http://nptel.ac.in/courses/112103174/15
............... (mainly from IIT Notes)
CNC part program contains a combination of machine tool code and machine-specific instructions. It consists of:
(a) Information about part geometry
(b) Motion statements to move the cutting tool
(c) Cutting speed
(d) Feed
(e) Auxiliary functions such as coolant on and off, spindle direction
In this lecture, first we will understand the coordinate systems of the machine tools and how they work.
1. CNC Machine Tool
Figure 7.1.1 Schematic of a CNC machine Tool
Figure 7.1.1 shows a schematic of a machine tool controlled by
a computer. It consists of a Machine Control Unit (MCU) and machine
tool itself. MCU, a computer is the brain of a CNC machine tool. It
reads the part programs and controls the machine tools operations. Then
it decodes the part program to provide commands and instructions to
the various control loops of the machine axes of motion. The details
regarding the construction and working of mechatronics based system have
already been studied in last lectures.
CNC systems have a limitation. If the same NC program is used
on various machine tools, then it has to be loaded separately into each
machine. This is time consuming and involves repetitive tasks. For this
purpose direct numerical control (DNC) system is developed. Figure 7.1.2
shows the schematic of a DNC system. It consists of a central computer
to which a group of CNC machine tools are connected via a communication
network. The communication is usually carried out using a standard
protocol such as TCP/IP or MAP. DNC system can be centrally monitored
which is helpful when dealing with different operators, in different
shifts, working on different machines.
Figure 7.1.2 Direct numerical control (DNC) system
2. Axes of CNC machine tool
In CNC machine tool, each axis of motion is equipped with a
driving device to replace the handwheel of the conventional machine
tool. A axis of motion is defined as an axis where relative motion
between cutting tool and workpiece occurs. The primary axes of motion
are referred to as the X, Y, and Z axes and form the machine tool XYZ
coordinate system. Figure 7.1.3 shows the coordinate system and the
axes of motion of a typical machine tool. Conventionally machine tools
are designated by the number of axes of motion they can provide to
control the tool position and orientation.
2.1 2-axis machine tool
Figure 7.1.3 Axes of motion of a machine tool
If the machine tool can simultaneously control the tool along
two axes, it is classified as a 2-axis machine. The tool will be
parallel and independently controlled along third axis. It means that
machine tool guided the cutting tool along a 2-D contour with only
independent movement specified along the third axis. The Z-axis control plane is parallel to the XY plane.
2.2 2.5-axis machine tool
Figure 7.1.4 Axes in 2.5-axis machine tool
In this type of machine tool, the tool can be controlled to follow an inclined Z-axis control plane and it is termed as 2.5-axis machine tool. Figure 7.1.4 explains the axes system in 2.5-axis machine tool.
2.3 3-axis and multiple axis machine tool
Figure 7.1.5 3-axis machine tool
In these CNC machine tools, the tool is controlled along the three axes (
X, Y, and
Z) simultaneously, but the tool orientation doesn’t change with the tool motion as shown in Figure 7.1.5.
If the tool axis orientation varies with the tool motion in 3D,
3-axis machine gets converted into multi-axis orientation machine (4-,
5-, or 6-axis). Figure 7.1.6 shows the schematic of tool motion in a
multi-axis CNC machine tool.
Figure 7.1.6 Multiple axes machine tool
3. CNC program structure
There are four basic terms used in CNC programming. These are a follows:
Character -> Word -> Block -> Program
- Character is the smallest unit of CNC program. It can have Digit / Letter / Symbol.
- Word is a combination of alpha-numerical characters.
This creates a single instruction to the CNC machine. Each word begins
with a capital letter, followed by a numeral. These are used to
represent axes positions, federate, speed, preparatory commands, and
miscellaneous functions.
- A program block may contain multiple words, sequenced in a logical order of processing.
- The program comprises of multiple lines of instructions, ‘blocks’ which will be executed by the machine control unit (MCU).
The address G identifies a preparatory command, often called
G-code. This is used to preset or to prepare the control system to a
certain desired condition or to a certain mode or a state of operation.
For example G01 presets linear interpolation at given feed but doesnot
move any axis.
The address M in a CNC program specifies miscellaneous
function. It is also called as machine function. These functions
instruct the machine tool for various operations such as: spindle
rotation, gear range change, automatic tool change, coolant operation,
etc.
The G and M codes are controller manufacturers’ specific. the G and M codes used for FANUC,
Japan controller. Other controllers such as SINUMERIC, MITSUBHISHI etc.
are also being used in CNC technology.
It is suggested to the readers to study the following G
and M codes for milling and turning operations. Programming exercises
will be carried out in the next lectures.
5. Ballscrew based linear drives
Fig.4.4.1 Ballscrew configuration
Ball screw is also called as ball bearing screw or recirculating
ballscrew. It consists of a screw spindle, a nut, balls and integrated
ball return mechanism a shown in Figure 4.4.1. The flanged nut is
attached to the moving part of CNC machine tool. As the screw rotates,
the nut translates the moving part along the guide ways. However, since
the groove in the ball screw is helical, its steel balls roll along the
helical groove, and, then, they may go out of the ball nut unless they
are arrested at a certain spot. Thus, it is necessary to change their
path after they have reached a certain spot by guiding them, one after
another, back to their “starting point” (formation of a recirculation
path). The recirculation parts play that role. When the screw shaft is
rotating, as shown in Figure 4.4.1, a steel ball at point (A) travels 3
turns of screw groove, rolling along the grooves of the screw shaft and
the ball nut, and eventually reaches point (B). Then, the ball is forced
to change its pathway at the tip of the tube, passing back through the
tube, until it finally returns to point (A). Whenever the nut strokes on
the screw shaft, the balls repeat the same recirculation inside the
return tube.
When debris or foreign matter enter the inside of
the nut, it could affect smoothness in operation or cause premature
wearing, either of which could adversely affect the ball screw's
functions. To prevent such things from occurring, seals are provided to
keep contaminants out. There are various types of seals viz. plastic
seal or brush type of seal used in ball-screw drives.
5.1 Characteristics of ball screws:
5.1.1 High mechanical efficiency
In ball screws, about 90% or more of the force used
to rotate the screw shaft can be converted to the force to move the
ball nut. Since friction loss is extremely low, the amount of force used
to rotate the screw shaft is as low as one third of that needed for the
acme thread lead screw.
