Method of digital automatic design of grinding operations carried out on CNC cylindrical grinding machines

. Based on the literature review, the problem is formulated, which consists in the absence of detailed recommendations for the design of grinding operations for CNC cylindrical grinding machines. This paper provides a solution to the problem described above using the developed method, which includes: the formation of initial data, the formation of grinding cycles, and the calculated determination of cycle parameters.


Introduction
Nowadays, grinding is one of the finishing operations for processing parts in order to achieve the specified design dimensions and high requirements.Grinding operations can be used not only to obtain high-precision operational dimensions, but also to fulfill shape tolerances due to spark-out processing modes.At the moment, the quality of the operations performed depends only on the experience and qualifications of the operator performing the processing, since the existing recommendations for choosing the characteristics of the grinding wheel and cutting conditions are mainly contained in the reference literature, but such recommendations are not enough to select effective cutting modes [1,2].Based on this, a problem is indicated, which consists in the absence of any detailed recommendations that allow designing grinding operations for CNC cylindrical grinding machines.
To solve the problem, it is proposed to develop and implement in the form of software a method for the automatic design of grinding operations carried out on CNC cylindrical grinding machines.The key feature of the method is that the process engineer will receive a toolkit that allows uploading the finished product drawing into the software, indicating the surfaces to be processed and their characteristics, and on the basis of this, obtaining a finished processing technology in the form of an automatic cycle and a control program.

Requirements to the method
For the correctness of the developed method of automatic design of grinding operations carried out on CNC cylindrical grinding machines, it is necessary to establish the main restrictions that will ensure the adequacy of the obtained results, specifically: 1. Automated design should be based on the input data, including: designated surfaces to be processed on a CNC cylindrical grinder, their characteristics (surface type, dimensions, accuracy degrees and roughness, technical requirements).
2. Automatic design should be carried out for all possible options for machined surfaces: cylindrical surfaces, end surfaces, conical surfaces, slots and teeth; with processing in the form of a cycle, and it means designing several possible cycles and choosing the optimal one from them.
3. When designing processing cycles, the physical nature of the process should be taken into account, including mechanics, thermal physics and dynamics, presented in the software in the form of high-precision mathematical models.
4. The result of automatic design is: a product processing technology and a file with a control program containing a set of frames for a CNC cylindrical grinding machine.

Stages of method development
Based on the requirements for the developed method of automatic design of grinding operations carried out on CNC cylindrical grinding machines, the design includes five main stages, graphically expressed in Figure 1: 1. Formation and input of initial data for processing design, including an indication of the processed surfaces and their characteristics.
2. Formation of all possible types of grinding cycles for each of the surfaces indicated for processing.
3. Calculation determination of the parameters of processing cycles in the course of deep digital modeling of the processing process, taking into account the physical nature of the process and internal, direct and inverse relationships.
4. Selection of the optimal processing cycle for each of the processed surfaces.5. Formation of the control program for a CNC cylindrical grinding machine.

Formation and input of initial data for design
At this stage, it is necessary to develop a convenient toolkit for representing the product in a computerized form, including information about the location of the surfaces to be treated and their characteristics (surface type, dimensions, accuracy degrees and roughness, technical requirements).In this case, it is supposed to take a file as a basis, which can be represented by a photograph, a scan or an exported drawing from a CAD system, and mark the surfaces to be processed and set their characteristics on it.As an example, let's consider the shaft drawing shown in Figure 2.After loading the source graphic file into the automatic design system, it is proposed to specify the following with the help of graphic tools: the zero of the workpiece coordinate system, the workpiece basing system, and the surfaces to be machined.Thus, the drawing with the indicated designations shown in Figure 3 includes: -zero of the workpiece coordinate system; -base surfaces -center hole in the left side of the shaft (B1) and center hole in the right side of the shaft (B2); -processed surfaces: a cylindrical surface with an end face on the right (S1); cylindrical surface with end face on the left (S2); slots (S3) and cylindrical surface (S4).Next, it is necessary to enter information about the basing, specifically: the type and subtype of basing, the Z and X coordinates.Among the possible basing schemes on CNC cylindrical grinding machines, there are generally four options: -installation in a three-jaw chuck, when the shaft is short and it is possible to fix it in the chuck; -installation in a three-jaw chuck and right center, when the shaft is long and it is possible to fix it in the chuck; -installation in the right and left centers along the center holes with the installation of a driver chuck; -installation in the left corrugated center and the right center.Thus, the B1 locating code can specify a three-jaw chuck, left center and driver chuck or left smooth center, and the B2 locating code can specify right center or nothing (Fig. 4).Thirdly, information about the surfaces being processed is entered: code, type and subtype of the surface, coordinates Z (beginning and end of the segment) and X (beginning and end of the segment), roughness of the cylindrical surface and in the presence of an end surface, as well as accuracy degree.Among the possible surfaces being processed may be the following types of surfaces: cylindrical short, cylindrical long, cylindrical and end, end, slotted and conical (Fig. 5).

