The milling technology of PCB CNC milling machine includes the selection of cutting direction, compensation method, positioning method, frame structure, and lower cutting point. All are important aspects to ensure milling accuracy.
First, the direction of the knife, compensation method
When the cutter cuts into the plate, one of the cut sides is always facing the cutting edge of the cutter, and the other side is always against the cutting edge of the cutter. The former has smooth surface and high dimensional accuracy. The main axis always turns clockwise. Therefore, whether it is a CNC milling machine with a fixed spindle movement or a fixed spindle movement of the table, it is necessary to use a counterclockwise cutting direction when milling the external outline of the PCB. This is commonly referred to as inverse milling. When milling the frame or slot inside the PCB, the down-milling mode is used. Milling plate compensation is when the milling machine automatically set the set value so that the milling cutter automatically to the center offset of the milling line set half of the milling cutter diameter, that is, the radius distance, so that the shape of the milling and the program to maintain consistency. At the same time, if the machine tool has the function of compensation, it is necessary to pay attention to the direction of compensation and the command of the use of the program, such as the use of compensation command errors will make the appearance of the circuit board more or less equivalent to the length and width of the milling cutter diameter.
Second, positioning method and cutting point
Positioning methods can be divided into two kinds; One is internal positioning, and the other is external positioning. Positioning is also very important for process makers, generally in the pre-production of PCB should determine the positioning program.
Internal positioning is a common method. The so-called internal positioning is to select the installation hole, insert and dial hole or other nonmetallic hole in the PCB as the positioning hole. The relative position of the hole should be on the diagonal and the hole of large diameter should be selected as far as possible. Do not use metallized holes. Because the difference in the thickness of the coating in the hole will affect the consistency of the hole you selected, and it is easy to cause damage to the coating in the hole and the edge of the hole surface when taking the board, the less the number of pins under the condition of ensuring PCB positioning, the better. Generally, the small board uses 2 pins, the large board uses 3 pins, and its advantages are accurate positioning, small deformation of the board shape, high accuracy, good shape, and fast milling speed. The disadvantages of various aperture types in the board need to prepare various diameter pins, such as there is no available positioning hole in the board, in the pre-production need to discuss with the customer to add positioning holes in the board, more cumbersome. At the same time, the different management of each kind of plate milling plate template is more troublesome and the cost is higher.
External positioning is another positioning method, which uses a positioning hole on the outside of the board as the positioning hole of the milling plate. Its advantage is that it is easy to manage, if the production standard is good in advance, the milling plate template is generally about fifteen. Because of the use of external positioning, the board can not be milling down at one time, otherwise the PCB is very easy to damage, especially the board, because the milling cutter and the dust suction device will bring the board out causing PCB damage and the milling cutter breaking. The piecewise milling method is used to leave the joint point, mill the plate first, and then the program pauses after the milling plate is finished, and then the plate is fixed with tape. The second stage of the program is executed, and the joint point is drilled off with a 3mm to 4mm drill bit. Its advantage is that the template is small and easy to manage, can be milling all the boards without mounting holes and positioning holes, small process personnel management is convenient, especially CAM and other early production personnel can be simplified, while optimizing the utilization of the substrate. The disadvantage is that due to the use of drill, the PCB shape has at least 2-3 raised points which are not beautiful, may not meet customer requirements, the milling time is long, and the labor intensity of workers is slightly greater.
Third. Frame and lower cutting point
The production of the frame belongs to the pre-production of PCB, frame design not only has an impact on the uniformity of electroplating, but also has an impact on the milling board, such as the design of the frame is easy to deform or produce some small blocks in the milling board, the resulting waste block will block the suction tube or break the high-speed rotating milling cutter, the frame deformation, especially when the external positioning milling board causes the finished board deformation. In addition, the tool point and processing sequence are selected well, so that the frame can maintain the maximum strength and the fastest speed. The choice is not good, the frame is easy to deform and make the PCB scrapped.