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High Speed Lifters and Plasma Starting Cycles
High Speed Lifters
With the introduction of "high-speed" lifters, ESAB and several other competitors have begun offering plasma stations with lift speeds in excess of 500 inches per minute. The primary reason for faster lifters is to reduce cycle time, and therefore increase production rates. However, due to the short stroke of these lifters (typically 9 inches or less) this theoretical maximum speed is actually achieved for only a fraction of a second, and only during positioning moves at the beginning and end of a cut. Therefore, making the slide even faster would have a negligible effect on cycle time. For this reason, ESAB has focused on optimizing the entire plasma starting cycle, and not solely on theoretical lift speeds. It is important to note that ESAB, because of our vertical integration, is the only cutting machine manufacturer capable of effecting changes in every aspect of the plasma starting cycle.
The B3 Lifter
Over the past few months, ESAB has begun shipping machines using the new B3 Motorized Lifter on both High Current and Precision Plasma torch stations. Both of these stations use the OMNI Torch Protection Device for crash protection. The version of this station used for PT-600 and PT-15XL High Current Plasma systems is being called the "High Speed OMNI" station. It uses the OMNI device to sense initial height also. The version of this station used for 100 and 200 Amp Precision Plasma is called the "Precision OMNI" station. It uses the OMNI device for crash protection and initial height sensing, but is also equipped with an "Electrical Soft-Touch" system. The Electrical Soft-Touch uses electrical conductivity between the torch nozzle and the plate to sense the initial touch, and also detect when the torch clears the plate. Thus, it is a dual mode initial height sensor, using whichever input first senses the touch.
The basic principals of initial height sensing, voltage height control, and collision detection have been employed by ESAB for more than 15 years. However, along with using this proven technology, ESAB continues to introduce new innovations to increase productivity for our customers.
Factors Affecting Plasma Starting Cycle
ESAB engineers often use the Vision CNC'™s built-in Diagnostic Oscilloscope function to make actual measurements of plasma starting cycles. These are helpful indicators of actual cycle times, which have aided in the optimization process. Below are the oscilloscope outputs during three typical plasma starting cycles. These represent actual starting cycle times that customers can achieve during production cutting, and not just laboratory results.
Full Up Position
This situation represents the first pierce in a nest, when the lift is in the Full Up position. This Plasma Start Cycle includes an initial height sensing starting from the top of the lift. All subsequent pierces will start from a Partial Up Position, and thus be shorter.
The green signal is the Plasma Start (M65) from the program. This is the beginning of a Plasma Starting Cycle.
The yellow signal is the ARC-ON Signal input from the plasma system. This signals when the arc has started.
Total start cycle time = 1.79 seconds
Partial Up Position
This situation represents most typical pierces throughout a nest of large parts. The lifter will do a partial Master Up at the end of the cut, so the next initial height sequence will not have to drive as far down. Using the encoder feedback, the lift is raised by only 50 mm for a partial Master Up.
Green signal is the Plasma Start.
Yellow signal is the ARC-ON.
Total start cycle time = 1.27 seconds
IHS Skipping
This feature can be used when cutting small holes or parts where the distance between pierce points is 50 to 60 mm. The machine skips the Initial Height Sensing, and automatically return to the same height that was used for the previous start.
Green signal is the Plasma Start.
Yellow signal is the ARC-ON.
Total cycle time = 0.64 seconds
Conclusion
The optimizations discussed above have resulted in the shortest plasma starting cycles in the industry. When making repetitive cuts on small parts, piercing can occur in less than one second. Even in the worst case scenario, when starting from the Full Up Position, the pierce occurs in less than two seconds. Therefore, competitive arguments regarding higher lift speeds are no longer valid. From now on the argument must focus on complete plasma starting cycle time, and have to include everything that takes place between the plasma start signal and the arc on signal. When the argument is presented in this way, the advantage of ESAB'™s integrated systems will become clear.
With the introduction of "high-speed" lifters, ESAB and several other competitors have begun offering plasma stations with lift speeds in excess of 500 inches per minute. The primary reason for faster lifters is to reduce cycle time, and therefore increase production rates. However, due to the short stroke of these lifters (typically 9 inches or less) this theoretical maximum speed is actually achieved for only a fraction of a second, and only during positioning moves at the beginning and end of a cut. Therefore, making the slide even faster would have a negligible effect on cycle time. For this reason, ESAB has focused on optimizing the entire plasma starting cycle, and not solely on theoretical lift speeds. It is important to note that ESAB, because of our vertical integration, is the only cutting machine manufacturer capable of effecting changes in every aspect of the plasma starting cycle.
The B3 Lifter
Over the past few months, ESAB has begun shipping machines using the new B3 Motorized Lifter on both High Current and Precision Plasma torch stations. Both of these stations use the OMNI Torch Protection Device for crash protection. The version of this station used for PT-600 and PT-15XL High Current Plasma systems is being called the "High Speed OMNI" station. It uses the OMNI device to sense initial height also. The version of this station used for 100 and 200 Amp Precision Plasma is called the "Precision OMNI" station. It uses the OMNI device for crash protection and initial height sensing, but is also equipped with an "Electrical Soft-Touch" system. The Electrical Soft-Touch uses electrical conductivity between the torch nozzle and the plate to sense the initial touch, and also detect when the torch clears the plate. Thus, it is a dual mode initial height sensor, using whichever input first senses the touch.
