Method for operating a machine tool

Abstract

In a first step of a method for operating a machine tool comprising a pump for conveying a cooling lubricant, a tool is introduced into a working spindle of the machine tool. In a second step, a learning process is carried out. The learning process comprises ascertaining a pump characteristic curve of the tool, determining an optimum operating point of the pump for the tool, and storing the optimum operating point of the tool in a tool table.

Description

[0001]

[0002] The present invention relates to a method for operating a machine tool and to a machine tool. In particular, the energy consumption of the machine tool is to be reduced by determining an optimal operating point of a pump for conveying a cooling lubricant.

[0003] When a workpiece is machined using a tool in a machine tool, mixed friction is the most common type of friction. Cooling lubricants (CMLs) reduce friction and can therefore reduce tool wear, workpiece heating, and energy consumption. CMLs therefore serve to cool the tool or workpiece and reduce friction between the tool and workpiece. Furthermore, in some machining processes, CMLs can be used to remove generated chips from the work environment by flushing them away, improve the dimensional accuracy of the workpiece, and ensure a better surface quality. Furthermore, the workpiece can be protected from corrosion. Conventional CMLs may contain water and oil as well as additives.

[0004] In the prior art, it is common practice to supply the coolant using a speed-controlled pump with a high, preset pressure of, for example, 50 to 100 bar, so that a sufficiently high flow rate is maintained at all times. However, it has been shown that a significantly lower pressure is often sufficient to ensure adequate cooling of the tool. This can significantly reduce the pump's energy consumption. Furthermore, it has been shown that the surface quality of the workpiece can be improved at lower pressures. Furthermore, the service life of the tool can be increased by operating at lower pressures.

[0005] The article by B. Denkena et al., "Energy efficient machine tools," CIRP Annals - Manufacturing Technology 69 (2020) 646-667, provides an overview of technologies for reducing the energy consumption of machine tools. In particular, it was shown that process cooling offers significant energy-saving potential. For example, considering the system characteristics of a machine tool with a variable-speed high-pressure pump, small changes in the coolant flow rate have a significant impact on the electrical pump power without affecting tool wear for a specific set of cutting parameters.

[0006] Although it is known in the art that switching the pump's pressure control to a volume control mode for delivering the coolant can save a significant amount of energy, this measure has not yet been applied in industry. One reason for this is that it is usually unknown what the actual required flow rate for a specific tool or machining process actually needs to be. Another reason may be the lack of a suitable flow sensor for measuring the coolant's flow rate.

[0007] The present invention is therefore based on the object of overcoming the problems known in the prior art and of providing a method for operating a machine tool that is improved compared to the prior art, as well as of providing an improved machine tool. In particular, the energy consumption of the machine tool is to be reduced.

[0008] A further object of the invention is to improve the surface quality of the workpiece after machining with a machine tool. Furthermore, the service life of a tool is to be increased.

[0009] The object is achieved according to the invention by a method for operating a machine tool according to claim 1, a method for operating a machine tool according to claim 9, and by a machine tool according to claim 10. Preferred embodiments of the present invention are the subject of the dependent claims, the attached drawings and the following description of exemplary embodiments.

[0010] The method according to the invention can be implemented essentially in any existing machine tool with numerical control, e.g., a CNC machine tool, for machining a workpiece with a tool. Such a machine tool typically has a work spindle into which a tool, such as a drilling or milling head or the like, is clamped.

[0011] The machine tool also has a pump for delivering a cooling lubricant (CLU) to cool the tool or workpiece. The pump can, in particular, be a speed-controlled high-pressure pump. Furthermore, the machine tool can have a flow sensor for measuring the volume flow of the CLU.

[0012] In a first step of the process, a tool is loaded into the work spindle of the machine tool. The tool can be removed, for example, from a tool magazine or the like, and this process preferably runs automatically.

[0013] According to one aspect of the invention, a learning process is performed in which a pump characteristic curve specific to the tool is determined. The pump characteristic curve can, for example, describe a relationship between a power consumption and / or a speed of the pump and a volume flow of the cooling lubricant through the tool.

