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Monday, February 6, 2017

Water jet cutting !!

A look inside the physics of achieving high-pressure water for waterjet cutting

The energy required for cutting materials is obtained by pressurizing water to ultra-high pressures and forming an intense cutting stream by focusing this high-speed water through a small, precious-stone orifice. There are two main steps involved in the waterjet cutting process.
  1. The Electric Servo Pump generally pressurizes normal tap water at pressure levels above 50,000 psi; to produce the energy required for cutting.
  2. Water is then focused through a small precious stone orifice to form an intense cutting stream. The stream moves at a velocity of up to 3 times the speed of sound, depending on how the water pressure is exerted.
The process is applicable to both water only and abrasive jets. For abrasive cutting applications, abrasive garnet is fed into the abrasive mixing chamber, which is part of the cutting head body, to produce a coherent and an extremely energetic abrasive jet stream.
To achieve these pressures, water is introduced into the unit by way of a booster pump and filter. This filtering process is very important as water must be clean before reaching ultra-high pressures in order to protect the high-pressure parts and provide a consistent cutting stream.
A water treatment system is sometimes needed to remove harmful minerals from the water. After being filtered, the water enters the high pressure cylinder where it is pressurized to the desired level.
The water is then carried to either an abrasive or straight-water cutting nozzle, depending on the application. The cutting nozzle can be stationary or integrated into motion equipment, which allows for intricate shapes and designs to be cut. Motion equipment can range from a simple cross-cutter to 2D systems and 3D machines as well as multiple axis robots. CAD/CAM software combined with CNC controllers translate drawings or commands into a digitally programmed path for the cutting head to follow.Cutting harder materials requires adding a fine mesh abrasive to the cutting stream. Various abrasive materials which can be used include olivine, garnet, and corundum with a particle size of between 50 to 120 mesh. When abrasive is required, TECHNI Waterjet provides an abrasive unit consisting primarily of an abrasive hopper, an abrasive feeder system, a pneumatically controlled on/off valve, and the abrasive cutting nozzle which contains the specialized mixing chamber.
The abrasive is first stored in the pressurized hopper and travels to a metering assembly, which controls the amount of particles fed to the nozzle. The abrasive is then introduced into the cutting stream in a special mixing chamber within the abrasive cutting head. Abrasive cutting allows harder materials to be cut at a faster rate by accelerating the erosion process. After the cut, residual energy from the cutting stream is dissipated in a catcher tank, which stores the kerf material and spent abrasive.

Relationship Between Increased Pressure and Cutting Speed

As pressure increases the power requirement increases proportionately and therefore with a given amount of available power the flow rate must be proportionately reduced, by using a smaller orifice, as shown in the commonly used formula P (Power) = p (pressure) x q (water flow rate).
For example, a 50% increase in pressure will require a 50% increase in power unless there is an equivalent reduction in flow rate (using a smaller orifice). Higher pressure gives an increase in cutting speed for a given amount of power, as higher pressures and lower volumes result in higher velocity of the water leaving the cutting head, which is a more efficient transfer of power to kinetic energy (the energy used in the cutting process).
This efficiency comes about because increasing velocity is a more efficient way of increasing the kinetic energy stored in the particles of abrasive hitting the work-piece. This is illustrated through another commonly used formula E=M V², where by increasing the velocity has a squared affect on the kinetic energy, compared to increasing the mass which has a linear affect. Therefore, in theory if we increase the pressure by 50%, but decrease the volume by 33% we use the same amount of power but get an increase in the velocity of 50%, which has the effect
of increasing the kinetic energy by 48.5%, as illustrated in the formula E=0.666 (33% reduction in water mass) x 1.5² (50% increase in velocity) therefore E = 1.485 (48.5% increase in kinetic energy). However, this illustration is only relevant for Water Only cutting, as the mass of the abrasive has not yet been taken into account.
Abrasive cutting dramatically increases the cutting capabilities of a waterjet by accelerating the abrasive particles at the work piece where each particle takes out a small gouge of the work piece material during impact. If all the abrasive particles were to hit the work piece in the same condition, but at the higher velocity, the same equation as above would be true. However, the major factor that affects what actually happens is that the abrasive particles get smashed to a very fine powder when hit by the high velocity water stream during the initial introduction of the abrasive to the stream, and more gets destroyed throughout the focussing tube 1 The intensity of the disintegration of the abrasive particles depends on the water pressure. The result is that at 60,000psi, only about 45% of the abrasive material reaches the work piece in an affective cutting condition. This % drops to about 22% (or less depending on the quality of abrasive) at 90,000 psi. The net result is that there is therefore only a very small net increase in cutting speed when pressure is increased, for an equivalent amount of power. This can be illustrated in simplified terms for 60,000psi as E=0.45 (45% effective garnet) x 1² therefore E=0.45, and 90,000psi as E=0.22 (22% effective garnet) x 1.5² (50% increase in velocity) therefore E=0.49, or a 9% increase in cutting speed for a 50% increase in pressure.

The Cost of Higher Pressures

Another important consideration before deciding to increase pressure is the significant increase in the capital cost, maintenance cost, consumable cost and increased machine downtime.
Pressure (also known as force or load) has a non-linear relationship with fatigue-related wear, and for many mechanical machine components, it has a cubed (x³) relationship. For example, the ISO formula for calculating bearing wear is
L = (C/P)3
L = life
C= rated load
P = actual load
That means that a 50% increase in pressure will reduce the design life of many mechanical components by about 70% or adversely by reducing pressure by 33%, say from 90,000psi to 60,000psi, the life of many components will increase by 330%.
As a result, in order to make pumps and cutting heads that last for reasonable amounts of time at extreme pressures like 90,000psi, manufacturers are forced to use very expensive exotic materials, because metal fatigue becomes the dominant failure mechanism.. The cost of components and consumables that experience high pressures over 66,000psi (such as dynamic and static seals, check valves, tubing and high- and low-pressure cylinders) are therefore typically 50-300% higher than standard waterjet components.
The other factor in the pricing of such components is competition, as only a few manufacturers are currently capable of producing those parts that are resistant to wear and failure at high pressures. As a consequence, the competition to bring down prices doesn’t yet exist.
For instance, a standard focussing tube rated to 60,000psi sells for approx. $100, while a 90,000psi rated tube sells for around $300. Moreover, even with the more exotic and expensive components designed for 90,000psi, their life remains well below that of traditional waterjet parts operating with up to 60,000psi. This means increased down time and higher maintenance labour costs, on top of the higher component prices and more frequent part replacements.

1 comment:

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