In mold manufacturing, common methods include machining and electrical discharge machining (EDM). Under certain circumstances, both methods may affect the test molding process and leave specific machining traces on the final product. Let’s first take machining as an example.
Machining is mainly used for hole processing, such as the drilling of ejector pin (tube) holes. However, the errors generated during machining can lead to a series of issues:
• It may cause the product to be out of tolerance.
• Problems like ejection deformation or even ejection penetration might occur.
• Uneven ejection force among multiple ejector pins (tubes) could result in failure to achieve automatic demolding.
• Defects such as flash, burrs, and excess material may also appear.
During test molding, it is not only difficult to correct these issues by adjusting process parameters, but sometimes the required precision cannot even be achieved. The precision of machining is mainly determined by the following aspects:
If the reverse clearance of the machine tool's feed drive is relatively large, when performing precision boring on a hole system, special attention must be paid to ensuring that the positioning direction of each hole is consistent. A one-way approach to the positioning point can be adopted to avoid the impact of reverse clearance errors in the transmission system or errors in the measurement system on positioning accuracy.
A short feed path can improve processing efficiency. According to common practices, people usually process 8 holes evenly distributed on one circumference first, then move on to holes on another circumference. However, for CNC machine tools with point-to-point control, high positioning accuracy and fast positioning speed are required. Therefore, such machine tools should arrange the feed path based on the shortest empty stroke to save processing time.
Note: This method is only applicable when the structure and size of the holes are completely the same (to facilitate subroutine programming); otherwise, it is rarely used.
Improper selection of machining allowance can cause troubles in electroplating, polishing, and other subsequent processes. For instance, if test molding fails and the mold needs to be corrected by polishing, a large amount of unnecessary working hours will be wasted.
• Machining Allowance: Refers to the thickness of the metal layer removed from the machined surface during the machining process.
• Process Allowance: Refers to the thickness of the metal layer that must be removed to complete a single process, i.e., the difference between the process dimensions of two adjacent processes.
• Total Machining Allowance: Refers to the total thickness of the metal layer removed from a specific machined surface during the process of converting a blank into a finished product. It equals the difference between the blank size and the design size on the part drawing, and the total machining allowance is the sum of all process allowances.
Machining allowance is divided into bilateral allowance and unilateral allowance:
• For planar processing, the allowance is unilateral, which equals the actual thickness of the removed metal layer.
• For rotating surfaces such as outer circles and holes, the machining allowance is bilateral (calculated in the diameter direction), meaning the actual thickness of the removed metal is half of the machining allowance value.
To gradually improve the machining quality of the workpiece, each process should generally cut into the normal metal structure below the surface to be machined, completely removing the rough surface layer (surface roughness Ra) and defect layer (depth Da) left by the previous process.
Additionally, the dimensional error Ta of the previous process directly affects the process allowance of the current process. Therefore, the allowance of the current process should include the dimensional error Ta of the previous process.
• When the relationship between form and position tolerance and dimensional tolerance follows the envelope principle, the dimensional tolerance controls the form and position tolerance, so the pa value can be ignored.
• When the form and position tolerance is independent of the dimensional tolerance (i.e., not following the envelope principle), the machining allowance must include the form and position error pa of the previous process. For example, if the axial straightness error (w) of a small shaft needs to be corrected in the current process, the machining allowance in the diameter direction should be increased by 2w.
Clamping error includes positioning error, clamping error (clamping deformation), and errors of the fixture itself. Due to the impact of clamping error, the surface to be machined of the workpiece deviates from its correct position. Thus, clamping error must be considered when determining the machining allowance.
A typical example: When grinding the inner hole of a workpiece by clamping its outer circle with a three-jaw self-centering chuck, the inaccurate centering of the chuck causes the workpiece axis to deviate from the spindle rotation axis by a value of e. This leads to uneven grinding allowance for the inner hole and may even result in no machining allowance on some local surfaces. To ensure sufficient machining allowance for all surfaces to be machined, the diameter allowance of the hole should be increased by 2e.
The size of the machining allowance has a significant impact on the machining quality, production efficiency, and economy of the part. Correctly determining the machining allowance is one of the key tasks in formulating process regulations—this is especially important for machining centers, as the size of all tools is adjusted based on the machining allowance of each process.
• If the allowance is too small: Due to the alignment error between the previous process and the machining center process, it may be impossible to cut off the defective metal layer, leading to scrapped parts. In some cases, it may also put the tool in harsh working conditions (e.g., cutting hard sand-included surfaces, which causes rapid tool wear).
• If the allowance is too large: It wastes working hours, increases tool consumption, and leads to unnecessary metal material waste.
On the premise of ensuring machining quality, minimize the machining allowance as much as possible. The value of the minimum machining allowance should be sufficient to remove the metal layer with various defects and errors, thereby improving the precision and surface quality of the machined surface.
Specifically, the following aspects should be considered:
1. For the final process, the machining allowance must ensure that the surface roughness and precision requirements specified on the drawing are met.
2. Take into account the machining method, the rigidity of the equipment, and the possible deformation of the part.
3. Consider the deformation of the part caused by heat treatment.
4. Factor in the size of the part to be machined: Larger parts tend to have more deformation due to cutting forces and internal stresses, so a relatively larger machining allowance is required.
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