ULTRASONIC MACHINING PROCESS

Ultrasonic Machining: USM

Ultrasonic Machining is finding wide application in machining of conducting as well as non-conducting materials. The process involves a tool vibrating with a very high frequency and a continuous flow of abrasive slurry in the small gap between the tool and the work surface. The tool is gradually fed with a uniform force. The impact of the hard abrasive grains fracture the hard and brittle work surface resulting in removal of work material in form of small wear particles which are carried away by the abrasive slurry. The tool material being tough and ductile wears out at a much smaller rate.

Material removal in USM is due to: 

i)                    Hammering of the abrasive particles on the  work surface by the tool

ii)                   Impact of free abrasive particles on work surface.

iii)                 Erosion due to cavitation

Following assumptions are made for determination of mrr: 

i)                    The rate of work material removal is proportional to the volume of work material per impact.

ii)                   Rate of work material removal is proportional to the number of particles making impact per cycle.

iii)                 The rate of work material removal is proportional to  the frequency (number of cycles per unit time)

iv)                 All impacts are identical

v)                  All abrasive grains are identical and spherical in shape. 

 So,  Q  =    v Z f 

Q is the material removal rate in mm³ /min.

'v' is the volume of  work material dislodged per impact (mm³ )

'Z' is the number of particles making impact per cycle.

f is the frequency of impact (number of impact per minute) 

Let us assume that the volume removed per grit (abrasive particle impact (indentation) ) be a hemispherical volume.

Here,   d = diameter of the abrasive particle

h = indentation caused by the particle on the  work surface

D =  diameter of the hemispherical volume of the work material being removed. 

If we see the figure (not to the scale) carefully, we can figure out: 

(D/2)² =   (d/2) ² –  ( d/2 – h) ²  

or,  D =  2 (dh) ^1/2

And now we can assume that the material removed is in shape of a hemisphere of diameter D.  So, the volume of material removed by impact of a single abrasive particle will be    v   =   (2Õ/3 )  (D/2) ³

                   =   (2Õ/3 )   (d h) ^3/2 

Process parameters:The important parameters that affect the process are : 

i)                    Frequency

ii)                   Amplitude

iii)                 Static Loading  (Feed Force)

iv)                 Hardness Ratio of The Tool & Work Piece

v)                  Grain Size

vi)                 Concentration of the abrasives in slurry 

Acoustic Head:  It produces vibration in the tool. It consists of a generator for producing high frequency electric current, a transducer to convert this into mechanical motion in the form of high frequency vibration, a holder to hold the head and a concentrator to mechanically amplify the vibration while transmitting it to tool.

 Most transducers used in acoustic-head work in magnetostriction. Vibration of frequency 15-30 KHz can be generated with low voltage supply and with simple cooling arrangement. Stamplings are used to reduce loss as in transformers. The dimensions are such chosen that the natural frequency coincides with the electric frequency. Almost all modern machines use the magnetostriction transducers made of Nickel stamplings, 0.1 – 0.2 mm in thickness.

 The main purpose of the concentrator is to increase the amplitude, to the level needed for cutting. Various types of concentrators are used

Feed Mechanism: It applies the working force during the machining operation. The basic types of feed mechanisms used are: 

               i)  Hydraulic/ Pneumatic Type

           ii) Spring Type

          iii)  Counter Weight Type

          iv) Spring Type

Tool: The tool is made of a strong but ductile metal. Generally stainless steel and low carbon steels are used for making the tools.  Aluminium and brass tools wear faster than steel tools. 

Abrasive Slurry: Boron carbide (B₄C), Silicon Carbide (SiC), Corundum (Al₂O₃), diamond and Boron Silicarbide are commonly used abrasives. Water is mostly commonly used fluid in the slurry. Benzene, glycerol and oils are also used. Mrr tends to decrease with increasing viscosity of the slurry. 

Summary of USM Characteristics: 

Mechanics of Material Removal:              Brittle fracture caused by impact of abrasive       

                                                                   grains due to tool vibrating at high frequency. 

Medium                                                      Slurry 

Abrasives                                                    B₄C, SiC, Al₂O₃, Diamond (100-800) grit size 

Vibration: 

Frequency                                                   15-30KHz 

Amplitude                                                  25-100mm 

Tool:

Material                                                     soft steel 

(Material Removal Rate) to

Tool Removal Rate Ratio                         1.5 for WC work piece, 100 for glass 

Gap                                                          25-40 mm 

Critical Parameters                                  Frequency, amplitude, tool material

                                                                grit size, abrasive material, feed force

                 slurry Concentration, slurry viscosity

Materials Application                               Metals & Alloys (Hard and Brittle), 

                                                                Semiconductors, non-metals, glass, ceramics 

Shape Application                                    Round & Irregular Holes, Impressions 

Limitations                                              Low mrr, tool wear, smaller depth of holes