Fig. 5. Table for entering information about surfaces being processed
Thus, performing these steps, the process engineer enters all the necessary data about the operation being carried out on the CNC cylindrical grinding machine.All further ac- tions on automatic processing design should be carried out by the developed software system [3].

Possible options for workpiece surface treatment in the form of machining cycles
After determining the basing schemes, processed surfaces and their parameters, the developed software starts to work.First of all, it is necessary to programmatically determine all the cycles that will be designed for each of the surfaces [4,5].To do this, we consider all possible options for the surfaces to be machined and give for each of the surfaces the possible options for machining cycles: 1. Cylindrical short surface: plunge grinding in a straight line or plunge grinding with oscillation; 2. Cylindrical long surface: multiple plunge grinding with a step, grinding per pass; 3. End face: straight plunge grinding or oscillation plunge grinding; 4. Left or right cylindrical and end face: grinding at an angle; 5. Slotted surface: multiple plunge grinding with a step, grinding per pass; 6. Conical surface: multiple plunge grinding with a step, grinding per pass.Each designed machining cycle includes three stages of processing with different cutting conditions: initial, finishing and sparking out [6].At the initial stage of processing, it is proposed to work with the maximum allowable cutting mode, which ensures maximum productivity and removal of most of the allowance, while the temperatures in the cutting zone cause hardening, burns, residual stresses and other temperature defects.At the finishing stage, processing is carried out with a cutting mode in which the defective layer formed after the initial stage is completely removed, while ensuring that the basic requirements for accuracy, burn-freeness and roughness of the machined surface are met.At the sparking out stage of processing, the surface roughness is formed, and elastic deformations are removed from the workpiece.
An analysis of the variations of all possible three-stage machining cycles for each type of machined surface made it possible to group the cycles according to their structure (Table 1) and identify typical machining cycles (Figure 6).As can be seen from Figure 6, typical machining cycles on a CNC cylindrical grinding machine are carried out according to the following machining schemes: -plunge grinding is carried out along a straight path or with oscillation with a change in the value of the radial feed from the largest for the initial stage to the smallest for the finishing stage and to the minimum for the sparking-out stage; -multiple plunge grinding with a step is implemented in the form of a series of radial translational movements of the tool to the workpiece with a certain step, on which the rough and finishing parts of the allowance are removed with variable feed, and at the final stage of sparking out, grinding is carried out with a longitudinal feed per pass; -longitudinal grinding per pass is carried out by removing the allowance with the movement of the tool along double working strokes with different depth of cut and feed for initial, finishing and sparking-out steps; -end face grinding is carried out progressively or with oscillation with a change in the cutting feed at the initial, finishing and sparking-out stages of the cycle; -grinding of the cylindrical surface and end face is carried out at an angle of 45° to the axis of the workpiece with varying feeds in the radial and longitudinal directions in accordance with the cutting mode at the initial, finishing and sparking-out stages.
Each of the presented typical grinding cycles on a CNC cylindrical grinding machine is designed according to the general method: 1.The control parameters are determined -the cutting mode elements on the technological operation (table 2).For each control parameter, the minimum and maximum values are determined, as a result of which a space of possible values of control parameters is formed, from which cutting modes for each of the machining steps will be selected [7].

