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Some problems of designing the main drive of milling machine.

Abstract: As a range of cutting speed increases, there is a tendency to increase rotational speed of spindles in design of the main drives of machine tools. Another requirement connected with the drive towards higher metal removal rates is that rotational speed be changed in a stepless way. Still another important requirement is that high (much over capacity of the driving motor) spindle speeds should be assured at a constant motor power. The paper presents the possibility of achieving constant value of power in wide range of rotational speed of milling machine spindle both for milling of steel and aluminum alloys. The two solutions of driving system have been used in analysis of possibilities of achieving constant cutting power at wide range of cutting speed. One of the possibilities is a driving system based on direct drive using electrospindles. Second option is an application of multiplying gear. The two driving systems have been compared. The load characteristics of example cutting processes has been taken into account.

Key words: milling machine, main drive, designing.

1. INTRODUCTION

The requirements given for the machine tool drives are the result of the tendencies of their development (increase of the cutting speed and efficiency of the cutting process). Other requirement is to ensure wide range of rotational speed for constant power. Low rotational speed at constant power is adequate for cutting such materials as steel, with high value of specific cutting resistance. Higher cutting speed is efficient when cutting aluminium alloys, which have lower value of specific cutting resistance. The fulfillment of such requirements can be done in two ways: using the multiplying gears or using electrospindles--currently used method. Both solutions are good, but they have also some disadvantages: the first solution enlarges the kinematic chain, which gears, belts and bearings are the source of noise and vibrations, while the electrospindles are acting as heat sources, which can influent the accuracy of the machine tool. The costs are also significant factor.

2. SHARPING THE CHARACTERISTICS OF MAIN DRIVE

The right choice of the drive systems requires accurate analysis and comparison of a load during cutting and characteristics of the driving motors, power supply units and gears. Properly designed driving system ensures possibility of realizing all cutting processes with high level of usage of machine tool possibilities.

A set of initial data used for designing machine tool load consists of:

* required rotational speed,

* power required for realizing the cutting process,

* cutting torque.

Accurate description of the initial data requires knowledge about the cutting processes, which will be conducted using a machine tool. The data are difficult to obtain, when the universal machine tool is designed; it is hard to predict the loading and machining plans, which are necessary to choose load characteristic of the drive system.

One of the most simple methods of the initial choice of the main drive motor is the maximal load method. In this method, the hardest cutting conditions are taken into account (rough cutting with big values of speed, depth of cut etc) and such, when high value of speed are applied. Using this method an increase of the machine tool properties has been achieved and the results are presented. In the paper, the improvement of the drive system of the machine tool is presented. The results have been achieved, when multiplying gear has been applied. Similar results can be achieved when the electrospindle is applied.

In order to obtain the loading characteristic of the main drive the calculations of the cutting power and cutting torque have been performed. The milling process has been analyzed, as it is one of the basic machining operations, which can be performed on the universal machine tools. In analysis the loading conditions for the St60-2 steel and Al99,8_F6 aluminum alloy have been considered. The two materials have very different specific cutting resistance.

For a choice of tools and cutting parameters the Sandvik Coromant catalogue has been used.

3. CHARACTERISTIC ANALYSIS OF THE MAIN DRIVE OF MILLING MACHINE

The analysis of the load of main drive of universal milling machine has been performed. The face milling in full material with the head d=50 mm and number of inserts z=4 has been chosen. For cutting steel and aluminum tool tip material (CT530) has been used, as advised by Sandvik Coromant catalogue. This material of the tool tip can be used in high speed cutting. The machining parameters used in the analysis of main drive loading for the milling machine are shown in table 1.

In order to obtain necessary power Pmax and cutting torque M, the maximal circumferential component of cutting force [F.sub.cmax] has been calculated. The material dependent parameter is the specific cutting resistance [k.sub.c]. This parameter is not constant, but it mainly depends on feed. During calculation of the cutting force value, the computer software and the catalogue of Sandvik Coromant has been used. Maximal circumferential component of cutting force [F.sub.cmax], necessary to calculate the cutting power [P.sub.max], can be described by equation:

[F.sub.cmax] = [k.sub.c] x [a.sub.p] x f [N] (1)

where: [k.sub.c]--specific cutting resistance, [a.sub.p]--depth of cut, f--feed.

Maximal cutting power [P.sub.max] for given cutting force [F.sub.cmax] and recommended cutting speed [v.sub.c] can be written in following way:

[P.sub.max] = [F.sub.cmax] x [v.sub.c] / 1000 x 60 [kW] (2)

The cutting torque M is described using following equation:

M = [P.sub.max] x 30 x d / [v.sub.c] [Nm] (3)

where d is the diameter of the milling head.

