28. Ballistic Cutting

28.1 Abrasive Jet Machining (AJM)

• The physics,

1. Fine particles (0.025mm) are accelerated in a gas stream (commonly air at a few times atmospheric pressure).

2. The particles are directed towards the focus of machining (less than 1mm from the tip).

3. As the particles impact the surface, they fracture off other particles.

 

• As the particle impacts the surface, it causes a small fracture, and the gas stream carries both the abrasive particles and the fractured (wear) particles away.

• Brittle and fragile work pieces work better.

• Material Removal Rate (mrr) is,

 

• Factors that effect the process are,

mrr

geometry of cut

roughness of surface produced

the rate of nozzle wear

• The factors are in turn effected by,

the abrasive: composition; strength; size; mass flow rate

the gas composition, pressure and velocity

the nozzle: geometry; material; distance to work; inclination to work

• The abrasive,

materials: aluminum oxide (preferred); silicon carbide

the grains should have sharp edges

material diameters of 10-50 micro m 15-20 is optimal

should not be reused as the sharp edges are worn down and smaller particles can clog nozzle.

• Gas jet,

mass flow rate of abrasive is proportional to gas pressure and gas flow

pressure is typically 0.2 N/mm2 to 1N/mm2

gas composition effects pressure flow relationship

• Nozzle

must be hard material to reduce wear by abrasives: WC (lasts 12 to 30 hr); sapphire (lasts 300 hr)

cross sectional area of orifice is 0.05-0.2 mm2

orifice can be round or rectangular

head can be straight, or at a right angle

 

• The relationship between head, and nozzle tip distance.

 

• Air drag also slows abrasive stream.

 

• Machines

 

• Summary of AJM characteristics

Mechanics of material removal: brittle fracture by impinging abrasive grains at high speed

media: Air, CO2

abrasives: Al2O3, SiC, 0.025mm diameter, 2-20g/min, non-recirculating

velocity = 150-300 m/sec

pressure = 2 to 10 atm.

nozzle: WC, sapphire, orifice area 0.05-0.2 mm2, life 12-300 hr., nozzle tip distance 0.25-75 mm

critical parameters: abrasive flow rate and velocity, nozzle tip distance from work surface, abrasive grain size and jet inclination

materials application: hard and brittle metals, alloys, and nonmetallic materials (e.g., germanium, silicon, glass, ceramics, and mica) Specially suitable for thin sections

shape (job) application: drilling, cutting, deburring, etching, cleaning

limitations: low metal removal rate (40 mg/min, 15 mm3/min), embedding of abrasive in workpiece, tapering of drilled holes, possibility of stray abrasive action.

28.1 Ghosh, A., Manufacturing Science, Ellis Horwood Ltd., Chichester, UK, 1986.

Problem 28.1 TRUE / FALSE: Water jet cutting can chip brittle work pieces.

28.2 High Pressure Jet Cutting

• Typical jet size <0.8mm faster than speed of sound.

• Typical fluid is water.

• Good for slotting.

• Used on,

metals

paper

cloth

wood

leather

rubber

plastics

frozen food

ceramics

• Main divisions or use,

jet cutting

destruction of brittle materials

separation of materials

• Typical pressures are 150-1000 MPa and use 8-80 KW.

• Velocities are 540-1400 m/s.

• Typical fluid volume is 0.5 to 2.5 l/min.

 

• Fluid Pressure Intensifier: hydraulics actuates smaller cylinders, that drive larger cylinders. The larger cylinders intensify air pressure.

• Nozzle: internal diameter reduces 40 to 160 times to the exit tip.

• The exit jet looks like,

 

• Cutting: If the material is brittle it will fracture, if ductile or erosive, it will cut well.

 

• Pulsed cuts can cut deeper, but feed rates must be reduced for acceptable cuts.

• The fluid used must have a low viscosity to minimize energy loses,

be non-corrosive

non-toxic

common and inexpensive

• Water most common, but additives such as alcohols, oils, products and glycerol are used, when they can be dissolved in water (ventilation may be required).

• The typical head is shown below. The orifice is often made of sapphire and ranges from 0.05” to 0.020”

 

28.3 Abrasive Waterjet Cutting (AWJ)

• 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,