DE19800033B4 - Method for compensating temperature-dependent changes in position on machine tools and machine tools therefor - Google Patents

Abstract

Method for compensating for temperature-dependent relative changes in position between a workpiece (10) and a processing tool (9) on machine tools with a control unit (19) which has a computing unit (15) with storage spaces, and with at least one length measuring device (12) and with means for measuring and transmitting temperature values of the length measuring device (12) and the workpiece (10) to the control unit (19), such that temperature-dependent changes in position of the machining tool (9) and workpiece (10) are compensated by the control unit (19), whereby to A surface temperature T _{wu of} the unprocessed workpiece (10) is measured at the beginning of the machining, at least at a later measuring time t ₁ , t ₂ , t ₃ , .... t _n a surface temperature T _{wb of} the machined workpiece (10) is measured in the machining _sequence and whereby both from temperature changes of each length measuring device (12) and from Tempe temperature changes of the workpiece (10) itself generated by the temperature measurements, correction signals and used for a target dimension correction to position conditions of a workpiece surface (18) and the processing tool (9) deviating from a reference temperature.

Description

The The invention relates to a method for compensating temperature-dependent relative changes in position between a workpiece and a machining tool on machine tools and a machine tool therefor.

wobei: L_bez = Länge bei der Bezugstemperatur,
L_T = Länge bei der Temperatur "T"
d = Werkstoff-Ausdehnungskoeffizient,
ΔT = Abweichung der IST-Temperatur von der Bezugstemperatur.The known physical relationship applies to compensating for temperature-dependent changes in position and length and for standardizing to a reference temperature T _bez of, for example, 20 ° C.

where: L _bez = length at the reference temperature,
L _T = length at temperature "T"
d = material expansion coefficient,
ΔT = deviation of the ACTUAL temperature from the reference temperature.

The temperature-dependent change in length limited measurement and tolerances that can be implemented in terms of control technology, especially for larger workpieces and / or at temperatures deviating from the reference temperature of 20 ° C, for example.

Through the DE 38 28 305 A1 it is known to keep parts of machine tools at a constant temperature by covering them with a network of temperature sensors and applying thermostatically controlled heating or cooling fluids. For this purpose, however, separate fluid channels must be provided in the components concerned, to which individually controllable heating or cooling devices are assigned. However, such a solution requires a large amount of construction work and has a considerable time constant. The workpiece temperature is not included in the control loops and is therefore not taken into account.

By the essay "Design the machine tool and its elements ", published in" Industrie-Anzeiger ", Essen, July 30, 1965, Pages 1446 to 1453, it is also known to be thermostatic regulated cooling and lubricant circuits to provide for heat distortion counteracting machine parts (e.g. Fig. 18). It is still specified by temperature influences contingent relocations during the Measure the machining and enter it into the control as correction values. An Invar rod is provided as an example in a headstock, their relative movement towards the headstock on a dial gauge is transmitted with an angle encoder (displacement encoder). Its electrical Output signal is fed to the controller, so that electrical compensation shifting at the interface is possible (Figure 21 and associated text on page 1452). The Invar rod serves as a length measuring device to some extent about their Relative movement indirectly as a temperature sensor, its temperature itself but is not measured. The workpiece temperature is also in this If not included in the control loop, it remains the same unconsidered.

By the essay by Pascher "compensation thermally induced displacements on machine tools (part 1) ", published in "HGF Brief Reports in the industrial indicator "no. 75, 09/19/1984, pages 55/56, it is also known machine temperatures to grasp the resulting displacements between tools and workpiece to be calculated and then active, for example with NC machines using the NC control, to compensate (page 55, left column, Paragraph 1; Page 56, middle column, picture 4, and right column, under "Experiment setup"). The workpiece temperature is not included in the control loop in this case either, remains so also disregarded.

By the next one EP 0 555 796 A1 For machine tools of the milling machine type, a thermal expansion compensation device is known, in which the temperatures of the machine stand, slide and machining head are determined by means of three temperature sensors, the temperature sensor of the slide being arranged above a front scale suspension, and the temperature in the storage area being determined by the temperature sensor of the machining head of the machining head is determined. The thermal expansions of the named components are determined by a computing device, and from this a correction value is calculated, which is applied to the compensation device in order to thereby To influence the movement of the machining tool to maintain close tolerances depending on the correction value. These interventions are based on the observation that during operation due to friction on bearings and guides, friction in the transmission, heat loss in the engine u. Like. The individual components expand by heat, which has a negative impact on the working accuracy of the machine (column 5, lines 15 to 23). The recording of the workpiece temperature itself and the consideration of the expansion of the workpiece is just as little described as the arrangement of appropriate temperature sensors for recording the workpiece temperatures before and during machining.

