December 10, 2012
Once custom-produced on a one-off basis for specific applications, large abrasive waterjet cutting tables have become part of many standard systems, thanks mostly to advanced linear-drive technology that makes the machines more affordable.
The abrasive waterjet (AWJ) cutting industry is experiencing a noteworthy trend toward larger and larger cutting tables. Very large cutting tables have been around for quite a while, but they have been expensive, custom-engineered products designed for specific applications and produced on a one-off basis. Standard, affordable, off-the-shelf cutting systems for use in machine shops and fabricating shops generally were available in the size range of 5 ft. by 10 ft. (1.5 m by 3.0 m) or smaller. However, in the last few years, table sizes available on standard machines have increased dramatically and now are readily available in sizes up to 13 ft. by 46 ft. (4 m by 14 m) at noncustom prices (Figure 1). This trend is driven by a combination of user demand and advanced linear-drive technology.
Fabricating shops of all sizes see the benefits of larger cutting tables. First is their ability to make larger parts in a single setup from very large panels of material stock. In addition, features can be cut into previously fabricated large panels, such as the window and door openings in a long side panel for a rapid-transit railcar.
For shops that produce large quantities of smaller parts, multiple large sheets of material stock can be placed on the table so that parts can be cut from one sheet while being removed from another, thus providing virtually continuous production.
Finally, large tables with an appropriate linear-drive system can be equipped with dual independently controlled bridges (Figure 2) to simultaneously cut two different parts from two different material thicknesses. This provides the manufacturing flexibility of two independent cutting tables at lower capital cost and with less required floor space.
In the past the benefits of a large cutting table were not enough to justify the capital cost of an expensive, custom-engineered and -manufactured system. The key limiting factor in meeting fabricators' demand for larger tables at a reasonable cost was the high cost of long, accurate linear-drive systems.
Very long linear positioning systems using traditional ball screw, rack-and-pinion, or belt drives could not meet shops' requirements for part accuracy without a substantial increase in cost compared to smaller cutting tables. However, this situation changed with the introduction of extremely precise noncontact linear encoders. The availability of such encoders in turn led to the development of linear-drive systems meeting the specific needs of AWJ cutting tables as shown in Figure 3 and discussed in detail in my article in the December 2009 issue of thefabricator.com. Besides offering the smooth, precise linear motion control required by an AWJ cutting table, these linear drives have the unique ability to be extended in length easily at modest incremental cost. The result has been entire families of X-Y tables on which the X-axis length can be extended in increments to meet the shop's requirements—essentially custom-length tables at a standard system price.
Developing very large tables for AWJ cutting requires several key engineering considerations. For structural reasons these tables all tend to be gantry systems with the gantry bridge spanning the shorter axis. Because AWJ cutting requires very little side load on the cutting nozzle, the gantry bridge does not need to be designed to handle the substantial side-force requirements of a traditional machine tool, which means that a structurally massive bridge is not required. Although nice from a cost standpoint, not needing a massive bridge brings its own design challenges. The AWJ cutting stream is extremely sensitive to vibration at the nozzle. Thus, careful attention must be paid to the gantry bridge's design to control vibration over the full range of cutting speeds.
An additional design consideration arises from the inevitable long distance between the high-pressure pump and the cutting nozzle for these very large tables. Using a simple small-diameter, high-pressure tubing whip from pump to nozzle just isn't practical when the nozzle has more than 46 ft. of X-axis travel. Not only are there mechanical support and safety issues for the whip, but the fluid pressure loss in such a long run of small diameter tubing can be substantial.
Pressure loss is particularly large for 90-KSI systems because the heavier wall thickness required for this tubing substantially reduces its inside diameter. If the inside diameter is cut in half, the cross-sectional area of the water passage is reduced by a factor of 4, and pressure drop in the tubing goes up dramatically. Tubing pressure losses reduce the effective power of the abrasive stream at the nozzle, and reduced power means reduced cutting speed. A scissor-style linkage system of larger-diameter 60-KSI tubing and large-passage, low-pressure-drop swivels results in much less power loss between the pump and the nozzle. This means faster cutting, lower maintenance costs, and greater system reliability.
It is not clear how far this trend toward larger cutting tables will go. However, with the availability of easily expanded, precision linear-drive systems, that decision clearly is in the fabricators' hands. If they need and want even larger cutting tables, the AWJ industry is ready to provide them.