[0014] The course of the pump characteristic curve can depend on a variety of parameters. The geometry of the tool, in particular, influences the pump characteristic curve. It is generally very time-consuming to determine the pump characteristic curve in advance, e.g., through simulation or numerical calculation. Therefore, it is advantageous to determine the pump characteristic curve empirically during the training process.

[0015] The pump's power consumption in watts can be determined, for example, based on the pump speed and / or pump current. The power consumption value (in watts) and / or the speed (in rpm) and / or the pump current (in amperes) can be recorded, in particular, by a pump controller. The pump's power consumption, pump speed, or pump current can also be referred to in the following as a generalized performance indicator or performance parameter.

[0016] An optimal operating point for the pump can then be determined for the tool. This operating point is specifically a point on the pump characteristic curve. The optimal operating point is characterized by the fact that it ensures sufficient cooling of the tool while minimizing energy consumption.

[0017] According to a preferred embodiment, a plurality of different optimal operating points can be determined based on different criteria, such as minimized energy consumption, sufficient cooling capacity, maximum tool life, improved workpiece surface quality, and the like. It is then possible to define an average of these criteria as the optimal operating point.

[0018] In a further step of the process, the optimal operating point of the tool is stored in a tool table. During subsequent machining of a workpiece with the same tool or a tool of the same or similar design, the optimal operating point can be read from the tool table and set. In particular, other machine tools can also access the tool table and set the optimal operating points when machining workpieces. For example, the tool table can be made available via a server or a cloud to a large number of machine tools, which may be located at geographically distant locations. For this purpose, the machine tools can be connected to a network via suitable interfaces.

[0019] This allows an optimal operating point to be learned for each tool or tool type. Relearning is no longer necessary for subsequent machining of workpieces. The optimal operating point can then simply be read from the tool table and set.

[0020] The optimal operating point can, for example, indicate an optimal pump speed, optimal pump power, optimal pump current, or the like. Thus, even a machine tool that does not have a flow sensor can be operated at the optimal operating point. A controller, in particular a pump controller of any suitable machine tool, can be configured for operation with optimal operating points, for example, by means of a software update, so that the method according to the invention can be easily retrofitted to existing machines.

[0021] Preferably, the learning process may include a step for recording a performance indicator of the pump (e.g., power consumption and / or pump speed and / or pump current). The performance indicator may, for example, be measured or output by a pump controller.

[0022] Furthermore, the teaching process can include a step for detecting a volume flow of the cooling lubricant through the tool. For this purpose, the machine tool can have a suitable flow sensor or volume flow sensor. According to a preferred embodiment, the teaching process can be carried out while the machine tool is idle, i.e., without machining a workpiece. In this case, the teaching process is a separate process that only needs to be performed once for each tool. The teaching process also does not need to be repeated for each machine tool. Thus, a machine tool that has a volume flow sensor can be used for the teaching process.

[0023] To record the pump characteristic curve, the pump can be operated during the training process at a variety of different performance parameters or pressure specifications and / or pump speeds and / or power values. A typical coolant pressure provided by the pump can be 40 to 100 bar, preferably 50 to 80 bar.

[0024] According to a further preferred embodiment, the learning process can be carried out using data acquired during the machining of a workpiece or during a plurality of machining operations. For this purpose, for example, when machining one or a plurality of workpieces by a machine tool or a plurality of machine tools, the pump parameters (performance indicator) as well as a measured volume flow of the coolant and the tool used can be recorded and stored. Subsequently, an evaluation can be performed for each tool or for each type of tool in order to generate a pump characteristic curve for each tool or for each type of tool.

[0025] A preferred teaching process as described above can, for example, be carried out on the basis of a large number of centrally collected and stored operating data from a large number of machine tools, so that a corresponding tool table can be generated centrally. As soon as an optimal operating point is available for a tool in use, this can then be used in subsequent machining. The centrally generated tool table can, in particular, be made accessible on a central data storage device for all machine tools in a network, so that the determined optimal operating points can be called up. This means that each time a tool is changed, the corresponding optimal operating point can be read out and set. The reading step can also be carried out in advance for each tool required to machine a workpiece.For machine tools that cannot automatically access the tool table, the respective optimal operating point can also be set manually by a machine tool user.

