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Ultrasonic Machining Hard And Brittle Materials hole Drilling

Ultrasonic machining is the removal of material by the abrading action of grit-loaded liquid slurry circulating between the workpiece and a tool vibrating perpendicular to the workface at afrequency above the audible range.
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  • M20-R

Ultrasonic Machining Hard And Brittle Materials hole Drilling


Introduction

In contrast, ultrasonic machining  is a non-thermal, non-chemical and non-electrical machining process that leaves the chemical composition, material microstructure and physical properties of the workpiece unchanged. Sometimes referred to as ultrasonic impact grinding (UIG) or vibration cutting, the UM process can be used to generate a wide range of intricate features in advanced materials.


UM is a mechanical material removal process that can be used for machining both conductive and non-metallic materials with hardnesses of greater than 40 HRC (Rockwell Hardness measured in the C scale). The UM process can be used to machine precision micro-features, round and odd-shaped holes, blind cavities, and OD/ID features. Multiple features can be drilled simultaneously, often reducing the total machining time significantly .

High-frequency, low-amplitude energy is transmitted to the tool assembly. A constant stream of abrasive slurry passes between the tool and workpiece. The vibrating tool, combined with the abrasive slurry, uniformly abrades the material, leaving a precise reverse image of the tool shape. The tool does not come in contact with the material; only the abrasive grains contact the workpiece.

In the UM process, a low-frequency electrical signal is applied to a transducer, which converts the electrical energy into high-frequency (~20 KHz) mechanical vibration (see Figure 2). This mechanical energy is transmitted to a horn and tool assembly and results in a unidirectional vibration of the tool at the ultrasonic frequency with a known amplitude. The standard amplitude of vibration is typically less than 0.002 in. The power level for this process is in the range of 50 to 3000 watts. Pressure is applied to the tool in the form of static load.

A constant stream of abrasive slurry passes between the tool and the workpiece. Commonly used abrasives include diamond, boron carbide, silicon carbide and alumina, and the abrasive grains are suspended in water or a suitable chemical solution. In addition to providing abrasive grain to the cutting zone, the slurry is used to flush away debris. The vibrating tool, combined with the abrasive slurry, abrades the material uniformly, leaving a precise reverse image of the tool shape.

Ultrasonic machining is a loose abrasive machining process that requires a very low force applied to the abrasive grain, which leads to reduced material requirements and minimal to no damage to the surface. Material removal during the UM process can be classified into three mechanisms: mechanical abrasion by the direct hammering of the abrasive particles into the workpiece (major), micro-chipping through the impact of the free-moving abrasives (minor), and cavitation-induced erosion and chemical effect (minor).2

Material removal rates and the surface roughness generated on the machined surface depend on the material properties and process parameters, including the type and size of abrasive grain employed and the amplitude of vibration, as well as material porosity, hardness and toughness. In general, the material removal rate will be lower for materials with high material hardness (H) and fracture toughness (KIC).


Parameters of Ultrasonic Machining:

The ultrasonic vibration machining method is an efficient cutting technique for difficult-tomachine materials. It is found that the USM mechanism is influenced by these important parameters. 

 Amplitude of tool oscillation(a0)

 Frequency of tool oscillation(f) 

 Tool material 

 Type of abrasive

 Grain size or grit size of the abrasives – d0 

 Feed force - F 

 Contact area of the tool – A 

 Volume concentration of abrasive in water slurry – C 

 Ratio of workpiece hardness to tool hardness; λ=σw/σt

Item

Parameter

Abrasive Boron carbide, aluminium oxide and silicon carbide 
Grit size(d0)  100 – 800
Frequency of vibration (f)  19 – 25 kHz 
Amplitude of vibration (a) 15 - 50 µm
Tool material Soft steel titanium alloy
Wear ratio  Tungsten 1.5:1 and glass 100:1 
 Gap overcut  0.02-0.1 mm


Although manufacturing technologies are well developed for materials like metals and their alloys, considerable problems still exist in the fabrication of hard and brittle materials including ceramics and glass. Their superior physical and mechanical properties lead to long machining cycle and high production cost. Ultrasonic machining (USM) using loose abrasive particles suspended in a liquid slurry for material removal is considered an effective method for manufacturing these materials. This work gives a brief overview of USM first and then mainly addresses the development of a simulation model of this process using a mesh-free numerical technique, the smoothed particle hydrodynamics (SPH). The crack formation on the work surface impacted by two abrasive particles is studied for understanding the material removal and the interaction of abrasive particles in USM. Experiments are also conducted to verify the simulation results. The SPH model is proven useful for studying USM and is capable of predicting the machining performance.


ultrasonic machining


Hard and brittle materials, such as glass, ceramics, and quartz crystal, are getting more and more attention in the recent years due to their superior properties like high hardness, high strength, chemical stability, and low density. High-performance products made of these materials play an important role in various industrial fields including semiconductor, optical components, aerospace, and automotive industries [1, 2]. However, considerable problems such as long machining cycle and high production cost still exist in the fabrication of hard and brittle materials. Particular difficulties are the production of micro−/nanostructures with high machining efficiency, high aspect ratios, and good surfaces possessing no residual stress and microcracks. Hence, there is a crucial need for developing precision and efficient micromachining techniques for these materials.


Nontraditional machining techniques such as electric discharge machining and laser beam machining have been proposed to machine hard and brittle materials. However, even these processes have prominent limitations that the machined surfaces are always subjected to heat-induced damages like recast layer and thermal stress. Ultrasonic machining (USM) is another alternative method for manufacturing both conductive and nonconductive hard and brittle materials. It is known as a total mechanical process without suffering from heat or chemical effects, so USM would not thermally damage the machining objects or appear to cause significant levels of residual stress and chemical alterations.


What's the Principle of ultrasonic machining?

      Through ultrasonic to achieve a very large impact acceleration (about 104-105 times the acceleration of gravity) under the action of a vibration frequency of 20-50KHz (ie, 2000-50,000 times per second), and the cutting direction of the machine is combined with the main motion of the machine. High frequency vibration, the material is first crushed and then removed.


      Ultrasonic milling is microscopically a pulse cutting. The effective cutting time of the tool is very short. The tool is completely separated from the workpiece more than 80% of the time, and the workpiece is intermittently contacted by the machined surface, which greatly reduces the cutting resistance of the tool and avoids the common cutting. The phenomenon of letting the knife phenomenon is greatly reduced on the surface residual stress of the workpiece.

Ultrasonic machining, or strictly speaking the "Ultrasonic vibration machining", is a subtraction manufacturing process that removes material from the surface of a part through high frequency, low amplitude vibrations of a tool against the material surface in the presence of fine abrasive particles. The tool travels vertically or orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm (0.002 to 0.005 in.).[1] The fine abrasive grains are mixed with water to form a slurry that is distributed across the part and the tip of the tool. Typical grain sizes of the abrasive material range from 100 to 1000, where smaller grains (higher grain number) produce smoother surface finishes



 Ultrasonic machining is suitable for machining of hard, brittle materials including:


Glass
Sapphire
Alumina
Ferrite
PCD
Piezoceramics
Quartz
CVD Silicon Carbide
Ceramic Matrix Composites
Technical Ceramics

  


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