Thursday, December 8, 2022

Introduction to Water Cutting Machines

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As we stated in a previous article on Plasma Cutting, there are numerous machining processes that, instead of using cutting tools, use different forms of energy. Thus, the use of the mechanical energy has given rise to one of the processes of greatest expansion in recent years, which has a series of advantages over others: the water cut or with water jet.

A priori, it may seem unusual that a Waterjet be able to cut wood, plastics, fabric, rubber, metals, glass, graphite, composite materials and a long etcetera to ceramic and granite in a single pass, without deforming or hardening the material, without producing heat, without releasing toxic gases or waste , with minimal burr formation and little material loss.

But in reality, the water cut highlights all these attributes, so it is well worth learning what it consists of and what machines are currently available.

The secret of achieving a water cut lies in directing a stream of water through a nozzle Very small diameter (0.1 – 0.4 mm) sapphire, ruby ​​or diamond driven by a pump capable of delivering pressures as high as 6000 bar (600 MPa) or more, and ultrasonic speeds. If to this device the contribution of a abrasive that is pre-mixed with water, the cutting power is increased and the variety of applications is extended to practically any material.

That is why, basically, they are distinguished two types of water cutting:

  • Pure water jet cutting.
  • Waterjet and abrasive cutting, where the abrasive fulfills the function of a cutting tool.

Either one method or the other, they generally use the same type of machine, the scheme of which we see in the figure below. The main difference between the two methods is that the cut with pure water it does not include the abrasive hopper and therefore, as we will see later, the cutting head has a different design.

Water-cutting-machine

Let’s see below some characteristics of the components of a water cutting machine.

Ultra high pressure water supply system

Originally, the water coming from a tank is driven by a compressor and enters at low pressure in the intensifier pump. This acts as an amplifier as it converts the energy of the low pressure hydraulic fluid into ultra high pressure water.

A hydraulic system provides hydraulic power to a reciprocating piston in the center section of the intensifier pump. A contact sensor, located at each end of the piston stroke, sends signals to electronic controls to move the directional control valve and reverse the direction of the piston. The intensifier pump, with a plunger on each side of the piston, generates pressure in both directions. While one side of the pump is on the intake stroke, the opposite side is outputting at ultra-high pressure. During the inlet stroke of the plunger, filtered water enters the high pressure cylinder through a check valve.

Once the plunger reverses direction, the water is compressed and exits as ultra-high pressure water and enters a pressure vessel (attenuator). The attenuator smooths out the pressure fluctuations of the intensifier pump and provides a constant and stable flow of ultra-high pressure water into the Cutting head.

Table and dimensions

The motion control table it is generally constructed of stainless steel and structural aluminum. Depending on the needs and the manufacturer, its dimensions vary from less than 2 meters for the X axis up to more than 12 meters for the Axis y, with a great diversity of intermediate measures. The maximum thickness of material that can be cut with these cutting equipment water cut is 200 mm (z axis), cutting speeds are around 20 meters / min and the average tolerance is usually ± 0.05mm / m.

The bottom of the table has cavities for sludge (a mixture of water and abrasive) to drain off. Generally, the cleaning process is carried out with continuous extraction systems that collect the sludge deposited in the cutting tank and separate the water for recycling.

Cutting head

Any machine water cut has at least one Cutting head (there may be several) and depending on the pure water or water and abrasiveThese heads present the designs that we see in the following figure.

Types-of-heads

Types-of-heads

The head (s) are mounted on arms that move along the XY plane of the table and in the Z direction, forming a cutting system. The most complex equipment has heads with up to 5 or 6 axes, which allow cutting at an angle of up to 90º and in three dimensions (3D) with infinite rotation, which saves time and maximizes the performance of these equipment.

Abrasive

The abrasive is made up of fine particles of garnet sand, a non-toxic natural material that can be landfilled or recycled for repeated use using accessory systems sold by some brands.

The abrasive power of the grains increases a thousand times that of water, so the amount of hard materials that can be cut can be understood by combining an ultra-high pressure jet and abrasive grains. Generally, a machine water cut uses 250 to 700 g of abrasive per minute.

Control system

The movement of this cutting system is computer controlled, so it can accept CAD / CAM files. Although it may seem otherwise, computerized operation of the cutting system is not complicated and an inexperienced operator should be able to learn how to operate and maintain a cutting machine. water cut after a minimum of one week of training, thanks to the diversity of nesting programs of CAD / CAM that the current market offers.

These programs allow the regulation of basic parameters such as, among others, the material properties, the jet flow, the cut tolerance and the average of time for machining, which determine the physical behavior of the head and the jet as they move over the material and cut it.

Water quality

Before installing a system water cutIt is very important to carry out a water quality analysis by a specialized company. The incoming water must be treated to remove the hardness or reduce the content of Total dissolved solids. A water treatment that produces a total solids content of less than 0.5 ppm is not recommended because the aggressiveness of purified water can damage the components of the intensifier pump. The treated water must have a pH value of 6 to 8.

Some manufacturers provide special water treatment equipment for the water cut, which not only eliminate hardness, but also condition the water and guarantee a constant pressure for its entry into the intensifier pump.

Teams water cut they use very little water (less than 4 liters per minute, depending on the size of the cutter head hole) and, as we mentioned earlier, the used water can be recycled through a closed loop system. Generally, the wastewater is clean enough to filter it down the drain.

Water cutting applications

Virtually all industries can make use of this innovative and versatile cutting technique, economical, precise and friendly with the environment. The applications of water cut include, among others, the aeronautical sector (alloys, insulators, carbon fiber, titanium), automotive (alloys, rubber, composite materials, leather) and metallurgical (steels, alloys, brass), as well as the construction industries (bars , ceramics, marble, beams), furniture (glass, mirrors, wood, fabrics), toys (foams, synthetic fibers, polymers, polypropylene), food (meats, pastries) and even advertising and art (methacrylate , polyethylene, PVC, vinyl).

The following video shows us some objects that can be achieved by cutting with water.

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