Method and apparatus for a displacement correction for a machine tool

The object of the invention is a machine for machining a work piece with at least one tool, having a ball screw, which is supported by a fix and a floating bearing.

For precision machining of work pieces, such as for milling, grinding, EDM machines, lathe etc., it is necessary that a table for workpieces is held over a nut on a ball screw and is positioned with a precision equivalent to the desired precision of the outcome of the work. As a rule, the position of the nut of the ballscrew is supported by one or more positioning devices.

The US 6958588 B2 discloses a machine for machining a work piece with at least one revolving or rotating tool has a work spindle with a shaft, whose temperature is monitored by means of at least one temperature sensor. The temperature sensor is detected in contact less fashion. The sensor is preferably a radiation-sensitive sensor, which detects the temperature radiation output. In triggering the positioning drive mechanisms, the control unit of the machine takes into account the temperature expansions of the shaft that result from the temperature changes in the shaft. The positioning drive mechanisms serve to position a tool, carried by the shaft, relative to the work piece. A machining precision is thus achieved that is independent of the temperature and temperature changes of the machine overall, of temperature changes in the coolant lubricant, and of temperature changes in the tool and the shaft, that can all be caused by the power converted at the machining site.

In the US 6456896B1 another apparatus and a method for thermal displacement correction for a machine tool is described. The position of a feed shaft is monitored, the mean moving speed and moving frequency of the feed shaft arc measured with every unit time for position correction, and a correction amount is determined from the speed and frequency according to an approximation formula and updated. A position correction amount for a

commanded position is determined from this correction amount, and the commanded position is corrected by the position correction amount. The position correction amount for the commanded position is determined from this correction amount, and the commanded position is corrected by this position correction amount. Since the correction amount is determined according to the approximation formula, thermal displacement correction can be effected at all times without requiring any sensor. When the correction amount changes substantially, the thermal displacement is measured by means of a sensor and used as the correction amount, whereby accuracy is improved. The frequency of measurement by means of sensors is reduced, so that the machining time can be reduced.

Based on the above the requirement of the invention is to create a device for compensation thermal caused displacement for generating a better accuracy and an improved better utilization of machining time in a simple way without any complexity.

The invention solves this problem in that way that the machine has a contact- free device for measuring the length of the extension of the ball screw.

Furthermore the invention is solved by a method for measuring and compensation a thermal displacement comprising the steps of: detecting the actual length of the axis of the ball screw, calculating the difference δl of the length of the extended axis and a reference length of the axis and compensating the difference value to the correct value.

A drift sensor is integrated contact-free at the end of the ball screw to measure its extension in real-time. Based on the delta value reported by this sensor and the actual position of the axis, a compensation value is generated along the axis direction. This value is combined to the target position calculated by the control unit.

This scheme assumes that at the end of the ball screw a floating bearing is used to allow the ball screw to thermally extend away of the fixed bearing.

Furthermore some means are available to initiate a calibration process for the planarity of the measurement surface. Because of the positioning of the sensor and the geometrical tolerances of the ball screw, the sensor typically reports different values based on the orientation of the ball screw.

The planarity issue is compensated for in order to avoid erroneous axes compensation values. This compensation must be active only when the ball screw does not rotate faster than a specified value. On faster speed the natural averaging which occurs on the drift sensors will iron out the unplanarity effect.

The ball screw section is divided by a pre-defined amount of same-sized sectors covering the 360° of the measuring section. To each sector is assigned a compensation value. Based on the circular orientation of the ballscrew, the compensation value for the matching sector is to be used to offset the value returned by the sensor.

The innovative ball screw extension compensation is applicable for any linear axe for example the X, Y and/or Z axes in milling, grinding, EDM machines or lathes etc. Below is the detail formula for calculating the compensation for X. The same formula is applicable for any other axes:

Xcomp ⁼ ((Xcurpos ⁺ Xorgoff)'Xscrewlength) XscrewK Xscrewext

where

Xc _o mp is the compensation value to be applied on the axis in μm

Xcurp _o s is the actual machine position on the axis in mm

Xorgo _ft is the distance between the O-position of the axis and the fix bearing in mm

Xscrewiength is the length of the ballscrew in mm

XscrewK is an extension gain

Xscrewext is the measured value on the drift monitoring sensor in μm

An example of a device according to the invention is shown in the drawings:

Fig. 1a is a device with a reference length of the ball screw and Fig. 2b is a device with an extended length of the ball screw

The ball screw 1 , driven by a motor 9, is connected with a nut 6 as shown in Fig. 1a and Fig. 1b. A table 10 with a workpiece 11 is connected with the nut 6There are two bearing seats 7, 8, each of which holds one bearing device 2, 3. The bearing device 3 is for example a roller bearing that supports the ball screw 1 rotatably but axially non-connected. The bearing device 2 is fixed connected with the ball screw 1.

At the end 5 of the ball screw a contact-free sensor 4 is situated. It measures the length of the ball screw 1. For example the sensor could be an inductive, capacitive, optical type etc.

The difference length δl shows the extension of the ball screw 1 between the reference length h and the extended length I ₂ .

The device described above works as follows:

For measuring the reference position the motor 9, which drives the ball screw 1 , has an incremental rotary encoder, which give the base to calculate the linear position of the nut 6. On the other hand the sensor 4 measures contact-free the extension of the ballscrew 1 , which is for example elongated by temperature, caused by the long lasting axis motion.

Both results, the position measured by the incremental rotary encoder and the extension of the ballscrew 1 measured by the sensor 4, are inputted in a control unit. The control unit calculates the value in a linear method, dependent of the position of the nut. The extension near the end 5 is larger than the extension near the motor 9.

In a next step the control unit generates a signal to the motor 9 for adjusting the correct position of the nut 6.