1 The question raised

Small holes (D<1Omm) are commonly found in punched holes and cast aluminum small machine seats of various micro-small thin plates. the materials of such parts have great plasticity. usually, a machine tap is used to tap at one time on a tapping machine, and the product quality is sampled with a thread gauge. the large end fails to pass, and the small end should pass easily or slightly tightly before it is qualified. the qualified rate should be greater than 99%. In production practice, it often happens that the sampling pass rate does not meet the requirements, which seriously affects the normal production.

Dissecting measurement of unqualified parts, found that there are two main reasons:

Thread diameter is small (bottom hole diameter is acceptable);

The thread surface is embedded with a sesame-like black hard object-accretion.

Through repeated test analysis, it can be seen that chip accumulation and bounce are the main causes of non-conforming products.

2 Formation and Prevention of Debris Oil and Recovery

In the metal cutting process, if the cutting speed is not very high, but also the formation of strip chips, the metal material on the workpiece will be cold welding and deposition in the front face to form a very hard wedge-chip.

In the process of metal cutting, due to the pushing of the rake face and the squeezing and rubbing of the flank face, the workpiece material in the cutting area produces elastic deformation and plastic deformation, and the phenomenon of elastic deformation recovery due to cutting force after cutting-elastic recovery.

The formation of chip accumulation when the chip flows out along the rake face, under the action of temperature and pressure, the bottom layer of the chip in contact with the rake face is subjected to great friction resistance, which slows down the flow rate of this layer of metal and forms a very thin stagnation layer. When the frictional resistance of the front knife to the stagnation layer exceeds the internal binding force of the material to be cut, a part of the metal will adhere to the vicinity of the cutting edge to form a chip accumulation tumor. The chip accumulation tumor will grow up continuously after formation. When the height is reached, when the stress caused by the cutting force exceeds the bonding strength of the chip accumulation tumor attached to the front knife surface, the chip accumulation pain will fall off. The above process is repeated after the chip tumor falls off or is taken away by the chip or embedded in the surface of the workpiece.

In the process of forming, the metal material is strengthened due to plastic deformation. Therefore, the hardness of the chip is higher than that of the workpiece material, which can replace the cutting edge for cutting and protect the cutting edge. At the same time, due to the existence of chip accumulation, the actual working angle of the cutting tool is increased, which makes the cutting brisk. However, the chip accumulation extends beyond the cutting edge and is constantly generating and falling off, which makes the actual cutting depth and cutting thickness constantly changing, affects the dimensional accuracy, and leads to the change of cutting force, causes vibration, and some chip accumulation or its fragments adhere to the processed surface, making the surface rough, cause defective or waste products. On the one hand, due to the influence of chip accumulation in small hole tapping, the size error is caused; on the other hand, due to the chip accumulation embedded in the thread surface, the gauge through end cannot pass through, resulting in unqualified products.

The main factors affecting the formation of the product tumor are: the performance of the workpiece material, cutting speed and cooling lubrication conditions, tool geometry parameters, etc. In the properties of workpiece materials, the greater the plasticity, the easier it is to form chip accumulation. For example, the processing of low and medium carbon steel and aluminum alloy materials, are easy to produce chip accumulation, to avoid chip accumulation, the workpiece material can be normalized or quenched and tempered, in order to improve its strength and hardness, reduce plasticity. Cutting speed is the main factor that affects chip accumulation through cutting temperature and friction when machining workpiece materials. When the cutting speed is very low, the cutting temperature is low, the internal binding force of the chip is large, the friction coefficient between the rake surface and the chip is small, and the chip is not easy to form. When the cutting speed increases, the cutting temperature increases, the friction increases, it is easy to form a chip, when the cutting speed is high, the friction is small, there is no chip formation. Therefore, when a certain speed under the occurrence of chip, can further reduce or increase the cutting speed to avoid the formation of chip. For the tapping machine, the speed of the change is limited to a few levels, so it is difficult to change the speed of the chip, the use of appropriate cutting fluid, can reduce the cutting temperature, reduce friction, can significantly reduce or avoid the chip. In the experiment, the method of watering or oil can reduce friction and reduce cutting temperature to avoid the generation of chip accumulation. However, the purchase and operation of water supply equipment has increased the production cost, and the lubrication of oil is greatly affected by human factors. Once neglected and forgotten, the quality cannot be guaranteed.

In the test, by changing the geometric parameters of the tool to cutting conditions, avoiding or reducing the generation of chip accumulation and rebound, increasing the rake angle of the tool, the extrusion between the chip and the rake face can be reduced, the friction force between the chip and the rake face can be reduced, and the friction force between the main flank face and the machined surface of the workpiece can be reduced. Therefore, increasing the rake angle of the tap or increasing the relief angle at the same time can reduce the extrusion and friction of the tap cutting edge on the cutting area of the workpiece, reduce the cutting force, and reduce the elastic deformation of the workpiece. The problem of unqualified thread diameter caused by the recovery of elastic deformation is also caused by the increase of the rake angle, and the cutting edge becomes sharp. The reduction of extrusion and friction makes the cutting temperature significantly lower and the influence of chip accumulation.

3 Concluding remarks

Production practice has proved that, by the processing cost and production conditions and other practical factors, the purchase of the material is generally no longer heat treatment, therefore, external conditions, such as changing the cutting speed, adding cooling lubricant, changing the tool angle, etc. have become the main measures of the impact of chip accumulation and elastic recovery on the quality of tapping. Among them, it is an important, economical and convenient method to sharpen the cutting edge by sharpening the rake surface of the tap to increase the rake angle, so that the impact of chip accumulation and spring-back.