INTRODUCTION
Machining time per work piece represents one of the most important parameters in the area of manufacturing technology with respect to attainable productivity and is the sum of cutting and non-cutting time for a specific work piece. One way to improve productivity is to reduce both these time parameters. The non-cutting times mainly comprises the time required for positioning and tool changes. Clearly increasing the positioning speed and acceleration of the machining tool by properly designing the drive, decrease non-cutting time and increase in productivity can be achieved. In a general machine tool the feed rate during machining is appropriate. During other movements the feed rate is high. In the Electro Mechanical drive system discussed here the feed unit is equipped with 2 motors one (brake motor) for rapid advance &other (standard motor) for slow feed as shown in fig .1b.It is designed in such a way that movement at feed rate is restricted to cutting length while all idle distance are traveled at rapid traverse rate.
The fine boring machine consist of bed, table, lead, screw, brake motor, slow feed motor & two spindle one at each side of the fine boring machine as shown in fig 1b.
FIGURE. (1b)
The main functions of table drive are the following (fig 1a):
1. Job that is fixed on table is moved from left to right fast called as rapid advance (RA).
2. Boring using right spindle take place slowly (SF).
3. Table is moved from right to left fast (Rapid Return-RR).
4. Boring take place left spindle slowly (SF).
5. Ttable brought to home position fast (RR).
FIGURE (1a)
CONCEPT OF ELECTRIC DRIVE
In many of the industrial applications an electric motor is the most important component. A complete production unit consists of primarily of three basic components; an electric motor, an energy transmitting drive and the working (or driven) machine.
An electric motor is source of motive power. An energy-transmitting device delivers power from electric motor to the driven machine (or the load) it usually consists of shaft, belt, chain, rope, etc. A working machine is the driven machine that performs the required production process. Examples of working machines are lathes, centrifugal pumps, drilling machines, boring machines, etc. An electric motor together with its control equipment and energy-transmitting device forms an electric drive. An electric drive together with its working machine constitutes an electric drive system. A ceiling fan motor with its speed regulator but without blades is an example of electric drive. Other examples of electric drives are: - a food mixer without food to be processed, a motor and Conveyer belt without any material on its belt. Some examples of electric drive systems are: a ceiling fan motor with regulator and also with blades, a food mixer with food to be processed, a motor and conveyer belt with material on its belt and so on.
Figure shows an electric drive system. The electric drive, consisting of electric motor, its power controller and energy-transmitting shaft
BORING
Boring is a machining process in which internal surfaces of revolution are generalized with a single point cutting tool. The term `boring` is applicable to enlarging an existing drilled or cored hole and it also includes machining of blind holes, holes with contoured bores and bores with steps, under cuts, etc.
FINE BORING MACHINE
Fine boring machines are precision boring machines built to machine components requiring high degree of accuracy and surface finish. The boring spindle, which is the heart of these machines, can be of a ball, roller bearing, hydrostatic, or air bearing type. On these machines, diamond tools can be use to achieve high degree of accuracy and surface finish.
ELECTRO MECHANICAL DRIVE SYSTEM FOR FINE BORING MACHINE
OPERATION OF DRIVE SYSTEM
The total drive system consists of a brake motor, slow feed motor, lead screw, guide ways and table. Brake motor is meant for rapid feed rate when table travels the idle distance. When the table has moved the idle distance, the brake motor will be switched off. Slow feed motor will be switched on. The table moves slowly. The sequence of operations is controlled by limit switches. The operations controlled by limit switches are explained below (refer fig.1c).
1, 2, 3, 4-LIMIT SWITCHES
FIGURE (1c)
First brake motor is switched on and Rapid table movement follows. Spindle motor is also on. Limit switch 1 is on. When the table moves left to right and meets limit switch by trip dog brake motor stops. Slow feed motor is on. Table is under slow feed job facing right side spindle. Limit switch 2 is on. When the t able moves and meets limit switch 2 slow feed motors is off. Spindle also off.
Next brake motor is switched on in the reverse direction. Table under rapid rate. Limit switch 3 is on. When table moving towards left touches limit switches 3 and brake motor stops. Slow feed motor is on. Table is under slow feed and job facing the left spindle. When table meets limit switch 4, slow feed is off. Brake motor is on. Table comes to home position. This is the cycle of events that tables placed in a fine boring machine drive system under operation.
GEAR ARRANGEMENT
The gear box is designed for transmitting power from brake motor to lead screw via sun and planet gear during rapid advance and transmitting power from slow feed motor to lead screw via change gear, bevel gears and sun & planet gear during slow feed. By varying the change gears during slow feed as many as 45 feeds can be obtained.