By the essay by Armbruster and Badur "measuring controls for CNC lathes "in" wt-Z. ind. Finished. ", 70 (1980), Seiten 511 to 514, it is known to go through a multi-stage machining process Turning before the last machining stage, the finishing machining, to interrupt and the temperature of the heated workpiece against insertion in the chuck for finishing a resilient temperature sensor to press the measured value to compare with the room temperature and the determined temperature difference, multiplied by the coefficient of expansion of a correction memory for the The movement of the tool. This is said to be automatic Tolerance field shift according to the temperature from the machine control are automatically adopted in the tool offset. To be sure thereby interim storage for the cooling of the workpieces avoided, however, the processing time is by unclamping and reclamping of the workpiece and extended the temperature measurement. Moreover changed the temperature again during clamping the workpiece. The measurement of the temperature of the clamped workpiece during the Editing and repeating the measurement to determine a Steady state of the workpiece temperature are not described, nor are they suggested. So far one Measurement on the clamped workpiece described, it is a matter of determining the geometric Location of the measuring surface.

The The invention is therefore based on the object of a method for compensating of temperature dependent changes in position on machine tools and a machine tool for this purpose, where also such changes in position between workpiece and tool considered are due to thermal shape changes of the workpiece during the Processing itself.

The solution the task is carried out in the procedure specified above according to the invention the features of claim 1 and at the beginning Machine tool by the features of claim 5.

By the object of the invention is the task in full solved, i.e. there will also be such changes in position between workpiece and tools are taken into account on thermal shape changes of the clamped workpiece even during processing back respectively are. In any case, it is enough this solution for little ones and average workpiece sizes and / or with sufficiently large ones Tolerance areas, if it can be assumed that the absorption of machining energy increased or due to the coolant emitted heat lower workpiece temperature to some extent evenly over that entire workpiece has spread.

The surface temperature T _{wb of} a workpiece results from the following general relationship: T wb = f (E, O / V, c, t) where: E = amount of energy transfer in the process,
O / V = surfaces: volume ratio,
c = heat capacity of the specific material,
t = process time.

Out This can be used for heating or cooling curves for the surface of a workpiece determine.

residual problems but remain with the following workpieces:

• a) with large Dimensions, especially large Distance between the core of the workpiece or the "cold" end and the measuring surface,
• b) with irregular cross sections,
• c) with unfavorable relationship from surface to volume,
• d) with large Material expansion coefficient "d",
• e) with poor thermal conductivity,
• f) for large Machining quantities per unit of time,

die Längenmaßkorrektur L_Twg bestimmt und als Korrekturfaktor auf die Steuereinheit aufgeschaltet wird, wobei d_w der Ausdehnungskoeffizient des Werkstücks, d_m der Ausdehnungskoeffizient der Längenmeßeinrichtung und L_Tm die Länge der Längemeßeinrichtung bei der Temperatur T_m und ΔT_m das Maß für die Temperaturänderung der Längenmeßeinrichtung ist.In order to additionally switch off at least one of these error sources and to be able to maintain even smaller tolerances, it is therefore proposed in the course of a further embodiment of the invention to "weight" the workpiece temperature, that is to say to determine a temperature value, by means of its computational recording and corresponding control of the machine which Tolerances are as small as possible. This is done by additionally measuring the surface temperature T _{wu of} the unmachined workpiece at the beginning of the machining, and measuring the surface temperature T _{wb of} the machined workpiece at least at a later measuring time t ₁ , t ₂ , t ₃ , ... t _n in the machining process and that this results in a weighted workpiece temperature ΔT _wg according to the relationship .DELTA.T wg = (1 - X) * T wu + X · T wb - reference temperature (1) is determined, where "X" is the process-dependent weighting factor and is 0 <X <1, and from this according to the relationship

the length dimension correction L _{Twg is} determined and applied as a correction factor to the control unit, where d _{w is} the expansion coefficient of the workpiece, d _{m is} the expansion coefficient of the length measuring device and L _{Tm is} the length of the length measuring device at temperature T _m and ΔT _{m is} the measure for the temperature change of the length measuring device is.

