The length-diameter ratio of micro-drilling increases significantly.

As we all know, the greater the length-diameter ratio of a drill bit, the greater its bending tendency. Reducing the length-diameter ratio can reduce the bending force, thus avoiding the bit breaking and increasing the aperture error. Deeper holes require a drill bit with a larger aspect ratio. Usually, holes with a depth of more than 3 times the diameter are "deep holes", and the hole depth of micro-drills generally exceeds this limit.

For example, a drill with a diameter of 3. 175mm processes a hole with a depth of 3 1.75mm, and the length-diameter ratio is10:1; The drill with a diameter of 0.508mm can process holes with a depth of 25.4mm, and its length-diameter ratio reaches 50: 1. Therefore, with the decrease of bit diameter and the increase of brittleness, deflection has become the root of many problems. In order to control the brittleness of the drill bit, the hardness and toughness of the tool matrix must be weighed.

Generally speaking, high-speed steel drill bits are allowed to have certain deflection and can bear corresponding bending force. However, this elastic deformability and low hardness of high-speed steel also reduce its wear resistance, thus limiting the life of the tool. On the other hand, cemented carbide has high rigidity and hardness, so it can make the tool life longer and the machining accuracy higher.

Joe Kueter, manager of drilling production department of M.A.Ford manufacturing company, pointed out that the high wear resistance of cemented carbide makes its micro-drilling speed three times that of high-speed steel, and its service life can be improved. At the same time, the high rigidity of cemented carbide helps to locate and maintain the hole size correctly. But cemented carbide is not omnipotent, and its high hardness will make it easy to crack. Peter Jones, the field sales engineer of Guhring Company, pointed out that using M35 cobalt high-speed steel as micro drill bit can achieve a good compromise between cemented carbide and ordinary high-speed steel (M2, M7). He said: "when cutting, the heat generated in the hole, combined with the rolling of the tool, makes the cutting edge dull and draws a passage, which eventually leads to the damage of the tool." The higher the cobalt content, the higher the heat resistance of M35, and the sharper the blade can be kept for a long time. "

In addition, the cemented carbide drill bit needs to be carefully installed and used, and accurate concentricity is particularly important, because the lateral load caused by eccentricity will cause the drill bit to crack. Larry Brenner, Senior Production Manager of Milling and Drilling Department of Mitsubishi Metal Materials (USA) Co., Ltd., suggested using micro-drills on machine tools (such as machining centers) where the drills rotate as much as possible. He pointed out that the spindle of the machining center can position the center line of the drill correctly, and the eccentricity of the workpiece on the lathe will lead to the bending of the drill. Therefore, if micro-drill is used on lathe, every factor affecting concentricity must be adjusted in advance, especially cemented carbide drill, because it can not adapt to bending deformation.

If micro-drill is used on lathe, it is best to re-bore the mounting hole of the tool holder, and adjust the concentricity of the drill bit and the workpiece to the best state by using the adjustable boring tool holder.

Brenner further pointed out that the jumping of the tool rest should be minimized. Therefore, the heat-shrinkable tool holder should be the first choice, followed by the hydraulic tool holder. The maximum runout of the end face of the tool holder sleeve is required to be in the range of 0.005 ~ 0.0076 mm