5.1.2 Low in wear
Because of rolling contact, wear is less than that
of sliding contact. Thus, the accuracy is high. Ball screws move
smoothly enough under very slow speed. They run smoothly even under a
load.
5.1.3 Thread Form
The thread form used in these screws can either be gothic arc type
(fig. 4.4.2.a) or circular arc type (fig. 4.4.2.b). The friction in this
kind of arrangement is of rolling type. This reduces its wear as
comparison with conventional sliding friction screws drives.
Fig. 4.4.2 Thread forms (a) Gothic arc (b) Circular arc
Recirculating ball screws are of two types. In one arrangement the
balls are returned using an external tube. In the other arrangement the
balls are returned to the start of the thread in the nut through a
channel inside the nut.
5.3 Preloading
Fig. 4.4.3 Double nut preloading system
In order to obtain bidirectional motion of the carriage without any
positional error, the backlash between the nut and screw should be
minimum. Zero backlash can be obtained by fitting two nuts with
preloading (tension or compression) or by applying a load which exceeds
the maximum operating load. Figure 4.4.3 shows double nut preloading
system. A shim plate (spacer) is inserted between two nuts for
preloading. Preload is to create elastic deformations (deflections) in
steel balls and ball grooves in the nut and the screw shaft in advance
by providing an axial load. As a result the balls in one of the nuts
contact the one side of the thread and balls in the other nut contact
the opposite side.
5.3.1 Effects of preload
- Zero backlash: It eliminates axial play between a screw shaft and a ball nut.
- It minimizes elastic deformation caused by external force, thus the rigidity enhances.
In case mounting errors, misalignment between the screw shaft and the
nut may occur this further generates distortion forces. This could lead
to the problems such as,
- Shortened service life
- Adverse effect on smooth operation
- Reduced positioning accuracy
- Generation of noise or vibration
- Breakage of screw shaft
5.4 Advantages of ball screws
- Highly efficient and reliable.
- Less starting torque.
- Lower co efficient of friction compared to sliding type screws and run at cooler temperatures
- Power transmission efficiency is very high and is of the order of 95 %.
- Could be easily preloaded to eliminate backlash.
- The friction force is virtually independent of the travel
velocity and the friction at rest is very small; consequently, the
stick-slip phenomenon is practically absent, ensuring uniformity of
motion.
- Has longer thread life hence need to be replaced less frequently.
- Ball screws are well -suited to high through output, high speed applications or those with continuous or long cycle times.
- Smooth movement over full range of travel.
5.5 Disadvantages of ball screws
- Tend to vibrate.
- Require periodic overhauling to maintain their efficiency.
- Inclusion of dirt or foreign particles reduces the life of the screws.
- Not as stiff as other power screws, thus deflection and critical speed can cause difficulties.
- They are not self-locking screws hence cannot be used in holding devices such as vices.
- Require high levels of lubrication.
5.6 Applications of ball screws:
- Ball screws are employed in cutting machines, such as machining
center and NC lathe where accurate positioning of the table is desired
- Used in the equipments such as lithographic equipment or inspection apparatus where precise positioning is vital
- High precision ball screws are used in steppers for semiconductor
manufacturing industries for precision assembly of micro parts.
- Used in robotics application where precision positioning is needed.
- Used in medical examination equipments since they are highly accurate and provide smooth motion.
1. Drives
Basic function of a CNC machine is to provide automatic and precise
motion control to its elements such work table, tool spindle etc. Drives
are used to provide such kinds of controlled motion to the elements of a
CNC machine tool. A drive system consists of drive motors and ball
lead-screws. The control unit sends the amplified control signals to
actuate drive motors which in turn rotate the ball lead-screws to
position the machine table or cause rotation of the spindle.
2. Power drives
Drives used in an automated system or in CNC system are of different types such as electrical, hydraulic or pneumatic.
- Electrical drives
These are direct current (DC) or alternating current (AC) servo motors. They are small in size and are easy to control.
- Hydraulic drives
These drives have l arge power to size ratio and provide
stepless motion with great accuracy. But these are difficult to maintain
and are bulky. Generally they employ p etroleum based hydraulic oil
which may have fire hazards at upper level of working temperatures. Also
hydraulic elements need special treatment to protect them against
corrosion.
- Pneumatic drives
This drives use air as working medium which is available in
abundant and is fire proof. T hey are simple in construction and are
cheaper. However these drives generate low power, have less positioning
accuracy and are noisy.
2.1 Spindle drives
Fig. 4.1.1 Schematic of a spindle drive
The spindle drives are used to provide angular motion to the
workpiece or a cutting tool. Figure 4.1.1 shows the components of a
spindle drive. These drives are essentially required to maintain the
speed accurately within a power band which will enable machining of a
variety of materials with variations in material hardness. The speed
ranges can be from 10 to 20,000 rpm. The machine tools mostly employ DC
spindle drives. But as of late, the AC drives are preferred to DC drives
due to the advent of microprocessor-based AC frequency inverter. High
overload capacity is also needed for unintended overloads on the spindle
due to an inappropriate feed. It is desirous to have a compact drive
with highly smooth operation.
2.2 Feed Drives
Fig. 4.1.2 Typical feed drive
These are used to drive the slide or a table. Figure 4.1.2 shows
various elements of a feed drive. The requirements of an ideal feed
drive are as follows.
- The feed motor needs to operate with constant torque characteristics to overcome friction and working forces.
- The drive speed should be extremely variable with a speed range
of about 1: 20000, which means it should have a maximum speed of around
2000 rpm and at a minimum speed of 0.1 rpm.
- The feed motor must run smoothly.
- The drive should have extremely small positioning resolution.
- Other requirements include high torque to weight ratio, low rotor
inertia and quick response in case of contouring operation where
several feed drives have to work simultaneously.
Variable speed DC drives are used as feed drives in CNC machine tools. However now-a-days AC feed drives are being used.
In CNC, usually AC, DC, servo and stepper electrical drives are
used. The various drives used in CNC machines can be classified as:
- Spindle drives to provide the main spindle power for cutting action
- Feed drives to drive the axis
3. Electrical drives
Fig. 4.1.3 Classification of motors
Electric drives are mostly used in position and speed
control systems. The motors can be classified into two groups namely DC
motors and AC motors (Fig. 4.1.3). In this session we shall study the
operation, construction, advantages and limitations of DC and AC motors.
