1. Technical Field
This disclosure is related, generally, to waterjet cutting systems, and, in particular, to a method and apparatus for controlling a standoff distance between a waterjet cutting head and a surface of a workpiece to be processed.
2. Description of the Related Art
Fluid jet or abrasive-fluid jet cutting systems are used for cutting a wide variety of materials, including stone, glass, ceramics and metals. In a typical fluid jet cutting system, a high-pressure fluid (e.g., water) flows through a cutting head having a cutting nozzle that directs a cutting jet onto a workpiece. The system may draw an abrasive into the high-pressure fluid jet to form an abrasive-fluid jet. The cutting nozzle may then be controllably moved across the workpiece to cut the workpiece as desired. After the fluid jet, or abrasive-fluid jet, generically referred to throughout as a “waterjet,” passes through the workpiece, the energy of the cutting jet is dissipated by a volume of water in a catcher tank. Systems for generating high-pressure waterjets are currently available, such as, for example, the Mach 4™ five-axis waterjet system manufactured by Flow International Corporation, the assignee of the present application. Other examples of waterjet cutting systems are shown and described in Flow's U.S. Pat. No. 5,643,058, which is incorporated herein by reference in its entirety.
Manipulating a waterjet in five or more axes may be useful for a variety of reasons, including, for example, cutting a three-dimensional shape. Such manipulation may also be desired to correct for cutting characteristics of the jet or for the characteristics of the cutting result. More particularly, as understood by one of ordinary skill in the relevant art, a cut produced by a waterjet has characteristics that differ from cuts produced by more traditional machining processes. These cut characteristics may include “taper” and “trailback,” as explained in more detail in Flow's U.S. Pat. No. 7,331,842, which is incorporated herein by reference in its entirety. These cut characteristics, namely taper and trailback, may or may not be acceptable, given the desired end product. Taper and trailback vary, depending upon the thickness and hardness of the workpiece and the speed of the cut. Thus, one known way to control excessive taper and/or trailback is to slow down the cutting speed of the system. Alternatively, in situations where it is desirable to minimize or eliminate taper and trailback while operating at higher cutting speeds, five-axis systems may be used to apply taper and lead angle corrections to the waterjet as it moves along a cutting path. A method and system for automated control of waterjet orientation parameters to adjust or compensate for taper angle and lead angle corrections is described in Flow's U.S. Pat. No. 6,766,216, which is incorporated herein by reference in its entirety.
To maximize the efficiency and quality of the cutting process, a standoff distance between where the waterjet exits the nozzle and a surface of the workpiece is preferably controlled. If the standoff distance is too small, the nozzle can plug during piercing, causing system shutdown and possibly damage to the workpiece. If the distance is too great, the quality and accuracy of the cut suffers. Systems for detecting and controlling such a standoff distance are known, and include, for example, direct contact type sensing systems and non-contact inductance type sensing systems. Examples of waterjet cutting systems including a sensing system for controlling a standoff distance are shown and described in Flow's U.S. Pat. Nos. 7,331,842 and 7,464,630, which are incorporated herein by reference in their entireties.
Known standoff detection systems, however, typically require direct contact sensing of the workpiece surface from which the desired standoff distance is to be maintained or positioning of a non-contact inductance type sensor proximate the surface. These types of systems therefore often include features which may limit, for example, the mobility and/or flexibility of the waterjet cutting system to traverse a workpiece in a particularly advantageous cutting path. In addition, components of these systems may be unavoidably exposed to spray-back which occurs when the waterjet first impinges on a surface of a workpiece or as the waterjet interacts with a structure beneath the workpiece during operation, thereby leading to potential wear and damage of the components.