METHOD FOR STEADY-STATE CONTROL OF CUTTING STATE

A method for steady-state control of cutting state, implemented based on computer numerical control (CNC) machine tools or cutting lathes. The method comprises: fitting parameters of a stored energy evolution model of a workpiece material to be machined; discretizing primary shear zone into multiple infinitesimals along normal direction of main shear plane; introducing equivalent cutting edge model, inputting pre-used cutting parameters, calculating strain and strain rate of each infinitesimal and analyzing temperature of each infinitesimal; deducing and solving differential equation of each infinitesimal of stored energy to position of the primary shear zone by taking initial shear plane of the primary shear zone as model boundary; determining application values of cutting parameters according to solved results; and, controlling and adjusting the cutting tool in the actual cutting process to cut the workpiece material to be machined with the application values.

TECHNICAL FIELD

The present invention belongs to the field of cutting technologies, and in particular, relates to a method for steady-state control of cutting state.

BACKGROUND

Descriptions herein only provide background techniques related to the present invention, and do not necessarily constitute the related art.

Metal cutting is a significant process for the formation of metallic materials and production of mechanical components, which is a complex thermal-mechanical coupling process encompassing the fields of machinery, materials and dynamics. The process includes a plurality of phenomena such as large elastoplastic deformation, a high temperature, a high strain rate, severe friction, and material failure. Therefore, performing research on the cutting mechanism to find out the relationship between the input (such as a machine tool system, machining parameters, tool parameters, and workpiece performance) and the output (such as the integrity of a machined surface and the service performance of components) of the cutting process is of great significance. The inventor found that, due to the complexity of the cutting process, most current researches on the cutting mechanism are limited to the empirical formulas and the phenomenological models, failing to provide a fundamental formation mechanism of a machined surface. The cutting process always includes input, output, storage, and dissipation of energy. The same is true for macroscopic deformation and structural transformation or microscopic dislocations, grain slip, recrystallization, and phase transformation. In addition, the research showed that the energy storage and dissipation of the machined surface greatly affects the performance and the surface integrity of the material.

SUMMARY

In the traditional research of the cutting mechanism, the stress field has directionality, excessive characteristic parameters are present, and the research process is complex. In view of the shortcomings of the prior art, the present invention provides a method and system for calculating a stored energy field of the primary shear zone during steady-state cutting, further provides a method for steady-state control of cutting state, to predict a cutting force, a cutting temperature, a chip morphology, and material properties by using a stored energy distribution of the primary shear zone, and obtain control parameters of steady cutting of a cutting tool according to the prediction results to implement a steady control of a working (cutting) state of the cutting tool in an actual machining process.

To achieve the foregoing objective, the present invention is implemented by the following technical solutions.

In a first aspect, the technical solutions of the present invention provide a method for calculating a stored energy field of the primary shear zone during steady-state cutting. The method includes steps of:

In a second aspect, the technical solutions of the present invention further provide a system for calculating a stored energy field of the primary shear zone during steady-state cutting. The system includes:

In a third aspect, the technical solutions of the present invention further provide a method for steady-state control of cutting state, comprising the following steps:

The technical solutions of the present invention have the following beneficial effects:

The spacing or dimensions between each part are exaggerated to show the position of each part, and the schematic diagrams are used only for illustrative purposes.

DETAILED DESCRIPTION

It should be pointed out that the following detailed descriptions are all illustrative and are intended to provide further descriptions of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those usually understood by a person of ordinary skill in the art to which the present invention belongs.

It should be noted that the terms used herein are merely used for describing specific implementations, and are not intended to limit exemplary implementations of the present invention. As used herein, the singular form is also intended to include the plural form unless the present invention clearly dictates otherwise. In addition, it should be further understood that, terms “include” and/or “including” used in this specification indicate that there are features, steps, operations, devices, components, and/or combinations thereof.

