CNC milling table itself is a positioning plate, it is a pin positioning, screw fixed aluminum alloy plate. Under the spindle of each CNC milling machine, there is a slot one positioning system on the working table. The milling pad is actually an intermediate positioning fixture, sometimes called "soft positioning". It is required that it can locate reliably and unload the board quickly, reduce auxiliary time and improve production efficiency. Before milling, the groove of the same size as the printed board is pre milled on the milling pad.Generally, the width of the groove is the diameter of the actual milling cutter plus 0. 5mm. The groove depth is 2. 5mm. In machining process, it is a path of milling cutter movement path. Due to the vacuum cleaner, a flow of air is generated in the groove to remove the chip, so that it can be processed more smoothly, prevent the chip from blocking the cutter chute and reducing the edge. When machining, insert the milling cutter into the groove 1. 5 to 2mm. This prevents the end wear and tear caused by the continuous cutting of the milling cutter into the plate, the reduction of the diameter and the reduction of the allowable end diameter in the manufacturing of the milling cutter, etc. resulting in the deviation of the processing size of the printed circuit board.
Before each batch production, a milling pad is installed on the NC milling table and a new nylon thread plug is screwed on. Drill holes on threaded plug and install locating pins. Cutting groove on the milling pad, milling deeper and wider, will be more conducive to the smooth flow of air, chip removal, so that the machined surface is more clean. However, the supporting surface is weakened, especially when the chip removal groove is near the locating pin, so the positioning is not firm.
Most milling insert plates are made of non-metal laminates. The material is relatively soft. When the pin is repeatedly loaded and unloaded, the locating hole is worn and expanded. Such as semi exclusive, expendable milling pad is working under such conditions.
Usually, the pin is pressed on the milling pad and the interference volume is 0.005 ~ 0.01mm. If the special milling pad or high density fiberboard is used as the milling pad, the tighter coordination is better. However, the interference of semi-special milling pad or consumptive milling pad is more than 0.007mm. When pins are pressed, it is possible to cut off some of the base materials in pin holes and form deep grooves or voids. The pins are repeatedly loaded and unloaded, and the pin holes of the laminates also produce lamination or fragmentation. In milling PCB, the pin force is mostly borne by the positioning pin. This lateral pressure squeezes the pin hole and adds defects in the hole, so that the pin one by one loosens and deflects. It directly affects the dimension of PCB, and can not guarantee strict tolerance.
The smaller the diameter of the locating pin, the greater the relative deflection. Therefore, large diameter holes should be used as positioning holes as far as possible. The location pin diameter and skewness also directly affect productivity. For example, a block of four blocks can be milled at a time. Because the pin diameter is small and the deviation is large, three pieces of milling have to be made, so that the efficiency is reduced by 25%.
During the processing, the positioning pin should be closely matched to make the processing plate reliable positioning. Instead of relying on adhesive tape or adhesive, it takes time to bond and solidify. Tight coordination also means strict tolerances, and the accuracy of the multi layer or high quality double-sided boards can be guaranteed.
Usually, the diameter of the drill bit is below 0, 0.005mm and 0 to 0.01mm. Most non-metal laminates have a certain shrinkage after drilling, usually 0.005 to 0.007mm. Such a deviation of 0.005 to 0.01mm hole with standard pin, can produce interference of 0.005mm. Therefore, it is not necessary to consider the special processing technology, and do not have to measure bit diameter one by one, select small diameter bit, can meet the requirements of pressure fit, saving time.
For example, a 1.6mm thick printed circuit board with tolerance tolerance of 0.013mm, four blocks per milling. Hole position tolerance +/-0.025mm, bore diameter difference + 0.025mm, use standard pin. So loading and unloading is difficult and time consuming. Because it is possible to lock the pin in the milling plate. In this case, it is pointed out that the matching of the pin and the plate requires a loose slip fit. Of course, if the clearance is too large, the machining accuracy and edge finish must be affected.
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