[0026] The optimal operating point can preferably be determined depending on the type and size of the tool. Accordingly, the optimal operating point can be stored in the tool table along with the tool type and size. For example, it can be specified that this is an optimal operating point for an M5 drill. Other exemplary tool types include (turning) chisels, milling tools, planes, rasps, and grinding tools, which can be available in a variety of sizes and / or geometries. Furthermore, the tools can be left- or right-rotating.

[0027] Preferably, each tool in a tool rest or tool magazine has a unique identification. This allows the optimal operating point to be stored in the tool table along with the identification. Modern machine tools can usually perform automatic tool changes. During this change, the optimal operating point can then be set from the tool table.

[0028] Furthermore, the optimal operating point can depend on the material of the workpiece being machined. Accordingly, the optimal operating point can be determined based on the material. For example, a hard material may require a higher cooling capacity and thus a higher volume flow of the coolant than a soft material. The optimal operating points for different materials can also be stored in the tool table. Thus, for a specific tool, a multitude of optimal operating points can be available for a corresponding variety of materials.

[0029] Furthermore, the optimal operating point can also depend on the machining process. For example, the desired or required cooling capacity can depend on the speed of the work spindle. Preferably, the optimal operating point can be determined and saved for different machining parameters and / or parameter ranges.

[0030] Preferably, the optimal operating point can be determined by executing an algorithm. The algorithm can be executed, in particular, by a control device of the machine tool and / or on a central data processing device, such as a server or a cloud.

[0031] A preferred method for machining a workpiece with a machine tool having a pump for delivering a cooling lubricant uses a predetermined optimal operating point. The method comprises a step of inserting a tool into a work spindle of the machine tool, a step of detecting an optimal operating point of the pump depending on the inserted tool or depending on the inserted tool and a workpiece material, and a step of machining the workpiece with the pump operating at the optimal operating point.

[0032] The step of determining the pump's optimal operating point may, for example, involve looking up the optimal operating point in the tool table. The tool table may be stored locally on the machine tool. Alternatively or additionally, the machine tool may access a centrally stored tool table (e.g., on a server or in the cloud) via a network.

[0033] Such a machining process can be performed on a machine tool without a flow sensor, since only the previously determined optimal operating point of the pump for the tool in use is read out and adjusted. Since similar tools of the same size have a similar geometry, an optimal operating point previously determined for this tool can be universally applied to any machine tool.

[0034] A preferred machine tool for machining a workpiece comprises a work spindle for holding a tool for machining the workpiece, a pump for delivering a cooling lubricant for cooling the tool, and a control device for controlling the machine tool. According to the invention, the control device is configured to carry out a method according to the invention as described above.

[0035] BRIEF DESCRIPTION OF THE CHARACTERS

[0036] Further advantageous embodiments are described in more detail below with reference to an embodiment shown in the drawings, to which the invention is not limited, however.

[0037] They show schematically:

[0038] Figure 1 Fig. 1 shows a measured pump characteristic curve for a milling tool with a diameter of 6 mm (R0.8).

[0039] Figure 2 shows a measured pump characteristic curve for a milling tool with a diameter of 3 mm (F0.2). Figure 3 shows a measured pump characteristic curve for a drill with a diameter of 2.5 mm.

[0040] Figure 4 Fig. 4 shows a measured pump characteristic curve for a drill with a diameter of 8.9 mm.

[0041] Figure 5 Fig. 5 shows a measured pump characteristic curve for a drill with a diameter of 14 mm.

[0042] Figure 6 shows a comparison of measured pump characteristic curves for various tools. This figure is taken from the article by B. Denkena et al., "Energy efficient machine tools," CIRP Annals - Manufacturing Technology 69 (2020) 646-667.

[0043] Figure 7 Fig. 7 illustrates a learning process for different tools.

[0044] Figure 8 Fig. 8 illustrates a learning process for different tools.

