Selecting a table for abrasive jet machining
Editor's Note: This article is the third in a series intended to help a prospective user evaluate abrasive jet machinery. The first article, Buying an abrasive jet machine, compared the abrasive jet process with other cutting processes. The second article, Software for abrasive waterjet machining, discussed software requirements. This article covers table construction and accessory hardware. The final article will discuss pumps.
Standard tables from 2 feet square up to 6 by 12 ft. are available, and custom tables of any size imaginable can be special-ordered from many manufacturers. Custom five-axis machines also are available for use in a variety of jet machining applications. Standard machines, basically XY tables, usually are used to cut parts from flat stock. This article discusses these standard tables.
Choosing a table size sometimes is easy because the parts being produced dictate the table size. Large parts need a large table. However, when making smaller parts, you should consider several factors to determine table size. If a whole sheet of material is to be cut into small parts, a table large enough to fit the stock size may be the best bet, but maybe not. On one hand, nesting on a single large sheet often can save material; on the other hand, loading and unloading a large machine is more difficult because of material weight and the operator's limited ability to reach across a large table.
If you are producing small batches of small parts from different materials (a task for which abrasive jets are particularly adept), you don't want to spend time loading and unloading partially cut, large sheets. A small table using stock sheared into , , or 1/8 sheet may be the most economic solution.
Business type can influence table selection. A job shop often will buy the largest table affordable to accommodate the surprise customer who wants an enormous part produced. Unless the job shop routinely produces large parts, this could be a costly strategy.
A shop with high production volume can buy a table large enough to fit two workstations so that one area can be loaded and unloaded while the other is cutting. A manufacturer can buy a table sized for its part or stock size without fear that a surprise big part will be needed.
No customer complains about consistent, precise parts. A job shop can service the widest range of customers with a high-precision table. High-precision tables can reduce or eliminate the need for secondary machining operations and lower total part cost. A manufacturer with a specific part or class of parts in mind may find the lower-cost, nonprecision table to be sufficient. Whatever your requirements, it is wise to know beforehand what you are purchasing
Manufacturers document the precision of their tables by several means. The ball bar test is one of the most relevant for predicting part accuracy, because the measurement is made very nearly at the location of the tool tip with the machine moving at its cutting speed. The ball bar test measures the errors in table travel as it traverses a commanded circle.
The measuring instrument is a slender, extensible bar with a precision length sensor that measures the length of the rod. At each end of the rod is a precision steel sphere (the ball) that rides in a cup. One cup is fixed to the machine at the center of the circle, and the other cup is mounted on the moving head of the machine. See Figure 1.
The table is commanded to move in a circle with the nominal radius of the bar. Variations from nominal length are sensed by the length sensor and recorded on a PC attached to the bar. The angular position around the circle is determined by the time after start of motion so that a plot of length variation versus position around the circle can be determined. Such a plot is shown in Figure 2.
The errors from a true circle are plotted as a function of the angular position of the bar. In the plot in Figure 2, very small errors are due to the fact that the position is commanded in 0.0005-in. steps all around the circle. A slightly larger error is at the points where the axes reverse direction. Note that the scale of the plot is greatly magnified to 0.0005 in. per division.
Several sources for the errors may show up as bar length variation. The sources can be divided into low-speed, or static, errors and high-speed, or dynamic, errors. A test run at low speed will show only the static errors.
Static errors may be caused by:
- Ball screw pitch errors
- Axis straightness and twist errors
- Squareness errors between the two axes
- Backlash in the mechanism
- Loose belts
- Flexibility in drive
Dynamic errors may be caused by:
- Axes vibration
- Servo following error caused by loads from
- High-pressure plumbing loads
- Direction reversal with a high integral gain
- Servo mismatch error
A single number representing the error found is the difference between the largest and smallest radius referred to as the circularity. The least part error you can expect from a table is the circularity. Beware of other measures of accuracy that may measure only static positioning accuracy or, at worst, only the lead error in the ball screws.
A second factor to consider is whether the reported accuracy of the table is the fundamental accuracy or the accuracy after error mapping and correction have been applied. Fundamental accuracy can be improved with software, whereas this step has already been taken in an error-mapped table.
