A portion of an article from Machine Shop Guide - September 1998)
By John A. Vaccari, Senior Editor / Richard Ward, Richel. Inc.
Here's how abrasive waterjet cutting works... High-pressure water, especially abrasive water, is a formidable cutting tool. As a fine jet of up to 60,000 psi, it can cut thin or thick, hard or soft materials, including metals, paper, plastic, foam, glass, ceramics, and composites. Parts of simple, complex or intricate design, internally or externally, can be cut burr-free at moderate to high speed with minimum kerf and without heat distortion or heat-affected zones.
Deionized and purified water, pressurized in an intensifier pump, enters a cutting head and is forced through a tiny orifice to form a coherent stream, which impinges on the workpiece. Because abrasive water is a much more potent cutting agent than pure water, abrasive can be introduced in a mixing chamber beneath the orifice and the stream exit through a nozzle. Thus the terms "waterjet" and "abrasive waterjet". US Jetting also markets a "direct injection" system: abrasive and water are premixed and the mixture is injected directly into the head.
Motion control can be manual or fully automated. Manual systems can be used for cutting profiles from flat stock by directing the workpiece about the jet of a fixed head with a water catcher below. They also can be used for trimming fixtured or guided, unfixtured formed parts, much like routing, with fixed or mobile heads. Automated systems range from fully contained CNC gantry machines for cutting profiles in nested programs from sheet and plate, including stacked stock, to multiaxis systems for 3D cutting. To boost productivity, multiple cutting heads can be used in cutting sheet and plate. Automated systems also can incorporate shuttle tables, loading and unloading equipment, water recirculation, self-cleaning water tanks and other auxiliaries. Motion-control components are protected from water and abrasive entry.
| Critical parameters
Water pressure, orifice/nozzle-size combination, pump power capacity and abrasive feed rate have a pronounced effect on cutting speed, cut-edge quality and part cost. The cut edge typically tapers, with kerf smaller at the bottom, or exit side, of the jet than at the top, or entry side. The finish of the cut surface can range from rough to quite smooth, depending on cutting parameters.
The greater the speed, the greater the taper and , in general, the greater the roughness, says Richard Ward, president of the waterjet consulting and outsourcing firm, Richel, Inc (Tallmadge, Ohio). Cut-edge surface finish, in increasing order of smoothness, is designated Q1 to Q5. For the Q1, the vertical striations, which are most pronounced along the bottom of the cut, may be as deep as 0.040 in. Such a rough finish could cause parts to "hang" rather than release from the workpiece unless pressure is supplied. For a Q5 finish, the striations may be but 0.002 inch. |
This variance, which may be even greater for stock more than 4 in. thick, depends largely on cutting speed. "In general, a Q2 can be cut at twice the speed of a Q5, and a Q1 at twice the speed of a Q2, " according to Ward. Thus, if a rough rather than smooth finish will suffice, parts can be produced at greater production rates.
The effect of water pressure and orifice/nozzle-size combination on cutting speed appears in the chart at left. Note that increasing pressure from 40,000 to 55,000 psi can permit a 57% increase in speed for cutting 0.5-in.-thick 304 stainless steel to a Q2 finish. Thus, if maximum speed, or production rate, is primary, the system should be equipped with a pump providing maximum pressure. Note also that cutting speed increases as the orifice/nozzle combination increases for the same pressure. For example, a 15/45 orifice/nozzle size - that is 0.015/0.045 in. diameter - results in a speed of 8.2 ipm versus 4.4 ipm for the 10/30 at the same pressure.
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Ward also emphasizes the advantage of high pump power for cutting multiple identical parts in nested programs with multiple cutting heads. "A system with a 150-hp pump and four heads," as shown on previous page, "may produce six times as many parts as a single-head system served by a low-capacity pump in the same time period." Although operating cost will increase, the additional cost is small compared with that of a single-head system. |
The quality, size, and quantity of abrasive used also influences cutting speed and cut-edge quality, Ward continues. The chart on the right illustrates that, without abrasive, cutting speed with various orifice/nozzle combinations is virtually nil for the 0.5-in.-thick 304 stainless. As abrasive (garnet in this case) is introduced, cutting speed increases. However, each orifice/nozzle combination has a limit for maximizing cutting speed. For example, as more abrasive is introduced in the 7/20, speed increases at first but then declines. Generally 80-mesh garnet is used, Ward adds, occasionally 50-mesh for rough cutting and 120-mesh for fine finishes. The 50-mesh cannot be used with very small orifice/nozzle combinations because the abrasive will lodge in the nozzle and block flow.
Straight-line cutting speeds at 50,000-psi pressure to achieve Q1 to Q5 finishes using two orifice/nozzle combinations appear in the table for four materials 0.125- to 2-in. thick.