3.1. DC motors
A DC motor is a device that converts direct current
(electrical energy) into rotation of an element (mechanical energy).
These motors can further be classified into brushed DC motor and
brushless DC motors.
3.3.1 Brush type DC motor
A typical brushed motor consists of an armature coil,
slip rings divided into two parts, a pair of brushes and horse shoes
electromagnet as shown in Fig. 4.1.4. A simple DC motor has two field
poles namely a north pole and a south pole. The magnetic lines of force
extend across the opening between the poles from north to south. The
coil is wound around a soft iron core and is placed in between the
magnet poles. These electromagnets receive electricity from an outside
power source. The coil ends are connected to split rings. The carbon
brushes are in contact with the split rings. The brushes are connected
to a DC source. Here the split rings rotate with the coil while the
brushes remain stationary.
Fig. 4.1.4 Brushed DC motor
The working is based on the principle that when a current-carrying
conductor is placed in a magnetic field, it experiences a mechanical
force whose direction is given by Fleming's left-hand rule. The
magnitude of the force is given by
F = BIL sinθ (4.1.1)
Where, B is magnetic field density in weber/m2
I is the current in amperes and
L is the length of the conductor in meter
θ is the angle between the direction of the current in the conductor and the electric field
If the current and filed are perpendicular then θ = 90°. The equation 4.1.1 becomes,
F = BIL (4.1.2)
A direct current in a set of windings creates a
magnetic field. This field produces a force which turns the armature.
This force is called torque. This torque will cause the armature to turn
until its magnetic field is aligned with the external field. Once
aligned the direction of the current in the windings on the armature
reverses, thereby reversing the polarity of the rotor's electromagnetic
field. A torque is once again exerted on the rotor, and it continues
spinning. The change in direction of current is facilitated by the split
ring commutator. The main purpose of the commutator is to overturn the
direction of the electric current in the armature. The commutator also
aids in the transmission of current between the armature and the power
source. The brushes remain stationary, but they are in contact with the
armature at the commutator, which rotates with the armature such that at
every 180° of rotation, the current in the armature is reversed.
Advantages of brushed DC motor :
- The design of the brushed DC motor is quite simple
- Controlling the speed of a Brush DC Motor is easy
- Very cost effective
Disadvantages of brushed DC motor :
- High maintenance
- Performance decreases with dust particles
- Less reliable in control at lower speeds
- The brushes wear off with usage
3.1.2
Brushless DC motor
Fig. 4.1.5 Brushless DC motor
A brushless DC motor has a rotor with permanent
magnets and a stator with windings. The rotor can be of ceramic
permanent magnet type. The brushes and commutator are eliminated and the
windings are connected to the control electronics. The control
electronics replace the commutator and brushes and energize the stator
sequentially. Here the conductor is fixed and the magnet moves (Fig.
4.1.5).
The current supplied to the stator is based on the
position of rotor. It is switched in sequence using transistors. The
position of the rotor is sensed by Hall effect sensors. Thus a
continuous rotation is obtained.
Advantages of brushless DC motor :
- More precise due to computer control
- More efficient
- No sparking due to absence of brushes
- Less electrical noise
- No brushes to wear out
- Electromagnets are situated on the stator hence easy to cool
- Motor can operate at speeds above 10,000 rpm under loaded and unloaded conditions
- Responsiveness and quick acceleration due to low rotor inertia
Disadvantages of brushless DC motor :
- Higher initial cost
- Complex due to presence of computer controller
- Brushless DC motor also requires additional system wiring in order to power the electronic commutation circuitry
3.2 AC motors
AC motors convert AC current into the rotation of a mechanical
element (mechanical energy). As in the case of DC motor, a current is
passed through the coil, generating a torque on the coil. Typical
components include a s tator and a rotor. The armature of rotor is a
magnet unlike DC motors and the stator is formed by electromagnets
similar to DC motors. The main limitation of AC motors over DC motors is
that speed is more difficult to control in AC motors. To overcome this
limitation, AC motors are equipped with variable frequency drives but
the improved speed control comes together with a reduced power quality.
Fig. 4.1.6 AC motor working principle
The working principle of AC motor is shown in fig.
4.1.6. Consider the rotor to be a permanent magnet. Current flowing
through conductors energizes the magnets and develops N and S poles. The
strength of electromagnets depends on current. First half cycle current
flows in one direction and in the second half cycle it flows in
opposite direction. As AC voltage changes the poles alternate.
AC motors can be classified into synchronous motors and induction motors.
3.2.1 Synchronous motor
Fig. 4.1.7 Synchronous AC motor
A synchronous motor is an AC motor which runs at constant speed
fixed by frequency of the system. It requires direct current (DC) for
excitation and has low starting torque, and hence is suited for
applications that start with a low load. It has two basic electrical
parts namely stator and rotor as shown in fig. 4.1.7. The stator
consists of a group of individual wounded electro-magnets arranged in
such a way that they form a hollow cylinder. The stator produces a
rotating magnetic field that is proportional to the frequency supplied.
The rotor is the rotating electrical component. It also consists of a
group of permanent magnets arranged around a cylinder, with the poles
facing toward the stator poles. The rotor is mounted on the motor shaft.
The main difference between the synchronous motor and the induction
motor is that the rotor of the synchronous motor travels at the same
speed as the rotating magnet.
The stator is given a three phase supply and as the polarity of the
stator progressively change the magnetic field rotates, the rotor will
follow and rotate with the magnetic field of the stator. If a
synchronous motor loses lock with the line frequency it will stall. It
cannot start by itself, hence has to be started by an auxiliary motor.
Synchronous speed of an AC motor is determined by the following formula:
N
s = Revolutions per minute
P = Number of pole pairs
f = Applied frequency
3.2.2 Induction motor
Induction motors are quite commonly used in
industrial automation. In the synchronous motor the stator poles are
wound with coils and rotor is permanent magnet and is supplied with
current to create fixed polarity poles. In case of induction motor, the
stator is similar to synchronous motor with windings but the rotors'
construction is different.
Fig. 4.1.8 Induction motor rotor
Rotor of an induction motor can be of two types:
-
A squirrel-cage rotor consists of thick
conducting bars embedded in parallel slots. The bars can be of copper or
aluminum. These bars are fitted at both ends by means end rings as
shown in figure 4.1.8.
