EP2817686A1

EP2817686A1 - Method for machining a material and apparatuses operating on the basis of said method

Info

Publication number: EP2817686A1
Application number: EP13706486.1A
Authority: EP
Inventors: Egbert SCHÄPERMEIER
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-02-24
Filing date: 2013-02-21
Publication date: 2014-12-31
Also published as: AT12948U1; WO2013124352A1

Abstract

The present invention relates to a method for machining a material with a controlled/regulated tool with a cutting edge, particularly by milling, in which the thermal diffusivity of the material, the chip thickness, the geometry of the cutting edge, and also the life of the tool and wear data that characterize the wear behaviour of the tool are taken as a basis, by means of a programmable control device, for continually calculating a cut and advance speed using the Péclet number and using said speed to control/regulate the tool.

Description

Method for machining a material and devices operating according to this method

description

Technical background

The present invention relates to a method for the machining of ductile materials with a geometrically defined cutting edge, in particular by milling. Furthermore, the invention relates to devices operating according to this method.

Ductile materials are machined, for example, by turning, drilling and milling.

State of the art

The control of such processes is still largely based on the empirical requirements that the operating personnel or work preparation from the

Cutting data recommendations of the tool manufacturer takes. Cutting data, which provide qualitatively and economically acceptable machining results, have so far been determined empirically. The influence of individual process parameters is not readily apparent.

Originally, the determination of such data was as follows:

The tool was made in a turning or milling machine at different

Cutting speeds and cutting depths used. Observations of

Cutting as well as the visual assessment of the resulting chips made it possible to describe the cutting conditions under which a tool performs best to predict tool wear. The results of such machining studies provide empirically collected data, which are summarized in tables and according to which the rotation or

Milling machines are usually set. This type of cutting data determination requires a lot of experience of the operator of the machine.

More recent methods try to simulate the chip flow with mathematical models, in order to get closer to the real conditions during machining in a more objective way and in order to pre-determine the optimal machine setting data. In addition, methods are known from the prior art, which are based on the development of cutting tools. By way of example, EP 592 541 A1 may be mentioned here. Here, an attempt is made to mathematically record the chip formation process using the artificial intelligence method. However, the model does not take into account the tool wear. The use in terms of calculation

economical cutting data is therefore only possible to a limited extent.

As a result, mathematical models of

Metal cutting operations developed to predict the shear angle, the resulting chip thickness and forces acting on the cutting edge. Most of these models include an elastoplastic material model, but the influence of temperature-induced diffusion is generally not considered. But this is the friction along the tool / chip interface, as well as the temperature and

speed-dependent properties of the material to be machined, and the mechanics of chip removal from the workpiece reproduced only incomplete.

In addition, the connection with the tool wear is missing here as well.

As another method, the finite element model for the cutting process to Strenkowski / Carroll and Usui has become known. However, the simulations with these models are only applicable for very low cutting speeds.

As a result, Iwatas developed a model for the fractional prediction of the Spans of the

Workpiece developed on the basis of the elongation at fracture of the considered steel grade.

To design a material model in which the thermoelastoplastic influence due to friction at the interface between tool and chip is taken into account Strenkowski and Mitchum have developed a material model that takes into account thermoelastopolastic factors resulting from friction at the interface between tool and chip. For the

Separation of the chip from the workpiece, a parting line criterion was introduced and a critical load dimension was implemented. Again, the coupling of the

Cutting data with tool wear.

Technical task

The state of the art thus still lacks an all-encompassing computational description of the physical phenomena in cutting processes. Thus, it is currently not possible to calculate economic cutting data purely by calculation as a function of any possible operating conditions of a tool. The sole method so far remains the experimental determination of economic cutting data, the value for the corresponding cutting speed of a tool depends on

The chip thickness,

The cutting width,

• the workpiece material and

• the service life.

Using the tool for a range of workpiece materials under various engagement conditions results in an extensive table work.

When programming the machining of a workpiece with different engagement conditions, therefore, the corresponding cutting speed must be taken from the tables for each individual step of machining, which is performed with this tool, and assigned to the machining step.

Based on this prior art, the present invention seeks to provide a method for the machining of ductile materials with geometrically defined

Cutting edge to provide that less effort for the determination and the application of qualitatively and economically acceptable cutting data in practice.

It is another object of the invention to provide a device suitable for carrying out the method and a method for their operation, which is easy to set and operate, and allows retrofitting existing systems.

The task is also to provide tool manufacturers with a method in which the wear data of his tools are deposited and thus the optimization of the use of these tools for the user for any application cost and without consulting and preliminary tests are feasible.

