Thin-walled frame parts occupy a large proportion in machining. In the development of customized CNC milling services, the machining of thin-walled frame parts has been facing deformation issues. These parts have thin structures and poor rigidity, lacking the support strength and stiffness required for cutting. During milling, they are easily deformed by cutting forces, resulting in the machining of the size of the serious overshoot.
Due to the structural and processing characteristics of thin-walled frame parts, and deformation of the parts, this article proposes the use of high-speed milling technology, combined with the reasonable choice of tools and cutting parameters, as well as improvements in fixture design. These measures effectively reduce vibration and machining deformation, and greatly improve machining efficiency and parts quality.
Structural Characteristics
Thin-walled frame parts have a complex structure, many machining elements, and many requirements for geometric tolerances. Their unique structural characteristics make them easy to produce deformation and deflection in machining. Parts deformation is mainly due to four aspects.
- Clamping Deformation: The parts produce clamping deformation.
- Cutting Force Deformation: Cutting forces cause the inner and outer walls to deflect during machining, resulting in uneven wall thickness. The thinner the wall, the more pronounced the deflection, leading to poor surface quality and greater dimensional deviations.
- Internal Stress Deformation: During machining, the removal of the material destroys the internal stress balance of the material, and after the clamping force is removed at the end of processing, the internal stress of the part leads to deformation.
- Thermal Deformation: Cutting heat induces thermal deformation in the parts.
Frame CNC Machining
Frames are vertically through structures that cannot bear radial forces, complicating clamping. Traditional methods involve CNC milling of the outer contour and upper end face, followed by side hole machining. The inner cavity of the frame is machined by wire cutting, which is time-consuming, produces high surface roughness, poses clamping challenges, and results in significant deformation. Additionally, multiple machining steps increase the turnaround time of the frames.
Through the structural analysis of the frame, the improved machining scheme is to complete the machining of all dimensions of the old machining scheme with one CNC milling process, including the wire-cutting process, with one CNC milling process on the five-axis CNC machine.
Based on the frame’s structural characteristics, the fixture base can employ a planar positioning method. The blank is fixed with three screws, with the frame’s outer shape and side holes machined first. The machine program then pauses, and a special pressure plate is used to secure the frame. The pressure plate’s inner cavity matches the frame’s outer shape, with a clearance of 0.02mm.
The use of the overall axial compression programme not only improves the rigidity of the frame, but also allows the inner wall of the pressure plate to play a supportive role for the frame, eliminating the phenomenon of tool letting during the machining process.
The high-speed milling method and tool parameter setting of the frame are basically the same as that of the bracket machining, and the tangential feed and spiral cutter method are adopted to ensure stable cutting conditions.
CNC Machining Program
To prepare a high-quality CNC machining programme, you need to make full use of the functions of the machine tool, the high-speed machining concept is implemented into the machining program. There are five key points in the preparation of CNC machining programmes for thin-walled frame parts.
(1) Use the layered milling programming method. Set the maximum depth of cut for each layer, processing in layers. Make full use of the uncut part as the most powerful support for the part to reduce machining deformation and improve tool durability.
(2) The milling adopts the smooth milling method to reduce the pulling phenomenon in machining and improve the surface quality of the parts.
(3) Setting a lower feed rate before milling the inner groove corner, so that the cutting volume per unit time is kept constant, avoiding overcutting and tool breakage due to the sudden increase of cutting volume and the inertia of the spindle when cutting the cavity corner.
(4) Half-finish machining of the cavity size directly to the drawing size, small-diameter extended milling cutter only play a role in clearing the corner, so as to reduce the small-diameter tool tendency to let the knife and vibration.
(5) Reasonable arrangement of the machining sequence of the parts is particularly important. If the machining order is wrong, it will bring a lot of disadvantages to the subsequent processing, and even cause parts scrapped.
Cooling Lubrication
During high-speed milling, 70%-80% of the heat is carried away by chips, about 5% is conducted to the part, and around 20% is absorbed by the tool. Therefore, choose a cooling method with good performance and fast coolant flow. Ideally, mist cooling should be used, but due to equipment limitations, a 5% emulsion is used for cooling.
Conclusion
Thin-walled frame parts are typically difficult to machine. Ensuring machining quality requires the use of high-speed cutting technology, a well-designed machining process, optimized cutting parameters, and well-designed fixtures and tools. Practical experience shows that using high-speed cutting technology can reduce the machining cycle by at least half compared to conventional CNC milling. Adjusting cutting parameters and optimizing milling depth can reduce internal stresses and deformation of the parts.
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Photo: iStock, inset photos client
