Computer numerical control (CNC) machining, as an efficient subtractive manufacturing technology, relies on numerous high-speed and precise machine tools to complete the fabrication of cnc mechanical parts. During this process, the material is gradually removed through various methods such as cutting, drilling, beveling and slotting to achieve the form of desired cnc mechanical parts.
In order to achieve the best processing effect, the characteristics of CNC machining need to be fully considered in the design stage. In this article, we will outline the diverse types of CNC machine tools and delve into several key design points.
Overview of Types of CNC Machine Tools
In the field of CNC machining, there are various types of CNC machine tools, each with its unique applications and advantages. The following are several common CNC machine tools and their characteristics:
Lathe: Specifically designed for manufacturing complex cylindrical shapes, it is favored by many designs due to its cost-effectiveness. In such machine tools, the material keeps rotating while the cutting tool remains stationary. The desired geometry is created by precisely controlling the movement and feed rate of the tool, as well as the rotational speed of the material.
Vertical milling machine: Its spindle axis is perpendicular to the machine bed. Unlike a lathe, the cutting tool here is moving. This design enables the vertical milling machine to perform a variety of cutting operations, such as drilling, milling and contour cutting.
Horizontal milling machine: Its cutting tool is installed on a horizontal spindle and is suitable for occasions where a large amount of material needs to be removed or where precision requirements are not high. This type of machine tool is widely used in fields such as mold manufacturing and mechanical parts processing.
Planer: Similar to a vertical milling machine, but with a different ratio of the working area to the machine. The cutting tool of the planer is perpendicular to the machine bed while the parts remain fixed. This design makes the planer more efficient when machining large parts.
During the CNC machining process, these machine tools are all equipped with various high-speed cutting tools for removing materials from solid metal or plastic blocks. These cutting tools usually have cylindrical handles and specific tip shapes, as well as limited cutting lengths. When the material is removed from the workpiece, the geometry of the tool is transferred to the part, so the internal features of CNC mechanical parts usually have fillet radii.
Material selection plays a crucial role in CNC machining
Because it directly affects the cost of the parts. When choosing materials, we must consider their hardness, rigidity, chemical resistance, as well as other mechanical and aesthetic qualities.
The commonly used metal materials in mechanical processing include steel, aluminium, brass and copper. Steel, as the most commonly used metal, its properties are determined by the content of carbon and alloys, providing a wide range of applications. Aluminium, renowned for its lightness and corrosion resistance, is often used in applications where weight reduction is required. Brass, favored for its high strength, high machinability and corrosion resistance, is often used in the manufacturing of hydraulic connectors and low-friction components. Copper has found its place in electrical products due to its excellent electrical conductivity. Meanwhile, its good corrosion resistance also makes it perform well in applications such as radiators, roofs and drainage ditches.
Tolerance and fit are indispensable considerations in the design process
Appropriate tolerance control can ensure the accuracy and interchangeability of parts, while reasonable fit design can optimize the performance and service life of parts. All these design points need to be comprehensively considered based on specific application requirements and material properties.
The geometric tolerances of multiple bearings on the rear axle of the vehicle need to be strictly controlled during the design process. Tolerance, as a key concept defining the acceptable range of any given dimension, plays a crucial role in mechanical design. If there is no clear overall tolerance standard, most mechanical workshops will adopt a standard tolerance of ±125 millimeters. However, for specific applications, defining more stringent tolerance standards is often crucial.
Fit, as a specific application of tolerance, is usually classified into three categories: clearance fit, transition fit and interference fit. Clearance fit allows two parts to slide or rotate during assembly, while interference fit requires overcoming greater resistance to complete assembly or disassembly. In the fit between the shaft and the bearing, transition fit or micro-clearance fit are common choices to ensure the design flexibility of the shaft.
In addition, the control of the pores during the installation of the casing is also of vital importance. If the shell needs to be rigidly installed on the frame, the pore size must be precisely controlled to meet the specific gap requirements. However, in more complex assembly scenarios, such as when multiple housings are connected through a long shaft, how to ensure the uniform clearance of the installation holes and the concentricity of the shaft bearings becomes a new challenge. At this point, the application of Geometric Dimensions and Tolerances (GD&T) becomes particularly crucial.
Next, we will explore the application of holes and threads in mechanical design and their correlation with tolerances and fits.