The required software, the creation of which is not difficult, standardizes on the one hand the length dimensions of the length measuring devices which deviate from L _bez (see the relationship given above for this), to the reference temperature of 20 ° C as an example, and on the other hand also the temperature of the workpiece according to the above relationship. It is generally assumed that the unprocessed workpiece, ie the workpiece that has been delivered for processing, has assumed the temperature T _wu over its entire cross section.

The determination of the weighting factor "X" is carried out in a particularly simple manner in that a workpiece-specific temperature curve K, which is influenced by the machining parameters causing a temperature change, is first determined and stored on the basis of a sample workpiece and which ranges from the initial surface temperature T _{wu of} the unprocessed workpiece to a virtual one Limit temperature T _g runs that this temperature curve K is assigned an equidistantly divided scale S for the weighting factor "X", whose zero point X = 0 is assigned to the initial surface temperature T _wu and whose end point X = 1 is assigned to the limit temperature T _g , that further at least one temperature _value T _wb1 , T _{wb2 is} determined from the temperature curve K at at least one measuring time t ₁ , t ₂ and _{plotted over} to the scale (S) and that there the at least one weighting factor "X ₁ ", "X ₂ " for the arithmetic operation determined according to formula (1) stored and called up for all other workpieces of the same type and in the computing unit ( 15 ) is used.

• a) mindestens eine, parallel zu einer Zustellrichtung des Bearbeitungswerkzeugs ausgerichtete, einen Linearmaßstab aufweisenden Längenmeßeinrichtung mit einem Temperaturfühler für die Erfassung der Temperatur der Längemeßeinrichtung angeordnet ist, und
• b) mindestens ein weiterer Temperaturfühler angeordnet ist, der zumindest vorübergehend mit dem Werkstück in Berührung bringbar ist, und daß die elektrischen Ausgänge der Temperaturfühler einer Recheneinheit für eine Temperaturbewertung und eine Zielmaßberechnung aus allen Temperaturmessungen aufgeschaltet sind, und daß der mindestens eine Ausgang der Recheneinheit der als CNC-Steuerung ausgebildeten Steuereinheit für die Zustellbewegungen des mindestens einen Werkzeugschlittens aufgeschaltet ist.

A particularly advantageous embodiment of the machine tool is characterized according to the further invention in that on the at least one tool slide

• a) at least one length measuring device, which is aligned parallel to an infeed direction of the machining tool and has a linear scale, is arranged with a temperature sensor for detecting the temperature of the length measuring device, and
• b) at least one further temperature sensor is arranged, which can be brought into contact with the workpiece at least temporarily, and that the electrical outputs of the temperature sensors of a computing unit for temperature evaluation and target dimension calculation are connected from all temperature measurements, and that the at least one output of the computing unit of the Control unit designed as a CNC control for the infeed movements of the at least one tool slide is connected.

An embodiment of the subject of the invention and its mode of operation are described below with reference to 1 and 2 explained in more detail.

It demonstrate:

1 a front perspective view of a machine tool designed as a grinding machine with a rotatable machine tool, which is designed as a grinding wheel, and a measuring and control unit and

2 a diagram for determining the weighting factor "X".

The The description below refers to an example of a grinding machine.

In 1 are parts of a grinding machine 1 shown, namely a stand 2 , on the transverse guide 3 in the direction of the double arrow 4 is horizontally movable. This lateral guidance 3 in turn carries a grinding spindle slide 5 that is in the direction of the double arrow 6 vertically movable on the cross guide 3 is stored. The grinding spindle slide 5 carries a spindle motor 7 with a grinding wheel spindle 8th and a machining tool 9 , which is designed as a grinding wheel that is out of engagement with a workpiece 10 located, which is designed as a ring and is rotatable about the axis "A". The workpiece 10 is in turn on a rotatable jig 11a a workpiece carrier 11 clamped.

Using the editing tool 9 (Grinding wheel) is both internal and external machining and machining of the workpiece surface 18 , here the upper end face possible, which is the measuring surface in the present case.