On basis of the calculations the power demand and the cutting torque were drawn.

The milling power demand diagram (fig. 1) for steel (the darker area) and aluminium (the lighter area) shows that the load curve for steel differs significantly from the one for aluminium. In case of steel, power demand occurs at lower rotational speeds than in case of aluminium. Thus a machine tool designed for machining steel will not be efficient for machining aluminium products since it is incapable of sufficiently high rotational speeds needed to achieve high productivity. Whereas a machine tool designed only for machining aluminium will not have a sufficiently high power in lower range of rotational speeds in which the power demand for machining steel is high.

[FIGURE 1 OMITTED]

The main drive motor power diagram (thick continuous 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=[n.sub.max]/[n.sub.p] ([n.sub.max]--the maximum motor speed, [n.sub.p]--the lowest rotational speed at which the motor attains the maximum power) equal to 3 was adopted.

The power demand diagram for the motor with the gear is represented by a thick dashed 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.

In order to compare the properties of the Mitsubishi SJ-P F7.5 drive equipped with the multiplying gear and the direct drives, the nominal power graphs P=f(n) for two electrospindles (delivered by Siemens) 1PH2254-6WB41 (thin continuous line) and 1PH2143-4WF42 (thin dashed line) were superimposed on the power demand graph. Chosen electrospindles have different range of rotational speed and comparable values of nominal power to the previously chosen SJ-P F7.5 motor. First electrospindle is efficient, and even better than the Mitsubishi SJ-PF7.5 motor for cutting steel, but it does not cover the power demand for cutting aluminum with low value of feed. Second electrospindle covers the demand for cutting aluminum, but it is not efficient for cutting steel with larger cross-section of chip (larger values of [a.sub.p] and [f.sub.z]).

Besides right choice of the motor power, the motor torque should be chosen correctly. Similarly to the power demand, the torque demand characteristic is different for cutting steel and aluminum. Cutting steel requires high values of torque at low rotational speed range, while for cutting aluminum lower load is required from the driving system, but cutting is performed in higher rotational speed range, similarly to power demand.

The cutting torque M is proportional to cutting power [P.sub.max], and motor torque is proportional to power. The conclusions of application of the Mitsubishi SJ-PF7.5 motor without the multiplying gear and with the multiplying gear with ratio i=3 and both electrospindles (fig. 1) will be similar. In the situation of milling (head diameter d=50mm, number of inserts z=4, feed range [f.sub.z]=0,1-1 mm/cutting edge, depth of cut [a.sub.p]=1-10 mm) the choice of the Mitsubishi motor with additional multiplying gear will fulfill the requirements in better way, than two presented electrospindles.

4. SUMMARY

The properties of tool materials enables efficient cutting in wide range of cutting parameters. However, the problems with designing the main drive system of the NC machines arises when one wants to fully exploit the range of possible cutting parameters. The difficulties grow, when the machine tool is going to be used for cutting range of materials, with various specific cutting resistance parameters. The paper presents the alternative solution for the main drive of milling machine, when the multiplying gear or the electrospindles are used.

The mechanical characteristics of motors show their rated power within the entire range of rotational speed, which corresponds to the most unfavourable motor 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.

Mechanical characteristics of the drive P=f(n) have been compared with the chosen range of cutting parameters only for one diameter of cutting head. Accurate design requires analysis of the power and torque demand for wider range of head diameters and rotational speed of spindles.

5. REFERENCES

Sandvik Coromant Internet catalogue. Aviable from : http:www.coromant.sandvik.com.pl

Krzyzanowski, J.; Nitek, W.; Wojciechowski, J. (2001). Rozwoj napedow obrabiarek skrawaj1cych (The development of machine tool drives). Napedy i Sterowanie, No. 5 (May,2001), pp.12-16

Mitsubishi General Katalog, March, 1999.

SIEMENS Sinumerik & Simodrive Catalog NC 06-2002--AC Motors--PM, PH, FM Main Spindle Motors

Wrotny, L.T. (1986); Projektowanie obrabiarek. Zagadnienia ogolne i przyklady obliczen,(Design of machine tools. General problems and examples of calculations), WNT, Warsaw
Table 1. Machining parameters for milling.

Machining parametrStock St60-2 A199,8_F6

Feed per cutting edge 0,1 1,0 0,1 1,0
[f.sub.z] [mm/edge]

Cutting depth [a.sub.p] 1-10
[mm]

Cutting speed [v.sub.c] 360 90 1,130 385
[m/min]
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Article Details
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Author:Wygladacz, P.; Wojciechowski, J.
Publication:Annals of DAAAM & Proceedings
Article Type:Technical report
Geographic Code:4EUAU
Date:Jan 1, 2005
Words:1917
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