SAG – Slitter and cut-to-length lines
SAG uses roll forming machines with flying shear system
Introduction
The production speed creates various design problems regarding mechanics and movement control
Gefran studied for SAG a highly dynamic control system.
Roll forming is a continuous bending process in which a strip of metal, usually steel or aluminum from 0.6 to 5 mm thick, passes through a series of roll tools mounted on consecutive units. Each unit performs part of the bending until the required profile is obtained.
Roll forming machines are used to make open profiles, such as open square, C, and Z shapes and combinations thereof, or closed profiles, such as pipes that may be round, oval, or square.
At the end of the line, the profile is cut to measure by a flying shear.
The customer
SAG, based in the province of Milan, specializes in the construction of slitter and cut-to-length lines, and roll forming lines for the production of welded pipes, open profiles, and corrugated sheets.
The process
This roll forming line is used to work 0.5 to 3 mm thick strips to produce round profiles (diameter up to 50 mm) or square profiles (40 mm side).
The strip is first bent in the forming section, then goes to the fin pass, where the profile is closed with the coil edges side by side. It is then welded longitudinally to obtain a round pipe.
The calibration section reduces the diameter of the pipe, after which it goes to the Turkish heads, which modify the profile section in order to obtain square, rectangular, round, or oval pipes.
The pipe is then cut to measure by a flying shear, which produces profile sections in a continuous process.
The shearing unit is mounted on a carriage that moves on linear guides parallel to the pipe to be sheared. The carriage first waits at a homing point while the pipe feeds, pushed by the bending section.
At the right moment the carriage accelerates until reaching the same linear velocity as the pipe and the correct shearing position. When the carriage reaches the synchronism, the cutter motor moves a circular saw toward the pipe and cuts it.
The problem
The maximum production speed of 160 meters/minute for a cutting length of 6 meters in continuous operation – compared to a standard of about 100 meters/minute – creates various design problems regarding mechanics and movement control.
With regard to movement control, the biggest problems concern the flying shear system: high accelerations, and the system’s ability to synchronize the carriage accurately and rapidly with the profile to be cut, require a highly dynamic control system.
To avoid excessive stress and wear on mechanical parts, the control system has to provide very smooth movements and precise torque.
The solution
For a precise cut, the carriage must be in the right position and its velocity must be perfectly synchronized. Any difference in velocity generates imperfections in the cut, and may cause stretching or mechanical breakdowns: the flying shear control firmware is specifically designed to adjust the reciprocal position of the carriage and the pipe to be sheared.
The carriage velocity profile is set by an electronic cam based on line speed and shearing length.
Positive and negative accelerations can be set with linear and sinusoidal profile in order to choose the best compromise between reducing the maximum torque required by the motor and the wear of mechanical parts.
The system is simple to commission because the GF-eXpress configurator lets you work on the system parameters of the inverter as well as on the specific parameters of the application.
Use of the Softscope digital oscilloscope with synchronous sampling (sample time 1 ms) also allows simultaneous run-time check of drive and application variables for optimization of the control system.