For detecting the transverse or horizontal movement or for reporting the position of the grinding spindle slide 5 serves a length measuring device 12 that have a linear scale 13 with scale division and optical scanning. To measure the temperature of the linear scale 13 and thus a first temperature sensor is used for the mathematical recording of its changes in length 14 whose output signal is a computer 15 is activated for temperature evaluation and target dimension calculation.

On the grinding spindle slide 5 is also by means of a vertical guide 16 a vertically displaceable second temperature sensor 17 arranged with the workpiece surface 18 can be brought into contact and its temperature is recorded. Also the output signal of this temperature sensor 17 is the calculator 15 switched.

To the grinding machine 1 belongs to a control unit 19 that run as CNC control and with the computer 15 is connected and contains the memory locations, input keys and a display, which, however, are not particularly shown. As already mentioned above, the grinding machine has 1 additional position sensors for the relative positions of other moving parts of the grinding machine 1 , Their output signals are also sent to the control unit 19 supplied, which is indicated by the arrows P ₁ to P _n .

Example:

A ring made of 100Cr6 steel with an initial temperature of 19.0 ° C, an outer diameter D _a of 400 (+0.2) mm, an inner diameter of 300 (-0.2) mm and an axial length of 170 mm is opened the dimension D _a = 400 (± 0.0) mm ground down. This material has an expansion coefficient d _w of 11.5 · 10 ^-6 · K ^-1 . The timing of the process was as follows:

Based on 2 it is now explained how the weighting factor "X" is determined: the process time "t" with the measurement times t ₀ , t ₁ and t _{2 is} plotted on the abscissa. The temperatures T _wu , 20 ° C, T _wb1 and T _{wb2 are associated} as ordinate values. From these measured values, the temperature curve "K" for the special workpiece described above can be obtained with sufficient accuracy 10 represent that asymptotically approaches a limit value "T _g ", the so-called steady-state temperature at which there is an equilibrium. A second ordinate is now entered in the diagram, the lowest value of which is "T _wu ", the initial surface temperature. This value corresponds to "X" = 0. The highest value is at the limit temperature "T _g " and corresponds to "X" = 1.

The distance between X = 0 and X = 1 is now divided equidistantly, and the ordinate values for t ₀ , t ₁ and t ₂ , ie, T _wu , T _wb1 and T _wb2 are now perpendicular to the second ordinate from X = 0 to X = 1, which results in the corresponding weighting factor "X". The case described above applies to the process of heating the workpiece by grinding energy.

In comparison, the following values result for the calculated temperature values:

"X" gives the following dimensional corrections in [μm] without and with weighting of the temperature at a final diameter D _a of 400 (± 0.0) mm:

The Method can be to other workpiece parameters, e.g. with other ratios of Workpiece surface: Transfer volume: X ^ O / V where "O" is the workpiece surface and "V" is the volume.

The temperature curve "K '" applies to another case where the workpiece 10 is cooled by heat of evaporation.

1

grinding machine

2

stand

3

width guide

4

double arrow

5

Grinding spindle slide

6

double arrow

7

spindle motor

8th

Grinding wheel spindle

9

processing tool

10

workpiece

11

Workpiece carrier

11a

jig

12

length measuring

13

linear scale

14

temperature sensor

15

computer

16

vertical guide

17

temperature sensor

18

Workpiece surface

19

control unit

A

axis

K

temperature curve

K '

temperature curve

P1 to P _n

arrows

X

weighting factor

Claims (7)

Method for compensating temperature-dependent relative changes in position between a workpiece ( 10 ) and a processing tool ( 9 ) on machine tools with a control unit ( 19 ), which is a computing unit ( 15 ) with storage spaces, and with at least one length measuring device ( 12 ), as well as with means for measuring and transmitting temperature values of the length measuring device ( 12 ) and the workpiece ( 10 ) on the control unit ( 19 ), such that the control unit ( 19 ) temperature-dependent changes in position of the processing tool ( 9 ) and workpiece ( 10 ) can be compensated, at the beginning processing a surface temperature T _{wu of} the unprocessed workpiece ( 10 ) is measured, at least at a later measuring time t ₁ , t ₂ , t ₃ , .... t _n in the machining process, a surface temperature T _{wb of} the machined workpiece ( 10 ) is measured and both from temperature changes of each length measuring device ( 12 ) as well as changes in the temperature of the workpiece ( 10 ) Correction signals are generated by the temperature measurements themselves and are used to correct the target dimension to the position of a workpiece surface that deviates from a reference temperature ( 18 ) and the editing tool ( 9 ) be used. A method according to claim 1, characterized by the use on grinding machines ( 1 ) with a grinding wheel as a processing tool ( 9 ) Method according to Claim 1, characterized in that, in order to maintain small tolerances, a weighted workpiece temperature ΔT _wg according to the relationship is additionally obtained from the measured temperature values (1) ΔT wg = (1 - X) * T wu + X · T wb - reference temperature is determined, where "X" is the process-dependent weighting factor and 0 <X <1, and from this according to the relationship

