Linear encoders improve machine tool accuracy

Heidenhain has said that machine tools not fitted with linear encoders for positioning may not be accurate enough for the increasingly high performance demands made upon them.

At the UK’s MACH 2008 machine tool exhibition last week, Heidenhain said that the thermal behaviour of machine tools is an important consideration, given that conditions during production are constantly subject to change This is why the difference in machining accuracy between using machine tools equipped with linear encoders, which fully compensate for thermal expansion of ballscrews, and those not relying on linear encoders was a major feature of Heidenhain’s demonstrations at MACH 2008 .

Heidenhain told manufacturingtalk that the machining requirement within the aerospace environment, for example, typically demands a significant cutting capability to remove large volumes of material from a single large workpiece.

On the other hand, subsequent finishing operations demand the maximum precision.

At the other extreme the increasing demand for flexibility in small batch production required by the SMEs (small to medium enterprises) means that the accuracy of the first machined component (right first time) and subsequent repeatability is crucial to profitability.

* Linear encoders – Heidenhain said that machine tools not fitted with linear encoders for positioning may not be accurate enough for the increasingly high performance demands made upon them.

The additional cost of specifying precision linear encoders is very reasonable, reasoned Heidenhain, when compared with the overall cost of a major capital investment in a machine tool.

It is easy to estimate the value of the resultant benefits in terms of productivity, accuracy, profitability and ‘competitive edge’.

In practice, changes in ballscrew length as a result of thermal drift, typically 100 micron/m within 20 min, can introduce significant errors to components produced on a machine tool equipped only with rotary encoders.

The reason is changes in the driving mechanics owing to wear or temperature increase resulting from high speeds and rapid acceleration are not compensated for in a semi-closed loop mode of operation.

If a linear encoder is used for measurement of the slide position, potential errors in the transfer elements of the machine tool have a lesser influence on the accuracy of the position measurement, which is why Heidenhain said that workpieces with tight tolerances can only be produced on machine tools that remain thermally stable despite very different machining operations.

Sealed linear encoders, such as Heidenhain’s LC 100 series, are protected from dust, chips and fluid ingress and provide absolute position measurement for measuring lengths up to 4240mm.

The position value is available from the encoder immediately upon switch-on, so there is no requirement to move the machine’s axes in order to establish a reference position.

This feature is particularly useful should machining be resumed following an unscheduled shutdown, said Heidenhain.

These considerations apply for linear and for rotary axes as, in the latter case, the absolute rather than incremental position can be measured with a speed-reduction mechanism connected to a rotary encoder on the motor or with a highly accurate angle encoder on the machine axis.