-
A wound rotor has a three-phase,
double-layer, distributed winding. The rotor is wound for as many
numbers of poles as the stator. The three phases are wired internally
and the other ends are connected to slip-rings mounted on a shaft with
brushes resting on them.
Induction motors can be classified into two types:
-
Single-phase induction motor : It
has one stator winding and a squirrel cage rotor. It operates with a
single-phase power supply and requires a device to start the motor.
-
Three-phase induction motor : The
rotating magnetic field is produced by the balanced three-phase power
supply. These motors can have squirrel cage or wound rotors and are
self-starting.
In an induction motor there is no external power
supply to rotor. It works on the principle of induction. When a
conductor is moved through an existing magnetic field the relative
motion of the two causes an electric current to flow in the conductor.
In an induction motor the current flow in the rotor is not caused by any
direct connection of the conductors to a voltage source, but rather by
the influence of the rotor conductors cutting across the lines of flux
produced by the stator magnetic fields. The induced current which is
produced in the rotor results in a magnetic field around the rotor. The
magnetic field around each rotor conductor will cause the rotor
conductor to act like the permanent magnet. As the magnetic field of the
stator rotates, due to the effect of the three-phase AC power supply,
the induced magnetic field of the rotor will be attracted and will
follow the rotation. However, to produce torque, an induction motor must
suffer from slip. Slip is the result of the induced field in the rotor
windings lagging behind the rotating magnetic field in the stator
windings. The slip is given by,
Advantages of AC induction motors
-
It has a simple design, low initial cost, rugged construction almost unbreakable
-
The operation is simple with less maintenance (as there are no brushes)
-
The efficiency of these motors is very high, as there are no frictional losses, with reasonably good power factor
-
The control gear for the starting purpose of these motors is minimum and thus simple and reliable operation
Disadvantages of AC induction motors
-
The speed control of these motors is at the expense of their efficiency
-
As the load on the motor increases, the speed decreases
-
The starting torque is inferior when compared to DC motors
1. Stepper motor
A stepper motor is a pulse-driven motor that changes the angular
position of the rotor in steps. Due to this nature of a stepper motor,
it is widely used in low cost, open loop position control systems.
Types of stepper motors:
- Permanent Magnet
- Employ permanent magnet
- Low speed, relatively high torque
- Variable Reluctance
- Does not have permanent magnet
- Low torque
1.1 Variable Reluctance Motor
Figure 4.2.1 shows the construction of Variable Reluctance motor.
The cylindrical rotor is made of soft steel and has four poles as shown
in Fig.4.2.1. It has four rotor teeth, 90⁰ apart and six stator poles,
60⁰ apart. Electromagnetic field is produced by activating the stator
coils in sequence. It attracts the metal rotor. When the windings are
energized in a reoccurring sequence of 2, 3, 1, and so on, the motor
will rotate in a 30⁰ step angle. In the non-energized condition, there
is no magnetic flux in the air gap, as the stator is an electromagnet
and the rotor is a piece of soft iron; hence, there is no detent torque.
This type of stepper motor is called a variable reluctance stepper.
Fig. 4.2.1 Variable reluctance stepper motor
1.2 Permanent magnet (PM) stepper motor
In this type of motor, the rotor is a permanent magnet. Unlike the
other stepping motors, the PM motor rotor has no teeth and is designed
to be magnetized at a right angle to its axis. Figure 4.2.2 shows a
simple, 90⁰ PM motor with four phases (A-D). Applying current to each
phase in sequence will cause the rotor to rotate by adjusting to the
changing magnetic fields. Although it operates at fairly low speed, the
PM motor has a relatively high torque characteristic. These are low cost
motors with typical step angle ranging between 7.5⁰ to 15⁰ .
Fig. 4.2.2 Permanent magnet stepper
1.3 Hybrid stepper motor
Hybrid stepping motors combine a permanent magnet and a rotor with
metal teeth to provide features of the variable reluctance and permanent
magnet motors together. The number of rotor pole pairs is equal to the
number of teeth on one of the rotor's parts. The hybrid motor stator has
teeth creating more poles than the main poles windings (Fig. 4.2.3).
Fig. 4.2.3 Hybrid stepper
Rotation of a hybrid stepping motor is produced in
the similar fashion as a permanent magnet stepping motor, by energizing
individual windings in a positive or negative direction. When a winding
is energized, north and south poles are created, depending on the
polarity of the current flowing. These generated poles attract the
permanent poles of the rotor and also the finer metal teeth present on
rotor. The rotor moves one step to align the offset magnetized rotor
teeth to the corresponding energized windings. Hybrid motors are more
expensive than motors with permanent magnets, but they use smaller
steps, have greater torque and maximum speed.
Step angle of a stepper motor is given by,
Advantages of stepper motors :
- Low cost
- Ruggedness
- Simplicity of construction
- Low maintenance
- Less likely to stall or slip
- Will work in any environment
- Excellent start-stop and reversing responses
Disadvantages of stepper motors :
- Low torque capacity compared to DC motors
- Limited speed
- During overloading, the synchronization will be broken. Vibration and noise occur when running at high speed.
2. Servomotor
Servomotors are special electromechanical devices that produce
precise degrees of rotation. A servo motor is a DC or AC or brushless DC
motor combined with a position sensing device. Servomotors are also
called control motors as they are involved in controlling a mechanical
system. The servomotors are used in a closed-loop servo system as shown
in Figure 4.2.4. A reference input is sent to the servo amplifier, which
controls the speed of the servomotor. A feedback device is mounted on
the machine, which is either an encoder or resolver. This device changes
mechanical motion into electrical signals and is used as a feedback.
This feedback is sent to the error detector , which compares the actual
operation with that of the reference input. If there is an error, that
error is fed directly to the amplifier, which will be used to make
necessary corrections in control action. In many servo systems, both
velocity and position are monitored. Servomotors provide accurate speed,
torque, and have ability of direction control.
Fig. 4.2.4 Servo system block diagram
2.1 DC servomotors
DC operated servomotors are usually respond to error signal abruptly
and accelerate the load quickly. A DC servo motor is actually an
assembly of four separate components, namely:
- DC motor
- gear assembly
- position-sensing device
- control circuit
2.2. AC servo motor
In this type of motor, t he magnetic force is generated by a
permanent magnet and current which further produce the torque. It has no
brushes so there is little noise/vibration. This motor provides high
precision control with the help of high resolution encoder. The stator
is composed of a core and a winding. The rotor part comprises of shaft,
rotor core and a permanent magnet.
