Welcome viewers, today we are going to start the first lecture of

Computer numerical control of machine tools and processes.

My name is A Roy Choudhury and I am a professor in the mechanical engineering Department of

IIT Kharagpur.

So 1st of all computer numerical control, what does it mean?

It means that control of a particular machine tool or a process is achieved by insertion

of or use of numbers, symbols, signals, letters, codes, words, instructions.

In other words in short, the communication with the machine in the form of a language.

Previously, machine and human interaction used to take place through physical devices

like cams, tracers, templates,so it was not very sophisticated, but now with the advent

of numerical control and computer numerical control, a sort of language communication

was started with machine tools and processes.

So in what way can this happen?

It can happen in the form of input to the machine, data input through numbers, letters,

codes, this data can be processed inside the machine through numerical calculations, logic

operations and execution of operations can be carried out through generated signals from

the computer or from some control circuit in the form of voltage signals, et cetera.

So what is the connection between numerical control, computer numerical control, etc?

So in the beginning what happened, there was no computer to start with and we had numerical

controlled machines in which numbers and alphabets were used in the form of codes to feed information

into a machine.

And for that, at first these machines were called or they were referred to as numerical

controlled machines.

Inside, they were primarily consisting of hard wired logic circuitry and after the advent

of computers, data entry became easier through the computers.

Hardwired circuitry,logic circuit, et cetera sometimes were replaced by software and control

inside the control loops the computer also used to participate in order to improve the

control characteristics, so it became known as computer numerical control or simply computer

control.

What is the area of application of computer control?

Do we understand that, say, in mass production, that is high-volume production, lots of pieces

made in a particular span of time.

Say, 1 lakh pieces in one year, so in mass production would CNC or computer numerical

control be suitable?

Here, surprisingly, it is not so, why ? - because mass production finds the application of fixed

automation, that means, “fixed” means machines which are, they can only carry out

certain operations and nothing else.

Special-purpose machines find wide applications in mass production together with automated

material handling devices, why so?

This is because in case of mass production there is not much change in part design over

extended periods of time.

There what we find is, a huge number of pieces have to be made again and again repeatedly

over long periods of time, so a computer, which is primarily meant for flexible communication

or changeable communication with machines, it does not find wide application in this

particular area.

Not much change is there with time, so we generally go for systems which are exactly

dedicated for that particular purpose.

Special-purpose machine, for example, camshaft grinding machine, you can only grind camshafts

and nothing else.

So, that is why in mass production, CNC is not very suitable, CNC’s strong point is

it can be changed very easily, why ? - because it is consisting of language instructions,

it is consisting of numerical calculations and logic operations inside the machine, these

things are not very difficult to change if the requirements change.

And why not fixed automation for low-volume production ? - by low-volume production, I

mean small lot and batch production and even piece production.

This is because fixed automation has dedicated machines, so if you dedicate some machines,

build them for very small number of pieces, after that part design becomes gets changed

to a different part, you have no use for that particular fixed automation, it becomes obsolete

that is why if you have low-volume production, you cannot implement fixed automation or dedicated

material handling system.

In that case, you cannot automate it with fixed automation.

In that case, CNC becomes highly applicable because if you are wanting automation, make

programs on the CNC machine which can produce those parts and when the part changes (part

design changes because it is low-volume small amounts, small numbers of different jobs one

after the other) - So when the parts design changes, make a separate program for the next

part and start running the machine where the cost for change over will be much-much less

in this case, so we understand that for low-volume productions for small lot and batch production,

etc CNC would prove to be highly appropriate.

Also, I think I should mention here at this moment that CNC also has the ability to manufacture

complex shapes, okay, without parts specific tooling.

Parts specific tooling means say form tools, if you want to make a very difficult profile,

make a form tool (conjugate profile) and machine that particular part, so you incur a lot of

costs for the tooling.

In CNC, it is not the tool shape that produces the conjugate shape on the part, but the tool

movements properly controlled by the computer can produce very complex shapes, this is an

added ability of the CNC machine.

Let us see how the control is achieved in case of CNC.

There you can find I have drawn a monitor to represent the computer and from the computer

there is some signal, which can directly control a motor, but is this a CNC machine?

It is not, I have intentionally drawn a conventionally controlled lathe, okay ? operator-run lathe

on which you will see some equipments like speed gearbox, feed gearbox, carriage, nuts,

et cetera, it is not a CNC machine.

So in order to achieve control over a machine first of all a computer has to be there, it

has to be interfaced with the machine and the conventional machine tool architecture

has to be changed.

So we will find some changes taking place in this particular architecture and apart

from that there should be the addition or incorporation of some devices which can control

the motion and the extent of motion, I mean rate of motion and the extent of motion.

And also, we have to ultimately write a program on the computer and execute it so that it

develops proper signals to the motor and other devices of the machine in order to control

it; this is how computer control is achieved.

So comes a question naturally what is a CNC program.