For convenience of description, the terms “above”, “below”, “left”, and “right” only indicate directions consistent with those of the accompanying drawings, are not intended to limit the structure, and are used only for ease and brevity of illustration and description, rather than indicating or implying that the mentioned device or element needs to have a particular orientation or needs to be constructed and operated in a particular orientation. Therefore, such terms should not be construed as a limitation on the present invention.

For the part of term explanation, terms in the present invention such as “mount”, “connect”, “connection”, and “fix” should be understood in a broad sense. For example, the connection may be a fixed connection, a detachable connection, or an integral connection, a mechanical connection, an electrical connection, a direct connection, an indirect connection by using an intermediate medium, an interior connection between two components, or interaction between two components. A person of ordinary skill in the art may understand specific meanings of the foregoing terms in the present invention according to a specific situation.

As described in the background, in the traditional research of a cutting mechanism, the stress field has directionality, excessive characteristic parameters are present, and the research process is complex. In view of the shortcomings of the prior art, the present invention provides a method and system for calculating a stored energy field of the primary shear zone during steady-state cutting, to predict a cutting force, a cutting temperature, a chip morphology, and material properties by using a stored energy distribution of the primary shear zone.

In a typical implementation of the present invention, the present example discloses a method for calculating a stored energy field of the primary shear zone during steady-state cutting. The method includes the following steps:

(1) Fitting parameters of a stored energy evolution model of a workpiece material based on stress-strain curves of the workpiece material in different deformation conditions, where the model is related to the temperature, strain, and strain rate. Stored energy Es may be represented by a dislocation density ρtotal. That is to say:

The dislocation cell wall thickness δ is 1.28×10−9 m, and the dislocation cell structure diameter is expressed as:

D
    cell
   
   =
   
    
     k
     cell
    
    /
    
     
      ρ
      total
     
    
   
  
  ,

(2) As shown in FIG. 1, a zone CDFE is the primary shear zone, CD is an initial shear plane, EF is a final shear plane, and AB is a main shear plane. In order to reduce the complexity of the analysis, the primary shear zone is discretized into N infinitesimals from CD to EF in a normal direction of AB, that is, N+1 planes. When N is large enough, a strain rate and a temperature in each infinitesimal may be assumed as constants.

(3) Introducing an equivalent cutting edge model, as shown in FIG. 2, define two ends of an actual cutting tool as an equivalent cutting edge, simplify actual 3D cutting into 2D cutting, and calculate distributions of a strain rate {dot over (γ)} and a strain γ of the primary shear zone according to a shear plane model:

A thickness of the shear zone is obtained according to the empirical formula of Oxley:

In order to obtain the shear angle, experiments are usually required to obtain a deformation coefficient. For simplicity, in the present example, an approximate shear angle formula of Merchant is used:

According to the distribution laws of the strain and the strain rate, the formula is substituted into a center position of the each infinitesimal, to obtain an average strain and an average strain rate of the each infinitesimal. The average strain and the average strain rate are respectively used as a strain feature value and a strain rate feature value of the each infinitesimal.

According to the heat conduction equation, the temperature value of the Kth plane is represented by a temperature value of a (K−1)th plane:

(4) Deriving a differential equation of stored energy versus location y by using the stored energy evolution model:

Since the strain rate and the temperature in the each infinitesimal are both regarded as constants, the differential equation in the each infinitesimal may be simplified. Finally, stored energy Es|k of the Kth plane is calculated by stored energy Es|K−1 of the (K−1)th plane:

The initial shear plane of the primary shear zone is used as a model boundary. Stored energy of a next plane is calculated in the each infinitesimal according to the foregoing formula, to obtain stored energy of all (N+1)th planes. The stored energy of all planes is used as the stored energy of the location in the primary shear zone. That is to say, the stored energy field distribution of the primary shear zone is obtained. The specific stored energy calculation process is shown in the process block diagram in FIG. 2.

(5) Predicting a stress field and a temperature field of the primary shear zone based on the stored energy field, and then analyze the cutting force, the cutting temperature, a chip forming law, and the material modification.