[0045] Figure 9 Fig. 9 illustrates an algorithm for determining an optimal operating point for a tool.

[0046] DETAILED DESCRIPTION OF THE INVENTION USING EMBODIMENTS

[0047] In the following description of a preferred embodiment of the present invention, like reference numerals designate like or comparable components.

[0048] Figs. 1 to 5 show exemplary pump characteristic curves for five different tools. After the tool shown is inserted, a pump for delivering a cooling lubricant (CLS) is operated at a variety of performance parameters, and the resulting volume flow Q of the CLS through the tool is measured.

[0049] In Figs. 1 to 5, the pump's power consumption P in watts is plotted against the flow rate Q in liters per minute. Three example operating points are represented by large dots in each pump characteristic curve. The upper point corresponds to a pump pressure control of 80 bar. The middle point corresponds to a pump pressure control of 40 bar. The lower point indicates an optimal operating point for the flow control.

[0050] Fig. 1 shows an example of the measured pump characteristic curve for a milling tool with a diameter of 6 mm (R0.8). Fig. 2 shows an example of the measured pump characteristic curve for a milling tool with a diameter of 3 mm (F0.2). Fig. 3 shows an example of the measured pump characteristic curve for a drill with a diameter of 2.5 mm. Fig. 4 shows an example of the measured pump characteristic curve for a drill with a diameter of 8.9 mm. Fig. 5 shows an example of the measured pump characteristic curve for a drill with a diameter of 14 mm.

[0051] The following Table 1 provides an overview of the determined parameters at the optimal operating point of the pump with volume control compared to conventional control at a pressure of 40 bar or 80 bar for the exemplary tools shown in Figs. 1 to 5. The values ​​correspond to the three points in Figs. 1 to 5, with the upper point corresponding to a pressure of 80 bar, the middle point to a pressure of 40 bar, and the lower point to volume control (see also labeling in Fig. 1).

[0052] Table 1:

[0053] As can be seen from the values ​​in Table 1, the required pump power P is significantly reduced with volume control compared to pressure control, while still achieving a flow sufficient to cool the respective tool. This significantly reduces energy consumption. Lower coolant pressure can also ensure a better surface finish on the machined workpiece and increase tool life. Furthermore, tests have shown that the service life of tools with volume control can be increased by up to 26%.

[0054] Surprisingly, it has also been shown that cooling and lubricating a tool with volume control can also significantly improve the surface finish or surface quality of the machined workpiece.

[0055] In summary, Table 2 below compares the surface quality of a workpiece machined with a 10 mm diameter drill at different pump power levels. A large number of holes, in this case six holes, were drilled adjacently into the workpiece at different pump power settings.

[0056] The holes and the surface of the workpiece were then measured.

[0057] The first column of Table 2 lists the measured variables or pump parameters. Measurements were taken at four different pump performance settings. The results are presented in the second through fifth columns of the table. Each entry in the table indicates how many of the six holes are outside the tolerance specified below.

[0058] The fourth row of Table 2 indicates how often the measured holes were outside the tolerances by more than 10 pm. It should be noted that a hole can violate multiple tolerance criteria simultaneously.

[0059] The results show that the volume control delivers optimal results at a flow rate of 6 l / min. The six holes were only outside the tolerance twice. In contrast, with the conventional pump control at 80 bar, the six drilled holes were outside the tolerance 16 times.

[0060] Table 2: Fig. 6 was taken from the article by B. Denkena et al. "Energy efficient machine tools", CIRP Annals - Manufacturing Technology 69 (2020) 646-667, with the English data translated. The source for the figure is the article "Energy Efficient Machining with Optimized Coolant Lubrication Flow Rates" by B. Denkena et al., Procedia CIRP 24:25-31. Fig. 6 shows a comparison of pump characteristic curves for different tools.

[0061] In Fig. 6, the pump pressure p is plotted against the flow rate Q. Additionally, the pump power corresponding to the pressure p is shown using the color of the curves. For the end mill with a 12 mm diameter, the power reduction from the usual pump pressure of 80 bar (point A) to 40 bar (point B) is shown as an example, which in this case amounts to 77%. The optimal operating point can be even at an even lower pump pressure of approximately 20 bar.