A CNC program is a sequence of commands just like we give a sequence of commands to someone

to carry out some operation, it is just a sequence of commands, it is written in suitable

language and it is meant for controlling the operations of a machine.

When executed, it makes a machine to carry out some motions and auxiliary operations.

Generally by motion I have meant here say fast motion to approach a particular point

and then say starting a linear cut at a controlled rate, then taking circular cuts either clockwise

or counter clockwise at a definite feed rate.

And also there can be auxiliary operations for tool changing for changing a job and bringing

in another pallet containing another blank like that.

And this way, when a computer program is executed it will ultimately lead to a part being successfully

produced from a blank.

And CNC program ultimately does not necessarily mean that it is only for controlling the operations

of a machine tool, but it can be used employed for controlling other general-purpose machines,

other general machines as well.



This is an example of a typicalsome command blocks taken from a CNC machine, what does

it show?

The 1st column shows the line number N006, N007, et cetera.

After that we have a number of codes placed one after the other, what are these codes?

For example, G90, G90 means that whenever there is a coordinate system existing, on

that coordinate system we are supposed to refer to the absolute X and Y coordinate values

of a particular point.

So we have a coordinate system in front of us and we can see that X and Y is written

there and in that X-Y coordinate space, G00 says G00 means that we are supposed to reach

a target coordinate position given by X20 Y30.

So it basically means move fast, G00 means rapid travels to the target point, no velocity

is mentioned because generally the machine moves at the highest possible velocity.

So move fast to the point X20 Y30.

After that we are supposed to move in the next command we are supposed to move to the

point Y50, why is X not mentioned because it is not changed, so X remains the same and

we and we change the Y coordinate to 50 from 30, which means we are now at the point 20,

50 and we have executed a vertical motion.

After this vertical motion at 20 50, after that it is written G01 X100 Y 100 F200, what

does this mean?

G01 is a code for linear motion that means straight-line motion at a particular feed

rate that means at a particular rate of motion, the rate of motion is mentioned as F feed,

it is called the feed word, 200 means if not otherwise defined 200 millimetres per minute,

so the target point is mentioned as X100 Y100, so we reach the target point 100, 100 from

the point 20, 50 at the rate of 200 millimetres per minute.

Next line is G02, G02 means circular interpolation clockwise, so we move from the present position

of 100, 100 to the point 140 , X140 okay and as Y is not mentioned, Y remains the same

and the radius with which we move is 30 millimetres okay.

So we reach this particular point at the same speed defined in the previous line to the

point 140, 100 from the point 100, 100with a circular radius of 30 millimetres in the

clockwise direction.

Is there any other possibility of reaching this particular point?

There is, but unless otherwise mentioned it will take this particular what you call it

minor arc, there is a major arc possible also with 30 millimetres clockwise that is not

taken.

M30 means end of program, so this is a typical example of command block of a CNC machine.

Now that we have started discussing on CNC machines, computers were not there throughout

the ages and we did not have NC machines also beyond in the past say 70 years back or even

more than that.

So at that time how were automatic operations executed on machine tools?

This is one example of mechanical automation, it means some sort of clockwork mechanised

motions were carried out without the help of computers, without the help of numerical

instructions, codes, language, et cetera, simply physical devices were used.

It means something like if you want to command someone, you can tell him or her or you can

push him or push her to do that particular task.

Physical devices, so here we see a cam, cam is a device, in which we have shown a disk

like cam, which has a profile on its periphery which deviates from a circle so that if it

is rotating, it will move anything in connection with it readily outward.

So here we have something called as follower which is pressed against the cam with the

help of a compression spring and then that cam move in a uniform circular motion in a

counter clockwise direction it will tend to push the follower, and then suddenly the follower

will fall back when the fall of the cam of the or drop of the cam along the radius takes

place.

So it will push the cutting tool also towards the left side continuously and then suddenly

it will retract, so this is the scheme of motion of the cutting tool which has been

planned with the help of this particular cam.

Is there a computer controlling it?

No.

Is there automatic motion getting repeated again and again and again?

Yes.

What is the purpose of this motion?

Maybe it is moving forward parting some job and then again retracting very fast, the job

is getting fed again, so this continues unless something wears out, either the tool wears

out or power goes off or the cam undergoes wear and tear, so this is a perfect example

of an automatic machine by mechanical control okay.

So this also existing without computers automation.

And it is a question given that is what should be the profile of the cam in order that we

will have a constant forward feed of the cutting tool, what should be the profile of a cam,

the answer is Archimedean spiral, you can try this out yourself.

So coming back to computer-controlled machines now, we have seen the example of mechanical

controls being achieved in case of automation.

So how do computer-controlled machines carry out this particular control?

In computer-controlled machines, instead of devices like cam, et cetera, there are programmed

instructions which go inside the machine and I mean when they are executed by the program

instructions, they develop digital signals which might be ultimately rotating motors

at a definite rate and this leads to achieving a definite extent of motion and achieving

a particular ratio of the of the axes velocities okay.