According to the foregoing technical solutions, the coefficient kcell in step (1) may be obtained in two ways. In the first way, the coefficient may be estimated as μ/200. In the second way, the dislocation density ρtotal and the dislocation cell diameter Dcell after the machining are measured by experiments, and then kcell=Dcell√{square root over (ρtotal)} is calculated.

The recovery factor r is a function of the strain rate and the temperature, which is expressed as:

According to the foregoing technical solutions, the primary shear zone in the steady-cutting process is discretized into N infinitesimals in a normal direction of the main shear plane, and a strain rate and a temperature in each infinitesimal are regarded as constants to simplify a solving process of a differential equation.

According to the foregoing technical solutions, a line connecting two end points of an actual cutting edge projected on a base plane is defined as an equivalent cutting edge, a cutting tool angle of the equivalent cutting edge is calculated according to a geometric relationship and is used as an actual cutting tool angle, and then a strain distribution and a strain rate distribution of the primary shear zone are calculated by using a normal rake angle of the equivalent cutting edge as an input parameter of the shear plane model.

According to the foregoing technical solutions, stored energy of the discrete planes in the primary shear zone in step (4) may be obtained by solving the following mathematical physical problem:

Boundary conditions are as follows: the initial shear plane is used as a model boundary, that is, an 0th plane in the (N+1) th planes obtained by division, and it is assumed according to the experimental results and the models that a temperature of the initial shear plane is a room temperature (25° C.), a strain and a strain rate are both 0, and stored energy is stored energy of an initial material.

A mathematical physical equation is:

The equation may be explained as follows: The stored energy of the Kth plane may be obtained by integration of the stored energy of the (K−1)th plane, wherein {dot over (γ)}AVE and T are respectively a feature strain rate and a feature temperature of a Kth element.

According to the foregoing technical solutions, the stored energy-based shear stress field prediction in step (5) is based on a mapping relationship between stored energy and a dislocation density and a mapping relationship between a shear flow stress and a dislocation density. The mapping relationship between a shear stress and a dislocation density may be expressed as:

In the formula, the first term to is a non-thermal stress independent of the strain rate during movement of a dislocation, the second term is a long-range stress of the movement of the dislocation, and the third term is the short-range stress during the movement of the dislocation. M is a Taylor coefficient, as is a constant generally having a value of 0.3-0.5, Tiu is a stress value required for the dislocation to cross an obstacle without the assistance of thermal activation, where a value thereof mainly depends on a strength of the obstacle during the movement of the dislocation, the constants p and q satisfy 0<p≤1 and 1≤q≤2, and values thereof depend on a shape of the energy barrier, ΔF=f2μb3 is Helmholtz free energy required by the dislocation to overcome the short-range barrier without the assistance of an external force, kb is a Boltzmann constant, T is an absolute temperature, and {dot over (ϵ)}p and {dot over (ϵ)}0 are respectively a plastic strain rate and a reference strain rate. According to the mapping relationship between stored energy and a dislocation density, a shear flow stress prediction formula based on the stored energy is obtained as:

According to the foregoing technical solutions, a normal stress κ perpendicular to the main shear plane is calculated based on a Hencky equation:

A normal stress KA at a free surface may be estimated by a stress at the free surface. Therefore:

A chip morphology prediction method is as follows: It is assumed the loading stage exists before the main shear plane and an unloading stage exists after the main shear plane AB. If a stored energy peak occurs at a location before AB, it may be considered that adiabatic shear occurs in the cutting condition and sawtooth chips are formed.

According to the foregoing technical solutions, a microhardness prediction formula based on the stored energy is:

A microscopic residual stress may be directly represented by the stored energy.

The present example fills up the gap in the existing cutting mechanism research technology. Based on the shear plane model in the traditional research of the cutting mechanism, the equivalent cutting edge model was first introduced to simplify three-dimensional cutting into two-dimensional cutting. Then, the established stored energy evolution model of the machined material and the strain rate distribution model, the strain distribution model, and the temperature distribution model of the primary shear zone during the cutting are substituted into the shear plane model, and the differential equation of stored energy versus location in the primary shear zone is obtained to obtain the stored energy distribution of the primary shear zone. Finally, the stress field and the temperature field of the primary shear zone are analyzed based on the obtained stored energy distribution, and the cutting force, the cutting temperature, and the material modification are further predicted. The stored energy field runs through the entire cutting process, and the cutting mechanism is explained more deeply, completely, and clearly in a simpler way, greatly promoting the research of the cutting mechanism.

