HEIDENHAIN: Accuracy of modern machine tools - full closed loop control compensates for thermal displacement errors

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The use of linear encoders in machine tools has a particularly outstanding effect.
Linear encoders in closed-loop control fully compensate for thermal expansion errors and other effects of the feed mechanism.
In particular, extremely high dimensional accuracy and contour accuracy can be achieved from the first part.

This demo will show you how closed loop control works and what it does.


This is a classic machine feed mechanism that includes:
• Motor
• Fixed bearing
• Recirculating ball screw
• and floating bearings.
The table is connected to the ball screw by a ball nut. It converts the rotational motion of the motor into a linear motion of the table.
The temperature of the ball screw will be measured.
Use a length gauge to determine the position repeatability of the workbench.
The counter shows the difference in position measured by the length gauge.
The linear encoder measures the position of the table in closed loop control.
Determine the different results of the accuracy in the following ways:
• Before working, the screw temperature is 20.8 °C.
• After 10 movements, we can see that the ball screw temperature at the table position increases.
• Although the temperature has increased by 2.1 degrees, the position measurement has not changed.
• No position error was detected by the length gauge after 10 movements.
This means that the linear encoder used on the feed axis maintains the exact position of the table.
In the indirect measurement of the semi-closed loop control, the table position is determined by the lead screw pitch and the rotary encoder.
• At this time, the screw temperature is 20.6 °C.
• The demo station now begins semi-closed loop control and clears the counter.
• The simulation starts and the table moves 10 times at 60 m/min.
• After 10 movements, the ball screw temperature increased by 2.1 degrees.

During the 10 movements, the temperature rise caused the table position to change by 6 μm.

This temperature effect can also be seen in the processing of an aerospace actual structural part.

This animation shows the machining process of the structural member. Under the semi-closed loop control, the heating during machining causes positional errors. The part has a dimensional tolerance of ±200 microns. During processing, the position error increases by more than 8 μm.
This positional error can be clearly seen in a small way: after machining 40 pieces, the first part is machined again, but the Z-axis feed is reduced by half. After the second machining under semi-closed loop control, the edges of 70 μm depth are clearly visible. However, the part is machined in a full closed-loop control under the same processing conditions, and the workpiece has no edges at all.

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