FIGURE (2a)
First when the brake motor rotates it transmits power to gear A. From gear A it is transmitted to B (spur gear) .The speed reduces. From B it is transmitted to gear C (sun gear) &C to D (planet gear) D to 9, 9 to 10 and 10 to 8. From 8 power is transmitted to lead screw 12 via 10A and 11 as shown in figure.
FIGURE (2b)
FIGURE (2c)
Now the lead screw rotates at rapid feed rate. When the lead screw rotates table gets linear motion and moves towards the spindle. When it reaches near the spindle, the brake motor is switched off and slow feed motor is switched on. The power is transmitted from standard motor (slow feed motor) to change gear 1. From spur gear 1 power is transmitted to spur gear 2 and speed is reduced considerably. Again the speed is reduced by transmitting via gears 3,4,5 and 6.Bevel gears 7 is fixed at the other side of spur gear. From bevel gear 7 power is transmitted to bevel gear 8 in perpendicular direction. From, bevel gear 8 power is transmitted through sun and planet gears C, D, 9, 10 and 10A, level to lead screw 12 and table gets slow feed.
CALCULATION OF RAPID FEED
The rapid rate is got from Brake Motor rotating at 1500 rpm. The power is transmitted from brake Motor to lead screw 12 via C, D, 9, 10A, 11 as shown in fig. 2.
From fig. 2 the gear ratio for rapid rate
= (ZA / ZB) * (ZC / ZD) * (Z9 / Z10) * (Z10A / Z11)
This is calculated as follows:
The gear ratio = (26/51)*(20/13)*(13/22)*(22/53)
= 0.192378
Pitch of screw = 8
No. of start = 2
Multiply the gear ratio by 1500, rpm of brake motor and pitch of lead screw thread and no. of start, we get rapid rate
= 0.192378*1500*8*2 mm/min
= 4617 mm/min
= 4.6 m/min
So the rapid rate at which the table will be moving is 4.6 m/min. During rapid rate only the brake motor will be switched on. The slow feed motor will be switched off. So there will be no power transmission to the gears meant for slow feed.
CALCULATION OF SLOW FEED
Slow feeds obtained from slow feed motor. Power is first transmitted from slow feed motor to gear 8 via, gears 1,2,3,4,5,6,7(ref fig 2). From gear 8 power is transmitted through sun and planet gears CD, 9, 10 then from 10 to 10A and 11 &12 leads screw.
The speed ratio after passing through sun and planet gear is got from table1. The type of planetary gearing as shown in fig3 is compound planetary drive with two sun wheels. From the table1, for the present application, drive no.2 is selected.
Among the fixed ratio drive possibilities,
The transmission ratio I = (rpm of driver /rpm of driven). Is got as
= Z10ZD/ (Z10ZD-ZCZ9)
(Refer drive no.2 in table 1& fig 2&3)
ZC = 20, Z10 = 22, ZD =13, Z9 =13
The speed ratio after passing through planet gear is
=22*13/ (22*13-20*13)
= 286/26 =1:11
Table 1 Fixed ratio drive possibilities (Compound planetary drive with two sun gears-fig 3)
Drive No. 1 2 3 4 5 6
Fixed Member 1 1 2 2 4 4
Driver 2 4 1 4 1 2
Driven 4 2 4 1 2 1
Transmission ratio I
=(Rpm of driver/rpm of driven) 1=(ZCZD/
Z10ZD) Z10ZD/(Z10ZD-ZCZ9) 1-(Z10ZD)/ZCZ9) (ZCZ9)/
(ZCZ9-Z10ZD) (Z10ZD)/
(ZCZD) (ZCZ9)/
(Z10ZD)
The speed is reduces 11 times after passing through sun and planet gear CD 9 &10.we can get as many as 45 feed rate using change gears and with and without reductor.
The gear ratio for slow feed can be obtained from fig.2 as
(Z1/Z2)*(Z3/Z4)*(Z5/Z6)*(Z7/Z8)*(1/11)
(The speed ratio after passing through sun and planet gear) *(Z10A/Z11)
In this (Z7/Z8)*(1/11)*(Z10A/Z11) is common for all 45 feeds.
They are (30/47)*(1/11)*(22/53)*(Z5/Z6)
Are change gears, which will be changed each time, when different feeds required. The different change gears starting from
(15/67) 0 (54/29) are shown in table 2.
As per table 2, first 15 feeds are obtained with gear pair.
Z1/Z2 = 41/41, Z3/Z4 = 41/41
Z5/Z6 change gears
By varying change gear we can get first 15 speeds.