Brief description of the invention

With regard to the method, this object is achieved by a method with the

technical features of claim 1 solved. Advantageous developments are the subject of the dependent method claims.

The solution according to the invention for these tasks is based on the following considerations, wherein the material removal is referred to below by the term "chip flow":

The peeling of the chip from a workpiece is a non-linear physical process. This automatically creates new surfaces and there are large, related to the chip formation deformations that lead to the generation of heat. Sources of this heat generation are the sliding contact along the

Tool surface and the permanent deformation of workpiece and chip.

In order to reshape the material to be cut off and push it away over the cutting surface, mechanical work is required, which is achieved by the cutting force. The mechanical energy is converted into heat energy. If the work done is converted accordingly, the volume of material removed per unit volume results in a certain amount of heat, the value of which corresponds approximately to the heat of fusion. There but the chip formation is in the range of milliseconds, the local exposure time of the tool is not sufficient to actually melt the abgespante material. However, it forms a temperature field, which changes the physical properties of the workpiece material in the forming zone.

The temperature field and the voltage field interact, additionally influenced by diffusion processes. Chip formation is thus a highly dynamic process, which can not only be described according to the laws of strength theory and not only according to the laws of plasticity theory.

The work performed during the material removal process leads directly to the generation of heat. The elevated temperature affects both the

Material properties of the workpiece as well as those of the tool. that

Part of the heat generated by the deformation of the chip, remains in this, as well as a proportion of that heat, which results from the detachment of the chip from the workpiece. Finally, a good part of the frictional heat remains in the chip. These amounts of heat are generated by the resulting chip and chip flow

removed.

The influence of the temperature on the chip formation is in the contact zone between the chip underside and clamping surface of the tool greatest, since here the

Frictional heat in addition to the heat produced by the forming work. The temperature at the underside of the chip influences the shear strength in this area, whereby this shear resistance is decisive for the power that has to be applied for pushing out the chip over the rake face.

With the help of the similarity mechanics as well as the power balance of the chip formation, a model for the heat generation and the heat flow can be set up for ductile materials, from which it emerges that the heat flow through the tool is negligible in practice.

In the similarity mechanics, the heat and mass transfer between two mutually moving bodies is described by the Peclet number Pe. The Peclet number Pe is a dimensionless metric. The Peclet number is formed from the ratio the relative velocity between the two bodies and the

Propagation speed of the temperature field orthogonal to the sliding surface.

The chip formation takes place with the cutting speed v _c and the chip thickness h. The thickness of the resulting chip h _spa n is greater than the chip thickness h by the compression factor λ. The chip thickness thus results from the product h ^* A. Due to the chip compression, the chip is pushed out over the rake face at a value which is lower than the cutting speed. This is the

Relative speed between chip and rake surface v _c / A.

The propagation speed of a temperature field depends on the

Temperature code of the body a divided by the distance from the sliding surface. In the present case, this distance corresponds to the chip thickness h _spa n.

For the Peclet number Pe one gets

a a

h * e

The product v _c ^* h is referred to as the chip removal volume and is a measure of the volume cut off per mm span width.

If two cutting operations with the same Peclet number occur, the temperature in the contact area between the chip underside and the chip surface is the same.

The extension force related to 1 mm chip width depends on steel

Flow chip formation from the temperature. For unalloyed and alloyed steels, the dependence of the extension force on the Peclet number can be represented in a curve as shown in FIG. In the diagram of Fig. 1 are plotted on the ordinate the Ausschubkraft and on the abscissa the Peclet number. The boundaries of these regions are marked by values of the Peclet number Pei-n and Pen-m, which are about the same for all materials with continuous chip formation, and which are at 6 to 8 and at 12 to 14, respectively.

The machinability of ductile materials can thus be characterized by the thermal conductivity of the material to be processed in the associated temperature range. As a result, the faster the chip absorbs the resulting heat, the higher the chip removal rate can be.

The illustration in FIG. 1 has three regions I, II and III. In the three areas, the contact conditions differ between the underside of the chip and the chip surface.

In area I, the temperature is lower than in areas II and III. For standard steels, the temperature in area I is below 721 ° C. The temperature is not enough to soften the chip bottom. The contact in this area consists only of the contact of the above microscopic irregularities of

Chip underside and clamping surface. When pushing out the chip, the edges of the chip lower side are sheared off and transported away into the hollows of the chip underside.

