Method of triangular straight thread on a CNC lathe
High-speed turning of triangular threads on CNC lathes typically involves the use of carbide tools, which offer better wear resistance and cutting efficiency compared to high-speed steel. When processing threads with separate roughing and finishing passes, it is possible to achieve superior surface finish and dimensional accuracy. The roughing tool’s cutting edge angle is generally set between 5°8.5' and 5°9', while the finishing tool has a slightly smaller angle, usually reduced by 1° to 1.5°. During roughing, the tool cuts to the full thread depth (i.e., the external diameter), and then the finishing pass is applied along the side of the tool, helping protect the cutting edge and saving time.
When grinding roughing tools, it's not necessary to sharpen the corners of the cutting edge. Instead, increasing the rake angle appropriately can help improve chip evacuation and reduce the profile angle. However, increasing the rake angle also reduces the wedge angle, which weakens the tool. To compensate for this, the clearance angle should be adjusted accordingly to maintain sufficient tool strength.
There are two main feeding methods for triangular threads: straight-in and oblique. The straight-in method involves cutting with both edges of the tool simultaneously, which leads to higher cutting forces, poor heat dissipation, and increased tool wear. This method is suitable only for small pitch threads (P < 2mm) or high-precision applications due to its tendency to cause chipping or breakage of the cutting edge. On modern CNC lathes, threading functions are often integrated into the system, with single-pass and multi-pass cycle commands like G32, G92, and G76 available. These commands allow for efficient programming and control over the cutting process.
For example, when machining an M50×4 external thread, the calculated thread diameter is 49.72mm, and the final diameter is 46.52mm. Using G92, the program would include multiple passes, each reducing the depth progressively. For a more complex thread, such as M30×2-6g, the program would involve setting up the tool path, specifying the feed rate, and using commands like G76 for compound cutting cycles. These commands simplify the programming process and ensure accurate thread profiles.
The oblique method involves feeding the tool diagonally along one side of the thread, using only one cutting edge. This method improves chip evacuation and allows for deeper cuts, making it ideal for roughing operations. However, the surface finish is coarser, so the final few passes should switch back to the straight-in method to ensure smoothness and accuracy.
In high-speed cutting, the cutting speed is typically set between 40 to 100 m/min. The tool must be made of hard alloy, such as YT15, to withstand the high temperatures and stresses involved. For large-pitch threads (P > 2mm) or harder materials, the tool edges should be ground with a width of 0.2–0.4 mm and a negative chamfer of -5°. Due to the high-speed nature, the workpiece material is compressed more, causing the thread profile angle to increase slightly. Therefore, the tool tip angle should be reduced by 0.5° relative to the desired thread angle to compensate for this effect.
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