JP6656945B2 - Compensation method for thermal displacement of machine tools - Google Patents

Description

  The present invention relates to a method for correcting a processing error of a work caused by thermal deformation of a machine tool in processing the work.

The machine temperature of a machine tool changes due to environmental temperature changes, driving heat generated during processing of a workpiece, processing heat, and the like, and thermal deformation occurs due to a change in the machine temperature, and the thermal deformation appears as a processing error of the workpiece.
For example, in a machine tool such as a lathe or a machining center, heat is generated in a bearing or the like due to rotation of a main shaft, and heat is also generated in a machined portion by a tool.
The heat is transmitted to a bed, a column, a headstock, a tool rest, and the like, and the temperature of the machine locally or entirely rises with the operation of the machine tool, which causes thermal deformation and causes an error in machining of the work.

  Therefore, in order to correct a processing error due to thermal deformation of a machine tool and a work, conventionally, the temperature of a headstock or a tool post is measured, and the relationship between the machine temperature and the processing error generated in the work is measured in advance. A correction formula for the processing error due to the machine temperature is obtained from the measured value and registered in the NC device, and a command value output from the NC device when actually processing a workpiece is converted into a machine temperature at the time by the correction formula. A processing error is reduced by correcting the position of a tool post based on a calculation result obtained by substitution.

However, when the machine tool becomes large and, for example, employs a large casting structure or the like for a frame or the like, the correction based on the real-time temperature measurement values of the bed and the components of the conventional machine tool requires a machine temperature. Since the time lag of heat transfer from the measurement site to the site directly affecting the processing error of the work is not taken into account, there is a problem that the processing accuracy is not necessarily improved.
Conventionally, in order to improve the correction accuracy, the temperature of a plurality of parts such as a headstock, a coupling case, a frame, etc. is measured, and a difference-like curve is generated by using a difference between the temperatures to obtain a correction value. However, since the number of measurement points increases and the coefficient increases, the correction value suddenly changes when an irregular temperature change occurs, and the correction accuracy becomes unstable.

The present applicant has proposed a correction formula including a correction term using a history value of a product of a spindle speed and a detected value of a machine temperature in Patent Document 1.
Although this method achieves an improvement in the accuracy of the processing error of the work, there is room for improvement in the delay of the heat transfer.

Japanese Patent No. 4271611

  SUMMARY OF THE INVENTION An object of the present invention is to provide a method for correcting a thermal displacement at a processing point due to thermal deformation of a machine tool, that is, a thermal displacement that can reduce a processing error of a work.

In the method for correcting a thermal displacement of a machine tool according to the present invention, a work command is corrected by correcting a feed command of an NC device by a correction value calculated based on a history value of measured data of a machine temperature and a delay value of a heat transfer. The tool position is corrected.
In this specification, the term “thermal displacement” refers to a displacement of a tool position caused by a thermal deformation of a machine tool.
Here, the delay value of the heat transfer means that the past data measured at predetermined intervals by a temperature sensor or the like is sequentially stored in a database, and based on the past temperature measurement value during processing of the workpiece. A coefficient that determines whether to calculate a correction value.

In the present invention, the history value of the measurement data of the machine temperature may be a history value of the measurement data of the temperature of a structure or / and a component of the machine tool.
Here, the structure refers to a mechanical structure such as a frame, a bed, a table, and a column of a machine tool, and the constituent part includes components provided in the machine tool such as a headstock, a tool rest, and a work support. Part.

  The thermal displacement correction method according to the present invention is based on a correction value calculated by using a coefficient corresponding to a delay value of heat transfer with respect to measured temperature data in the past for a predetermined period of time from the time of processing a workpiece. Since the feed command is corrected, the processing accuracy of the workpiece is improved as compared with the conventional correction method.

Further, in the present invention, temperature measuring means such as a temperature sensor is attached to each of the different parts constituting the machine tool, for example, the front and rear of the headstock, each part of the frame, the coupling case, etc., at predetermined time intervals. The measured temperature measurement data may be sequentially stored, and a past temperature measurement value of a measurement site having a strong correlation between a temperature change and thermal deformation may be adopted according to a processing method and processing content of the work.
By doing so, the correction coefficient can be reduced as compared with the conventional correction based on the difference between a plurality of parts, and a sudden change in the correction value due to a temperature change is reduced, and the correction of the thermal displacement is stabilized.

4 shows an example of a temperature measurement value in the present invention in which a delay in heat transfer is considered. 5 shows an example of a conventional real-time temperature measurement value.

An example of the method for correcting thermal displacement according to the present invention will be specifically described below, but the present invention is not limited thereto.
The measured temperature T ₀ by a temperature sensor attached to the headstock front rotating at a high speed of the machine tool in FIG. 1 shows an example employing a measurement temperature of 60 minutes ago against during processing of the workpiece.
The correction value δ ₀ of the thermal displacement is obtained by an equation of δ ₀ = α ₀ × T ₀ + β ₀ (constant).
The suffix in the formula indicates a measurement temperature point.
α ₀ refers to the slope (correction coefficient) when the relationship between the adopted past measured temperature T ₀ , thermal displacement, and the measured temperature T ₀ temperature is approximated by a linear function.
In the case of the example shown in FIG. 1, the arithmetic unit of the control program of the NC device has a storage unit for temperature data, and sequentially stores values measured by a temperature sensor attached to the headstock, for example, every three minutes.
The number of data to be stored may be sequentially deleted for a predetermined time or more in consideration of practicality, recording capacity, and the like, for example, up to 240 minutes before.
Although FIG. 1 uses the past measurement data 60 minutes before the current processing, how much past measurement data is used is preferably measured and selected in advance.
Further, when the history is lost for some reason, interpolation may be performed using other remaining measurement data.

FIG. 2 shows a graph of a change in the real-time measured temperature at each measurement site of the machine tool.
Curve 1 shows the case where the temperature sensor was attached to the rear part of the headstock. Similarly, curve 2 shows the coupling case, curve 3 shows the side of the hydraulic tank of the frame, and curve 4 shows the case where the temperature sensor was attached to the center of the rear part of the frame. It is an example.
As described above, since the correction based on the measured temperature in real time does not consider heat transfer, the measured temperature increases from the beginning of the processing, and the correction amount is added before actual thermal deformation occurs.
Therefore, the accuracy is lower than the correction method according to the present invention shown in FIG.

In the present invention, as described above, among the past measured temperature data stored in the storage unit at regular intervals at predetermined intervals, the measured temperature at each part of the machine tool is determined based on the past coefficient based on the delay coefficient. May be determined in advance and stored in the storage unit.
In this case, a correction value can be obtained by selecting a part suitable for the processing method and processing conditions of the work and a delay coefficient.
For example, arithmetic expressions are stored and registered in the headstock rear portion, such as a correction formula δ ₁ = α ₁ × T ₁ + β ₁ , and a correction formula δ ₂ = α ₂ × T ₂ + β _{2 in} a frame rear center portion. It can be read out at the time of machining a workpiece and issue a selection command to be used for executing a program.

Claims (1)

The tool position during work machining is corrected by correcting the feed command of the NC device with the correction value calculated based on the history value of the measurement data of the machine temperature and the delay value of the heat transfer ,
A plurality of arithmetic expressions for calculating a correction value based on the history value of the measurement data and the delay value of the heat transfer are stored in the storage unit for each of different parts or / and components of the structure constituting the machine tool. ,
A method for correcting thermal displacement of a machine tool, comprising: means for selecting an arithmetic expression suitable for machining conditions from the plurality of arithmetic expressions .