Small tables may not require a foundation for accuracy, because it is possible to build a structure that is stiff over the length of the machine. Large tables are somewhat flexible, and the floor is an important element of the structure. Large tables (4 ft. by 8 ft. and larger) often are grouted or shimmed to the floor. For maximum precision, a thick, stiff foundation should be poured.
Sealing and Protection
The abrasive jet process, which includes large quantities of water and abrasive, is not friendly toward machinery. These elements can damage the abrasive jet machine and other nearby machinery. The best protection for the table axes are bellows that completely surround the linear bearings and ball screws and are sealed at the ends. But these bellows are impractical for very long axes, especially when the axes must be supported at one or more midpoints. In this case, look for a rigid enclosure with a downward-facing lip seal that is opened by passage of the carriage.
A worse configuration is U-shaped bellows sealed by their weight on a surface. Such bellows always have garnet under them, which is often blown under by a conscientious operator cleaning his machine with a blowgun.
Underwater cutting is a simple strategy for protecting the other machinery in your shop. With underwater cutting, the contamination is similar to that caused by grinding machinery. If cutting underwater is not possible, consider putting all of your abrasive jet machines in a common room isolated from the rest of the shop.
A machine can be designed in multiple ways to move a nozzle around a workpiece. Some of these are shown in Figure 3.
Figure 3ashows a moving beam table in which the nozzle is affixed to the front of a beam that moves in and out across the tank. This design provides excellent access to the tank for material loading, but an area equal to the tank size is required behind the table to clear the back of the beam. The required floor size is double that of the machine design shown in Figure 3b, and for that reason, the moving beam configuration is used for small machines only.
Figure 3b's cantilever design provides almost the same access to the tank as the moving beam design with half the footprint, and it is suitable for even very large machines. The cantilever design provides access for working on plates larger than the table, and when the back beam is supported only at the two ends as shown, a long-plate workpiece can be fed under the beam.
Figure 3cshows a structure in which all of the mechanism is above the operator's head. It provides excellent operator access to the tank for removing cut parts by hand. Material can be loaded from the front with either a fork truck or overhead crane. This design also provides plenty of space for additional tilt axes and often is used for building full five-axis tables.
The major disadvantage of this design is that the long Z axis makes the nozzle position sensitive to twist and bending errors in the overhead beams. Also, the long structural path between the table bearings and workpiece provides opportunity for errors caused by machine deflection.
Figure 3dshows what is perhaps the most inherently accurate construction. The bearings are very close to the plane of the work piece minimizing the errors discussed previously. Access to the tank is equal for material loading, but not quite as good for manual unloading.
Here are some factors to consider when choosing a machine type:
- Can the material easily be loaded with a fork truck or overhead crane? Do enclosures, guards, or machine structures interfere?
- Can the table accommodate work on a sheet larger than the machine tank?
- Are the bearings and ball screws well-protected from water and grit? Can an operator blow grit into the bearings?
- Is the Z axis relatively short?
- Can underwater cutting be done?
Many accessories are available for abrasive jet machines. You likely would purchase some of them with the machine, but may later find that useful new attachments have become available or that a new project requires additional accessories. It then becomes important that your table be compatible with the desired accessory. Some tables are designed to accept a range of accessories; others are not. Some available accessories include:
- Programmable Z axis for following materials that are not flat.
- Automatic drill heads for piercing materials that would otherwise delaminate.
- Edge- and hole-finding devices for accurately referencing existing parts to the machine.
- Tilting head for controlling taper in the edge of the part.
- Joystick or pendant for moving the machine from a distance from the control.
- Automatic garnet removal system for keeping the tank clean.
- Garnet recycle system for reclaiming a fraction of the spent garnet.
- Water recycle system for cases in which draining to municipal systems is prohibited.
- Various fixturing and clamping devices.
- Rotary axes for tube cutting and production of small, 3-D parts.
Remember, when choosing a table size and design, consider your type of work. Consider part size, stock size, precision, run quantity, and material handling.