56. ABRASIVE WATERJET CUTTING (AWJ)

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Basic principle a narrow, focused, water jet is mixed with abrasive particles. This jet is sprayed with very high pressures resulting in high velocities that cut through all materials. The water jet reduces cutting forces, and virtually eliminates heating. The basic cutting mechanism is erosions.

Non-contact and no tool wear.

Good for materials that cannot stand high temperatures of other methods for stress distortion or metallurgical reasons.

Developed in 1960s.

Typical pressures are 10-100 Kpsi - lower pressures are good for soft materials metals need higher pressures. The required jet pressure decreases with the use of harder abrasives.

Steel plates over 3" thick can be cut.

The energy in the beam can be expressed with Bournoulli's equation,

The basic system uses,

Filters - purifies the water to extend system life

Compressors/Intensifiers - increases water pressure

Water delivery - tubes and fittings to deliver intensified water

Abrasive hopper - to deliver abrasive

Orifice/Mixing Chamber/Refocusing Nozzle - to mix the high pressure water, and abrasives

Cutting nozzle - to direct the jet

NC Gantry - to position the cutting head

Catcher - to stop the spent jet

The velocity of the stream is up to 285 fps (or 1950 mph) about 2.5 times the speed of sound. The cutting energy is proportional to the root of the velocity.

The pressures may also have transient spikes up to 3 times the base cutting pressure.

Typical materials,

Typical cut width (Kerf loss) is 0.030" and above.

Typical orifice sizes are 0.007,9,13,15 in.

The effective jet range is up to 8" for hard materials. Pressure falls off after 1".

The jet will have a well behaved central jet surrounded by a fine mist.

Typical process jet variables are,

- pressure

- diameter

- standoff

- abrasive feed rate

- material feed rate

The greater the amount of energy delivered the,

- thicker the cut

- smoother the finish

- harder the material

Multiple pass cutting involves making a cut that does not fully penetrate, and does not chip edges, and making subsequent passes. This saves energy but will result in degraded surface quality.

Advantages,

- surface finishes can be as good as conventional machining

- no temperature changes

- very little scrape produced

- negligible cutting forces, therefore little or no fixturing

- fragile/brittle materials can be cut

The water filter,

- uses filters to remove microscopic particles that might damage the orifice and other high pressure parts.

- a traditional pump is used to drive the water through 10 micron, 1 micron and 0.5 micron filters.

- typical flow out is 1 gallon per minute at normal tap pressure.

Intensifiers,

- basically a small piston driven by a larger hydraulic piston. The opposing cylinders change a large differential volume for a large differential pressure.

- as the hydraulic unit in the center pumps in both directions, a high pressure is generated in the water cylinders at either end. Check valves allow water flow in and out as appropriate.

- the pressures generated by the intensifier can be adjusted by modifying the hydraulic pressures.

- roughly 2Hz.

Accumulator - acts as a pressurized reservoir for the water.

Tubing and Fittings,

- between the accumulator, and the movable head, a variable dimension delivery system is required.

- at lower pressures flexible rubber hoses would be used, but at these pressures, a coiled stainless steel tube is often used. The ends of the tubes are connected with high pressure swivels.

- a protective sheath is placed about the tubes to prevent damage in the instance of leaks. Flow valves are also used to reduce the chances of damage.

Mixing head,

- mixes water and abrasive, and focuses for cutting

- combines orifice, mixing chamber, refocusing and nozzle

- Single jet - side feed heads are suited to tight/small applications because of simple head geometry. Some problems with mixing head.

- multiple jet, center feed. Good mixing characteristics, but hard to manufacturing.

- mixing tubes are often made of tungsten carbide or similar materials

Abrasives,

The catcher,

- stops the jet after it has passed the cutting surface.

- reduces press, noise, dust, increases safety.

- a water filed tube can be used.

- the jet should be dispersed within the length of the tube (up to 24") shorter tubes need hard materials at the far end.

- some mechanisms use a tank with a 2" steel plate for the bottom.

General parameters,

- a larger orifice results in a rougher surface finish

- surface finish > 100 micro in. (Ra) up to ????

- deeper cuts give rougher surface finish

- cutting speeds < 1 ipm up to 5 ipm

- higher cutting speeds give better finishes

- surface quality degrades at bottom side first

- the surfaces tend to have waves, probably caused by intensifier

- the jet flares from 0.1" to 1"

- faster cutting speeds result in more flaring

Generally,

- slower feeds than laser

- faster than wire EDM

- slower than plasma cutters

Cost is typically $20 to $40 per hour for operation mainly as a function of abrasives.

Advantages,

- can cut traditionally hard to cut materials, eg. composites, ceramics, glass

- no special tooling required

Disadvantages,

- hourly rates are relatively high

- flaring can become large

- not suitable for mass production because of high maintenance requirements

Typical machining conditions,