a length correction L _{Twg is} determined and applied to the control unit as a correction factor ( 19 ) is activated, where d _{w is} the expansion coefficient of the workpiece ( 10 ), dm is the coefficient of expansion of the length measuring device ( 12 ) and L _Tm the length of the length measuring device ( 12 ) at temperature T _m and ΔT _m the measure for the temperature change of the length measuring device ( 12 ) is. Method according to Claim 3, characterized in that, in order to determine the weighting factor "X", first using a sample workpiece ( 10 ) a workpiece-specific temperature curve (K) which is influenced by the machining parameters causing a temperature change is determined and stored, which is based on the initial surface temperature T _{wu of} the unprocessed workpiece ( 10 ) runs up to a virtual limit temperature T _g that this temperature curve (K) is assigned an equidistantly divided scale (S) for the weighting factor "X", whose zero point X = 0 is assigned to the initial surface temperature T _wu and whose end point X = 1 the limit temperature T _g is associated with the fact that at least one surface temperature T _wb1 , T _{wb2 is also} determined from the temperature curve (K) at at least one measurement point in time t ₁ , t ₂ and _plotted over the scale (S) and that there the at least one weighting factor " X ₁ "," X ₂ "determined for the arithmetic operation according to formula (1), saved and for all other workpieces ( 10 ) of the same type and in the computing unit ( 15 ) is used. Machine tool with a control unit ( 19 ), which is a computing unit ( 15 ) with memory locations to compensate for temperature-dependent relative changes in position between a workpiece ( 10 ) with a workpiece surface ( 18 ) and with a processing tool ( 9 ) with at least one on one opposite a stand ( 2 ) and the workpiece ( 10 ) movable tool slide ( 5 ) arranged processing tool ( 9 ), and with at least one length measuring device ( 12 ), as well as with at least one temperature sensor ( 17 ) with the workpiece surface ( 18 ) can be brought into contact, as well as with means for transmitting temperature-dependent changes in length of the length measuring device ( 12 ) and temperatures of the workpiece surface ( 18 ) on the control unit ( 19 ), both of each length measuring device ( 12 ) as well as the workpiece ( 10 ) at least one temperature sensor each ( 14 . 17 ) is assigned, by means of which temperature measurements with an electrical output signal can be generated, and that the computing unit ( 15 ) and the control unit ( 19 ) are switched in such a way that from the temperature measurements on the at least one length measuring device ( 12 ) and from at least two measured temperature values T _wu , t ₁ , t ₂ , t ₃ , .... t _{n of} the workpiece surface ( 18 ) electrical correction signals can be generated at the beginning and during the machining process, which are used to correct the target dimensions to the positional conditions of the workpiece surface that deviate from a reference temperature ( 18 ) and editing tool ( 9 ) can be used. Machine tool according to claim 5, characterized by the design as a grinding machine with a grinding wheel as a processing tool ( 9 ). Machine tool according to claim 5, characterized in that on the at least one tool slide ( 5 ) a) at least one, parallel to an infeed direction of the processing tool ( 9 ) aligned, a linear scale ( 13 ) having length measuring device ( 12 ) with a temperature sensor ( 14 ) for recording the temperature of the length measuring device ( 12 ) is arranged, and b) at least one further temperature sensor ( 17 ) is arranged, which at least temporarily with the workpiece ( 10 ) can be brought into contact and that the electrical outputs of the temperature sensors ( 14 . 17 ) a computing unit ( 15 ) for a temperature evaluation and target dimension calculation from all temperature measurements, and that the at least one output of the computing unit ( 15 ) the control unit designed as a CNC control ( 15 ) for the infeed movements of the at least one tool slide ( 5 ) is activated.