Waterjet basics

THE FUNCTION PRINCIPLE OF THE MICRO ABRASIVE WATERJET

The principle of the micro abrasive waterjet cutting head is described schematically in the illustration to the left. The main parts of the cutting head are a straight inlet tube that stabilizes the stream of incoming pressurized water, a primary nozzle or orifice, a mixing chamber with a feeding port for abrasives, and a mixing tube or focusing tube.  The waterjet formed in the orifice is gradually broken up into small drops moving at high velocity through the cutting head.  In the mixing chamber, these drops strikes the abrasive particles (usually garnet grit) which are fed into the cutting head and an energy transfer accelerate them to high speeds.

 

Forming a particle stream

Abrasives and water are then focused to form a fine particle stream in the mixing tube.  The micro abrasive waterjet is a stream of particles without any real core of water.  By volume the mixture is ca 4 % water, 1 % abrasive and the rest is air.  It is highly critical that both nozzles should be aligned to avoid losing power in the process.  The method extends the same equipment as that used for pure waterjet cutting, with the addition of an abrasive feeder and a cutting head.

 

The erosive power of an abrasive water jet makes it considerably more powerful than a water-only jet. The diameter of the particle beam, and thus the width of the cut, is established by the diameter of the focusing tube, which is usually about 0.2 – 0.3 mm, for the micro abrasive waterjets of today, although dimensions as small as 0.05 mm has been tested experimetally.  The cutting result depends on a large number of parameters that influence both power supply and efficiency, which in their turn determine cutting speed and cut quality.  Much of the skill of the operator lies in his or her ability to understand and to master these parameters and the resulting cut quality.

 

Cut quality classification

With appropriate selection of cutting speed and parameter settings, the abrasive waterjet process can produce parts with a choice of cut surfaces that ranges from rough separating cuts to precise taper-free cuts. An industrial praxis for cut quality classification has become widely accepted among abrasive waterjet systems manufacturers and operators. Although this may vary slightly among practitioners, cut quality is usually divided into five classes ranging from an extra rough quality index, q = 1, to an extra fine quality index, q = 5. For micro abrasive waterjets these classes are extended to q=9 which represent a taper-free cut.

 

Parameters influencing accuracy and cut quality

The finished part accuracy and cut quality that can be achieved with the abrasive waterjet cutting process depend on a combination of machine and motion control errors, to which is added  errors related to workpiece stability (flatness, fixturing, temperature rise, movement due to residual stress release), process errors (the jet), and the influence of the material being cut.

 

working principle

Machine and motion control issues include:

  • Machine squareness:  The coordinate axes must be squared in order to achieve square parts. All deviations in straightness, flatness and parallelism will ultimately affect the machine accuracy and, thereby, the shape of each part.
  • Positioning accuracy: describes the accuracy with which the machine can reach a given coordinate (i.e. a precise predetermined location) on the cutting table. Positioning accuracy is seldom related to the speed of the machine.
  • Repeatability: describes the capacity of the machine to return repeatedly to the same point on the cutting table.
  • Dynamic precision: depends on machine stability combined with the capability of the control system to follow the desired contour exactly. When contouring in small radii and corners, the dynamic forces act on the machine members. To minimize dynamic forces the component weight/stiffness ratio is important. Regarding dynamic forces, it should be pointed out that micro abrasive waterjet cutting has smooth motions and speeds are relatively low why dynamic forces are usually relatively moderate. Therefore, high dynamic precision may be expected.

How to obtain ultra high precision?

The precision of a machine tool is a matter of design and component quality and full utilization of it ultimately depends on the skills of the operator. Much can be done, both mechanically and in the machine control system to obtain good accuracy.

 

When designing the Finecut WMC500II machine the above points has formed the design criteria. In the design of machine tools the machine members are made to high tolerances. The control system has capability to compensate for remaining geometrical deviation. Therefor to obtain excellent positioning accuracy the machines are calibrated after assembly. Calibration is preceded by optimizing the servo parameters to obtain accurate motion. Then lasers are used to measure any geometrical errors, and the results are fed into the control. To further improve accuracy a volumetric compensation is performed.

 

 

 

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