Cnc systems with a constant structure and system software implementation of algorithms
Absolute positioned system. A CNC drive system that retains its grid position even when turned off, then on. These machines feature linear feedback devices.
Ball-screw. A backlash compensating, linear drive system in which a split nut is forced laterally both directions against a screw. Balls roll through the circular channel between nut and screw.
Closed loop. A circuit that includes monitors to send a progress signal back to the CPU.
CPU – central processing unit The component that decodes the program, sends the drive commands to the servo motors, and monitors and adjusts progress.
Feedback. The signal returning from the drive system to the CPU.
Hard limits. Physical switches that verify location of an axis position.
Homing the machine. Sending all axes to limit switches in order to achieve a zero base for the encoders. Generally required only on older machines.
Initialization. Turning on the control, depending on sophistication, routines must be followed to prepare for machining. Newer machines load software and ready themselves; older units need more preparation.
Machine home. A never-changing position where the machine must be driven to refresh its feedback signal at initial start-up.
MCU. The master control unit.
Open loop. A budget drive system that does not feature any provision for feedback signal processing.
Servo error. An unwanted condition in a closed-loop system where the axis progress exceeds the tolerance from the expected result.
Servo motor. A highly controllable motor with predictable power, speed, and acceleration curves based on input energy.
Soft limits. Optical sensors that verify proximity to hard limit switches.
Stepper motor. A drive motor that moves a given part of a rotation, given one pulse of energy.
Five Axis Drive Components
Using five electrical/mechanical components, a CNC drive performs the same tasks as using muscles to move and position your arm. The controller can keep a cutter hugging the true programmed shape from within 0.001 or 0.0005 in. for heavy cuts down to an amazing 0.0002 in. on light loads, depending on machine construction and the nature of the shape and volume of metal being removed.
The control constantly compares physical tool position to the expected position. If the cutter’s progress doesn’t fall within a preset lag tolerance, the controller has one or two options, depending on basic architecture within its operating system.
More commonly, it can wait a preset time, until the physical cutter location catches up to the expected position before issuing another axis drive command. Second, on more powerful machines, the control can increase the axis drive force when the cutter enters heavier cuts and starts to lag behind, and it can back off on the axis drive as the cutter breaks into lighter work sections. The system performing these marvels is a closed-loop, linear, servo axis drive.
That mouthful is shortened to a CNC axis drive! It’s responsible for the cutting motions and for the positioning accuracy. Like your muscle movements, five components are required (Fig. 3.1):
CNC Drive |
Muscle Motion |
Controller unit |
Brain – processing in/outgoing data |
Axis drive relay card |
Synapses – triggering muscles |
Translation axis screw |
Tendons pulling |
Controllable drive motor |
The muscle |
Feedback device |
Sensory nerves reporting progress |
Figure 3.1 – A closed-loop CNC axis drive system.
Mechanical Axis Translation
The heart of the precision drive is mechanical: the backlash eliminating, recirculating ball-screw, and preloaded split nut. Its purpose is to move, position, and reverse axis motion with no backlash.
The CNC brain is sometimes called the master control unit (MCU), or the central processing unit (CPU). This component has four or five functions, depending on whether or not it is connected to an outside computer. For our study of axis systems, the controller’s CPU converts stored program commands or keyboard entries by the operator into drive commands, then sends them to the axis motor relays.
The CPU issues a low-energy command (usually 12 V DC) to relay devices that work as electricity amplifiers and switches. At the same time, it is also processing incoming feedback from the axis drive sensors to determine what action is required to maintain the programmed feed rate.
Axis Drive Relays. An individual CNC controller can be used to drive a variety of different CNC machines. To do so, the controller doesn’t directly command the motors to move.
Drive relays make it possible for a single control model to move any kind of motor, big or little, servo or stepper (studied next). One control is able to operate a small offload mill or it can be wired into a giant horizontal mill with over 100 hp per axis, because of the axis relays.
Wired between the controller and the motor, the intermediate relay card receives the low-energy control command, then amplifies it and configures it to the motor’s needs. At the relay, the control command triggers high-energy electrical current configured into a direct current stream or as pulses, depending on the kind of motor being driven.
Drive Motors – Servo or Stepper. Different from manually operated machines, that often use one motor to drive several functions through gears, belts and clutches, CNC drive motors are highly controllable and predictable: one motor per axis. Given an exact amount of input energy, they reliably move the same way each time. They accelerate and decelerate on an exact time/load curve.
There are two kinds of popular drive motors, servo motors and stepper motors. (A few machines such as precision grinders, use hydraulic motors powered by pumps but they are far less common.).
Direct Current Servo Motors – More Power for Industrial Machines. Electric servo motors are stronger but also more costly. They produce accurate RPM based on continuous input energy. As the input energy increases, servo motors spin faster or drive harder when pushing axis drives.
Steppers - Moving in Small Increments. Steppers are the second most common drive muscle. They are generally used in smaller machines. Steppers do just that, they move in tiny jumps rather than spinning as servos do. Grasping the shaft of an unmounted stepper and spinning it, you’d feel it click or pulse as the motor rotates through small magnetic arcs.
Using the CPU’s clock function to coordinate timed pulses to the drive relay, after reading the programmed feed rate, the controller determines how fast the pulses must be sent. The faster they are issued, the faster the motor rotates (up to its maximum speed).
Each pulse from the axis relay, rotates the motor an exact arc. For example, common steppers rotate through one full circle with 15, 30, or 45 pulses per revolution, depending on their construction. So the motor moves 240, 120, or 80 per pulse.
That, in turn, translates to an exact rotation of the ball-screw.