In the process of manufacturing metal parts, sometimes we encounter small hole processing with strict tolerance requirements (such as H7). Even when CNC processing is more popular, there are still difficulties in processing. Now, Wcustommachining introduces you a practical small hole processing technology that has been verified by us – hole-extrusion by steel ball .
Hole-extrusion by steel ball uses steel ball as the main tool to complete the finishing of the inner surface of metal parts. This process is to use a steel ball with a diameter slightly larger than the diameter of the metal part, pass through the inner hole of the metal part at a certain speed, and use the steel ball and high precision, high hardness and low surface roughness to extrude the inner hole of the component, the material Produce permanent deformation to improve the surface quality, surface hardness, dimensional accuracy and production efficiency. In the production process of Wcustommachining, the spherical hole was used to extrude the inner hole processing parts for many times, and it was found that the inner hole processing effect is better when the inner hole length of φ2 ~ 35mm is less than 10 times the diameter of the hole and the hole wall is larger than 7mm.
- Basic requirements before and during extrusion
- For holes with a diameter of more than 20 mm, it is best to use a press, and less than 20 mm is best to use a bench drill or a CNC milling machine.
- The material of metal parts can be uniform and refined by heat treatment to improve the processing performance, but the hardness should be controlled below 32HRC to ensure the service life of the extruded steel ball.
- The surface roughness value of the inner hole of the metal parts before extrusion should be as low as possible, which is beneficial to the improvement of the accuracy after extrusion. Generally, the surface roughness value of the inner hole before extrusion should be controlled between R = 3.2-1.6um.
- Metal parts should have sufficient shape error and position error before extrusion. Generally, the accuracy of H8 level can be improved to H7 level after extruding the hole. Since the extrusion hole is the movement of the steel ball along the processing track of the previous process, the shape error can be partially or completely repaired, but the position error cannot be repaired, so it is recommended to use CNC milling machine processes bottom holes to ensure hole spacing.
- The extrusion process requires the hole walls to be clean and lubricated. Extruded aluminum parts should be lubricated with oil or saponified oil, and extruded steel parts should be lubricated with kerosene.
- It is better to control the extrusion speed at 2-10m/min.
- Selection of extrusion allowance
Hole-extrusion processing is mainly used for finishing processing. Based on the mechanism of permanent deformation of metal materials, the selection of extrusion hole allowance is very important. Please refer to the attached table.
Commonly used hole-extrusion process allowance experience parameter table (unit: mm)
Since the selection of the steel ball diameter is related to the workpiece wall thickness, surface roughness, material properties, process lubrication, extrusion speed and other factors, a lot of tests are still needed to improve the extrusion process. However, when each production unit adopts the extrusion process, when the extrusion allowance is determined, steel balls with different diameters can be selected for testing before the formal extrusion to find the appropriate steel ball diameter.
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