Precautions For Milling Surface Of Die Casting Mold | Rough Milling, Semi Precision Milling & More

Problems that must be paid attention to when milling curved surfaces: rough milling, semi precision milling, precision machining, ball end milling cutter in milling curved surfaces, avoiding vertical cutting, etc.

Precautions For Milling Surface Of Die Casting Mold

1. Tool selection
Most of the tools used by CNC machine tools in processing molds are the same as general tools. Milling cutters with indexable cemented carbide inserts that do not regrind with machine clamps are often used. Because there are many cavities formed by curved surfaces in the mold, it is often necessary to use ball head cutters and ring cutters (that is, the end milling cutter tip is in the shape of circular arc chamfer).

2. Matters needing attention in assembling milling machine fixture
According to the feeding mode during milling, the milling machine fixture is generally divided into linear feeding type, circular feeding type and copying milling fixture. So what should be paid attention to when assembling these fixtures for the milling machine?
– First, in order to improve the stability of the fixture of the milling machine installed on the machine tool and the anti vibration performance under dynamic conditions, the structure of various devices should be compact, the center of gravity of the fixture should be as low as possible, and the contact area between the fixture and the machine tool workbench should be large.
– Second, the milling machine has high productivity (short cutting time). When designing the fixture, it is necessary to consider how to quickly install the workpiece to shorten the auxiliary time. Generally, the fixture is also provided with elements to determine the position and direction, so as to quickly adjust the relative position of the fixture and the machine tool.
– Third, when assembling the milling machine fixture, the chip outflow and cleaning convenience should be considered. A large number of chips are generated during milling, and there should be enough chip removal space. The chip discharge port and chip discharge groove should be considered for large fixtures; The parts and spaces that are not easy to remove chips shall be provided with protective covers. When cutting fluid is used in machining, the flow direction and recovery of cutting fluid shall be considered in the specific design of clamp. For heavy-duty milling machine clamps, lifting rings shall also be set on the clamp for easy handling.

3. Problems needing attention when milling curved surface
(1) Rough milling. During rough milling, according to the allowance given by the surface to be machined, the end milling cutter shall be used to mill the contour surface layer by layer. This rough milling has high efficiency. The rough milled surface is similar to the terrace on the hillside. The height of the step depends on the rough milling accuracy.
(2) Semi finish milling. The purpose of semi finishing milling is to mill off the steps of “terraces”, so that the surface to be processed is closer to the theoretical surface. The ball end milling cutter is generally used to reserve about 0.5mm machining allowance for the finishing process. The row spacing and step spacing of semi finishing can be larger than that of finishing.
(3) Finishing. Finally, the theoretical surface is processed. When finishing curved surface with ball end milling cutter, row cutting method is generally used. For parts with good openness, the turning back point of line cutting shall be outside the curve table, that is, when programming, the surface shall be extended outward. For the part surface with poor opening, due to the change of cutting speed during turning back, it is easy to leave tool marks caused by pauses and vibrations on the machined surface and the blocking surface. Therefore, during machining and programming, the feed speed shall be reduced during turning back, and the turning back point of the machined surface shall be slightly away from the barrier surface during programming. The intersection line between the surface and the barrier surface shall be processed separately by a root cleaning procedure, so that the processed surface and the barrier surface can be smoothly connected without large tool marks.
(4) When the ball end milling cutter is milling curved surfaces, the cutting speed at the tip is very low. If the ball cutter is used to mill relatively flat curved surfaces perpendicular to the surface to be processed, the surface quality cut by the ball cutter tip is relatively poor. Therefore, the spindle speed should be properly increased. In addition, cutting with the tip should be avoided.
(5) Avoid cutting vertically. There are two kinds of flat bottom cylindrical milling cutters. One is that the end face has a center hole, and the end edge is not the center. The other is that the end face has no center hole, and the end edge is connected and passes through the center. When milling curved surfaces, the end milling cutter with a center hole must not feed vertically downward like a drill, unless a process hole is drilled in advance. Otherwise, the milling cutter will be broken. If the end tool without a center hole is used, the tool can be fed vertically downward. However, since the blade angle is too small and the axial force is large, it should be avoided as much as possible. The best way is to feed the tool obliquely downward, and then cut laterally with the side edge after reaching a certain depth. When milling the groove surface, the process hole can be pre drilled for cutting. Although the effect of vertical feeding with ball end milling cutter is better than that of flat end milling cutter, it is better not to use this cutting method because the axial force is too large and the cutting effect is affected.
(6) In the process of milling curved surface parts, if it is found that the heat treatment of the part material is not good, there are cracks, and the structure is not uniform, the processing shall be stopped in time to avoid wasting labor time.
(7) When milling the curved surface with complex mold cavity, it generally requires a long period. Therefore, the machine tool, fixture and tool should be properly inspected before each start-up milling to avoid failure in the process, affecting the machining accuracy and even causing waste.
(8) When milling the die cavity, the trimming allowance shall be properly controlled according to the roughness of the machined surface. For parts difficult to be milled, if the surface roughness is poor, more trimming allowance shall be reserved; For the parts that are easy to be machined such as plane and right angle groove, the roughness value of the machined surface should be reduced as much as possible to reduce the workload of filing and avoid affecting the accuracy of the cavity surface due to large-area filing.