In order to ensure that the micro-hole processing has good dimensional accuracy, roundness and straightness, the most important prerequisite is to use a machine tool with high feed accuracy and good stability. In particular, it should be noted that the combined errors of the spindle, fixture and tool should be greatly suppressed.
The machine tool suitable for small diameter hole processing is equipped with a special drilling attachment, which can perform high-precision small diameter hole processing. The device has a guide rail that can be adjusted in the horizontal and vertical directions, which is convenient for coaxiality with the main spindle. The spindle and the ドリルビット axis are straightened using a special instrument, which allows the two axes to be adjusted when rotating at a low speed. Compared with the general processing method in which only the drill bit rotates, the method of rotating the spindle and the drill bit axis at the same time can obtain a higher coaxiality.
There are roughly two types of shank shapes for micro drill bits, one is a small straight shank and the other is a thick rod straight shank. The tool material is still mainly cobalt high-speed steel. In recent years, coated drills and carbide drills have appeared in large numbers. In particular, fine-grained carbide has greatly improved its wear resistance and toughness. At present, WC ultrafine-grained carbide with a particle size of less than 0.5μm has been developed, with a bending strength of 500kg/mm2 and a hardness of 92.5HRA. It is used to make micro drills, and the cutting performance is very ideal.
Before drilling, the spare drill bit should be carefully checked with a microscope. If a drill bit with low grinding accuracy is used, it will not only reduce the accuracy of the processed hole, but also cause the drill bit to break in severe cases.
In terms of drill bit size accuracy, if it is processing materials such as stainless steel or iron-nickel-cobalt alloy, the cutting edge height error is required to be about 0.001-0.02mm. This can prevent edge wear and extend tool life.
The parts to be processed should have high coaxiality, verticality and good surface roughness, especially at the entry and exit of the drill bit. If the accuracy is poor, the cutting edge wear will increase and even cause the drill bit to break. The entry surface is preferably ground or polished to improve the processing accuracy of the hole. In addition, the drill point angle of the center drill must be consistent with the drill point angle of the drill bit used.
The core thickness of the micro drill bit is relatively large and the cutting edge is relatively small. Suppose it is processed according to the general peripheral speed calculation. In that case, it will not only lose the dynamic balance of the drill bit, reduce the accuracy of the processed hole, but also easily cause the drill bit to break. Therefore, factors such as hole diameter and depth must be considered when processing micro holes, and appropriate processing conditions must be determined for different workpiece materials. If the tool used is a cobalt high-speed steel drill, its rotation speed should be 1000-1500m/min. If it is a carbide drill, the centrifugal force during high-speed rotation causes less vibration on the blade tip due to the high longitudinal elastic modulus. Therefore, high-speed cutting can be used.
The cross-sectional area of the micro drill is small, and the smaller the diameter, the lower the drill rigidity. Therefore, the clamping system selected is required to maintain high clamping rigidity and runout accuracy during high-speed rotation, and the selected machining center must also have high-precision positioning characteristics. Under normal circumstances, when the L/D ratio of small-diameter hole processing reaches more than 3 times, the cutting depth should be determined as 20%-30% of the drill diameter.
For the cutting fluid used for small-diameter hole processing, special attention should be paid to its lubrication performance. Cutting fluid with good lubricity is easy to adhere to the drill groove, and it is easier to reach the processing part through the drill groove, which can reduce friction, reduce tool wear, and also facilitate chip removal. If a spray device can be used, it will be more beneficial for chip removal and improving processing efficiency.
