To achieve CNC and automation of rebar straightening machines and threaded rebar straightening machines, accurate measurement of rebar length and automatic cutting of rebar to the required length are essential. This paper studies these two issues and achieves satisfactory results on a rebar straightening and cutting machine produced by our school's machinery factory. The relative error in cutting rebar is less than 0.5%, overcoming the problem that the original equipment could not be used for straightening and cutting small-diameter rebar. The device features automatic length detection, automatic synchronous movement of the lower cutter table, and automatic counting of the number of rebars cut.
1. Working principle of steel bar straightening and shearing
Figure 1 shows the mechanical transmission principle diagram of the rebar straightening and cutting machine. The rebar 1 to be straightened is straightened in the rotating straightening cylinder 3 under the drive of the feeding roller 2 and the traction roller 4. The straightened rebar moves forward through the ports of the upper and lower cutter tables. When the front end of the straightened rebar has not reached the stop 7, although the hammer head 8 moves up and down reciprocating under the drive of the inertia wheel 9, it does not contact the upper cutter table 10, so the rebar is not sheared. When the front end of the straightened rebar reaches the stop 7, the upper cutter table 10 and the lower cutter table 5 move forward through the pull rod 6. At this time, the reciprocating hammer head 8 will shear the rebar (the length of the sheared rebar is L) through the upper and lower cutter tables, completing the straightening and shearing of one rebar. Continuous movement can complete continuous rebar straightening and shearing.
2. Inspection of rebar length
The working environment of a rebar straightening and cutting machine is harsh, with significant vibration during operation. When inspecting rebar dimensions, in addition to ensuring accuracy, it is also essential to meet requirements such as strong anti-interference capability, reliable operation, and long service life. Existing rebar straightening and cutting machines use contact measurement for dimension inspection, where the rebar moves, driving an elastic friction wheel to rotate. The friction wheel is then connected to a pulse generator, and the rebar length is calculated from the pulse generator's recorded values. Due to the intense vibration of the machine and the rebar during operation, the friction wheel experiences severe wear. Failure to replace the friction wheel regularly will affect the inspection accuracy and cause cutting errors. For these reasons, a non-contact inspection method must be adopted for rebar length inspection. Through comparative experiments, a scheme that alternately applies magnetic signals to the rebar proved feasible and met the design requirements.
Figure 2 shows the electrical working principle diagram of the rebar length detection and rebar cutting control. Control parameters are input to the microcontroller via the keyboard, and the digital tube displays the set parameters and machine operating status (including the number of rebars cut).
To achieve non-contact rebar length detection, an excitation head and a magnetic signal reader are installed at the exit of the tool holder to enable non-contact detection. At the start of operation, the microcontroller outputs a high level on port P1.0, driving the excitation amplifier. The current output by the excitation amplifier generates a magnetic field on the excitation head. This magnetic field magnetizes the straightened rebar into a magnetic signal with an S polarity. When the rebar with the magnetic mark moves to the position of the reading head, an induced pulse voltage is generated on the reading head. This pulse voltage signal is amplified and input to port P3.0 of the microcontroller. When the microcontroller detects a high level on port P3.0, port P1.0 outputs a low level. This means the excitation amplifier changes the current direction of the magnetic head, magnetizing the rebar into a magnetic signal with an N polarity. Thus, for each change in the magnetization direction of the reinforcing bar detected by the reading magnetic head, the polarity of the excitation magnetic head also changes accordingly, and the counter automatically increments by one. As the reinforcing bar moves forward, it carries a continuous magnetization signal with alternating magnetic polarities. The length of one polarity magnetization region is the distance between the excitation magnetic head and the reading magnetic head. From the figure, the length of the cut reinforcing bar can be calculated as: L = (N+1)BA (1)
In the formula, N represents the counter value, B represents the distance between the excitation head and the reading head, and A represents the distance between the cutter head and the excitation head.
The distance A between the cutter head and the excitation head is fixed, while the distance B between the reading head and the excitation head is adjusted according to the shearing length of the reinforcing bar.
3. Principle of Cutter Speed Tracking and Rebar Shearing Control
When the microcontroller calculates that the straightened steel bar has reached the specified length using formula (1), the pull rod needs to be activated to move the upper and lower cutter tables. Existing CNC steel bar straightening machines use the electromagnet's attraction action to move the pull rod. This method has two characteristics: (1) When the steel bar moves at a high speed and the steel bar to be cut is short, the electromagnet needs to be activated frequently, and the coil is very easy to overheat and burn out the electromagnet. (2) The hammer position is uncertain, resulting in the cut steel bar length exceeding the tolerance. Therefore, the movement of the cutter table cannot be controlled by the action of the electromagnet.
Stepper motors have advantages such as high positioning accuracy and easy speed control. If a continuously rotating stepper motor is used, the elastic lever on the output shaft can be pushed at a constant speed, ensuring precise cutting of the rebar to the required length. Figure 3 shows the transmission principle diagram. The hammer head moves up and down driven by an inertia wheel. When the rebar travels the required cutting distance, the hammer head may not be at its lower limit position. If the cutting head moves at the same speed as the rebar (equivalent to the cutting head gripping the rebar), cutting the rebar when the hammer head is at its lower limit position ensures precise cutting. Assume that within one cycle of the hammer head's movement, the maximum distance S of the lever satisfies...