Digital encoder can be of optical or magnetic type. It gives
digital signals, which are in proportion of rotation of the shaft. The
details about optical encoder have already discussed in Lecture 3 of
Module 2.
Advantages of servo motors
- Provides high intermittent torque, high torque to inertia ratio, and high speeds
- Work well for velocity control
- Available in all sizes
- Quiet in operation
- Smoother rotation at lower speeds
Disadvantages of servo motors
- More expensive than stepper motors
- Require tuning of control loop parameters
- Not suitable for ha zardous environments or in vacuum
- Excessive current can result in partial demagnetization of DC type servo motor
Machining centers are used to carry out multiple operations like
drilling, milling, boring etc. in one set up on multiple faces of the
workpiece. These operations require a number of different tools. Tool
changing operation is time consuming which reduces the machine
utilization. Hence the tools should be automatically changed to reduce
the idle time. This can be achieved by using automatic tool changer
(ATC) facility. It helps the workpiece to be machined in one setup which
increases the machine utilization and productivity. Large numbers of
tools can be stored in tool magazines. Tool magazines are specified by
their storage capacity, tool change procedure and shape. The storage
capacity ranges from 12 to 200. Some of the magazines are discuseed as
follows.
1. Tool turret
It is the simplest form of tool magazine. Figure 4.6.1 the
schematic of a turret with a capacity to hold twelve tools. It consists
of a tool storage without any tool changer. The turret is indexed in the
required position for desired machining operation. Advantage of the
turret is that the tool can easily be identified, but the time consumed
for tool change is more unless the tool is in the adjecent slot.
Fig. 4.6.1 Tool turret
2. Tool magazines
Tool magazines are generally employed in CNC drilling and milling
machines. Compared to tool turrets the tool magazines can hold more
number of tools therefore proper management of tools is essential.
Duplication of the tools is possible and a new tool of same type may be
selected when a particular tool is worn off. The power required to move
the tools in a tool magazine is more in comparison with that required in
tool turrets. The following are some of the tool magazines used in
automation.
2.1 Disc or drum type
2.2 Chain type
2.3 Disk or drum type
2.1 Disc type magazine
Fig. 4.6.2 Drum magazine
The disc type tool magazine rotates to get the desired tool in
position with the tool change arm (Fig. 4.6.2). Larger the diameter of
the disc/drum more the number of tools it can hold. It has pockets where
tool can be inserted. In case of drum type magazine which can store
large amount of tools, the pockets are on the surface along the length.
It carries about 12 to 50 tools. If the number of tools are less the
disc is mounted on top of the spindle to minimize the travel of tool
between the spindle and the disc. If the tools are more then, the disc
is wall mounted or mounted on the machining center column. If the disc
is column mounted then, it needs an additional linear motion to move it
to the loading station for tool change.
2.2 Chain type magazine
Fig. 4.6.3 Chain magazine
When the number of tools is more than 50, chain type of magazines
are used (Fig. 4.6.3). The magazine is mounted overhead or as a separate
column. In chain magazines the tools are identified either by their
location in the tool holder or by means of some coding on the tool
holder. These types of magazines can be duplicated. There can be two
chain magazines: one is active for machining and the second magazine is
used when the duplicate tool is needed since the active tool is worn
out.
2.3 Rack type magazine
Rack magazines are cost-efficient alternative to usual tool
magazine systems (Fig. 4.6.4). Set-up time can be optimized by utilizing
the racks' capacity of up to 50 tools. The high storage capacity of up
to 400 tools permits a large production capacity of varying work pieces
without tool changes. They can also be used to store work pieces.
3. Automatic tool changing
Fig. 4.6.5 Automatic tool changer
The tools from the magazines and spindle are exchanged by a tool
changer arm (Fig. 4.6.5). The tool change activity requires the
following motions:
- The spindle stops at the correct orientation for the tool change arm to pick the tool from the spindle.
- Tool change arm moves to the spindle.
- Tool change arm picks the tool from the spindle.
- Tool change arm indexes to reach the tool magazine.
- Tool magazine indexes so that the tool from the spindle can be placed.
- The tool is placed in the tool magazine.
- The tool magazine indexes to bring the required tool to the tool change position.
- Tool change arm picks the tool from the tool magazine.
- Tool change arm indexes to reach the spindle.
- New tool is placed in the spindle.
- Tool change arm moves back to its parking position.
3.1 Advantages of automatic tool changer
- Increase in operator safety by changing tools automatically
- Changes the tools in seconds for maintenance and repair
- Increases flexibility
- Heavy and large multi-tools can easily be handled
- Decreases total production time
http://engineersedge.com/edm.shtml
Electrical Discharge Machining, EDM is one of the
most accurate manufacturing processes available for creating
complex or simple shapes and geometries within parts and
assemblies. EDM works by eroding
material in the path of electrical discharges that form an arc
between an electrode tool and the work piece. EDM
manufacturing is quite affordable and a very desirable
manufacturing process when low counts or high accuracy is
required. Turn around time can be fast and depends on
manufacturer back log.
The EDM system consists of a
shaped tool or wire electrode, and the part. The part is connected
to a power supply. Sometimes to create a potential difference between the
work piece and tool, the work piece is immersed in a dielectric
(electrically nonconducting) fluid which is circulated to flush
away debris.
The cutting pattern is usually
CNC controlled. Many EDM machine electrodes can rotate
about two-three axis allowing for cutting of internal cavities.
This makes EDM a highly capable manufacturing process.
EDM comes in two basic types:
wire and probe (die sinker). Wire EDM is used primarily for shapes
cut shapes through a selected part or assembly. With a wire EDM machine, if a
cutout needs to be created, an initial hole must first be drilled
in the material, then the wire can be fed through the hole to
complete the machining. Sinker (die sinking EDMs are generally used
for complex geometries where the EDM machine uses a machined
graphite or copper electrode to erode the desired shape into
the part or assembly. Sinker EDM can
cut a hole into the part without having a hole pre-drilled for
the electrode.
