Linear Scales vs Rotary Encoders on CNC Axes
Share
A CNC control needs to know where each axis is. There are two places to measure it: at the motor, or at the axis itself.
Rotary encoders: semi-closed loop
Most machines measure position with an encoder on the servo motor. The control counts motor turns and multiplies by the ball screw lead to work out where the table is. It's simple, durable and cheap, and modern encoders resolve far finer than the machine can cut.
The weakness is everything between the motor and the table: coupling wind-up, ball screw lead error, backlash and, above all, heat. The encoder can't see any of it.
The heat problem
A steel ball screw grows about 12 µm per meter for each degree Celsius, and as an axis works, the screw, nut and support bearings warm up. A 600 mm screw that warms by 8 °C grows about 0.06 mm. With a motor encoder, that error goes straight into the part. Warm-up cycles, hollow cooled screws and thermal compensation reduce it.
Linear scales: fully closed loop
A linear scale is a precision glass or steel scale mounted along the axis, with a reading head on the moving part. The control closes the position loop on the scale, so screw growth, lead error and wear no longer decide where the table ends up. The scale's own accuracy does, which for good scales is a few microns over a meter.
- Benefits: accuracy that holds as the machine warms up, over long travels, and as the screws wear.
- Costs: the scales themselves, installation and protection. Scales need sealed housings and usually an air purge to keep coolant and chips out.
- Tuning: closing the loop at the table makes servo tuning more sensitive to backlash and friction in the drive.
When scales pay off
- Mold and die work with long cycles, where the machine warms up during the job.
- Large machines with long ball screws.
- Tight-tolerance production where the last part of the day must match the first.
For short-travel machines doing general work, a good encoder, a warm-up routine and regular backlash checks are often enough.
Retrofitting
Scales can be added to many machines, but the control has to support them and the axis needs room to mount them. Check both before buying.
See ball screw accuracy grades and signs of a worn ball screw.