Some problems of designing the main drives of universal machine tools
JAN WOJCIECHOWSKI, PRZEMYSLAW WYGLĄDACZ
Institute of Production Engineering and Automation, Wrocław University of Technology, Łukasiewicza 5, 50-371 Wrocław
Abstract: The problem of fitting the characteristic of the universal machine tool’s main drive to anticipated machining tasks is considered. The power demand and the cutting torque demand for turning and milling are analysed. It is shown that by employing a multiplying gear one can obtain a constant machining power in a wide range of spindle speeds whereby both steel and aluminium alloys can be efficiently machined.
Keywords: machine tool, main drive, designing
1. Introduction
As the range of cutting speed increases, there is a tendency to increase the rotational speed of spindles in the design of the main drives of machine tools. In the years 1989–1999, the maximum spindle speed increased ten times.
Another requirement connected with a drive towards higher metal removal rates is that rotational speed be changed in a stepless way to maintain the optimum cutting speed on the one hand and to minimize selfexcited vibration on the other one [1]. Still another important requirement is that high (much over the capacity of the driving motor) spindle speeds should be assured at a constant motor power. The power is in-dispensable for efficient machining at both low and high spindle speeds. The efficient machining of materials characterized by high specific cutting resistance, such as steel, requires a low spindle speed at a constant power, whereas by increasing this speed one can efficiently machine such materials as aluminium and its alloys as well as other materials with a much weaker cutting resistance but requiring a much higher cutting speed. The highspeed electrospindles used today are often incapable of meeting the above requirements. The latter, however, can be met by incorporating additional gears, both reducing and multiplying the motor speed, into the main drive. The possibilities of applying of such gears in the main drives of selected universal machine tools are explored in this paper.
2. Shaping main drive characteristics
The design of the main drive of machine tool consists in the selection of proper commercially available subassemblies. In order to choose a suitable drive unit, onemust analyse and compare the required load curves for the intended machining proc-esses with those of a potential motor equipped with a power supply unit and possibly a gear transmission [2]. The drive should be capable of performing all the intended tasks at a high degree of machine tool production potential utilization. The drive’s load curves can be determined if one has such input data as:
− the rotational speeds required,
− the power needed to realize the machining process,
− the cutting torque.
To precisely determine the input data one must know the machining tasks which a given machine tool is to perform. In the case of universal machine tools, it is not easy to foresee what procedures will be applied to workpieces. The latter could be used to determine the load characteristic.
One of the methods of preselecting a motor for the main drive is the method of maximum loads [2], in which the worst possible drive loading conditions that may occur during the operation of the machine tool are selected. This means that roughing operations in which large-diameter layers are cut and operations involving high rotational speeds are considered. The choice of machining parameters should take into ac-count the machine tool’s load-bearing structure, its ways and the machining parameters permissible for the cutting tool materials.
Fig. 1. Specific cutting resistance kc of different materials
The method of maximum loads is applied here to show how by incorporating a multiplying gear into the kinematic chain of the main drive the machine tool’s functional properties can be improved. In order to determine the main drive’s load characteristic, the machining power and the torque were calculated. Turning and milling, as the main ways of machining by universal machine tools, were investigated. Much heavier loads than the ones associated with, for example, drilling are involved in the above processes.
The machining of objects made of steel St60-2 and aluminium alloy Al99,8_F6 (DIN) was analysed since the latter are the most commonly machined materials with diametrically different cutting resistances (Figure 1).
Sandvik Coromant’s catalogues were used for the selection of cutting tools and machining parameters for the exemplary procedures [3].
3. Analysis of main drive characteristics
The load characteristics of the main drives of universal machine tools: a milling machine and a lathe were analysed. In the case of the milling machine, face milling by a cutter with a typical diameter d = 50 mm and number of inserts z = 4 was chosen. In-sert material CT530 (ensuring a high cutting speed) recommended by Sandvik Coromant was used for machining both steel and aluminium [3]. In the case of the lathe, a roller with diameter d = 50 mm was straight turned and the insert material was CT630.
The machining parameters used in the analysis of main drive loading for the two machines are shown in the Table.