In area II, the chip underside is heated to such an extent that the unevenness of the two contact surfaces penetrates more strongly. The volume of the sheds sheared when pushing out the chip increases. As a result, part of the sheared material will remain in the troughs of the rake face, tending to weld. If the weld is too high, the coating dissolves again. This process is periodic and leads to uncontrolled Abtragsbedingungen.

In area III, the temperature has risen so much that the chip underside adapts to the unevenness of the rake face due to the softening, and thus none

Residues on the rake surface leaves.

The tool steels can not be used in the range above 721 ° C for reasons of wear resistance. Can therefore be used economically only in the area I. On the other hand, the hard cutting materials can be used at temperatures that allow processing in the area III. These cutting materials are underused, as long as they are used in area I. Because of the uncontrolled

Machining conditions, which also cause increased wear, the machining in area II must be avoided in general.

The temperature in the area of the cutting surface of the tool also determines the service life of the tool. For curves of constant service life T, the dependency illustrated in FIG. 2 results for region III:

In this diagram, the Peclet number is plotted logarithmically against the chip thickness. For a given constant tool life, the logarithmic graph shows a linear dependence between Peclet number and

Chip thickness. The straight line for a service life of the tool of 15 minutes is to be determined by lifetime tests.

Pe 15

It is defined by the two wear characteristics C15 and x.

The two values are exclusively tool-specific and apply unchanged to the machining of ductile materials.

For values of the service life T, which deviate from 15 minutes, is general

The wear constants provide the Peclet number for a given chip thickness, which gives the tool life T.

Compared to turning and drilling, the cutting edge is machined during milling

intermittently. In each case, a commaspan is lifted from each cutting edge with each revolution of the tool. The Peclet number is thus during the machining process even with temporally constant engagement conditions of the tool in the

Workpiece variable due to the change over time in the chip thickness. From the lateral engagement of the tool in the workpiece, the chip thickness h can be calculated, which determines the wear of the tool. This is the largest thickness of the Kommaspans h _max , which corresponds to the feed / tooth s _z in the case of the full cut of a milling cutter.

D _F * π * v _f

z = -

v _c * z

D _F = cutter diameter

v _f = feed rate

v _c = cutting speed

z = number of teeth

The cutting speed is the relative speed between

Tool cutting edge and workpiece. The feed is the movement of the tool in relation to the workpiece.

From graph 2 it is possible to read off the associated Peclet number Pebearb for this chip thickness s _z and for the desired tool life T. The equation for the Peclet number returns the value for the cutting speed v _c after conversion

The illustration in FIG. 2 reveals that the value for the Peclet number is greater the greater the maximum chip thickness for a particular machining case is selected for a given tool life. Since the removal rate is proportional to the value of the Peclet number, the method eliminates the need to choose the chip thickness as large as possible.

The chip thickness stands for the load of the cutting edge. For this is from the

Tool manufacturer recommended a limit s _zm ax. With the value s _{z to} edit _{z ma} x, the cutting speed v _C can then be calculated. The feed rate v _{f bea} rb is obtained with the previously selected or

ascertained values. s z edit * V c edit * z

Thus, all data for the machine setting are available.

The method according to the invention thus has the following advantages in the processing of ductile materials compared to the conventional procedure according to the prior art:

• The wear data of a tool can be determined with just a few puncture tests and are the same for machining different ductile materials

• The definition of economic cutting data is purely computational based on the specifications

- Wear data of the tool

- Thermal characteristics of workpiece material

- Lateral delivery of the tool

- life

• The programming of machining programs is simplified and generalized

- Only intervention conditions are programmed for each tool used - The cutting speeds are calculated continuously during the milling process

- The machining program can be used without modification for machining the same workpiece made of different ductile materials.

• Machining with changing engagement conditions during one

Processing step can be regulated with regard to constant levels. For example, the present invention manifests itself in

• a procedure for the machining of ductile materials with geometric

defined cutting edge, in particular by milling, characterized by the following steps:

- For the workpiece material to be machined, the value of

Determined thermal conductivity,

- A suitable tool for machining this workpiece material tool is selected

- The wear data for this tool are determined by random tests

- The conditions of engagement between tool and workpiece are determined

- The desired tool life is selected and

- Based on these specifications is by means of a programmable

Control device the optimal cutting speed and the optimal feed rate is calculated continuously from the changing during machining engagement conditions and passed to the control of a processing device, or

• a device for the machining of ductile materials with geometric

defined cutting edge with a programmable control device, characterized in that the programmable control device is adapted to detect the following values as input variables:

- the value of the thermal conductivity of the material to be processed

Workpiece material,

- the wear data of the selected tool

- the engagement conditions between tool and workpiece

the desired service life of the tool and that the control device is further designed to continuously calculate the optimum cutting speed and the optimum feed speed on the basis of these values and to transmit them to a control device of the device, or

• a program logic for the control of a cutting device for ductile materials with geometrically defined cutting edge, characterized in that they are based on

the thermal conductivity of a workpiece material to be machined,

the characteristic of a tool selected for machining this workpiece material,

- the wear data of this tool, - The initial predetermined and in the course of processing constantly changing engagement conditions between the tool and the workpiece

- and the desired tool life of the optimum cutting speed and the optimum feed rate continuously calculated and passed to the control of the cutting device, or.

