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Research on Ultrasonic Milling Technology

Release time:May 07, 2019 Visits:

Key words: ultrasonic; ultrasonic machining; milling

In the ultrasonic milling process, the tool also performs the rotary motion while the ultrasonic vibration is high-frequency, and at the same time, under the control of the CNC system, each layer performs the feed motion in the X and Y directions. Abrasive particles continuously hammer, impact, polish and scrape the surface of the workpiece of hard and brittle materials under the combined action of the three movements of the tool. When the tool, the abrasive grain and the workpiece are not in contact, the material removal is mainly caused by the abrasive impact, and the rotary motion of the tool is weak: when the three are in contact, the abrasive directly acts on the surface of the workpiece, with the processing During the process, a part of the abrasive particles is pressed into the tool due to the pressure of the tool, and the exposed portion is scribed on the surface of the workpiece, and under the action of the rotary motion of the tool, a circular trajectory is drawn to form a curved dent on the surface of the workpiece. . Another part of the abrasive grains rolls between the workpiece and the tool, which produces a rolling effect, causing microcracks on the surface of the workpiece. After the crack propagates, the surface of the workpiece is brittle and chipped to form chips.

2. Ultrasonic milling

2.1 Ultrasonic milling machine

In ultrasonic machining, the force between the tool and the workpiece is very small. The machining machine only needs to realize the working feed motion of the tool and adjust the relative position between the tool and the workpiece. Therefore, the machine tool structure is relatively simple, generally including the frame supporting the vibration system. Work surface, feed mechanism, and bed body. The domestic CSJ-2 ultrasonic machining machine is one of the representative products, as shown in Figure 1. The vibration system is mounted on a guide rail that can move up and down. The guide rail is positioned by two sets of rolling guide wheels, so that the guide rail can move up and down flexibly and reliably.

2.2 Ultrasonic milling

The downward feed of the tool and the pressure applied to the workpiece depend on the weight of the vibration system. In order to adjust the pressure, there is a balance weight that can be added or subtracted behind the machine tool. In addition, there are heavy hammer lever loading, spring clamping, Pressurized by hydraulic or pneumatic loading.

At the same time, the feed motion of the tool is also strong when the tool, the abrasive grain and the workpiece are in contact, and the effect is weak when it is not in contact. Since the tensile strength of the hard and brittle material is smaller than the compressive strength, when the abrasive grains are pressed, microcracks are generated at the maximum tensile stress of the hard and brittle material processing surface, and the rotation and feed motion of the tool further promote the crack propagation. . When the criss-cross cracks expand and cross each other, the portion surrounded by the crack breaks and collapses out of the small fragments. This is the basic process of cutting and surface formation during milling of hard and brittle materials. It can be seen from the above analysis that the following three material removal mechanisms are also included in the ultrasonic milling process: 1. Impact: As the tool rotates, the abrasive particles of the tool end face impact different parts of the machined surface; 2. Abrasion: rotation of the tool The movement of the movement and the tool causes the abrasive particles to scrape the tiny grooves on the surface of the workpiece; 3. Ultrasonic cavitation.

Therefore, we can divide the whole process into two stages: 1. When the tool, the abrasive grain, and the workpiece are not in contact, only the ultrasonic vibration is considered, and the influence of the tool rotation and the feed motion is ignored. 2. When the tools, abrasive grains, and workpieces are in contact, consider the combined effects of ultrasonic vibration, tool rotation, and feed motion. The total material removal rate in ultrasonic milling is the sum of the two-part removal material, where the material removal in the first part is less and the removal of the second part is the primary removal.

3. Ultrasonic milling processing material removal

3.1 Processing material removal model

In ultrasonic machining, a large number of free abrasive particles continuously impact the workpiece under the ultrasonic vibration of the tool, and each abrasive particle can be regarded as a small indenter. In the ultrasonic milling process, the tool performs the rotary motion, and when the ultrasonic vibration is performed, the situation will be different from the conventional ultrasonic vibration. The form of the indentation and the crack generated in the workpiece are as follows: When the tool retracts into the next cycle that pushes the abrasive against the workpiece, the tool rotates to a new position, and the abrasive particles slide and roll accordingly. Therefore, most of the indentations generated in a tool impact cycle overlap with the indentations of the previous cycle. The transverse crack ring formed by the indentation is surrounded by the two rings, causing a large amount of scratching in the cutting area, resulting in material removal.

