CNC and DNC

ugliestmysticAI and Robotics

Nov 14, 2013 (3 years and 8 months ago)

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C+ NC= CNC


NC with dedicated computer is known as CNC


Software controlled


MCU

of

CNC

is

based

on

a

dedicated

microcomputer




High program storage capacity


Various forms of program input


Punched Tape


Magnetic Tape


Floppy Disk


RS
-
232


Industrial keypad for manual entry


Program editing within MCU


Saves time



Fixed cycles & programming subroutine


Reduces the length of program


Saves execution time


Saves space


Easy to use


Interpolation facility


Straight line, circular, helical, ..


Easy Machine set up


Compensation


Tool Radius & length


Acceleration & deceleration calculations


Helps to reduce tool marks on the surface due to high
feed rates




Communication interface


With peripheral devices (Conveyor belt, Robot..)


Diagnostics


Start
-
up diagnostics


Malfunction


Tool life monitoring


Preventive maintenance


Prog. Check (syntax & logic)



Consists of


CPU


Memory


I/O interface


Controls for machine axes


Sequence Control (Miscellaneous controls )


Central Processing
Unit

(CPU)

Memory

-
RAM: Operating
System


-

ROM: Part Program

I/O Interface

-
Operator Panel

-
Tape reader


Machine Tool Control

-
Position Control

-
Spindle Control

Sequence Control

-
Coolant

-
Tool Changer

-
Automatic Chucking


Memory

-
RAM: Operating
System


-

ROM: Part Program

I/O Interface

-
Operator Panel

-
Tape reader


Central Processing
Unit

(CPU)

Machine Tool Control

-
Position Control

-
Spindle Control

Sequence Control

-
Coolant

-
Tool Changer

-
Automatic Chucking


System bus


Operating system software


Control program


Execute program


Machine interface software


Auxiliary functions


Communication


Application software


NC Programs


DNC= Direct NC or Distributed NC



Developed in 1960s


Controlling

number

of

machine

tools

by

a

single

main

frame

computer

through

direct

connection

and

in

real

time


MCUs

of

Individual

Machines

were

free

from

punched

tape

reader


MCUs

are

connected

to

Main

frame

computer

by

BTR
:

Behind

tape

reader


Central

Computer

supplied

one

block

of

instruction

to

MT

each

time

on

demand
.


2

way

communication

between

central

computer

&

MT



Components of Direct NC:


Central Computer


Bulk Memory


Number of MTs


Communication links


MT1

MCU

MT2

MCU

MT3

MCU

Central Computer

Bulk memory

NC Programs

Telecommunication lines


Advantages


High reliability


Elimination of tape & tape reader


Control no. of MTs with single computer


Improved computational capabilities


Improved prog. storage


Limitations


No

alternative

way

of

prog
.

Entry

in

case

of

Central

Comp
.

Failure


Slow compared to Distributed NC



Central

Computer

is

connected

to

MCUs

of

MTs


MCUs are computer themselves


Central

Computer

uploads

complete

program

in

MCU

on

demand


2 way communication


Data & information flow


Can have variety of configuration


(a) Switching network


(b) LAN


Flow of Information
from Central Computer
to MT


NC Part Prog.


List of tools


Machine set up
instructions


Operator Instructions


Production schedule


Flow of Information
from MT & Shop floor to
C. Comp.


Piece counts


Machining time


Tool life infor.


Machine
uptime/Downtime


……

Data Switching
Box

DNC

Computer

MCU

MT

MCU

MT

MCU

MT

MCU

MT

MCU

MT

DNC: Switching Network

DNC Computer

Satellite Computer

MCU

MT

MCU

MT

Satellite Computer

MCU

MT

MCU

MT

DNC: LAN


6.4



Applications of NC


Advantages & Disadvantages


Comparison

between

conventional

machines/NC/CNC/DNCs


Aerospace


Machinery


Electrical


Fabrication


Automotive


Instrumentation


Mold making

Right Hand coordinate
system


Group I: Machines with rotating tool


Vertical Milling M/c, Drilling M/c, Vertical Boring M/c


Group II: Machines with rotating workpieces


CNC Turning, Horizontal Boring M/c


Group III: Machines without rotating cutter/
workpiece


CNC Shaper , Planer


Assign only Z & X


Y is perpendicular & as per right hand rule


+Z

+Y

+X


Group I


Along rotating spindle


+Z : Takes tool away from workpiece



Group II


Along rotating spindle


+Z : Takes tool away from workpiece



Group III


Perpendicular to the work holding surface


+Z : Takes tool away from workpiece



Group I


Along the principal motion of tool/WP


+X : Towards right when looking spindle from top



Group II


Along the principal motion of tool/WP


+X : Towards right when looking spindle from front



Group III


Along the principal motion of tool/WP


+X : Along Principal direction of movement



Perpendicular to X & Z


Direction: As per Right hand rule


Turning center cutters


Machining center cutters


T
ypes of cutters used on CNC turning

c
enters


Carbide
s

(and other hard materials) insert turning

and
boring tools


Ceramics


High Speed Steel (HSS) drills and taps



V


used for profiling, weakest
insert, 2

edges per side.


D


somewhat stronger, used for
profiling

when the angle allows it, 2
edges per side.


T


commonly used for turning
because it

has 3 edges per side.


C


popular insert because the same
holder

can be used for turning and
facing. 2 edges

per side.


W


newest shape. Can turn and
face like

the C, but 3 edges per side.


S


Very strong, but mostly used for

chamfering because it won’t cut a
square

shoulder. 4 edges per side.


R


strongest insert but least
commonly

used.



Most machining

centers
use some

form of HSS or

carbide insert endmill

as
the basic cutting

tool.


Insert endmills cut

many
times faster

than HSS, but
the


HSS endmills leave a

better
finish when

side cutting.



Facemills flatten large

surfaces quickly and
with

an excellent
finish. Notice

the
engine block being

finished in one pass
with a

large cutter.



Ball endmills (both
HSS and insert)

are
used for a variety of
profiling

operations
such as the mold
shown

in the picture.


Slitting and side
cutters are used

when
deep, narrow slots
must be

cut.



Drills, Taps, and Reamers


Common HSS tools such as
drills,

taps, and reamers are
commonly

used on CNC
machining centers.

Note that a
spot drill is used instead

of a
centerdrill. Also, spiral point or

gun taps are used for through
holes

and spiral flute for blind
holes. Rarely

are hand taps used
on a machining

center.



Drills, Taps, and Reamers


Common HSS tools such as
drills,

taps, and reamers are
commonly

used on CNC
machining centers.

Note that a
spot drill is used instead

of a
centerdrill. Also, spiral point or

gun taps are used for through
holes

and spiral flute for blind
holes. Rarely

are hand taps used
on a machining

center.



All cutting tools must be held in a holder
that fits in the spindle.

These include end
mill holders (shown), collet holders, face
mill

adapters, etc.

Most machines in the
USA use a CAT taper which is a modified

NST 30, 40, or 50 taper that uses a pull stud
and a groove in

the flange. The machine
pulls on the pull stud to hold the holder

in
the spindle, and the groove in the flange
gives the automatic

tool changer something
to hold onto.

HSK tool holders were
designed a number of years ago as an

improvement to CAT tapers, but they are
gaining acceptance

slowly.



In

telecommunications,

RS
-
232

(Recommended

Standard

232
)

is

a

standard

for

serial

binary

data

signals

connecting

between

a

DTE

(Data

Terminal

Equipment)

and

a

DCE

(Data

Circuit
-
terminating

Equipment)
.

It

is

commonly

used

in

computer

serial

ports
.