And also it results in attainment of programmed feed velocity along a cutter path.

Why be so much concerned about the program ratio of axes velocities, because that is

ultimately the particular parameter which determines the path taken by the tool, which

ultimately defines the profile of the path being machined.

Here there are no physical devices but the path of the tool defines the profile of the

job.

So we have been talking about digital signals, binary logic, logic gates, digital circuitry,

quite a lot, what do we exactly mean?

Many of you might be conversant with digital circuitry, digital logic.

In case you do not have formal introduction to this particular subject, we will be taking

up in the next lecture some aspects of digital logic and hopefully you will be able to follow

the later part of the other lectures which involve some discussion on logic circuitry.

What we have shown here is that digital circuitry will be employed in many aspects of CNC control

like data input, data storage, data processing, interpolation, motion execution, et cetera.?

In the figure we have shown 2 typical logic gates and how do we read them?

Like a and b if there are 2 inputs which can only take up 2 values like say high value

and low value, AND gate is a gate which controls the relation mainly in such a way that c is

going to get a high value only if both a and b are simultaneously high, this is called

AND gate, that means say x and y if both are high, then only z is high, z is the output.

In the same way OR gate is a little more lenient, it says that if a is high and b is high or

any of anyone of them is high, then the output is high.

In 3 cases, a and b both are high, a is high, b is high, in these 3 cases the output will

be high, when both are low then the output will be low, so we will have some more discussions

on logic gates, they are not exactly mathematical operations but logic operations.

Now that we have gone into the discussions of CNC machine, let see what is the architecture

of the control unit that is what is there inside which has all the controls.

The control unit is frequently referred to as machine control unit, which contains 2

different modules; one is the Data processing unit and another is the Control loops unit,

what does the Data processing unit do?

It is concerned with data entry, data processing that means all sorts of calculations involving

the data and also interpolation.

Interpolation means that you are instructing a machine to move on a say circular path or

a linear path from one point to another, but you are not instructing it about what sorts

of velocities and positions it is supposed to attain in between, so somebody has to do

the calculation and there is a device called interpolator inside the data processing unit,

which does all this calculation and finds out the intermediate positions and velocities

of the tool okay, in order to cover the intermediate path between 2 program points.

The Control loops unit, it contains all the devices for achieving required motion along

an axis for example, it might involve the actual motor, the prime mover, the lead screw

nut, the gearbox, all these things are together referred to as a Control loops unit.



And now that we are talking of, we had referred to the modifications of the conventional machine

tools in order to achieve CNC machining, so let’s have a look at that what are these?

For example, the machine has to be more robust and rigid for the same power level why because

deflections et cetera should be less so that errors are less.

Are deflections there in other machines?

There is but there are physical devices to ensure that the movement et cetera are exactly

carried out.

In CNC we are depending upon the accuracy of the movement, et cetera in order to achieve

a particular accuracy on the job.

Backlash elimination has to be there, motions here have to be completely free of backlash

and for that a number of devices need to be incorporated for example, the recirculating

ball screw-nut mechanism.

Gearboxes which establish definite ratios of motion between axis of motion in a particular

machine, these gear boxes limit the ratios of between axes speed and therefore, they

are generally eliminated, but gearboxes for attainment of a particular range of speeds

that has to be there.

Feedback is sometimes necessary where the prime mover is not capable of carrying out

exact motions as required by the program.

And generally from a complex or compound chains or structures, we generally go for simple

kinematic chains, which mean we will be discussing these things in more detail.

And motors with lower time constants are preferred which have faster response, interpolators

are necessary in case of continuous control.

And we need to have machines where overshoots and sluggish response can be avoided that

means that means the dynamic constant of the machine have to be so designed so that overshoots

are less and sluggish response is avoided, et cetera.

So in this machine, we want to implement computer control, we can see that there is a chain

moving from the top up to the spindle and there is also power flowing this way so that

the same motor through gearboxes is controlling 2 motions, this has to be removed and a separate

motor has to be put for the feed motion, so one motor for the feed motion, one motor for

the speed motion and yet another motor would be required for a cross feed, one for Cross

feed, one for longitudinal feed okay.

For example, you would see a milling machine in which we have 3 motions X, Y and Z and

there is a motor which is also providing the cutting speed.

Instead of this, on the CNC machine we will find one motor for X, one motor for Y, one

motor for Z and also separate motor for a rotation of the cutter.

So finally let us look at the advantages of the CNC machine.

It is more flexible, it is possible to incorporate automation in low-level production, we have

the ability to cut complex profile and in many applications we can attain higher productivity

and accuracy.

The disadvantage is that the initial investment is very high and required skill level of machinist,

operator, et cetera is high.

There is one practice question, main advantage of CNC machining over fixed automation is

flexibility, accuracy, speed, none of the other, and the answer is flexibility, thank

you very much.