In order to establish a material removal rate model, it can be assumed that the workpiece material is an ideal brittle material, and the material removal is a brittle fracture method: all abrasive particles have the same size, are evenly distributed in the suspension, and all the abrasive grains in each ultrasonic vibration cycle are Participating in cutting, the ultrasonic amplitude, frequency, and diameter of the machine are constant, regardless of the loss of tools during processing. Based on the interaction between the single abrasive particles and the workpiece, the volume of the material removed by a single abrasive in a vibration cycle is given, and then the sum of all the abrasive removal materials is calculated as the material removal rate of the whole process. The abrasive particles impact the surface of the workpiece under the ultrasonic vibration of the tool end surface, and form an indentation on the surface of the workpiece. Each indentation causes a circular transverse crack with a depth of Ch from the surface of the workpiece. When the transverse crack propagates to the surface of the workpiece or two adjacent transverse cracks meet each other. At the time, a layer of workpiece material having a thickness of Ch is removed.

In the actual ultrasonic machining, the tool head moves downward from the highest point of the amplitude, and the abrasive particles are contacted at time a to push the abrasive grains downward. At the time b, the abrasive grains reach the lowest position. Due to the resistance of the workpiece, the abrasive particles can The maximum impact depth is reached at 6. The tool then moves in the opposite direction, and at time c the tool comes out of contact with the abrasive particles. Therefore, due to the up and down vibration of the tool, the tool, the abrasive grains and the workpiece are in contact for only a part of the time in one vibration period, so that the abrasive grains impact and scratch the surface of the workpiece.

As processing parameters such as processing static pressure, abrasive particle diameter, tool rotation speed, and ultrasonic vibration frequency increase, the material removal rate increases. Simply considering the rotational motion of the tool, assuming that the tool, the abrasive particles and the workpiece are in contact, the rotational speed of the abrasive particles is consistent with the rotational speed of the tool. Theoretical analysis, in actual ultrasonic milling, because the abrasive particles and the tool are not rigidly connected, the grinding The grain has sliding and rolling relative to the tool. The formula should also be multiplied by a scale factor of less than one or a reduction factor to match the actual machining process. Regardless of the machine feed motion, the workpiece material is mainly removed due to the impact of the abrasive particles in the working fluid hitting the surface of the workpiece and scratching the surface of the workpiece. Although theoretically speaking, due to the feed of the machine tool, the actual sliding distance between the abrasive grains and the workpiece is small, but due to the existence of the combined force, the stress of the workpiece in the advancing direction of the abrasive grains increases, which provides a crack for the generation and expansion of the crack. Advantageously, the possibility of crack initiation and expansion of the brittle material is increased, and the material removal rate is increased. Therefore, machine tool feeding is a non-negligible factor in the removal of ultrasonic milling materials.

3.2 Influence of layer thickness on material removal rate

Tool speed, machine feed rate, layer thickness, tool amplitude and static pressure are all important process parameters in ultrasonic milling. The greater the layer thickness, the higher the material removal rate, but the layer thickness and the feed rate are mutually constrained. Generally, the layer thickness is increased, and the feed rate is appropriately reduced. Conversely, the layer thickness is reduced. Increase the feed rate. The layering thickness has a great influence on the forming precision. This layered processing method inevitably causes a step effect on the contour of the cavity. The smaller the layer thickness, the higher the processing precision, but the processing efficiency is lowered. The maximum layer thickness is also restricted by factors such as the material properties of the workpiece. If the layer thickness is too large, the tool will bend due to excessive pressure in the axial direction during processing, which may make the processing impossible. The optimum layer thickness and feed rate need to be determined according to different workpiece materials in the experiment.

4. Summary

This paper mainly analyzes the ultrasonic milling processing technology, and analyzes the principle of material removal and the factors affecting the removal of ultrasonic milling technology. It has certain value for the promotion and practical application of ultrasonic milling.

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