[0062] Fig. 7 illustrates the teach-in process. On the left, Fig. 7 shows a tool magazine with a variety of different tools. The teach-in process can be performed for each tool in the tool magazine to determine the pump's optimal operating point for that tool, which is then entered into the tool table. In the example shown, the optimal operating point can be entered into the tool table as a correction parameter K for the usual operating point of, for example, 80 bar. This correction parameter K is preferably a dimensionless number less than 1 (e.g., the quotient of pressure at the optimal operating point and 80 bar).

[0063] Fig. 8 illustrates a setup for performing the teach-in process and determining the pump characteristic curve. A tool is clamped in the work spindle. A speed-controlled pump delivers the coolant to cool the tool. A volume flow sensor is arranged in the flow path of the coolant, which measures the volume flow Q. The pump control system sequentially sets a variety of different pump parameters (or performance indicators). The determined values ​​can then be plotted as a pump characteristic curve and evaluated to determine the optimal operating point, which is then entered into the tool table.

[0064] Fig. 9 illustrates an exemplary method for determining the optimal operating point. The left-hand diagram of Fig. 9 shows an example pump characteristic curve, where the pump pressure p is plotted against the flow rate Q. A straight line is drawn through the starting point and the end point of the characteristic curve. The slope of this straight line can be understood as the global relationship between pressure p and flow rate Q for the tool and is therefore a tool-specific parameter.

[0065] In the next step (see diagram on the right in Fig. 9), the straight line is shifted parallel until it tangentially touches the pump characteristic curve at a single point. This point of contact is defined as the optimal operating point for the tool and can be saved accordingly in the tool table.

[0066] The exemplary method described with reference to Fig. 9 for determining the optimal operating point from the pump characteristic curve can in particular be stored and executed as an algorithm in a control device of a machine tool or can also be executed by a computing device of a server or a cloud.

[0067] The features disclosed in the above description, the claims and the drawings may be important both individually and in any combination for the realization of the invention in its various forms.

Claims

PATENT CLAIMS 1. A method for operating a machine tool with a pump for conveying a cooling lubricant, the method comprising: Changing a tool into a work spindle of the machine tool; and Carrying out a learning process, comprising: Determine a pump characteristic curve of the tool; Determine an optimal pump operating point for the tool Saving the optimal operating point of the tool in a tool table.

2. The method of claim 1, wherein the learning process further comprises: Recording a performance indicator of the pump; and Recording a volume flow of the cooling lubricant through the tool, whereby the pump characteristic curve describes a dependency between a performance indicator of the pump and a volume flow of the cooling lubricant through the tool.

3. The method according to claim 2, wherein the teaching process is carried out while the machine tool is idling without machining a workpiece and the pump is operated at a plurality of pressure settings.

4. The method of claim 2, wherein the teaching process is performed using data acquired during machining of a workpiece.

5. The method according to claim 4, wherein the teaching process is performed using data from a plurality of machine tools.

6. Method according to one of claims 1 to 5, wherein the optimal operating point is determined depending on a type of tool.

7. The method according to claim 4 or 5, wherein the optimal operating point is determined depending on a type of tool and / or depending on a material of the machined workpiece.

8. The method according to any one of claims 1 to 7, wherein the optimal operating point is determined by executing an algorithm.

9. A method for machining a workpiece with a machine tool having a pump for conveying a cooling lubricant, the method comprising: Changing a tool into a work spindle of the machine tool; Determining the optimum operating point of the pump depending on the tool being used; or Determining an optimal operating point of the pump depending on the tool used and the material of the workpiece; and Machining the workpiece with the pump operating at the optimum operating point.

10. A machine tool for machining a workpiece, comprising: a work spindle for receiving a tool for machining the workpiece; a pump for delivering a cooling lubricant for cooling the tool; and a control device for numerically controlling the machine tool, characterized in that the control device is configured to carry out a method according to one of claims 1 to 9.

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