Getting the best accuracy out of machine tools

Thermal accuracy of machine tools is becoming increasingly important considering the strong varying operating conditions in manufacturing. Especially with small production batches that require constantly changing machining tasks, a thermally stable condition can’t be reached. At the same time, the accuracy of the first workpiece is becoming very important for the profitability of production orders. Constant changes between drilling, roughing and finishing operations contribute to the fluctuations in the thermal condition of a machine tool. During the roughing operations, the milling rates increase to values above 80%, whereas values below 10% are reached during finishing operations.The increasingly high accelerations and feed rates cause heating of the recirculating ball screw in linear feed drives. Position measurement in the feed drives therefore plays a central role in stabilising the thermal behaviour of machine tools.

Thermal stability of machine tools

Solutions for avoiding thermally induced dimensional deviations of workpieces have become more crucial than ever for the machine tool building industry. Active cooling, symmetrically designed machine structures and temperature measurements are already common practice.

Thermal drift is primarily caused by recirculating ball screws on feed axes. The temperature distribution along the ball screw can rapidly change as a result of the feed rates and the moving forces. On machine tools without linear encoders, the resulting changes in length (typically: 100μm/m within 20 min.) can cause significant flaws in the workpiece.

Position measurement on feed drives
The position of an NC feed axis can be measured in principle through the ball screw in combination with a rotary encoder or through a linear encoder.

If the slide position is determined from the pitch of the feed screw and a rotary encoder, then the ball screw must perform two tasks: As the drive system it must transfer large forces, but as the measuring device its expected to provide highly accurate values and to reproduce the screw pitch. However, the position control loop only includes the rotary encoder.

Because changes in the driving mechanics due to wear or temperature cannot be compensated, this is called semiclosed-loop operation. Positioning errors of the drives become unavoidable and can have a considerable influence on the quality of workpieces.

If a linear encoder is used for measurement of the slide position, the position control loop includes the complete feed mechanics. This is therefore referred to as closed-loop operation. Play and inaccuracies in the transfer elements of the machine have no influence on the accuracy of the position measurement. Measurement accuracy depends almost solely on the precision and installation location of the linear encoder.

This basic consideration applies both for linear axes and rotary axes, where the position can be measured with a speedreduction mechanism connected to a rotary encoder on the motor, or with a highly accurate angle encoder on the machine axis.

Using angle encoders produces significantly higher accuracy grades and reproducibility.

Summary
The successful fulfilment of manufacturing orders requires machine tools with high thermal stability. Machine accuracy must be maintained even under strongly varying load conditions. As a consequence, feed axes must achieve the required accuracy over the complete traverse range even with strongly varying speeds and machining forces. Thermal expansion in the recirculating ball screws of the linear feed axes adversely affects accuracy and varies depending on the velocity and load.

Position errors of 100 μm and more may result within 20New Balance