For example, the workpiece material is nickel-based alloy inconel718, the cutting tool is a cemented carbide cutting tool, and parameters of the cutting tool are as follows: a rake angle of 0°, a tool cutting edge angle of 90°, a tool cutting edge inclination of 0°, and a tool tip arc radius of 0.8 mm. The following cutting parameters are used:

Cutting

Depth

Group
speed
Feed
of cut

A method for calculating a stored energy field of the primary shear zone during steady-state cutting of the nickel-based alloy inconel718 includes the following steps:

1) Fitting parameters of a stored energy evolution model of the material based on stress-strain curves of the workpiece material in different deformation conditions, where the model is related to a temperature, a strain, and a strain rate;

Stored energy Es may be represented by a dislocation density ρtotal. That is to say:

For the inconel718 workpiece material, α=0.5, μ=80000 MPa, b=2.56e−7 mm, and χ=0.6. A total dislocation density is

The dislocation cell wall thickness δ is 1.28×10−9 m, and the dislocation cell structure diameter is expressed as:

D
    cell
   
   =
   
    
     k
     cell
    
    /
    
     
      ρ
      total
     
    
   
  
  ,

By fitting results of stress-strain curves at different strain rates and different temperatures, the following is obtained:

2) In order to reduce the complexity of the analysis, the primary shear zone is discretized into N infinitesimals from an initial shear plane CD to a final shear plane EF in a normal direction of a main shear plane AB, that is, N+1 planes, where when N is large enough, a strain rate and a temperature in each infinitesimal may be assumed as constants, and in consideration of calculation efficiency and calculation accuracy, N=200.

3) Introducing an equivalent cutting edge model, simplify actual 3D cutting into 2D cutting, and calculate distributions of a strain rate j and a strain y of the primary shear zone according to a shear plane model:

A thickness of the shear zone is obtained according to the empirical formula of oxley:

In order to obtain the shear angle, experiments are usually required to obtain a deformation coefficient. For simplicity, in the present example, an approximate shear angle formula of Merchant is used:

According to the distribution laws of the strain and the strain rate, the formula is substituted into a center position of the each infinitesimal, to obtain an average strain and an average strain rate of the each infinitesimal. The average strain and the average strain rate are respectively used as a strain feature value and a strain rate feature value of the each infinitesimal.

According to the heat conduction equation, the temperature value of a Kth plane is represented by a temperature value of a (K−1)th plane:

4) Deriving a differential equation of stored energy versus location y by using the stored energy evolution model:

Since the strain rate and the temperature in the each infinitesimal are both regarded as fixed values, the differential equation in the each infinitesimal may be simplified. Finally, stored energy Es|K of the Kth plane is calculated by stored energy Es|K−1 of the (K−1) th plane:

The initial shear plane of the primary shear zone is used as a model boundary. Stored energy of a next plane is calculated in the each infinitesimal according to the foregoing formula, to obtain stored energy of all (N+1)th planes. The stored energy of all planes is used as the stored energy of the location in the primary shear zone. That is to say, the stored energy field distribution of the primary shear zone is obtained. FIG. 3 shows a stored energy field result obtained according to the cutting parameters in the table.

5) Predicting a stress field and a temperature field of the primary shear zone based on the stored energy field, and then analyze the cutting force, the cutting temperature, a chip forming law, and the material modification.

A shear flow stress prediction formula based on the stored energy is obtained as:

A stress field distribution of the primary shear zone is predicted according to the stored energy field result, as shown in FIG. 4. A temperature field distribution of the primary shear zone is predicted according to the stored energy field result, as shown in FIG. 5.