In order to determine the required power Pmax and the cutting resistance torque M, first maximum cutting component force Fcmax was calculated. Specific cutting resistance kc determines the material properties which affect the above force. The resistance is not constant and depends mainly on the rate of feed. A computer program and relevant data contained in the Sandvik Coromant catalogue were used to calculate the force Fcmax [3]. Maximum cutting component needed to calculate the machining power Pmax is expressed by this relation:
Fcmax = kc · ap · f [N], (1)
where:
Fcmax – the maximum cutting component force,
kc – the specific cutting resistance,
ap F the depth of cut,
f – the feed.
Maximum machining power Pmax for force Fcmax and recommended cutting speed vc is written as:
，
where:
Pmax – the maximum machining power,
vc – the cutting speed.
The cutting resistance torque M can be expressed by:
，
where:
M – the cutting resistance torque,
d – the tool’s or the workpiece’s diameter for respectively milling and turning.
On the basis of the calculations the power demand and cutting torque diagrams for the machining parameters shown in Table 1 were drawn.
The milling power demand diagram (Figure 2) for steel (the darker area) and aluminium (the lighter area) shows that the load curve for steel differs significantly from that for aluminium. In the
case of steel, power demand occurs at lower rotational speeds than in the case of aluminium. Thus a machine tool designed for machining steel will not be efficient in the machining of aluminium products since it is incapable of sufficiently high rotational speeds needed to achieve high productivity. Whereas a machine tool intended only for machining aluminium will not have a sufficiently high power in the lower range of rotational speeds in which the power demand for machining steel is high.
The main drive motor power diagram (broken line) was superimposed on the power demand diagram. The Mitsubishi SJ-P F7.5 motor was selected since most of its power demand diagram is under the line demarcating the motor operation area. It became apparent that as regards its power characteristic the motor quite well met the requirements in the case of steel. Unfortunately, quite a large portion of the power demand area for machining aluminium was outside the motor operation field. In order to expand the latter, a multiplying gear was employed. Such a gear ratio was selected so as to obtain the widest possible range of spindle speed at a constant power. Gear ratio i equal to 3 was adopted:
，
where:
nmax – the maximum motor speed,
np – the lowest rotational speed at which the motor attains the maximum power.
Fig. 2. Power demand versus spindle speed diagrams for milling steel and aluminium using Ø50 mm face milling cutter at different machining parameters ap and fz. Power diagrams for Mitsubishi SJ-P F7.5 motor with and without gear with i = 3
The power demand diagram for the motor with the gear is represented by a solid line. Thanks to the gear the main drive’s operation field expanded and covered the rest of the area of possible loads for machining aluminium.
Besides fitting the motor power characteristic to the power demand curve one should also choose proper motor torque characteristics. And so a torque diagram was made in a way similar to that of the power diagram for the machining parameters adopted (Figure 3).
Spindle speed n(rpm)
Fig. 3. Torque demand versus spindle speed diagrams for milling steel and aluminium using Ø50 mm face milling cutter at different machining parameters ap and fz. Torque diagrams for Mitsubishi SJ-P F7.5 motor with and without gear with i = 3
As in the case of power demand curves, the torque demand characteristic for steel is different from that for aluminium. The machining of steel requires high torques at lower rotational speeds (the darker area), while the machining of aluminium results in much lighter loading of the main drive, but at higher rotational speeds (the lighter area). The situation is similar to that for power. A machine tool for machining steel will not be fully utilized when used for machining aluminium and vice versa.
Also here the motor mechanical characteristic (broken line) is superimposed on the torque demand curve. The motor’s torque versus its speed extends over a considerable part of the field corresponding to the possible drive loads during the machining of steel, but covers only a small part of the area representing the resistance during the machining of aluminium. This means that at limited machining parameters (the rate of feed and the depth of cut are located below the broken line) it is possible to machine steel and aluminium at insufficient speeds. When additional gear with the gear ratio i = 3 is employed, the range of useful spindle speeds (solid line) increases, which, in turn, results in the reduction of the spindle torque Mi relative to the motor torque M:
，
where:
Mi – the spindle torque,
M – the motor torque,
η – the efficiency of the gear,
i – the gear ratio.
Although the torque decreases nearly three times, the drive operation range in-creases sufficiently to meet almost fully the torque demand for the machining of alu-minium. This is due to the (5–8 times) lower specific cutting resistance.
