Requirements for post-processing
Post processing is an important step in CNC machining, which involves further processing of the processed workpiece, such as anodizing, heat treatment, painting/spray painting, deburring, etc. Aluminum alloys are usually anodized and have almost no effect on tolerances, as the thickness of anodize is approximately 6 μ m. Steel parts that require heat treatment (such as quenching, tempering, annealing, etc.) will change the internal structure and mechanical properties of the workpiece, thereby affecting the changes in the size and shape of the workpiece. Usually, size correction (such as straightening or grinding) is required after completing the heat treatment. Powder coating/spray painting is a process of spraying workpieces, with difficult tolerance control and relatively large tolerances.
4. Selection of process
If there are no design defects in the parts, different machining equipment will have different tolerances for different parts. For example, cylindrical and conical parts are suitable for lathe equipment. If CNC milling equipment is used, the tolerance will be much different. If it is a complex curved surface part, five axis equipment can be used for machining to achieve good accuracy standards. The positioning accuracy of machine tools is usually between ± 0.005 mm and ± 0.02 mm.
Other factors that affect machining accuracy:
1. Geometric errors of machine tools
The geometric accuracy of CNC machine tools directly affects the machining accuracy. For example, wear or improper installation of components such as guide rails, screws, bearings, etc. can lead to geometric errors in machine tools, thereby affecting the accuracy of machined parts. We need to regularly maintain and calibrate the machine tool to ensure geometric accuracy.
2. Processing tool factors
The material, shape, degree of wear, and installation accuracy of cutting tools all have an impact on machining accuracy. Tool wear can cause changes in cutting force, which in turn affects machining dimensions and surface quality. Choosing appropriate cutting tools and regularly replacing them can further ensure machining accuracy.
- Fixtures and Fixtures
The rigidity and clamping method of the fixture will affect the positioning accuracy and stability of the workpiece. Improper clamping may cause deformation or displacement of the workpiece, thereby reducing machining accuracy. Optimize the clamping method to ensure the stability of the workpiece.
4. CNC machining parameters
The setting of machining parameters such as cutting speed, feed rate, and cutting depth directly affects machining accuracy. Unreasonable parameters can lead to excessive or insufficient cutting force, thereby affecting machining quality. Reasonably setting processing parameters can reduce cutting force fluctuations.
5. Thermal deformation during processing
The cutting heat and environmental temperature changes generated during the machining process can cause thermal deformation of the machine tool, cutting tool, and workpiece, thereby affecting machining accuracy. Control the processing environment temperature to reduce thermal deformation. For example, both five axis and wire cutting workshops require a constant temperature and humidity environment for processing to ensure progress.
6. Control system error
The accuracy of the numerical control system, the response speed of the servo drive system, and the accuracy of the feedback device all affect the machining accuracy. For example, hysteresis or feedback errors in servo systems can lead to dimensional deviations in machining. So it is necessary to improve the accuracy and response speed of the CNC system.
7. Environmental factors in processing
Environmental factors such as vibration, humidity, and dust can also indirectly affect the accuracy of CNC machining. A high-precision lathe must have a good foundation and a constant temperature and humidity workshop environment to ensure accuracy. Taking five axis CNC equipment as an example, the machining workshop must be on the first floor, and the workshop foundation must be well laid to prevent the accuracy impact caused by machine tool sinking and ground vibration. A good foundation can also eliminate equipment resonance, as machine tool vibration can increase the roughness of the machined surface.
8. Human factors
If the CNC operator operates improperly, it may directly lead to the scrapping of the workpiece, so qualified employment is the most basic requirement and also the guarantee of quality.