CNC machining is widely used in the mechanical manufacturing industry, being a complex application technology that relies on engineering techniques and information technology. CNC milling processes are a crucial part of CNC machining. This article analyzes the process steps for parts in CNC milling services, choosing appropriate equipment according to the processing flow to ensure the quality of the machined parts.
1.Positioning of Parts
In the mechanical processing of workpieces, the rational selection of positioning references is essential for ensuring dimensional accuracy and mutual positional accuracy. Positioning references determine the correct position of the workpiece on the machine tool or in the fixture and are divided into rough references and finish references.
Rough references use unprocessed surfaces as positioning references, while finish references use machined surfaces. The principles for selecting rough and finish references vary: rough references are generally chosen for ease of clamping, while finish references are selected based on the principle of coinciding references and ease of clamping.
2. Clamping of Parts
When selecting fixtures for CNC machining, two basic requirements should be met: first, ensure that the fixture’s coordinate direction is relatively fixed with the machine tool’s coordinate direction; second, coordinate the size relationship between the part and the machine tool’s coordinate system.
Choose suitable fixtures based on a comprehensive consideration of factors such as the size and shape of the part, the material of the blank, and the tools. For parts with relatively regular and simple shapes and smaller dimensions, a general-purpose CNC milling fixture such as a flat vise is sufficient for clamping.
3. Selection of Cutting Tools
The selection of cutting tools is one of the crucial aspects of CNC machining processes. The rational and optimized selection of tools directly impacts the machining efficiency of the machine tool and the quality of the part. When programming, the choice of cutting tools typically involves a comprehensive consideration of factors such as the workpiece material, the machining capabilities of the machine tool, and the process requirements.
There are many types of CNC milling cutters, and different tools are chosen based on various features and sizes. For finish machining, the diameter of the selected tool should be smaller than the smallest corner radius of the internal contour. For example, you can choose carbide end mills (with diameters of 16mm, 10mm, and 1mm), twist drills (with diameters of 11.8mm and 5.1mm), reamers, taps, etc. Tools for rough machining and finish machining should be separated.
4. Selection of Cutting Fluid
Cutting fluid plays a crucial role in cooling, lubricating, cleaning, and preventing rust, thereby enhancing the effectiveness of the cutting process. The commonly used types of cutting fluid include emulsions, aqueous solutions, and cutting oils. When selecting cutting fluid, it is essential to consider the tool material, workpiece material, machining requirements, machine tool type, and machining method to make an appropriate choice.
5. Selection of Cutting Parameters
The key factors in CNC milling parameters include milling speed, feed rate, milling depth, and milling width. The selection of these parameters is closely related to the productivity, machining accuracy, and surface quality of the milling process.
For rough machining, it is advisable to choose large milling widths and depths, higher feed rates, and optimal cutting speeds to remove the material as quickly as possible. In semi-finishing and finishing operations, the milling width and depth should be determined first, followed by smaller feed rates and higher cutting speeds to ensure the part meets the dimensional accuracy and surface quality specified in the part drawing.
The sequence for selecting cutting parameters during rough machining is: milling depth → milling width → feed rate → cutting speed. The milling depth should be chosen based on the depth of the part features, and the tool (the cutting edge length of the end mill should be greater than the milling depth) should be selected accordingly. The milling width should be approximately 35% of the tool diameter. The feed rate and cutting speed should be chosen based on the tool’s load capacity and the characteristics of the machine tool.
For semi-finishing and finishing, the sequence is: milling depth → milling width → feed rate → cutting speed. The milling depth and width should be determined based on the dimensional tolerances specified in the part drawing. The feed rate and cutting speed should be selected based on the required surface roughness and the characteristics of the machine tool.
6. Deburring Workpieces
After finishing machining, workpieces often have sharp edges. Traditionally, burrs and sharp edges are removed and chamfered using tools like scrapers, oilstones, and saw blades after the workpiece is taken off the machine. However, this method can result in uneven chamfers and may scratch the workpiece surface during deburring.
To achieve a more aesthetically pleasing and standardized finish, automated programming software can be used to chamfer sharp edges with a chamfer tool. The chamfer size should be C0.1-C0.2. When programming, it is important to set a safe distance based on the chamfer tool’s radius to avoid interference and overcutting. Therefore, it is advisable to select a chamfer tool with a slightly smaller diameter.
For sharp edges that cannot be machined with a chamfer tool, manual chamfering is necessary. During manual chamfering, ensure the direction is consistent, the pressure is even, and the chamfer size matches the chamfer size of the chamfer tool as closely as possible.