A predicted stress is used as an input and is substituted into a main cutting force prediction formula. A comparison between a predicted result and an experimental result of the cutting force is shown in the table. It may be learned that the predicted result is quite similar with the experimental result.

Group No .:
Predicted  value (N)
Experimental value (N)

A chip morphology prediction method is as follows: it is assumed the loading stage exists before the main shear plane and an unloading stage exists after the main shear plane AB. If a stored energy peak occurs at a location before AB, it may be considered that adiabatic shear occurs in the cutting condition and sawtooth chips are formed. In consideration of a hysteresis effect of the adiabatic shear, a critical speed at which the sawtooth chips are generated is about 30 m/min, which is quite in line with the experimental result. Hardness variations of the cut shear zone may be obtained based on a mapping relationship between micro-hardness and stored energy, and a microscopic stored energy distribution of the cut shear zone may be further obtained according to the stored energy distribution.

In a typical implementation of the present invention, the present example discloses a system for calculating a stored energy field of the primary shear zone during steady-state cutting. The system includes:

The infinitesimals are a series of discrete infinitesimals generated in a normal direction of the main shear plane on the primary shear zone.

It may be understood that, a pre-processing module may be an existing processor. The processor is connected to a memory in which the program codes of the fitting unit, the infinitesimal generation unit, and the conversion unit in the present example are burned. Alternatively, the pre-processing module includes three processors. The three processors each are connected to a memory in which the program codes of the fitting unit, the infinitesimal generation unit, and the conversion unit in the present example are burned. An output conversion module is further connected to the pre-processing module for reading, analyzing, organizing, assembling, converting, and drawing results outputted by the pre-processing module, and mapping various attributes to inherent names and attributes.

The solution unit is configured to establish a differential equation and a definite solution condition for a mathematical engineering model according to the mathematical engineering model, a numerical discrete algorithm, and a numerical solution method that are preselected, perform calculation of discretized regions for continuous time physical quantities and continuous space physical quantities, and establish an algebraic equation by the numerical solution method to form a solution result.

It may be understood that the solution unit may be an existing processor.

It may be understood that, in the present example, the system described in Embodiment 1 may be used for calculation. Specifically, in the pre-processing module, the fitting unit is configured to perform step (1), the infinitesimal generation unit is configured to perform step (2), the conversion unit is configured to perform step (3), and the solving module is configured to perform step (4) and step (5).

In a typical implementation of the present invention, the present example discloses a method for steady-state control of cutting state, including the following steps:

The method for the steady-state control of the cutting state in an actual cutting process, specially comprises the following steps:

1) Fitting parameters of a stored energy evolution model of the material based on stress-strain curves of the workpiece material in different deformation conditions, where the model is related to a temperature, a strain, and a strain rate;

Stored energy Es may be represented by a dislocation density ρtotal, which is:

Use a workpiece material of inconel718 as an example.

For the inconel718 workpiece material, α=0.5, μ=80000 MPa, b=2.56e−7 mm, and χ=0.6. A total dislocation density is

The dislocation cell wall thickness δ is 1.28×10−9 m, and the dislocation cell structure diameter is expressed as:

D
    cell
   
   =
   
    
     k
     cell
    
    /
    
     
      ρ
      total
     
    
   
  
  ,

By fitting results of stress-strain curves at different strain rates and different temperatures, the following is obtained:

2) In order to reduce the complexity of the analysis, the primary shear zone is discretized into N infinitesimals from an initial shear plane CD to a final shear plane EF in a normal direction of a main shear plane AB, that is, N+1 planes, where when N is large enough, a strain rate and a temperature in each infinitesimal may be assumed as constants, and in consideration of calculation efficiency and calculation accuracy, N=200.

3) Introducing an equivalent cutting edge model, simplify actual 3D cutting into 2D cutting, and calculate distributions of a strain rate j and a strain y of the primary shear zone according to a shear plane model:

A thickness of the shear zone is obtained according to the empirical formula of oxley:

In order to obtain the shear angle, experiments are usually required to obtain a deformation coefficient. For simplicity, in the present example, an approximate shear angle formula of Merchant is used:

According to the distribution laws of the strain and the strain rate, the formula is substituted into a center position of the each infinitesimal, to obtain an average strain and an average strain rate of the each infinitesimal. The average strain and the average strain rate are respectively used as a strain feature value and a strain rate feature value of the each infinitesimal.