The basic principals of initial height sensing, voltage height control, and collision detection have been employed by ESAB for more than 15 years. However, along with using this proven technology, ESAB continues to introduce new innovations to increase productivity for our customers.
Factors Affecting Plasma Starting Cycle
- Slowdown Distance - As previously discussed, the faster lift will reduce the time it takes to complete the initial height positioning movement, as well as the retract movement at the end of cut. However, lift speeds over 500 ipm have negligible additional benefit. Where added benefit can be gained is by keeping the lift moving at full speed for as much of the IHS movement as possible. ESAB’s Precision OMNI station uses an encoder to track the actual vertical position of the lift. By comparing the lift position to the known height of the table, and taking into account the known thickness of the plate, we can drive the lift down at full speed until it is within a few millimeters of the plate surface. Only then does the lift switch to low speed, in order to accurately sense the plate surface. This feature takes advantage of ESAB'™s SDP files to automatically set the Initial Height and the slowdown point based on plate thickness.
- Initial Height Sense Time - The time it takes to sense the plate and then retract to the proper Initial Height. Systems that use a flip-down probe may take longer if they have to retract first in order to make room for the probes. Systems that touch the plate with the torch can be inaccurate if they only use a timer to set the retract height. ESAB'™s Precision OMNI system uses an encoder to measure the actual retract height, which also allows faster retract speeds since the encoder ensures accuracy.
- Pre-flow Time - The length of time the system has to wait for the plasma to pre-flow gases before firing. With ESAB'™s integrated plasma system, this is taken care of during the IHS Cycle and/or positioning motion. Therefore, there is no additional pre-flow delay after the torch has found initial height.
- Plasma Firing Time - The time it takes to start the plasma arc. ESAB'™s integrated plasma system is fully controlled by the CNC, unlike competitive systems where the CNC simply issues a start command and then waits for an arc-on signal. This allows ESAB to simultaneously control the sequencing of both the height control and the plasma system, and also to optimize the arc starting sequence for minimum starting time. The result is that the plasma system starts faster, reducing cycle time and increasing productivity.
- Plasma Retract Distance - The distance the lifter retracts at the end of a cut. This is often a simple timer that sets how long the lifter runs up after the plasma shuts off. On ESAB'™s Precision OMNI, the retract distance is CNC controlled using the encoder. The typical retract distance is less than two inches. This can be customized using ESAB'™s SDP files to automatically set the optimum retract distance for the type of parts and material being cut. This shortens cycle time by reducing the length of time that the machine waits for the lift to travel up and down between cuts.
- In addition to optimizing the Plasma Start Cycle itself, ESAB can further reduce cycle time for some applications by skipping the IHS cycle completely. This feature can be used when cutting small holes or parts where the distance between consecutive pierces is very short. IHS Skipping can be activated by M-Code in the part program, or set up to automatically skip every other IHS cycle. This causes the machine to skip the Initial Height Sensing, and automatically return to the same height that was used for the previous start, using the encoder height.
ESAB engineers often use the Vision CNC'™s built-in Diagnostic Oscilloscope function to make actual measurements of plasma starting cycles. These are helpful indicators of actual cycle times, which have aided in the optimization process. Below are the oscilloscope outputs during three typical plasma starting cycles. These represent actual starting cycle times that customers can achieve during production cutting, and not just laboratory results.
Full Up Position
This situation represents the first pierce in a nest, when the lift is in the Full Up position. This Plasma Start Cycle includes an initial height sensing starting from the top of the lift. All subsequent pierces will start from a Partial Up Position, and thus be shorter.
The green signal is the Plasma Start (M65) from the program. This is the beginning of a Plasma Starting Cycle.
The yellow signal is the ARC-ON Signal input from the plasma system. This signals when the arc has started.
Total start cycle time = 1.79 seconds
Partial Up Position
This situation represents most typical pierces throughout a nest of large parts. The lifter will do a partial Master Up at the end of the cut, so the next initial height sequence will not have to drive as far down. Using the encoder feedback, the lift is raised by only 50 mm for a partial Master Up.
Green signal is the Plasma Start.
Yellow signal is the ARC-ON.
Total start cycle time = 1.27 seconds
IHS Skipping
This feature can be used when cutting small holes or parts where the distance between pierce points is 50 to 60 mm. The machine skips the Initial Height Sensing, and automatically return to the same height that was used for the previous start.
Green signal is the Plasma Start.
Yellow signal is the ARC-ON.
Total cycle time = 0.64 seconds
Conclusion
The optimizations discussed above have resulted in the shortest plasma starting cycles in the industry. When making repetitive cuts on small parts, piercing can occur in less than one second. Even in the worst case scenario, when starting from the Full Up Position, the pierce occurs in less than two seconds. Therefore, competitive arguments regarding higher lift speeds are no longer valid. From now on the argument must focus on complete plasma starting cycle time, and have to include everything that takes place between the plasma start signal and the arc on signal. When the argument is presented in this way, the advantage of ESAB'™s integrated systems will become clear.
