• Fundamentals of NC
• Advantages of NC
• Classification of NC
– PTP- Contour
– Incremental – absolute
– Open loop – closed loop
• Punched Card
Manufacturing Systems Evolution
• Construction of simple production machine started in
• Fixed automatic mechanisms and transfer lines for
mass production came along at this century
• Introduction to Numerical control NC in 1952
• The logical extension of NC – CNC
• Industrial robots were developed simultaneously with
• First commercial robot in 1961
• 1969 Mechatronics –
• FMS with CAD / CAM
Fundamentals of Numerical Control
• Fundamentals of Numerical Control
Definition – EIA
Electronics Industries Assoc.
• Numerical Control is a system in which
actions are controlled by the direct insertion of
numerical data at some point. The system
must automatically interpret at least some
portion of the data.
Types of Numerical Control
• Conventional Numerical Control (NC)
– Data is sent to the machine tool by means of
punch cards or tapes. The reader at the machine
performs no calculations or interpolations.
• Computer Numerical Control (CNC)
– The idea of computer numerical control is to
position a computer right at the machine tool.
• Part Program- Numerical data on a punched
• The block contains , in coded form , all
information for processing a segment of work
piece like Segment length, cutting speed,
• NC replaces the manual actions of the skilled
• Part Programmer V/s Skilled operator.
• APT – Automatically Programmed Tool
• BLU- Each unit corresponds to the position
resolution of axes of motion referred to Basic
• BLU is also known as ‘increment size’ or ‘bit
• The part dimensions are expressed in part
programs by integers
• To calculate position command in NC, the
actual length is divided by the BLU value.
• To move 0.7 inch in positive X direction , with
BLU =0.001, the position command is X+700
• Axis of motion – An axis in which the cutting
tool moves relative to the work piece.XYZ
• Machine tools- drilling,milling,lathe
• Rotary motions also possible.
NC machine parts
– Data Processing Unit
• To decode the information received from tape, process
it, and provide data to CLU
– Control Loops Unit.
• Operated Drive attached.
• Machine tool
Advantages of NC
First NC- 1952.
Accuracy- Measuring Time
70-80 % Time wasted on measurements
A Further saving of time is achieved while passing
from one operation to another during the
machining of the work space.
• Rate of production decreases in conventional
• Contour cutting in 3 D or 2D can not
performed by manual operation
• Two Hand wheels required for this operation
CNC - Disadvantages
• High initial investment
• A long preparation time for each production
• Inflexibility of the process. Machine is planned
to make a certain fixed cycle of operation. If
the part configuration changed , The machine
adjustment must be rebuilt or altered.
• A big stock of parts is required for the process.
NC in Flexible Manufacturing System
Production in low quantity
Eg . Aircraft
Machines should be economical.
When a new product required , only the part
program has to b chgange.
Advantages of NC
1. A Full flexibility ; a part pgm is needed for producing a
2. Accuracy is maintained through the full range of
speeds and feeds.
3. A shorter production time
4. Easy adjustment of the machine. Less time than other
5. The possibility of manufacturing a part of complicated
6. Need for highly skilled & Experienced labor is avoided
7. The operator has free time
Disadvantages of NC
• A relatively high cost
• More complicated maintenance- Special
maintenance crew is desirable.
• Highly Skilled & properly Traned part
programmer is needed
Classification of NC
• According to type
– Point to point / contouring
• According to Structure
– NC/ CNC
• According to programmed method
– Incremental / absolute
• According to type of control loops
– Open loop / closed loop
• Drilling machine
• Work piece is moved along axis of motion
until center of hole is drilled is exactly beneath
• Drill is automatically moved to the work
piece, hole is drilled , and the drill moves out
in a rapid traverse speed.
• In countering or continuous-path systems, the
tool is cutting while the axis of motion are
• Eg. milling machine
• Interpolator contained in the DPU of
contouring systems provide the proper
velocity commands for each axis.
Straight Cut Systems
• PTP machine equipped with milling tool.
NC & CNC
• NC- Hard ware based
• CNC- software based
• The digital control in NC employs voltage
• 1 pulse = BLU (NC)
• Pulses to actuate motors.
– Stepper- open loop
– DC motor – closed loop
• 1 bit = BLU (CNC)
• Simplest hardware configurations – emulates
hardware based NC and transmit output pulse.
• 1 bit =1 pulse =BLU
• The main difference in the operation between NC
and CNC is in the way that the punched tape is
– NC – read 1 block and cut
– CNC – store complete data , stored in memory before
Absolute - Advantages
• With an incremental system , each time the
work is interrupted the operator must restart
the part pgm and repeat the entire operation
• Dimensional data can change at any time.
• One reference point
Incremental - Advantages
• Inspection of the part pgm is easy. Sum of
position commands must be zero
• The performance of the incremental system can
be checked by a closed-loop tape. This is a
diagnostic punched tape which test the various
operations and performance of the NC Machine
• Mirror image programming is facilitated with
incremental systems. The sign of the
corresponding position commands is changed
from + to - . No new calculation required for the