According to the heat conduction equation, the temperature value of a Kth plane is represented by a temperature value of a (K−1)th plane:

4) Deriving a differential equation of stored energy versus location y by using the stored energy evolution model:

Since the strain rate and the temperature in the each infinitesimal are both regarded as fixed values, the differential equation in the each infinitesimal may be simplified. Finally, stored energy Es|K of the Kth plane is calculated by stored energy Es|K−1 of the (K−1)th plane:

The initial shear plane of the primary shear zone is used as a model boundary. Stored energy of a next plane is calculated in the each infinitesimal according to the foregoing formula, to obtain stored energy of all (N+1)th planes. The stored energy of all planes is used as the stored energy of the location in the primary shear zone. That is to say, the stored energy field distribution of the primary shear zone is obtained. FIG. 3 shows stored energy fields result obtained according to the cutting parameters in the table.

5) Predicting a stress field and a temperature field of the primary shear zone based on the stored energy field, and then analyze the cutting force, the cutting temperature, a chip forming law, and the material modification.

A shear flow stress prediction formula based on the stored energy is obtained as:

A stress field distribution of the primary shear zone is predicted according to the stored energy field result, as shown in FIG. 4. A temperature field distribution of the primary shear zone is predicted according to the stored energy field result, as shown in FIG. 5.

(6) Using a predicted stress as an input and is substituted into a main cutting force prediction formula, to analyze and judge the cutting force prediction results.

Wherein, the main cutting force Fc prediction formula is as follows:

When the predicted cutting force, cutting temperature, and/or chip morphology obtained from a certain prediction calculation meet the predefined steady-state requirements of the cutting state, the pre-used data of the cutting speed and cutting depth input in the certain prediction calculation are used as application values of steady-state control of a cutting tool in the actual cutting process.

A chip morphology prediction method is as follows: it is assumed the loading stage exists before the main shear plane and an unloading stage exists after the main shear plane AB. If a stored energy peak occurs at a location before AB, it may be considered that adiabatic shear occurs in the cutting condition and sawtooth chips are formed. Hardness variations of the cut shear zone may be obtained based on a mapping relationship between micro-hardness and stored energy, and a microscopic stored energy distribution of the cut shear zone may be further obtained according to the stored energy distribution.

(7) In the actual cutting process, the application values of the cutting speed and cutting depth can be input into a computer numerical control (CNC) machine tool, or operating corresponding control levers on a regular cutting lathe by an operator according to the application values of the cutting speed and cutting depth, the cutting tool can be controlled and adjusted to perform a steady-state machining on the workpiece material to be machined according to the input application values during the actual cutting process, and ultimately obtain the expected material properties of the machined workpiece material.

It can be understood that the predefined steady-state requirements for cutting states described in the present example may vary depending on the workpiece material to be machined, as well as the type and model of the cutting tool. Those skills in the art can set and adjust them based on experience, actual needs, and/or special requirements. The present invention is not particularly limited.

It can be understood that the example can adopt the example described in Example 1, but is not limited to the example described. Specifically, the workpiece material to be machined is the nickel-based alloy inconel718, the cutting tool is a cemented carbide cutting tool, and parameters of the cutting tool are as follows: a rake angle of 0°, a tool cutting edge angle of 90°, a tool cutting edge inclination of 0°, and a tool tip arc radius of 0.8 mm. The following cutting parameters are used:

Cutting

Depth

Group 
speed 
Feed 
of cut

The cutting force, the cutting temperature, and the forming law of the cutting chip, etc., are predicted. Based on the prediction results, and taking the adiabatic shear hysteresis effect into consideration, the critical velocity for generating serrated chips is approximately 30 m/min.

The above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. A person skilled in the art may make various alterations and variations to the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.