EDM Power System. The discharge energy during EDM is provided by a direct current pulse power
generator. The EDM power system can be classified into RC, LC, RLC, and transistorized types. The
transistorized EDM power systems provide square waveform pulses with the pulse on-time usually
ranging from 1 to 2000 msec, peak voltage ranging from 40 to 400V, and peak discharge current ranging
from 0.5 to 500 A. With the RC, LC, or RLC type power system, the discharge energy comes from a
capacitor that is connected in parallel with the machining gap. As a result of the low impedance of
plasma channel, the discharge duration is very short (less than 5 msec), and the discharge current is very
high, up to 1000 A. The peak voltage is in the same range of transistorized power systems.
The transistorized power systems are usually used in
die-sinking EDM operations because of their lower tool wear. Capacitive
power systems are used for small hole drilling, machining of advanced
materials, and micro-EDM because of higher material removal rate and
better process stability. WEDM power generator usually is a
transistor-controlled capacitive power system that reduces the wire
rupture risk. In this power system, the discharge frequency can be
controlled by adjusting the on-time and off time of the transistors that
control the charging pulse for the capacitor connected in parallel with
the machining gap.
Key EDM System Components: The machining gap between tool and work piece during EDM must be submerged
in an electrically nonconductive dielectric fluid. In die-sinking EDM, kerosene is often used as
a dielectric fluid because it provides lower tool wear, higher accuracy, and better surface quality.
Deionized water is always used as a dielectric fluid in WEDM to provide a larger gap size and lower
wire temperature in order to reduce the wire rupture risk. This fluid also serves to flush debris from the
gap and thus helps maintain surface quality.
Copper and graphite are commonly used as die-sinking EDM tool materials because of the high
electrical conductivity and high melting temperature and the ease of being fabricated into complicated
shapes. The wire electrode for WEDM is usually made of copper, brass, or molybdenum in a diameter
ranging from 0.01 to 0.5 mm. Stratified copper wire coated with zinc brass with diameter of 0.25 mm
is often used.
In the traditional die-sinking EDM process, the tool is fabricated into a required shape and mounted
on a ram that moves vertically. The spark discharges can only occur under a particular gap size that
determines the strength of electric field to break down the dielectric. A servo control mechanism is
equipped to monitor the gap voltage and to drive the machine ram moving up or down to obtain a
dischargeable gap size and maintain continuous sparking. Because the average gap voltage is approximately
proportional to the gap size, the servo system controls the ram position to keep the average gap
voltage as close as possible to a preset voltage, known as the servo reference voltage.
In a WED machine, the wire
electrode is held vertically by two wire guides located separately above
and beneath the work piece with the wire traveling
longitudinally during machining. The work piece is
usually mounted on an x-y table. The trajectory of the relative movement
between wire and work piece in the x-y coordinate
space is controlled by a CNC servo system according to a preprogrammed
cutting
passage. The CNC servo system also adjusts the
machining gap size in real time, similar to the die sinking
EDM operation. The dielectric fluid is sprayed from above and
beneath the work piece into the
machining gap with two nozzles.
 |
The power generators in WED machines usually are
transistor-controlled RC or RLC systems that
provide higher machining rate and larger gap size
to reduce wire rupture risks. In some WED machines,
the machining gap is submerged into the dielectric
fluid to avoid wire vibration to obtain a better accuracy.
The upper wire guide is also controlled by the CNC
system in many WED machines. During machining,
the upper wire guide and the x-y table
simultaneously move along their own preprogrammed trajectories
to produce a taper and/or twist surface on the work
piece.
Advantages of EDM :
- Complex shapes that would otherwise be difficult to produce with conventional cutting tools
- Extremely hard material to very close tolerances
- Very small work pieces where
conventional machining tools may damage the part from excess cutting
tool pressure.
- There is no direct contact
between tool and work piece. Therefore delicate sections and weak
materials can be machined without any distortion.
Disadvantages of EDM :
- Relatively s low rate of material removal.
- Additional lead time and cost used for creating electrodes for ram/sinker EDM.
- Reproducing sharp corners on the workpiece is difficult due to electrode wear.
- Electrical power consumption is high.
- Material mustbe electrically conductive
Mechanical Design Considerations:
- Relax the surface-finish for
the part, if feasible. This allows the manufacturer to produce
the part with fewer passes, at a higher current level and a
higher metal-removal rate.
- Design or prepare the part such that the
amount of stock removed by EDM is relatively small. Use
traditional machining techniques to remove the bulk of the
stock with the finishing operations performed by EDM. This
significantly reduces the amount of time and cost for each
part.
- The EDM manufacturer should
consider fixtures such that several parts can be stacked and
machined simultaneously or a single part can have several EDM
operations performed simultaneously.
- When existing holes are to be
enlarged or reshaped by EDM, through holes are preferred to
blind holes as they permit easier flow of dielectric fluid past
the area being machined
- There will be some degree of materials exchange
between the EDM wire / probe and the base material. Specifya cleaning
procedure is galvanic corrosion is a concern.
- The minimum internal corner radius of cut feature
will dictate the maximum wire diameter that can be used. Obviously, the
wire diameter needs to be at less than double the minimum inside corner
radius. However, one also has to account for the amount of final
overcut, plus a small amount of ???maneuvering??? room, so that the CNC
can generate the corner. This is analogous to CNC contour milling, in
which accurate internal corner radii are generated by machine motion,
rather than just plunging an end mill into a corner and accepting the
result. Usually, ???the bigger, the better??? for wire diameters up to
.010???. It is important to note that the new ???twin wire??? machines
can employ a different strategy for these conditions, however, most of
us do not have this luxury. Recommendations for small diameter wires
include:
??? High Tensile Brass wire for .006??? diameter
??? Steel Core wire from .002??? to .004??? diameter
??? Moly wire from .002??? to .004???
??? Tungsten wire from .0008??? to .002???
Dimensional Accuracy (+/- 0.0005
inches per inch)
Feature Profile accuracy of .0003 is
obtainable with cutting path
Features to feature
true position of .002 is reasonable and down
to .001 is possible when geometry requires removal and
reattachment of wire.
Unit-3
ELECTRICAL
ENERGY BASED PROCESSES
3.1
INTRODUCTION
Electrical
energy based processes என்பது electrical energyயை direct ஆக பயன்படுத்தி materialயை cut செய்து தேவையான அளவு மற்றும் வடிவம் கிடைக்கிறது.