A similar analysis was carried out for turning. As in the case of milling, power de-mand (Figure 4) and torque diagrams (Figure 5) were drawn.
Fig. 4. Power demand versus spindle speed diagrams for straight turning of Ø50 mm steel or aluminium roller at different machining parameters ap and fz. Power diagrams for Mitsubishi SJ-P F7.5 motor with and without gear with i = 3
Also here the load curves for machining steel differ much, both with regard to power and torque, from those for machining aluminium. By extending the range of useful rotational speeds for the main drive through the adoption of an additional gear one can meet the power and torque demand in a much wider range of possible loads at given parameters. The Mitsubishi SJ-PF7.5 motor without a transmission gear or at i = 1 enables the machining of steel at lower cutting speeds (operation at machining parameters located under the broken line representing the motor characteristic is pos-sible), but no rotational speeds sufficiently high for machining aluminium at recom-mended parameters are attained. Neither a high-speed motor will ensure a sufficient
Fig. 5. Torque demand versus spindle speed diagrams for straight turning of Ø50 mm steel or aluminium roller at different machining parameters ap and fz. Power diagrams for Mitsubishi SJ-P F7.5 motor with and without gear with i = 3
torque at the lower rotational speeds used for machining steel (the area of possible loads located under the solid line representing the characteristic of the motor with an additional gear). But the combination of the characteristics, which is possible thanks to the additional gear in the main drive’s kinematic chain, increases the machine tool’s operating capability by extending the range of useful spindle speeds.
4. Conclusions
The following conclusions can be drawn from the above analysis:
• The mechanical characteristics of motors show their torque or rated power within the entire range of rotational speed, which corresponds to the most unfavourable mo-tor operating conditions: the motor may operate under the maximum permissible load for an extended time. This occasionally occurs during the drilling of deep boreholes, but prolonged operation under variable loading occurs much more often in practice. Then an ED (an index showing the permissible percentage of maximum load time in a specified time interval) characteristic is superimposed on the power demand curve. Such loading occurs in most universal and special-purpose machine tools [2].
• When designing a drive, one can choose a motor with a lower power but one must bear in mind the range of machining parameters will decrease. The parameters can be read from the diagram (feasible parameters are under the line representing the maximum power – Figures 2 and 4). One must take into account the costs of using
larger motors and electronic power supply devices. Thus the permissible machining parameters can be specified during the preliminary selection of the main drive.
• Motor overload is allowable for a short time only (depending on the motor ther-mal operating conditions), but the motor cannot be loaded with a torque higher than the peak catalogue torque since it will be stopped (its protection will be actuated or the motor may be damaged).
To verify the preselected motor one should do thermal stability calculations for it.
Due to their properties contemporary tool materials can be machined in a very wide range of machining parameters. But the existing NC machine tool main drive designs usually do not allow one to fully exploit this possibility. In order to extend the func-tionality of a machine tool, one can incorporate an additional gear into the main drive’s kinematic chain. This will expand the range of rotational speed at a quite good power and torque characteristics and make the machining of materials with widely dif-ferent cutting resistances more efficient. The solution proposed is suitable for two ma-chine tools: one for machining materials at low speeds and high cutting resistances and the other for machining at high speeds and low cutting resistances. The introduc-tion of an additional gear has the advantage that it reduces the power demand whereby a smaller power supply unit is needed.
The gear ratio can be changed by mechanically reconfiguring the machine tool. The most convenient solution is to control the gear through the machine tool’s control system since the latter when analysing the set machining parameters will select the proper gear ratio.
References
[1] Krzyżanowski J., Nitek W., Wojciechowski J.: Rozwój napędów obrabiarek skrawają-cych (The development of machine tool drives), Napędy i sterowanie, 2001, No. 5, pp. 12–16.
[2] Wrotny L. T.: Projektowanie obrabiarek. Zagadnienia ogólne i przykłady obliczeń (De-sign of machine tools. General problems and examples of calculations), WNT, Warsaw, 1986.
[3] The Internet catalogue of Sandvik Coromant products: http://www.coromant.sandvik.com/pl.
[4] Mitsubishi General Catalogue, March 1999.