For example the feed rate 129 mm/min is obtained from
(41/41)*(41/41)*(15/67)*(30/47)*(1/11)*(22/53)*1500
(Rpm of slow feed motor)*8 (pitch of the lead screw)*2(no of start -double start)
= 129 mm/min
Next 15 feed rates are obtained by putting
Z1/Z2 = 18/64, Z3/Z4 = 15/67
Z5/Z6 change gears
By varying change gears we can get next 15 feeds. For example the feed rate 8 mm/min.is obtained from
(18/64)*(15/67)*(15/67)*(30/47)*(1/11)*(22/53)
For example the feed rate 36mm/min is obtained from
=8 mm/min
Table 2 Different Slow Feeds
Change Gear 5 Change Gear 6 Slow Feed MM/Min
Number of Teeth Z5 Number of Teeth Z6 Z5 / Z6 Range without Reductor and with 41/41*41/41 Range with Reductor 18/64 with gear pair
15*67 41*41
15 67 15/67 129 8 36
18 64 18/64 162 10 45
22 60 22/60 212 13 59
26 57 26/57 263 16 74
29 54 29/54 310 19 87
32 51 32/51 363 23 102
35 48 35/48 421 26 118
37 45 37/45 475 29 133
41 41 41/41 578 36 162
43 39 43/39 637 40 180
45 37 45/37 703 44 200
48 35 48/35 792 49 222
51 32 51/32 921 57 259
54 29 54/29 1076 67 302
Next 15 feed rates are obtained by keeping
Z1/Z2 = 18/64, Z3/Z4=41/41
Z5/Z6 change gears
By varying change gears we can get next 15 feed rates.
For example the feed rate 36 mm/min is obtained from
(18/64)*(41/41)*(15/67)*(30/47)*(1/11)*(22/53)*1500*8*2
=36 mm/min
CYCLE TIME OF FEED
The cycle times of doing the five operations are calculated as follows. The job selected is shown in fig 4. The component taken for the boring is the cylinder of one type of I.C. engine. The bore 500.05 is made on fine boring machine. The cylinder is having a length of 70 mm.
Cutting speed = 90 m/min for material cast iron and cutter carbide tip
Figure 4
Details of the component bored in fine boring machine
Material: cast iron
Component name: cylinder for I.C Engine
All dimensions are in mm
Cutting speed = (DN/1000)
= 90 m/min
Dia of bore D = 50 mm
N = (90*1000)/ (*50)
= 600 rpm.
Feed rate for fine boring 0.05 mm/rev. (normal feed for fine boring operation)
Feed /min = 600*0.05
= 30 mm/min
Bore length = 70 mm
Machining time = (70/30)*60
= 140 secs.
Rapid rate = 4600 mm/min
Rapid travel = 300 mm
Time for rapid travel = (300/4600)*60
= 4 sec.
Time for feed
Table from home position to right = 4secs
Right side boring = 140 secs
Right to left = 8 secs
Left side boring = 140 secs
Left to home position = 4 secs
Total = 296 secs
Cycle time for fine boring using Electro Mechanical Drive System = 296 secs
COST OF DRIVE SYSTEM
1. Raw material cost Rs. 47530
2. Bought out Rs. 28150
3. Imported Rs. 3295
4. Sub contract Rs. 1960
Labour cost
5. Manufacturing Rs. 1, 02160
6. Assembly cost Rs. 25520
7. Pattern cost Rs. 25000
Production cost =1+5+6+7 = 200210
Factory cost Rs. 200210
Cost price 10% added Rs. 220231 (200210+20021)
Bought out + imported + sub contract = 33405
Total cost Rs. 253636
Cost of Electro Mechanical Drive System = Rs. 2, 53,636
CONCLUSION
The electro mechanical drive system is designed to operate at rapid feed rate of 4.6 m/min and 45 m/min slow feeds. Since the non-cutting time is reduced considerably the over all cycle time is reduced. So the productivity is increased. The cost of drive system is justifiable. For stepped drive this is the best drive. The only the advantage is that the gear changing is little tiresome when change in feed rate is required unlike step less drive where speed changing is very easy. This same design can be tried in other special purpose and other machine tools also.
REFERENCE
1. Dr.P.S.Bimbhra, Power Electronics text book, Khanna Publishers
(2003), Pg-460
2. R.Henry Xavier and Dr K.V.Thyagarajan, Manufacturing Technology Today, volume-3, August 2004, Page No-13 to 17.
3. T.P.S.Iyer, Production Technology,HMT Publication Ltd,New Delhi
No comments:
Post a Comment