• a program logic for the regulation of a cutting device for ductile materials with geometrically defined cutting edge, characterized in that they are based on

the thermal conductivity of a workpiece material to be machined,

the characteristic of a tool selected for machining this workpiece material,

- the wear data of this tool,

- The initial predetermined and in the course of processing constantly changing engagement conditions between the tool and the workpiece

- And the desired tool life of the optimal

Cutting speed and the optimum feed rate continuously calculated and passed to the control of the cutting device on.

embodiment

In the following the invention will be explained in more detail with reference to an embodiment.

In a workpiece made of stainless steel (X5CrNi18-10), a V-groove with a width of 20 mm is produced by means of a milling cutter.

To prepare or carry out the milling is a computer-controlled

Device used. This consists of • a software for calculating the cutting speed and the feed speed

• an interface to a processing unit for transfer and

Further processing of the two calculated values.

The software generates a user interface in a computer via which the following values are specified.

• workpiece o thermal conductivity

• Tool o Cutter diameter o Number of cutting edges o Maximum chip thickness o Tool life o Wear indexes

• Interference of the tool with the workpiece o Lateral infeed and calculates the cutting speed o the feed rate using the formulas given above o

It is mathematically checked whether the selected specifications result in a treatment whose Peclet number is above the critical value of 15, and thus a

provides economically justifiable processing.

The processing unit can be o A pure output unit for the visualization of the calculated results o A CAM / CAD program o The control of the machine selected for machining

Device is used for carrying out the method according to the invention, will be explained in more detail below by way of example.

• The material-specific thermal diffusivity a in this case is 8 mm ² / s

• The tool is an end mill.

DF = 20 mm

z = 6

Szmax = 0.24 mm

• Its service life is set to 240 minutes.

• The wear indicators are

C = 550 x = 0.65

• From the values available, v _{c is} calculated according to the above equation: v _c = 223 m / min

• Based on this information you get the required

Feed rate vf:

vf = 5,125 mm / min The calculated values are used to calculate the Peclet number Pe = 1 1 1, 5

Thus, the machine setting data results in a processing in the area III.

Claims

claims

1 . Method for the cutting of a material with a controlled / controlled tool, in particular by milling, characterized in that on the basis of the thermal conductivity of the material, the chip thickness, and the tool life and wear data, the

Characterize the wear behavior of the tool by means of a

programmable controller means a cutting and

Feed rate with the aid of the Peclet number continuously calculated and used for controlling / regulating the tool.

2. The method according to claim 1, characterized in that the tool has a cutting edge, and that also the geometry of the cutting edge of the calculation of the cutting and feed rate is used.

3. The method according to claim 1 or 2, characterized in that the

Wear data for the selected tool by 2 to 7, preferably 2 to 5, random tests are determined.

4. The method according to any one of the preceding claims, characterized in that the cutting and feed rates of the tool is controlled to a constant time chip volume.

5. Device for the machining of a material with a controlled / regulated

Tool, comprising a programmable control device, characterized in that the programmable control device thereto

is designed to record the following values as input variables:

the thermal conductivity of the material,

- Wear data, the wear behavior of the tool

characterize, - the chip thickness, and

- The life of the tool and that the control device is further adapted to continuously calculate based on these values using the Peclet number cutting and feed rate and to transmit the calculated cutting and feed rates to a control device of the device for controlling the tool.

6. The device according to claim 5, characterized in that the input variables also include the geometric structure of the tool.

7. Apparatus according to claim 5 or 6, characterized in that the

Input sizes also include the lateral engagement width.

8. Method of processing data necessary for the control of a

Device for machining a material with a controlled tool are relevant, characterized in that a in a

Data processing device stored program means based on

Temperature conductivity of the material, the chip thickness, the tool life and wear data that characterize the wear behavior of the tool, a cutting and feed rate of the tool continuously calculated and outputs the calculated cutting and feed rates to a control unit of the cutting device, wherein the calculated cutting and feed rates are used to control the tool.