Examples:
1.
Electrical
Discharge Machining (EDM)
2.
Wire Cut Electrical
Discharge Machining (WCEDM)
3.2 ELECTRICAL
DISCHARGE MACHINING (EDM)
3.2.1 Workng Principle Of EDM:
இம்முறையில் tool–க்கும் work piece-க்கும் இடையில் உருவாகும் powerful electric spark-கினால் metal-ஆனது நீக்கப்படுகிறது. இதில் tool ஆனது cathode-ஆகவும் work piece ஆனது
anode-ஆகவும் செயல்படுகிறது.
இம்முறையில் electric spark மூலம் உருவாகும் அதிக வெப்பத்தில் workpiece-ஐ சூடுபடுத்தி அடிபகுதியில் இருந்து உலோகம் erosion மூலம் நீக்கப்படுகிறது. இதில்
ஒரு தொட்டியில் உள்ள fixture-ல் workpiece
பொருத்தபட்டிருக்கும். இந்த
தொட்டியில் mineral oil,white
spirit அல்லது
paraffin போன்ற
மின் கடத்தா திரவம்(dielectric fluid) நிரப்பபட்டிருக்கும். D.C. supply-ன் +ve முனையுடன் workpiece இணைக்கபட்டிருக்கும்.
Workpiece-ல் உருவாக்கபட வேண்டிய வடிவத்திற்கு ஏற்ப tool வடிவமைக்கப்பட்டிருக்கும். Copper, Brass, Tarssfan அல்லது Graphite ஆல் tool செய்யப்பட்டிருக்கும்.
Tool-க்கும் work piece இடையில் சுமார் 0.005mm முதல் 0.05mm
வரையிலான குறுகிய இடைவெளி இருக்குமாறு ஒரு servomotor
மூலம் தானாகவே tool நகர்த்தப்படும்.
D.C. supply கொடுத்தவுடன் workpiece மற்றும் tool க்கு இடையே உள்ள சிறிய இடைவெளியில் electric spark உருவாகிறது. இதனால்
அந்த இடத்தில் சுமார் 120000C வரை வெப்பம் உண்டாகிறது. இந்த
அதிக வெப்பத்தில் work piece ன் சிறிய பகுதி வருகிறது. ஒரு
வினாடிக்கு ஆயிரக்கணக்கில் உருவாகும் இந்த spark களில் அதிக விசையால் உருகிய உலோகம் மிக நுண்ணிய துகள்களாக பிரிக்கப்பட்டு நீக்கப்படும். அதிக
நேரத்தில் அதிக அழுத்ததுடன் செலுத்தப்படும் dielectric fluid இந்த உலோக துகள்களை அடித்து சென்றுவிடும்.
3.2.2
Advantage:
1. மின்கடத்தும்
பொருட்கள் அனைத்தையும் machining செய்யலாம்.
2. 0.2 micron அளவுக்கு துல்லியமாக machining செய்யலாம்.
3. Workpiece-ல்
stress உருவாவதில்லை.
4. சிக்கலான
வடிவங்களையும் ஏற்படுத்தலாம்.
5. விரைவான
செயல்முறை.
3.2.3
Disadvantage:
1.அதிக electric power தேவை
2.மின் கடத்தும் பொருட்களை மட்டும் machining செய்ய இயலும்.
3.Tool
அதிக
அளவு தேய்மானம் அடையும்.
Application:
1. Nozzle களில் சிறிய துளைகளை ஏற்படுத்துவதற்கு இம்முறை பயன்படுகிறது.
2. Tungsten carbide, Stelite போன்ற எளிதில் உடையும் தன்மை கொண்ட கடினமான பொருட்களை machining செய்ய இம்முறை உதவுகிறது.
3. Tool மற்றும் cutter களை கூர்மையாக்க பயன்படுகிறது.
4. Workpiece இரண்டாக வெட்டி பிரிக்கப்படுகிறது.
3.2.4
Dielectric Fluid
v Dielectric
fluid ஆனது electricity யை conduct செய்யாது. In electric discharge machining process ல் workpiece மற்றும் tool ஒன்றையொன்று சார்ந்து காணப்படும். இதில்
dielectric fluid medium நடைபெறும். Dielectric fluid பொதுவாக petroleum, hydrocarbon fluid, paraffin, white spirit, transformer oil, kerosene, mineral போன்றவற்றிற்கு கலந்து பயன்படுகிறது.
v Dielectric
fluid ஆனது process நடைபெறும் பொது tool and workpiece ஐ அரிக்காது.
v Dielectric
fluid ஐ choose செய்வது
அந்த workpiece ன் size, type of shape, tolerance, metal removal rate and surface finish ஆகியவற்றை பொருத்தது.
3.2.5 Function of Dielectric Fluids:
1. இது
insulating medium ஆக
பயன்படுகிறது.
2. Spark உருவாகும் region ஐ cool ஆக வைத்து கொள்கிறது. Tool and workpiece ஐ பாதுகாப்பாக வைக்கிறது.
3. Workpiece erode ஆகும் போது particles ஐ remove செய்ய பயன்படுகிறது.
4. இது
constant resistance வைத்துக்கொள்ள பயன்படுகிறது.
Tool
materials and Tool wear:
1. The tool material பொதுவாக metallic material ல் செய்யப்படுகிறது. (Copper, Brass, Copper-tungsten).
2. இது
non-metallic materials and ஒருங்கிணைந்த
metallic and non-materials.
3. Copper,yellow brass,alloys af
zinc,copper tungsten,silver tungsten,tungsten carbide போன்றவை tool material ஆக use ஆகிறது.
4. Commercial application க்கு copper fine machining பயன்படுகிறது.
3.2.6
Metal Removal Rate:
v Metal
removal rate என்பது
metal ஆனது time யை பொருத்து எவ்வளவு volume metal remove ஆகிறது.
v Metal
remove rate, current
density, poor
finish பொருத்து
இருக்கும். EDM process நடைபெறும் பொழுது first அதிகம் metal remove செய்வதற்கு அதிக power தேவை. Surface finish செய்ய 10w power செய்கிறது.
3.2.7
Factors affecting MRR:
1. Dielectric fluid க்கு உள்ள force ஐ
பொருத்து metal remove ஆகும்.
