3. INTRODUCTION
• SPEED BOX : A change-speed gearbox is a set of movable or constant
gears which allows the speed ratio between input and output shafts
to be changed either manually or automatically.
• FEED BOX : A mechanism of a metal-cutting machine; used
for changing the feed. It consists of interchangeable gear drives in a
housing.
• CONTROL SYSTEMS : The control systems of a machine tool are
meant to generate controlling movements which are essential for
carrying out a machining process in accordance with the technical
specifications of the part being machined.
4. Functions of Control Systems
• Changing speed and feed
• Providing working and auxiliary motions
Requirements of Control Systems
1. Reliability against failure to respond; the greater the degree of automation of
the control system, the higher should be its reliability.
2. Fast response to the information provided by the feedback device; again, the
greater the degree of automation, the faster should be the control system
response.
3. Accuracy of the system, which depends upon the accuracy of the feedback
device, control member and transmission.
5. Control Systems
for Changing
Speeds and Feeds
Control System
with Centralized
Control
Simple
Centralized
Control
Preselective
Centralized
Control
Selective
Centralized
Control
Control System
With Individual
Control
Types of Control Systems for Changing Speeds and Feeds
6. Speeds and Feeds Changing Systems with Simple
Centralized Control
• These systems may be single lever or multiple lever.
• Multiple lever systems are unwieldy, inconvenient and are also more time
consuming. In comparison, single lever systems are more efficient, but they are
also more complicated and expensive.
• The use of single lever systems is generally economically feasible only for
machine tools on which the machining time of typical operations is rather small.
• However, from the consideration of limiting operator fatigue, single lever systems
must invariably be preferred, irrespective of economic feasibility, for machine
tools on which the speeds and feeds are required to be changed frequently.
7. • The mechanism consists of hand wheel
11, bevel gears 10 and 9, spur gears 1 and
2, drum cam 4 which has two curved
profiles a and b cut on it, and levers 5 and
8 which are the operative members.
• At one end of the levers are pins 3 which
move in the curved profile of drum cam
4, while at their other end are blocks 6
mounted on eccentric pins“.
• Wheel 11 can be rotated and locked in
positions at which operative members
engage the appropriate gears for a
particular speed value.
Speed-Changing Mechanism of a Jig Boring Machine
8. Feed-Changing Mechanism Of A Horizontal Milling Machine
• When hand wheel 14 mounted on shaft 11 is
rotated, this rotation is transmitted to shaft 17
through gears 10-9-8.
• Bevel gear 7 mounted on shaft 17 rotates disc
cam 6.
• This cam has three curved grooves cut on its
faces; two of these, a and b, are visible in the
diagram, while the third c is on the opposite
face.
• Pins 18 mounted on levers 3, 5, and 19 move in
the grooves.
• Blocks 4 mounted at the ends of levers, which
incidentally are the operative members, fit into
the grooves of gear blocks and shift them.
9. • On the periphery of disc cam 6 are 12 V-shaped recesses into which fixing
pins loaded under the action of spring 2 can enter.
• The tension of spring 2 can be regulated by nut 1.
• Thus, corresponding to 12 different feed values, disc cam 6 can be fixed in
12 different positions.
• The number of teeth of the gears indicate that disc cam 6 completes one
revolution in the same time as hand wheel 14 completes three.
• Therefore, the transmission ratio between gear 15 mounted on shaft 16
and the indicator scale 13 mounted on shaft 11 is also kept equal to
three. Thus, one revolution of disc cam 6 corresponds to one revolution
of indicator scale 12.
• The indicator is marked for the 12 values of feed and these are indicated
on the scale.
10. Speed and Feed Changing Systems with Preselective
Control
• If the machining time of operations carried out on a machine tool is small, it
becomes desirable to cut down. as far as possible, the non-productive time,
which includes the time required for changing speeds and feeds.
• This is possible by providing a preselective control system of speed and feed
regulation on the machine tool. In a preselective system, the greater part of the
work involved in changing speed or feed is carried out while an operation is going
on, thus obtaining an overlapping of the nonproductive and machining times.
• Then when the operation is over, the speed or feed is rapidly changed by a simple
motion of a lever or the pressing of a button.
• Preselective systems are generally mechanical and hydraulic. However, the latter
are more efficient in operation, and, therefore, more commonly used.
11. Speed and Feed Changing Systems with Selective Control
• In a simple control system, the change from one value of speed or feed involves
passing through all the intermediate values. The main drawbacks of such a
control system are:
• Large auxiliary time.
• Excessive wear of sliding gears, jaw clutches, etc.
• If the number of speed or feed steps is large, the adverse effect of these
drawbacks on economic machining and proper machine tool exploitation can
become significant. In such cases, it is desirable to employ control systems which
permit changing of feed and speed from one value to another without having to
pass through the intermediate values. Such systems are known as selective
control systems.
• The working principle of selective control systems is explained through the
example of a four speed gear box using single lever control (Fig. 7.8).
12. • In this gear box, to engage one of the four available speeds of output
shaft 4. selector disc 2 is pulled out by means of lever I so that it does
not press upon racks 3.
• The disc is now turned to the position where an arrow or index
indicates the required speed.
• When the selector disc is now released, it pushes the corresponding
pair of racks. The pinions placed between the racks rotate and move
the appropriate forks to shift the gears.
