Taper left on a part was by far the biggest complaint from our customers across the world. As we have learned in prior posts, the faster you cut through a material with an abrasive waterjet, the greater the v-shaped taper. At the time, the only solution was to slow down your cut speed. The problem was, slowing down only minimized the taper – but rarely get rid of it; and slowing down costs shops a lot of money per part.

Flow had some experience with taper control in very tough applications.  Under a highly confidential (at that time) program, Flow was involved in cutting thick glass for space telescopes such as the Hubble telescope.  Flow scientists, including Steve Craigen and Mohamed Hashish, found out that the only way to cut thick sections with the required accuracy was to tilt the jet to correct for the taper. Because cutting this thick glass has very limited geometries, the taper angles were determined from experiments and hard coded into the CNC.  So, when I spoke to Glenn to see if he could make a mathematical model that would accurately predict the taper angle and stream lag on a part at any speed, he said the technical team at Flow had considered the problem in the past, and he thought we could. I asked him if we could then eliminate taper with a 5 axis wrist, on the fly, for any pattern on any material or thickness using those mathematical predictive models embedded within the FlowMaster Intelligent PC Control. He said, it probably was possible, but “…it will be really difficult to do.” I said great, because that means we can do it and we can have a ground breaking product our customers will love.

A few years later we launched Dynamic waterjet. Glenn still works on FlowMaster (the PC based intelligent control) and Dynamic waterjet today, and he I have chatted about this conversation many times since then. Dynamic waterjet is on 80% of all abrasive waterjet machine tools we provide today and has significantly expanded waterjet usage due higher production rates and improved part accuracy.

People often ask me to explain what, exactly, is Dynamic waterjet – and why they should consider it.

How Dynamic Waterjet works

Dynamic waterjet, as you might have guessed already, accurately predicts the V-shaped taper angle and stream angle on a part – and provides the +/- 10 degree wrist articulation to eliminate the taper from the part by pushing twice the taper to the scrap side of the material. The illustration should help you visualize this if you’re having trouble getting it from my inadequate description. A Dynamic waterjet is, therefore, a 5 axis machine, even when cutting a flat part. The wrist is needed to tilt the head over while traveling around the part geometry to eliminate the taper from the part wall. Dynamic waterjet also deals with stream lag, where the jet exits the bottom of the part behind the top of the part. It does so by angling the head forward slightly. However, this compensation is not as dramatic, or obvious to the eye, as the angling of the head to remove taper.

The waterjet programmer does not calculate these 5 axis wrist moves. The mathematical model calculates the angle for the programmer, and applies it to the cut path automatically. There is no difference in programming a conventional abrasive waterjet or a Dynamic waterjet – the tricky stuff happens behind the scenes in the computer program – quite slick.

Benefits of Dynamic Waterjet

The result of angling the head to make the part wall straight is two-fold. First, it allows the part to be far more accurate. Dynamic waterjet will achieve a finished part accuracy of metals, stone, glass, composite (nearly any material) of +/- 0.003”. With some effort you can often get it down to +/- 0.001”. This precision would be held on materials up to around 2 or 3 inches thick, beyond that the tolerance will get a bit worse. This is 4 to 6 times better than a conventional waterjet. Now, those of you experienced in cutting with abrasive waterjets know that you can cut a good part with a conventional waterjet. By slowing way down you can get a finished part of perhaps +/- 0.004 to 0.006 inch. Maybe even better if your programmer and operator are extremely talented and experienced. But the sacrifice of speed drives the cost per part up. This brings us to the second benefit.

The second benefit of Dynamic Waterjet is the ability to cut a part at a much faster cut speed. As I mentioned above, you can improve the accuracy of a part with a conventional non-tilting cutting head by slowing down. Taper usually cannot be eliminated on a conventional non-tilting system, but it can be reduced. At 12-20% of maximum cut speed you can reduce the taper on the part to a degree of angle or so. Obviously shops don’t want to cut slower. It costs more, and they can’t get as much production off the machine in a shift. Dynamic waterjet allows you to hold a tolerance at much higher speed because you can cut a part at 30%, 50%, even 80% of maximum cut speed while holding tolerance. Cutting at 60% of maximum cut speed is very common in industry today on a Dynamic waterjet machine resulting in a high quality part, yet if we tried cutting at 60% with a conventional machine the part would exhibit significant taper and inside corner damage. Of course, the edge is rougher when cutting at higher speed whether cut with Dynamic or not, but for many applications, in fact most applications, shops find that the waterjet edge is generally so good that they can cut at high speed, hold tolerance, and please their customers.

Not all tilting heads are the same

The technology behind Dynamic waterjet is patented, as you probably assumed. Why then, are there other systems out there that offer taper reduction? The idea of tilting a head to remove taper from a part is not novel. What is novel, and what is a major part of the patent issued, is the ability to change the angle within any part path dynamically with speed. Slow down and speed up to come into and out of a corner or arc and the taper changes – so a fixed taper machine will start delivering tolerance errors on the part. Dynamic waterjet compensates for taper and stream lag dynamically with speed change. After 100,000 R&D hours, and a few prototypes, Mr. Erichsen and his team released Dynamic waterjet to the market in 2001 to the delight of waterjet shops around the world. And he was right. It was hard. But very much worth the effort.

Hope this was helpful and/or interesting. As always, please let me know what you think in the comments.
chip

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