Waterjet Cutting Dynamics
Unlike machining utilising rotating, reciprocating or band tools, waterjet cutting utilises focussed energy to cut or shape a work-piece. Without a shape defined tool the cutting process become a multi-dimensional process where the cutting surface becomes a factor of not only the tool shape but a factor of time as well.
This is best demonstrated in the figure below which shows the water and abrasive mixture as it exits the mixing tube or nozzle and impacts the work-piece. The energy is highest as the jet stream impacts the part, the jet loses energy while cutting the material and thus the impact diameter of the jet reduces the deeper the jet penetrates. If the jet is kept stationary the penetration will increase in size until it is uniform through the depth of the part. Therefore the time dependence as noted above. This is what happens during the piercing process or the beginning of a cut.
Now as the nozzle proceeds to move forward the same process is taking place continuously and is therefore dependent on how fast the nozzle travels. As the nozzle moves the jet stream loses energy and the effective diameter of the jet is reduced, this causes the cut surface to taper inward. The slower the nozzle moves forward the less pronounced the taper effect on the part cut surface.
Another effect that is seen, which as with taper, becomes more pronounced with thicker materials is called jet lag or trailback. The effect is the formation of trail marks on the cut surface as the nozzle travels and due to the time dependence of the jet stream to transfer the energy to the part and effectively cut the material. The effect of this trailback is shown in the illustration below.
Any abrupt change in the nozzle travel direction, such as corners, causes further phenomena such as corner kick marks and corner wash out. Due to the jet lag or trailback it is important for the nozzle to allow the lag to catch up with the impact point otherwise the corner will not be cut completely. Therefore the nozzle needs to pause briefly for the lower lag to catch up with the entry point which often results in marks in the cut material due to this pause.
With Dynamic water jet these time dependent cutting factors are addressed by the ability of the cutting head to swivel in two dimensions to compensate for the energy loss of the jet stream. The figure below shows a Flow Dynamic XD cutting head, with the B and C harmonic drive actuators that control this motion, this is the same cutting head Waterjet Cape Town use to compensate for taper and jet lag.
By tilting the head towards the part being cut, by the same angle as the energy loss of the jet stream, the head is compensating for the induced taper on the part. The dynamic head also tilts the cutting forward with regards to direction of travel to compensate for the jet lag or trailback ensuring faster cutting speeds. The dynamic functioning of the cutting head becomes a function of very complicated mathematical numerical equations which is especially evident when changes in direction or corners are cut in a part. The travel slows down to allow more time to fully cut the corner while the head tilts backward and outward as the corner is cut to the precise geometry. As the travel accelerates from the corner the cutting head again tilts to compensate and eliminate all the unwanted effects accosted with waterjet cutting. On small parts this makes for very accurate and fine cut geometry which enables the Flow waterjet to be used in fine applications. With large or thick parts high accuracy is achieved with faster cutting speeds ensuring taper elimination and corner integrity.
Have a look at the video in our YouTube channel for a very good visual description of Dynamic Waterjet Cutting.
