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Simplified Chinese Setup Rotary Axis
There are two types of rotary axes commonly used on CNC machines. The maximum and minimum travel settings will differ depending on which type of rotary axis is being configured.
The first and most common type is a 4th axis, typically used on a mill. This is usually configured as the A axis and rotates the workpiece around a single axis, while the Z axis moves along the length of the workpiece in a manner similar to a lathe.
The second type is used as part of a 5-axis machine, where the A and B axes work together to position the workpiece in three-dimensional space for machining.
Rotary Axis Types
Rotary 4th Axis
Used on milling machines to rotate the workpiece around a single axis.
5-Axis Configuration
Uses both A and B axes to orient the workpiece in multiple directions for complex machining.
Rotary Axis Behaviour
Rotary axis settings are angular, not linear. Using extremely high step rates combined with large gear reductions does not improve accuracy and often results in a rotary axis that moves very slowly. Breaking one full rotation into an excessive number of steps offers no practical benefit.
For most applications, a value between 8000 and 10000 pulses per revolution is more than sufficient. Higher pulse counts are only beneficial when rotating very large diameters, such as workpieces one to two metres in diameter.
How to Calculate Rotary Axis Settings
Before configuring the rotary axis, gather the following information:
The number of motor steps per revolution, noting that most stepper motors are 1.8-degree motors with 200 steps per revolution.
The gear reduction ratio between the motor and the rotary axis output, using a ratio of 1 if no gearing is present.
The microstepping value configured on the motor drive.
The rotary axis settings are calculated as follows:
Motor Degrees per Revolution must be set to 360, as this value is fixed.
Drive Pulses per Revolution is calculated by multiplying motor steps per revolution by the gear reduction ratio and the microstepping value.
Maximum Feedrate must be set to a value suitable for the rotary axis and is measured in degrees per minute, not RPM.
Acceleration depends on the capability of the drives and must be determined through testing on the machine.
Maximum and Minimum Travel values depend on how the rotary axis is used. A mill-based 4th axis can use large values to allow continuous rotation, while a 5-axis machine may require limited travel to prevent contact with hard stops.
Backlash must be measured on the machine and entered accordingly.
My Rotary Axis Moves Slowly
Slow rotary axis movement is commonly caused by confusion between linear feedrates and rotary feedrates. Rotary axes move in degrees per minute, and the machine will always move at the speed of the slowest axis involved in the move.
A linear feedrate that appears reasonable can result in extremely slow rotary motion. For example, a feedrate of 10 inches per minute can take approximately 36 minutes for a rotary axis to complete a full rotation. This can give the impression that the axis is not moving at all.
Understanding rapid rotary motion behaviour is strongly recommended.
Additional resources
https://www.ganotechnologies.com/cnc/rapidrotary/
Homing Sensor
A homing sensor may be installed on the rotary axis to establish a home position. Homing sensors may be mechanical, optical, magnetic, or proximity types.
The homing input must normally display a Low state on the F1 screen and change to High when triggered. If the logic is reversed, the input logic can be toggled using the spacebar.
Hard Limits
Hard limits are not suitable for rotary axes because the axis passes the switch once every full revolution. If rotary axis travel needs to be restricted, maximum and minimum travel limits must be used instead.