Determination of technological limitations
To determine the cutting conditions for each of the machining steps, technological limitations are imposed on the space of control parameters.These limitations are different for each of the steps.Consider the technological limitations for each of the machining steps [8].
At the initial stage, most of the allowance is removed at the maximum allowable cutting mode, which ensures maximum productivity.During processing, under the action of the pressure force of the tool, the workpiece is elastically deformed (pressed from the tool) and a so-called spin is formed, which affects the accuracy of processing.As a result of intensive cutting, defects appear on the surface of the workpiece -burns, residual stresses, microcracks [9].Based on this, the limitations on the initial stage are: -limitation on the permissible grinding power, which ensures effective removal of the allowance; -limitation on elastic deformations of the workpiece, which can be eliminated at subsequent stages; -limitation on the depth of the defective layer in order to remove it at subsequent stages; -limitation on intensive wear of the tool and grain friability.At the finishing stage of processing, the remaining part of the allowance is removed, elastic squeezes and the defective layer that appeared on the surface of the workpiece at the initial stage are eliminated.Technological limitations at this stage are:  limitation on elastic deformations of the workpiece, which must be removed from it, once in the tolerance for machining;  limitation on the depth of the defective layer in order to remove it at subsequent stages.
At the sparking-out stage, it is necessary to ensure dimensional accuracy and specified requirements for surface roughness, while the allowance is practically not removed.Technological limitations are:  limitation on the elastic squeezing of the workpiece, which should tend to zero;  limitation on the depth of the defective layer, which should not be present on the workpiece;  limitation on surface roughness, which must meet the requirements.To calculate the cutting conditions for each of the machining stages, taking into account the technological limitations imposed on them, it is necessary to predict with high accuracy the process parameters (cutting forces, elastic deformations of the workpiece, machining accuracy, temperature in the cutting zone and on the surface, surface roughness) during digital processing simulation.
Based on the described method of the formation of typical machining cycles, when designing, it is necessary to take into account technological limitations: on the allowable grinding power; on the tool friability; on the depth of the defective layer; on elastic deformations of the workpiece; on surface roughness.For the computational design of machining cycles, it is necessary to determine the permissible control parameters, taking into account technological limitations, i.e., to solve the problem of assigning the maximum allowable cutting modes, taking into account the fulfillment of the requirements according to the given criteria.
To understand the necessary structure of the digital model of the machining process, let's consider the nature of the occurrence of each of the technological limitations.The technological limitation on the allowable grinding power arises based on the processing conditions at the initial stage, when it is necessary to remove the rough part of the allowance with the maximum allowable feed.In this case, it is necessary to check whether the grinding power does not exceed the permissible power according to the machine's passport.The calculated grinding power directly depends on the cutting force and the speed of tool's rotation.The rotation speed of the tool is given, so it is necessary to predict the cutting force from the entire grinding wheel that occurs under given machining conditions.It is also worth noting that the cutting force of a grinding wheel is the total value of the cutting forces from the action of each single abrasive grain.
A technological limitation on tool friability also occurs at the roughing stage, when grinding is carried out at the maximum cutting mode, as a result of which each abrasive grain acts on the workpiece with the maximum cutting force.If the cutting force from a single abrasive grain exceeds the force of holding the grain in the bond, such a grain can be chipped out of the grinding wheel, resulting in an unacceptable intense tool friability.To predict the possibility of this process occurring, it is necessary to model the cutting force on a single abrasive grain.
Technological limitation on the depth of the defective layer, including burns, microcracks and residual stresses, and resulting from the high thermal stress of the grinding process.When grinding, many abrasive grains are involved in the cutting and plastic deformation process, resulting in a large amount of heat.The released heat is concentrated in the thin surface layers of the workpiece, when cooled by a lubricating-cooling process medium, defects occur.To assess the occurrence of thermal defects, it is necessary to predict the thermophysical processes that occur during grinding, specifically, the heat release from individual abrasive grains, taking into account the specifics of their interaction with the workpiece and the distribution of heat in the surface layers of the workpiece.
The technological limitation on elastic deformations of the workpiece is the key to ensuring the accuracy of machining.During machining, the grinding wheel acts on the workpiece with a cutting force, as a result of which the workpiece is elastically deformed (pressed from the tool) depending on the degree of its rigidity.Elastic pressing of the work-piece from the tool leads to the fact that the circle removes a smaller allowance than planned, and the resulting size will go beyond the tolerance field, as a result of which the requirement for machining accuracy will not be met.To predict elastic deformations, it is necessary to model the cutting force and the susceptibility of the workpiece to elastic deformations.
The technological limitation on surface roughness is aimed at meeting the requirements for the finish of processing [10].During cutting, many abrasive grains form a certain roughness on the workpiece, the magnitude of which depends on the cutting mode.It is necessary to evaluate the roughness parameters at the design stage to ensure that the requirements for the finish of processing are met.

Conclusion
Thus, a method of digital automatic design of grinding operations carried out on CNC cylindrical grinding machines has been developed with maximum consideration of the process features.The developed toolkit will take over the design, starting from the formation of initial data and ending with the output of the control program, taking into account 5 different typical grinding cycles and the technological limitations imposed on them.Based on the developed method, it is planned to develop a module for digital automatic design of grinding operations carried out on CNC cylindrical grinding machines with further approbation of the results in the existing production.

Fig. 1 .
Fig. 1.Method of automatic design of grinding operations carried out on CNC cylindrical grinders

Fig. 2 .
Fig. 2. Graphical representation of the shaft drawing considered as an example

Fig. 3 .
Fig. 3. Graphical representation of the shaft drawing with the designation of the basing scheme, machined surfaces and the zero of the workpiece coordinate system

Fig. 4 .
Fig. 4. Graphical representation of the shaft drawing with the designation of the basing parameters

Table 1 .Fig. 6 .
Fig. 6.Graphical representation of the allowance division scheme in typical machining cycles: a) plunge grinding; b) multiple plunge grinding with a step; c) longitudinal grinding per pass; d) end face grinding; e) grinding at an angle; I -allowance for the initial stage; II -allowance for the finishing stage; III -allowance for the sparking out stage; S -feed path

Table 2 .
List of control parameters for each typical machining cycle