CNC automatically compensates for thermal effects

Machine tool builder has introduced a system for predicting thermal movement and automatic compensation within the CNC of machining centres and lathes, without operator commands.

Humans have a finely tuned, autonomic response that keeps body temperature constant, without the person being aware of it.

The Japanese machine tool manufacturer, Okuma, has sought to emulate nature by incorporating into its latest machining centres and lathes a new system for predicting thermal movement and compensating for it automatically within the CNC, without the need for commands from the operator.

NCMT, the UK agent for these machines, describes the measures that have been taken.

Starting point for the new technology, which has patents pending, is to avoid the expense of a temperature controlled room and assume that the machine will be used in a general factory environment subject to temperature change of up to 8C.

The premise is that the machine structure and spindle will undoubtedly expand and contract, even with cooling, and therefore they must be designed so that thermal displacement is minimal, symmetrical and predictable, allowing compensation to be applied accurately.

The goal was to hold displacement under 10 microns in all linear axes, despite repeated temperature changes up to 8 deg C.

Okuma has achieved this on its vertical and horizontal machining centres incorporating the company's so-called thermo active stabiliser (TAS), there being one type for the spindle (TAS-S) and another for the overall machine construction (TAS-C).

Similar results have been obtained on the company's horizontal- and vertical-spindle turning centres, including the MacTurn lathes with B-axis and tool changer.

A number of factors in modern-day manufacturing have combined to make thermal compensation more important.

Spindle speeds are faster, up to 25,000 rev/min on Okuma machines, and feed rates are higher - even the manufacturer's ballscrew slideways are achieving 90m/min - so more heat is generated within the machine.

This is in addition to the heat transmitted to the machine structure by swarf, coolant and perpheral items such as the electrical control cabinet.

At the same time, components are becoming more complex and drawing tolerances are ever tighter, so the need for high accuracy machining has never been greater.

Every operator has been inconvenienced by thermal instability in a machine tool, never being quite sure how long it will take to warm up, or taking a year or more to learn exactly how the machine is going to behave during the day.

So manually-applied compensation has frequently been needed for high accuracy machining.

Okuma's simple, inexpensive solution has been not only to make their machines thermally symmetrical, but also to position heat sensors at strategic points around the machine and feed back temperature information to the control.

Here the information is interpreted into linear displacement figures for the three orthogonal axes, based on a calibration map created by Okuma during three months' temperature cycling and testing of each machine in its range.

Temperature tracking delays are factored into the equation and the increment of compensation has been reduced 10-fold from the typical one-micron step to 0.1 micron.

The control is therefore able to adjust the program continuously in real time to take account of the predictable movement in each linear axis, at the same time employing conventional cooling of ballscrews and spindle, according to model, to help keep the temperature stable.

Even the cooling system pipework is symmetrical.

It is the combination of temperature sensing and cooling of a thermally symmetrical spindle and box-type structure, whose expansions and contractions are predictable, together with the application of correct, real-time compensation, that is at the heart of Okuma's breakthrough.

The technique is more accurate and responsive than column cooling alone and has the additional advantage of reducing power consumption and maintenance of the coolant pump and pipework.

Looking at the spindle in more detail, heat generated by the bearings may be reduced by cooling, but not eliminated.

So to keep thermo-symmetry, the oil-air coolant nozzles are equi-spaced around the outside and uniform temperature distribution is maintained within the head using twin cooling jackets, one around the spindle bearing and another around the motor.

Okuma has added an extra, non thermally related compensation technology, also patented, to the spindle whereby growth of BT tapers owing to centrifugal force at high speeds is compensated for automatically.

(HSK and other face-and-taper tooling are not so affected by this problem).

Insofar as machine structure is concerned, conventional measures adopted by other manufacturers for heat management have included using materials with a low coefficient of thermal expansion, applying heat insulating materials and circulating coolant to minimise temperature change.

Although these measures have resulted in a certain degree of improvement in accuracy, none has been completely satisfactory.

The problems of cost and the effect of the time lag in heat exchange between the casting and coolant remain.

Okuma, on the other hand, has developed a simple, block stacking construction for the column in which the right and left sides have roughly equal mass.

With this balanced assembly, the machine grows and shrinks predictably as the ambient temperature changes.

Similarly, the front and back of the machine move symmetrically and predictably as temperature changes, by virtue of a swarf shield on the front of the column to prevent temperature transfer and careful positioning of the electrical cabinet at the rear.

Both are isolated from the machine frame to prevent direct heat transmission.

As a postscript, NCMT points out that Okuma is uniquely placed in the machine tool sector to effect an integrated approach to minimising and managing thermal displacement, and then compensating for what movement there is, by virtue of the manufacturer producing not only the machine, but also the control system, spindle, servo drives, encoders and linear scales.

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