Threaded holes on cnc mechanical parts are one of the key elements in mechanical design. During the drilling process, the depth needs to be strictly controlled to avoid the necessity of using professional tools, unless it is truly necessary to level the bottom hole. For extension holes, although holes can be drilled from both sides of the part respectively, it is necessary to pay attention to the possible mismatch problem that may occur at the meeting point of the two holes. This can be solved by fixtures, but it will increase the cost. When drilling at the edge, it is necessary to ensure that the entire diameter of the drill bit is contained within the part to avoid drill bit breakage and surface finish issues.
When creating threads, there are three main methods to choose from: cutting taps, forming taps and thread milling cutters. For the type of threaded holes, through holes are the best choice as they allow the tap to pass through the part completely and ensure precise thread formation. If blind holes are used, the thread length needs to be increased by five times the nominal diameter of the bottom of the hole. In addition, choosing the appropriate thread size is also crucial, as smaller taps may increase the risk of breakage during the production process.
When planning the thread depth, only the necessary length should be processed to reduce costs and ensure the accuracy of the parts. At the same time, providing detailed drawing information is also indispensable to ensure that the threads included in the quotation are consistent with your design and to avoid confusion and misunderstanding.
Finally, chamfering and fillet are also factors that need to be considered in cnc mechanical parts design.
They play a transitional role at the edges and corners of the parts, which helps to improve the appearance quality and practicality of the parts.
Chamfering, as an important technique in cnc machining, involves beveling at the intersection of two sharp edges. This step is designed to simplify the assembly process, such as enabling bolts to be inserted into holes more smoothly, while reducing the risk of injury that may occur when handling sharp edges.
Fillet refers to the rounding treatment at the inner or outer corners of a part, which is usually determined by the radius of the cutting tool. During the machining process, it is crucial to keep any radius on the part larger than the tool radius, as this will make the cnc machining process smoother and more efficient.
In addition, it is necessary to distinguish between chamfering and deburring. The mechanic will remove burrs by breaking the edges of the parts, but if specific dimensions are required, the material will be chamfered. When chamfering, the edge should be kept at 45°, unless there are special requirements.
The design of the internal rounded corners is also of vital importance. To use large-diameter cutting tools more efficiently, the internal fillet should be as large as possible. However, it should be noted that its radius should be at least one-third of the cavity depth to prevent the tool from breaking.
The five main steps of CNC machining
CNC machining typically consists of four basic steps. Regardless of the machining process adopted, the following procedures must be followed:
►Step One: Design the CAD model
The first step in CNC machining is to create a 2D or 3D model of the product. Designers usually use AutoCAD, SolidWorks or other CAD (Computer-Aided Design) software to build precise product models. For more complex parts, 3D modeling can present product features more clearly, such as tolerances, structural lines, threads and assembly interfaces.
►Step 2: Convert to CNC-compatible format
CNC machine tools cannot directly recognize CAD files. Therefore, CAM (Computer Aided Manufacturing) software, such as Fusion 360, Mastercam, etc., need to be used to convert CAD models into CNC-compatible numerical control codes (such as G codes). This code is used to guide the machine tool to execute precise parameters such as the cutting path, feed rate, and tool movement trajectory to ensure the accuracy of the processing.
►Step 3: Select the appropriate machine tool and set the processing parameters
According to the material, shape and processing requirements of the CNC mechanical parts, select the appropriate CNC machine tools (such as CNC milling machines, lathes, grinders, etc.). Then, the operator needs to carry out the following preparations:
Install and calibrate the cutting tools
Set parameters such as processing speed, feed rate and cutting depth
Ensure that the workpiece is firmly fixed to prevent deviation during the processing
►Step 4: Perform CNC machining
Once all the preparatory work is completed, the CNC machine tool can perform processing tasks according to the preset numerical control program. The processing is fully automated. The cutting tool cuts the material along the set trajectory until the part is formed.
Step 5: Quality inspection and post-processing
After processing is completed, quality inspection of the cnc mechanical parts is required to ensure that their dimensional accuracy and surface quality meet the design requirements. The detection methods include:
Dimensional measurement: Dimensional inspection is carried out using calipers, micrometers or coordinate measuring machines (CMM)
Surface finish inspection: Check the surface roughness of the cnc mechanical parts to see if additional polishing or spraying treatment is required
Assembly testing: If cnc mechanical parts need to be assembled with other components, an assembly test should be conducted to ensure compatibility.
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