Study of spindle drives for boring machines

This paper examines the main requirements for the electric drive systems of a class of boring machines with digital program control. On this basis, a methodology for choice of the spindle drives is offered. The algorithm takes into account the specific features of the technological processes, the treated materials, the tools used and their wear, as well as the mechanical gear types. The experimental studies of the implemented electric drives for the spindle are presented and discussed. The research carried out and the results obtained can be used in the development of such drives for the studied class of machine tools.

algorithms in drive systems with dual-zone speed regulation.
In [9,10] are presented technical possibilities and mathematical models for kinematic system diagnostics of machine tools. In [11] a DC electric drive system with dual-zone speed regulation is presented, where control shift is a function of the motor back EMF voltage. Using an appropriate vector-matrix description of the controlled object, in [12] an optimal modal speed controller for the first zone is synthesized, as well as an adaptive optimal modal controller of the back EMF voltage for the second speed zone. To improve the electric drive performance an adaptive speed controller with switchable structure is developed [13]. In the second zone, the controller parameters adapt to the decreasing magnetic flux. Such an approach provides for better static and dynamic characteristics of the driving system.
Nowadays, spindle drives with AC motors are used applying vector control [14,15]. The main advantage of such electric drives relates to the easier maintenance of the AC motors because they are brushless.
In this paper, the main features for the respective drives of a type of boring machines with digital program control are formulated and on this basis, a methodology for selection of spindle electric drives is developed. In choosing suitable spindle drives, a number of essential factors were taken into account, namely: the technological process features, the processed materials, the tools used and the mechanical gears. Some dual-zone spindle drives with DC and AC motors are presented, illustrating the practical application of the offered approach. Experimental studies of the implemented drives related for the spindle are presented and discussed.

Features of the drive system
The simplified block diagram of the drive system for a studied class of boring machines is shown in Fig. 1, where the following notations are used: DPCdigital program control device; ED1 -ED10electric drives; G1 -G8mechanical gears; L1 -L8loads. Each of the presented drive subsystems has specific features and requirements that need to be taken into account in the process of design, study and practical application.
The feed drives of the x, y, z, a, and c coordinate axes are used for positioning of the tool and the workpiece at the desired locations and they participate in the machining process. The main requirements for them can be formulated as follows: -smooth speed regulation in a wide range; -good dynamics; -high position accuracy; -formation of the necessary position cycles; -providing the required torque; -reversible speed and torque control; -compensation of the disturbances; -easy maintenance; -reliability; -economy. The spindle drive is involved in the machining process and it should meet the following requirements: -dual-zone speed regulation (by constant torque and constant power, respectively); -high maximum speed; -sufficient torque; -oriented braking with high accuracy; -reversible speed control. The main requirements for the auxiliary drives can be formulated as follows: -solid angular positioning of the workpiece with high precision; -a subsystem providing choice of the desired tool; -dosing lubrication of the machine coordinate axes; -cooling subsystem with option for automatic and manual braking and starting from the control panel in order to reduce energy consumption.
The system for choice of instruments from the auxiliary drives relates to the spindle drive in the studied type of boring machines.
The development of appropriate spindle electric drives includes selection of motors, power converters, sensors, controllers and mechanical gears. This problem is very important in terms of performance, technoeconomic parameters, reliability, etc.

Selection of spindle drives
The design of spindle electric drives for boring machines includes the following stages:  development of a methodology for optimal choice of dual-zone speed drives;  performing calculations, according to the respective procedures of the methodology;  technical and economic analysis of the possible types of electric drives, taking into account the catalog data;  modeling and computer simulation of the relevant dynamic and static modes of operation;  development of a stand for experimental studies;  experimental determination of the necessary parameters for modeling;  optimization and tuning of the respective control loops;  computer simulation with various settings of the control loops to verify the used control algorithms;  detailed experimental study to evaluate the actual performance of the drive system.

