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X-Y Table Designs

• Integrated table/gantry
• Integrated table/cantilever
• Floor-mounted gantry systems with separate cutting tables
• Floor-mounted cantilever systems with separate cutting tables
• Features common to all quality systems

In order to make precision parts, an abrasivejet system must have a precision X-Y table and motion control system. Tables fall into four general categories:

1. Integrated table/gantry systems
2. Integrated table/cantilever systems
3. Floor-mounted gantry systems with separate cutting tables
4. Floor-mounted cantilever systems with separate cutting tables

Each type of system has its benefits and drawbacks.

Integrated table/gantry system

The integrated table/gantry system uses, guides for the gantry beam that are integrated into the cutting table. Because of this the X-Y motion system and the material support table are part of the same overall structure any unwanted relative motion between them is eliminated. In this type of system, the floor is not a vital part of the system structure. This system is typically more accurate than the more traditional separate gantry and table.




OMAX Model 60120


Advantages of this system:

• Inherently better dynamic accuracy (and thus potentially better parts accuracy) than the separate gantry system because the floor is not part of the structure and relative unwanted motion or vibration between the table and X-Y structure is eliminated
  
• System accuracy can be built at the factory and does not require extensive on-site set-up and alignment
  
• Less floor space is required for a given table size because the external support frame is eliminated
• Well-adapted to the use of multiple nozzles for large production runs

Drawbacks to this system:

• More expensive to build than the traditional separate frame system
  The table/tank structure must be fairly complex to insure precision alignment of the gantry rails
  
• Loading material onto the table can be difficult because the gantry beam may interfere, unless the gantry can be moved completely out of the way, as in OMAX designs
  
• Because the gantry beam is moved at both ends, a very high-quality electronic or mechanical system must be employed to ensure that both ends move precisely in unison or accuracy will suffer
  
• Careful attention must be paid to control of vibration in the gantry beam or dynamic accuracy will suffer

Integrated table/cantilever system

The integrated table/cantilver system employs a fixed X-axis that is structurally integrated with the support system used to hold the material being cut. The cantilever Y-axis is then attached to an X-axis carriage.





An integrated table/cantilever system


Advantages of the table/cantilever system:

• Inherently better dynamic accuracy (and thus potentially better parts accuracy) than the separate gantry or cantilever system because the floor is not part of the structure and relative unwanted motion or vibration between the table and X-Y structure is eliminated
  
• System accuracy can be built at the factory and does not require extensive on-site set-up and alignment
• Less floor space is required for a given table size because the external support frame is eliminated
  
• Better access to the cutting table because the external frame or rails of a gantry system are eliminated
  
• Eliminates the need for a precision dual-drive system required by a gantry beam - the cantilever beam needs to move at only one end

Drawbacks to the integrated table/cantilever system:

• May be more expensive to build because the integrated table/tank structure may be more complex
• Y-axis is limited in length to under five feet because of structural and vibration considerations
• Cantilever beam must be carefully designed to control vibration or dynamic positioning accuracy will suffer. On larger systems, a vibration damping system on the Y-axis is almost required. It's critical that this type of system be evaluated for dynamic accuracy using a ball bar check or some other dynamic measurement. The system may have excellent static accuracy but make very inaccurate parts due to vibration if not designed properly.
• Not as well-suited to multiple-nozzle applications as a gantry beam, but multiple-nozzle configurations are possible

Floor-mounted gantry with separate cutting table

A floor-mounted gantry with a separate cutting table is the most common approach used by waterjet system manufacturers. A framework that supports the X-Y motion system is secured directly to the floor and straddles a separate cutting table and catcher tank. The nozzle(s) is mounted to a carriage which moves along a gantry beam that straddles the table. The gantry beam is supported on each end by a guide system and is moved by ball screws, rack and pinion assemblies or drive belts located at each end. The parallel drive mechanisms are either operated by two electronically-coupled drive motors or by a single motor driving a mechanically-coupled drive system.




Floor-mounted gantry system


Advantages of the floor-mounted gantry with separate cutting table:

• Easy to manufacture a wide range of sizes using a modular approach
  
• Well-adapted to the use of multiple nozzles for large production runs
  
• The table/tank structure can be structurally simple
  
• Well-adapted to special table configurations needed for rapid material handling (e.g., shuttle changers) typical of production operations

Drawbacks to the floor-mounted gantry with separate cutting table:

• Requires extremely careful on-site setup by skilled technicians in order to assure that the cutting table and the X-Y structure are aligned
  
• Parts accuracy is limited because of the potential for relative motion and vibration between the cutting table and the separate X-Y structure (floor vibration in particular can be a problem)
  
• The overall system accuracy will be affected by any long-term movement of the floor; alignment needs to be checked periodically
  
• Because the gantry beam is moved at both ends, a very high-quality electronic or mechanical system must be used to ensure that both ends move precisely in unison; otherwise, accuracy will suffer
  
• Careful attention must be given to analyzing and properly controlling vibration modes in the table, gantry beam, and X-Y support structure. It's critical that this type of system be evaluated for dynamic accuracy using a ball bar check at full cutting speed or some other dynamic measurement; the system may have excellent static accuracy but make very inaccurate parts due to vibrations if not designed properly..

Floor-mounted cantilever system with separate cutting table

This type of system uses a floor-mounted X-axis and a cantilevered Y-axis mounted to the X-axis carriage. The nozzle mounts to a carriage on the Y-axis. The cutting table is totally separate from the X-Y motion structure.




Floor-mounted cantilever system


Advantages of the floor-mounted cantilever system with separate cutting table:

• Better access to the cutting table because the external frame or rails of a gantry system are eliminated
• Eliminates the need for a precision dual-drive system required by a gantry beam; the cantilever beam needs to moved at only one end
• The separate table/tank can be structurally simple and low in cost

Drawbacks to this system:

• Y-axis is limited in length because of structural and vibration considerations
  Cantilever beam must be carefully designed to control vibration or dynamic positioning accuracy will suffer. On larger systems a vibration damping system on the Y-axis is almost a must. It's critical that this type of system be evaluated for dynamic accuracy using a ball bar check or some other dynamic measurement. The system may have excellent static accuracy but make very inaccurate parts due to vibrations.
• Requires careful on-site setup by skilled technicians who can assure that the cutting table and the X-Y structure are aligned
• Parts accuracy is limited because of the potential for relative motion and vibration between the cutting table and the separate X-Y structure. (floor vibration in particular can be a problem)
• The overall system accuracy will be affected by any long-term movement of the floor; alignment needs to be checked periodically
• Not as well-suited to multiple-nozzle applications as a gantry beam, but multiple nozzle configurations are possible

Features common to all quality systems

Regardless of the system type, some features should be common to all quality systems:

• Precise X-Y motion with servo-driven precision-ground, zero backlash ball screws and precision-ground machine-tool quality linear guides or motion systems using precise linear encoders
• Careful attention to vibration control during design
• A pump module that is completely separate from the X-Y table and motion control system to ensure that no pump vibrations are transmitted to the table or nozzle.
• Manufacturer's documentation of dynamic accuracy and repeatability via a Renishaw Ball-Bar Test or equivalent. At least four widely-spaced locations on the X-Y table should be tested at a nozzle speed of at least 100 inches per minute.
• Easily replaceable material support slats.
• Fully sealed X-Y guides and ball screws; bellows should not have horizontal pleats that can collect dirt and debris.
• Provision for rapidly raising and lowering tank water level to permit cutting material underwater, thus minimizing noise and dust.