Abrasive Jet Machining| Unconventional Machining Process

Introduction

In abrasive jet machining (AJM) a focused flow of abrasive grains of Al2O3 or SiC carried by high-pressure gas or air at a high velocity is made to impinge on the work surface throughout a nozzle of 0.3- to 0.5-mm diameter.

The process differs from sandblasting (SB) in that AJM has minor diameter abrasives and a more finely controlled delivery system. The work piece material is removed by the mechanical abrasion (MA) action of the high-velocity abrasive particles. AJM machining is excellent suited for machining holes in super hard materials. It is typically used to cut, clean, peen, deburr, deflash, and etch glass, ceramics, or hard metals.

Machining system

In the machining system shown in Fig. 1, a gas (nitrogen, CO2, or air) is supplied under a pressure of 2 to 8 kg/cm2. Oxygen should never be used because it causes a aggressive chemical reaction with work piece chips or abrasives. After filtration and regulation, the gas is passed through a mixing chamber that contains abrasive particles and vibrates at 50 Hz.

From the mixing chamber, the gas, along with the entrained abrasive particles (10–40 μm size), passes through a 0.45-mm-diameter tungsten carbide nozzle at a speed of 150 to 300 m/s. Aluminum oxide (Al2O3) and silicon carbide powders are used for deep cleaning, cutting, and deburring.

Magnesium carbonate is recommended for use in light cleaning and etching, while sodium bicarbonate is used for fine cleaning and the cutting of soft materials. Commercial-grade powders are not suitable because their sizes are not well classified. They may contain silica dust, which can be a health hazard. It is not practical to reuse the abrasive powder because contaminations and worn grit will cause a decline of the machining rate. The abrasive powder feed rate is controlled by the amplitude of vibrations in the mixing chamber. The nozzle standoff distance is 0.81 mm. The relative motion between the work piece and the nozzle is physically or automatically controlled using cam drives, pantographs, tracer mechanisms, or using computer control according to the cut geometry required. Masks of copper, glass, or rubber may be used to concentrate the jet stream of abrasive particles to a confined location on the work piece. Intricate and precise shapes can be produced by using masks with corresponding contours. Dust removal apparatus is incorporated to protect the environment.

image

Fig.1 AJM system.

Material removal rate

As shown in Fig.2, the abrasive particles from the nozzle pursue parallel paths for a short distance and then the abrasive jet flares outward like a fine cone. When the sharp-edged abrasive particles of Al2O3 or SiC hit a brittle and fragile material at high speed, tiny brittle fractures are created from which small particles dislodge. The lodged out particles are carried away by the air or gas. The material removal rate MRR, is given by

VRR=KNda3V3/2clip_image0043/4

where K = constant

N = number of abrasive particles impacting/unit area

da = mean diameter of abrasive particles, μm

ra = density of abrasive particles, kg/mm3

Hw = hardness number of the work material

n = speed of abrasive particles, m/s

The material removal rate, cut accuracy, surface roughness, and nozzle wear are influenced by the size and distance of the nozzle; composition, strength, size, and shape of abrasives; flow rate; and composition, pressure,

and velocity of the carrier gas. The material removal rate is primarily dependent on the flow rate and size of abrasives. Larger grain sizes produce greater removal rates. At a particular pressure, the volumetric removal rate increases with the abrasive flow rate up to an best value and then decreases with any further increase in flow rate. This is due to the fact that the mass flow rate of the gas decreases with an increase in the abrasive flow rate and hence the mixing ratio increases causing a decrease in the removal rate because of the decreasing energy available for material removal.

image

fig,2 AJM terminology

The typical material removal rate is 16.4 mm3/min when cutting glass. Cutting rates for metals vary from 1.6 to 4.1 mm3/min. For harder ceramics, cutting rates are about 50 percent high than those for glass. The minimum width of cut can be 0.13 mm. Tolerances are normally ±0.13 mm with ±0.05 mm possible using good fixation and motion control. The produced surface has a random or matte texture. Surface roughness of 0.2 to 1.5 μm using 10 and 50 μm particles, respectively, can be attained.

Taper is present in deep cuts. High nozzle pressures result in a superior removal rate, but the nozzle life is decreased. Table summarizes the overall process characteristics.

Abrasives

Type Al2O3 or SiC (used once)

Size Around 25 μm

Flow rate 3–20 g/min

Medium

Type Air or CO2

Velocity 150–300 m/s

Pressure 2–8 kg/cm2

Flow rate 28 L/min

Nozzle

Material Tungsten carbide or sapphire

Shape Circular, 0.3–0.5 mm diameter

Rectangular (0.08 × 0.51 mm to 6.61 × 0.51 mm)

Tip distance 0.25–15 mm

Life WC (12–30 h), sapphire (300 h)

Operating angle Vertical to 60° off vertical

Area 0.05–0.2 mm2

Tolerance ±0.05 mm

Surface roughness

0.15–0.2 μm (10-μm particles)

0.4–0.8 μm (25-μm particles)

1.0–1.5 μm (20-μm particles)

Applications

  • 1. Drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing, and radiusing
  • 2. Deburring of cross holes, slots, and threads in small precision parts that require a burr-free finish, for example hydraulic valves, aircraft fuel systems, and medical appliances
  • 3. Machining complex shapes or holes in sensitive, brittle, thin, or difficult-to-machine materials
  • 4. Insulation stripping and wire cleaning exclusive of affecting the conductor
  • 5. Micro-deburring of hypodermic needles
  • 6. Frosting glass and trimming of circuit boards, hybrid circuit resistors, capacitors, silicon, and gallium
  • 7. Removal of films and delicate cleaning of uneven surfaces because the abrasive stream is able to follow contours

Advantages and limitations of AJM

Advantages

  • _ Because AJM is a cool machining process, it is best suited for machining brittle and heat-sensitive materials like glass, quartz, sapphire, and ceramics.
  • The process is used for machining super alloys and refractory materials.
  • _ It is not reactive with any work piece material.
  • _ No tool changes are required.
  • _ Intricate parts of sharp corners can be machined.
  • _ The machined materials do not experience hardening.
  • _ No initial hole is required for starting the operation as required by wire EDM.
  • _ Material utilization is high.
  • _ It can machine thin materials.

Limitations

  • _ The removal rate is slow.
  • _ Stray cutting can’t be avoided (low accuracy of ±0.1 mm).
  • _ The tapering effect may occur especially when drilling in metals.
  • _ The abrasive may get impeded in the work surface.
  • _ Suitable dust-collecting systems should be provided.
  • _ Soft materials can’t be machined by the process.
  • _ Silica dust may be a health hazard.
  • _ Ordinary shop air should be filtered to remove moisture and oil.

Related Topics:

Leave a Reply

Your email address will not be published. Required fields are marked *