Methodology algorithm
When choosing an electric drive, it is essential to define the heaviest processing mode of operation for the machine to optimize the power, speed and the price. Fig. 2 shows the simplified block diagram of the developed algorithm for selection of spindle drives for the studied boring machines.
The following notations are used in this section:  (1) Equation (3) Equation (4) Equation (5) Equation (6) Calculation of bm  . The tabular data used in this methodology are taken from [16].
The spindle speed in boring process is determined by the expression [16]: Minimum speed, which is selected for the spindle motor: The boring feed speed is calculated using the following equation [16]: The maximum power needed to perform boring process, distributed between both feed drive and spindle drive without taking into account the tool wear, is calculated by the next expression [16]: The maximum power needed to perform boring, distributed between both feed electric drive and spindle electric drive with taking into account the tool wear, is determined with the equation [16]: The power required only for the spindle drive is determined as follows [17]:

Practical application
The offered methodology for spindle drives selection takes into account the specific features of the technological processes, the processed materials, the tools used and their wear, as well as the mechanical gear coefficient. It gives opportunity for choice of motors, power converters and sensors. Fig. 3 shows a diagram, illustrating the elements of some of the used electric drives for the studied machines. The notations used are as follows: DPCdigital program control device; 1position sensor for z axis; 2 motor for z axis; 3coupling between the motor and the ball screw for z axis; 4ball screw for z axis; 5guides for z axis; 6speed sensor for the spindle drive; 7spindle motor; 8coupling between the spindle motor and gearbox; 9instruments drive; 10boring tool; 11position sensor for x axis; 12motor for x axis; 13coupling between the motor and the ball screw for x axis; 14precision bearings for x axis; 15driven mechanism (work desk for the workpiece); 16processed workpiece; 17guides for x axis; 18ball screw for x axis; Some examples of using this methodology for selection of spindle electric drive for cutting materials with different hardness are presented below.  Table 1.
Boring operation modes can be as follows: singleedge cuts, multi-edge cuts, step-boring, reaming.
The choice of boring tools goes through the following several stages: definition of the operation type; selection of a boring system; definition of the boring diameter and hole requirements; choosing of the entering angle; selection of an adaptor; selection of inserts for the tools.
The calculations performed according to the presented methodology have the same input data for materials of different hardness, in order to compare and analyze the obtained results.
The selected motors must have power about 10% greater than the calculated one, in order to compensate for the allowable wear over time. These obtained values are used for the motor choice from the respective technical catalogs. As a result of the calculations made for these two materials, appropriate DC and AC electric drives were chosen from [14,15,18].
Some of their basic parameters are presented in Table 2.

Experimental studies
A stand for experimental research of electric drives for machine tools has been developed, equipped with the necessary measuring and visualization devices. Based on the formulated requirements, the developed methodology, the calculations performed, and the selected electric drives, detailed experimental studies were carried out at different operating modes and settings of the respective controllers. Fig. 4 illustrates the experimental study of a dualzone spindle electric drive for boring machines. Fig. 5 shows a modernized instruments drive, where the following indications are used: 1induction motor for instruments drive; 2sensor for selection of position; 3springs; 4pins; 5hydraulic cylinder; 6instruments; 7hydraulic valves; 8sensor used for control of instrument drive from developed ladder diagram.

Fig. 5. A modernized instruments drive.
The following figures present some oscillograms obtained for the implemented DC, synchronous and induction drives in the studied class of machine tools. Fig. 6 shows time diagrams obtained experimentally for both zones of speed regulation for a DC drive at different reference speeds and tuning of the control loops. Fig. 6a presents a time diagram for the first speed zone with direct reverse. In this case, the reference speeds are 60 rad/s and -60 rad/s, respectively. Fig. 6b shows a time diagram for the second zone at different working speeds.     Based on the studies carried out with various DC and AC electric drives the following practical inferences can be drawn:  The implemented DC motors have very good tuning qualities and provide the necessary static and dynamic characteristics for spindle drives. Their disadvantage is the presence of the brush collector.  Electric drives with AC motors have easier operational maintenance however their price is relatively higher.

Conclusion
The main features of the drive system for a type of boring machines with digital program control are analyzed and formulated. On this basis, a methodology for optimal selection of spindle drives is offered. The developed algorithm takes into account the technological process, the treated materials, the tools used and the mechanical gear. Examples for choice of spindle electric drives with DC and AC motors are shown, illustrating the practical application of the described methodology.