Quality Waterjet Newsletter 6/7/2005

A Tip for Lead-in/Lead-Out in Abrasive Waterjet Cutting

During cutting, the stream of abrasive waterjet is deflected backward, creating a “jetlag” --- the bottom position of the jet lags behind the top position. When the jet comes across an edge, the jet skips, leaving a triangular section uncut. In fact this phenomenon leaves a “bump” on an otherwise perfectly round hole. To eliminate this “bump”, people have tried all kinds of lead-in/lead-out techniques. Typically the jet is started outside of the contour and it cuts toward the contour along a straight line or an arc (lead-in). When the jet finishes cutting the contour, it cuts away from the contour along a straight line or an arc (lead-out).

Andersson and Johansson (*) studied three methods of lead-in/lead-out. The first one is to start the jet on the contour and end it at the same spot. In the second method, the jet leads in along a straight line perpendicular to the contour and the cut ends at the lead-in point on the contour. The third method is similar to the second one, but the jet moves so rapidly that it does not cut through the material until it has reached the contour, and the cut ends on the contour some distance past the lead-in point. They also studied the effect of slowing down as the cut ends. Among their findings, the first method is the most effective one in terms of eliminating the “bump”. However, this method is often not acceptable because it leaves a piecing mark on the top surface. The intention of the third method was to leave the piecing mark on the scrap while gaining the same benefit of the first method --- eliminating the “bump”. It appeared to do well on both accounts. However this method uses extra time when the jet cuts past the lead-in point and it also appears to alter the geometry of the contour slightly. Among these three methods, the second method appears the most practical one. It did very well in minimizing the “bump” when it was combined with a slow down as the cut was ended.

When you think about it, the second method does make sense. When the jet makes a 90 degree turn at the lead-in point, because of the “jetlag”, the jet “pre-cuts” part of the section which otherwise will become the triangular uncut section the jet will skip. By properly slowing down the jet when it comes to the end point, it can make the jet profile match the “pre-cut” section and completely eliminate the “bump”.

* Andersson, U., and Johansson, B., “Cutting quality improvement by start and stop procedures for closed-contour cutting”, Proceedings of the 16th International Conference on Water Jetting, Aix-en-Provence, France, October 16-18, 2002.

Book Review – Part One

Positive displacement, reciprocating pumps can be classified as:

·      Power pumps or direct acting pumps

·      Horizontal or vertical pumps

·      Single acting or double acting pumps

·      Piston, plunger or diaphragm pumps

·      Simplex, duplex or multiplex pumps.

In the high pressure and ultra high-pressure fields, intensifiers and plunger pumps are most often used to provide pressures in the 10,000 to 40,000 psi ranges.  A reciprocating pump is a positive displacement and not a kinetic machine like a centrifugal pump so it does not require velocity to achieve pressure. It is an advantage to obtain high pressure at low velocity for large flows and slurry applications. A reciprocating pump has high efficiencies in the range of 85% to 94% with a 10% loss through belts, gears, bearings, packing and valves.

Excerpt from Chapter 2, Pump Design, of High Pressure Pumps & Systems by Michael Gracey, with the courtesy of the author.


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