Linear digital scale encoders increase accuracy

Edited by the Engineeringtalk editorial team Sep 3, 2001

The change from rotary to linear digital scale encoder technology on Cincinnati VMC vertical machining centres and Hawk TC CNC lathes has improved positioning accuracies by almost 60%

The change from rotary to linear digital scale encoder positioning technology on the Cincinnati range of VMC vertical machining centres and Hawk TC CNC lathes has improved positioning accuracies by almost 60 per cent.

Not only has this development led to highly consistent machining, but it has also enabled Cincinnati Machine UK to lay claim that its highly competitive price/performance ratio machines are now in a class of their own in world markets.

The linear scales form part of a control and drives package supplied by Heidenhain (GB) of Burgess Hill, that creates a closed-loop positioning system.

The Heidenhain package is also connected into electronic thermal compensation that monitors changes in the machine structure and provides instant feed back of relative data to the machines' Heidenhain TNC 426 and TNC 410 contouring control systems.

According to Pete Robbins, Sales Product Engineer at Cincinnati Machine's Birmingham build operations: "The fitting of linear scales to Hawk CNC lathes was a milestone.

It made us the first major machine tool builder to provide such high levels of cylindrical turning accuracy on a volume built production lathe.

This gave us an immediate competitive edge throughout the world, from the cold of Alaska to the heat of Arizona." The advantage of linear scale technology is that it accommodates most mechanical transmission errors in the drive system of the machine tool, the effects of temperature changes, friction and any variation in slide positioning.

Rotary encoders are less expensive but have limitations because they are mounted with the motor and are detached from the cutting zone.

While rotary encoders are perfectly acceptable for most turning operations, they will only provide indirect control of the linear position of the slide, relative to the position of the rotor in the servo motor.

However, as axis speeds rise, problems arise from this arrangement which cannot account for any increased heat generation in the ballscrew and effects from thermal distortion that will occur.

The Heidenhain LS 176C encapsulated linear scales, which are tolerant of high vibration, are now fitted as standard on the X and Y-axes and as an option on the vertical Z-axis of the Arrow and Lancer machining centres.

Meanwhile, the Heidenhain LS 476C linear scale is fitted to the X-axis of the Hawk lathe to provide high accuracy repeatability and positional stability when performing cylindrical turning and boring.

Heidenhain is regarded by Cincinnati as the 'industry standard' where high accuracy is required.

Its scales provide consistently high levels of linear and contouring accuracy, dramatically reducing the need for extended warm-up cycles and possible reject workpieces.

Jeremy Bridgwater, Cincinnati Machine's Product Development Supervisor, explains: "Linear scales eliminate the cold start error effect on component sizing.

By the time the machine axes have reached thermal stability, expansion of the ball screw can be as much as 0.1 to 0.12 mm over a 1,000 mm of travel.

Even after a short break such as lunch, there can be quite an aggressive cool down and warm up of the ballscrew when machine recommences causing inaccuracies of 50 to 70 microns." Linear scales are particularly advantageous during long machining cycles, such as when producing large moulds on a machining centre.

Here, thermal expansion can cause significant changes in machining accuracy and surface finish.

Cincinnati's development team also maintains that the mounting position of the scales on the machine structure is important.

Because Cincinnati has been able to design-in the mount position, a positional accuracy of three microns is now achievable compared to seven microns when rotary encoder feedback was used under stable thermal conditions.

But, as Pete Robins points out: "Lathes are even being bought to hard turn and replace grinding machines which means users now demand ever-greater stability and tolerance holding capability." In concluding, Cincinnati reckons that leading the pack of machine builders in adopting Heidenhain linear scales to eliminate the effects of thermal expansion and to influence higher positional accuracy has improved customer confidence leading to repeat orders in many instances.