2. Capacitance increase ஆகும் போது metal remove ஆகும்.
3. Work – tool இடையெ உள்ள gap பொருத்து optimum level increase ஆகும் then decrease ஆகும்.
4. Spark discharge time பொருத்தும் ஒரு optimum value பொருத்தும் increase ஆகும்.
3.3 WIRE CUT ELECTRO DISCHARGE MACHINING
3.3.1
Construction
v Thin
wire ஆனது (0.2 to 0.3mm) brass
(or) molybdenum ஆனது.
v இந்த wire ஆனது இரண்டு roller இடையில் stretch செய்யப்பட்டுள்ளது.
v Moving
wire ஆனது
complex shape யும்
cut செய்ய
உதவுகிறது.
v Workpiece ஐ
control செய்ய
ஒரு unit
உள்ளது
v Workpiece
table, wire drive
section ஆனது
wire move செய்ய
பயன்படுகிறது.
v Power
supply unit
3.3.2
Working
* எந்த
workpiece Machine செய்யவேண்டுமோ
அதை table ல் வைக்கவும்.
* Workpiece ஐ சிறிய hole போடவும்,
wire use செய்து
hole போடவும்.
* Dielectric fluid யை workpiece tool க்கும் pass செய்கிறோம் pump use செய்து
* D.C. power supply கொடுப்பதன் மூலம் workpiece க்கும் tool க்கும் இடையில் spark produce ஆகும்.
* Workpiece க்கும் tool க்கும் இடையில் போதுமான அளவு voltage வந்தவுடன்
ஒரு பெரிய spark produce ஆகும்.
* Spark ஆனது (10-30 microseconds) வந்து செல்லும் current ஆனது (15-500)Amp/mm2 அதனால் ஒரு second ற்கு 1000 spark produce ஆகும்.
இதன் காரணமாக temperature ஆனது 10000◦C ஆக அதிகரிக்கிரது.
* High pressure, high temperature காரணமாக workpiece melt ஆகிறது, erod ஆகிறது. Air உடன் கலக்கிறது. இவ்வாறு
metal remove செய்யப்படுகிரது.
* Remove செய்யப்படும் material ஐ dielectric fluid மூலம் வெளியேற்றப்படுகிறது.
3.3.3 Features of wire cut EDM
process
i.
Manufacturing electrode
இந்த
process ல்
wire ஆனது molybdenum மற்றும் brass செய்யப்படுகிறது. இந்த
thin wire tool உருவாவதற்கு
other tool cutting winding process செய்யத்
தேவை இல்லை. ஒரு
expensive tool material தேவை
இல்லை
ii.
Electrode wire
Machining செய்யும் போது constant ஆக wire ஐ fed கொடுக்கிறது. அதனால்
tool wear ஆகாது.
iii.
Surface finish
சிறிய wire (10-30mm) பயன்படுகிறது. Chips and gas produce ஆவதில்லை. அதனால்
அதிக surface finish கிடைப்பதில்லை.
iv.
Complicated shapes
Program use செய்வதன் மூலம் complicated shape மற்றும் சிறிய shape யும் வரையலாம்.
v.
Time utilization
All the machine motion wire cut EDM
process ஆனது
NC
machine மூலம்
control செய்யப்படுகிறது. இதன்
மூலம் ஒரு நாள் முழுவதும் process செய்யப்படுகிறது.
vi.
Straight holes
இந்த wire ஆனது ஒரு குறிப்பிட்ட tension ல் இழுத்து கட்டப்பட்டுள்ளது. அதனால்
wire ஆனது break, vibrate, taper hole,
barrel shaped hole ம் Produce ஆகும். Initial ஆக plan செய்வதன் மூலம் program choose செய்வதன் மூலமும் material rejection குறைக்கலாம்.
vii.
Economical
Program மூலம் process செய்வதனால் ஒரு batch production செய்யலாம்.
viii.
Cycle time
Cycle time die manufacturing குறையும் ஏனெனில் process ஒரு
machine ல்
நடைபெறுவதால்.
ix.
Inspection time
Single piece தயார் செய்யும் போது அந்த inspection time குறையும்,
position accuracy கிடைக்கிறது.
3.3.4
Disadvantage
* விலை
அதிகம்
* cutting rate குறைவு
* இது
பெரிய workpiece க்கு பொருந்தாது
3.3.5 Application :
Gear tools, dies, rotors, turbine blades and cams for small to
medium size batch production.
3.4
Difference between EDM and Wire Cut EDM:
WCDEDM
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EDM
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Thin wire brass molybdenum use செய்துTool செய்யப்படுகிறது.
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விலைமதிப்புள்ள tungsten silver அதனால் தான் tool செய்யப்படுகிறது.
|
முழு workpiece யும் dielectric fluid- ல் மூழ்கக்கூடாது. Remove செய்ய வேண்டிய இடத்தில் மட்டும் dielectric fluid கொடுக்க வேண்டும்
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முழு workpiece யும் dielectric fluid ல் மூழ்க வேண்டும்.
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Complex 2D profile எளிதாக manufacturing செய்யலாம்
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Complex 2D profile manufacturing செய்வது கடினம்
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In EDM, or electrical discharge machining, the equivalent of tiny lightning bolts perform the material removal. Though slow in terms of metal removal rate, EDM is capable of machining complex shapes in hard materials. Mold and die makers, as well as makers of jet engine components, rely on EDM routinely. The process includes an electrode and a workpiece, both submerged in dielectric fluid. Current flows between the workpiece and electrode, repeatedly creating tiny plasma zones with temperatures around 10,000 degrees C. The high temperatures produce localized, instantaneous melting of the material. Though the process may seem violent, it occurs on such a tiny scale that the resulting metal removal can be precisely controlled. The electrode in EDM takes different forms. Wire EDM machines use a thin wire to cut with electricity, with the wire advancing into the hard metal part as if it was a slow-motion saw blade. By contrast, ram EDM machines, which are also called “die sinkers,” use electrodes that are custom machined into 3D shapes. The EDM process then produces a cavity in the part that is the opposite or female version of the “male” electrode form. Similar to the ram EDM machine is the small-hole EDM machine, or “hole popper.” On this machine, the electrode is a cylinder used to machine a hole. Often these machines are used simply to provide starter holes for wire EDM, but the technology may also be used to machine finished holes in materials that are too difficult to drill.
