ULTRASONIC MACHINING (USM)

ULTRASONIC MACHINING (USM)

Ultrasonic machining is a kind of grinding method. Abrasive slurry is pumped between tool and work, and the tool is given a high frequency, low amplitude oscillation, which, in turn, transmits a high velocity to fine abrasive particles, which are driven against the workpiece. At each stroke, minute chips of material are removed by fracture or erosion.   The general arrangement of ultrasonic machining is shown in Figure below. The equipment consists of a transducer, a tool holder and the tool. The linear oscillatory motion of the tool is obtained by magnetostrictive transducer, which converts electric energy into mechanical energy. The transducer consists of a stack of nickel laminations that are wound with a coil. When a high frequency current is passed through the coil, changes in the electromagnetic field produce longitudinal strains in the laminations.  These longitudinal strains are transmitted to the tool through a tool holder. The tool oscillates linearly with

an amplitude of about 0.05 mm at ultrasonic frequencies of from 15 to 25 kHz. Power supply is: Potential volts = 220; current = 12A. 

Ultrasonic machining:-

The tool whose shape is essentially re-produced in the workpiece is also subjected to the impact-fracture action and should be made of a soft ductile material that is easily machined, for example, unhardened steel, copper or brass. The tool is ordinarily 0.075 to 0.10 mm smaller than the cavity it produces. The tool is brazed, soldered or fastened mechanically to the transducer through a tool holder. For selecting tool holder material, factors to be considered are : Conductivity, how well the material can be brazed and fatigue properties. The material for tool holder can be : titanium alloys, Monel, Aluminium, stainless steel etc. Important factor in tool fastening is that no relative motion and hence energy loss occurs. So, soldering and brazing have proved more satisfactory than mechanical clamping. The tool feed rate is about 0.1 mm/s maximum.

The abrasive used in USM can be aluminum oxide, boron carbide, or silicon carbide grains in slurry which also carries away debris. The slurry can be made in water, which also acts as a coolant. The usual combinations are: boron carbide in water and silicon carbide in paraffin. Grain size of abrasive particles ranges from 200 to 1000.

This method involves brittle fracture and obviously works only on relatively brittle materials. So, USM has been applied very successfully to hard, refractory, difficult to machine materials which are quite brittle, for example, ceramics, borides, ferrites, carbides, glass, precious stones, hardened steel, cermets and some super alloys etc. It is used chiefly for drilling holes, engraving, cavity sinking (carbide wire drawing dies), slicing and broaching etc. Hole diameter as small as 0.01 mm can be produced.

Tolerance and surface finish depend upon grit size. More practical values for tolerance are + or - 0.001 mm/mm.  Of course, tolerance of + or - 0.0005 mm/mm can be obtained. Usual values for surface finish are 0.25 to 0.50 u m Ra. Metal removal is about 3 mm3/s.

The difference between conventional grinding and ultrasonic machining also known as grinding is that whereas in the former, the motion of the grinding grit is tangential to the surface of workpiece, in USM (USG), the motion of grinding grits is normal to the work surface. Advantages of the process are: no thermal stresses, low tooling costs and the use of semiskilled workers for precision work.