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PowerPoint to accompany
Technology of Machine Tools
6th Edition
Krar • Gill • Smid
CNC Turning Center
Unit 77
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
77-2
Objectives
• State the purpose and functions of
chucking, turning, and turning/milling
centers
• Identify the applications of computer
numerical control (CNC) for turning
centers
• Name the machining operations that
may be performed simultaneously
77-3
CNC Turning Center
• In mid-1960, 40% all metal-cutting
operations performed on lathes
– Not very efficient
• Research led to development of numerically
controlled turning centers and chucking
lathes
– Could produce round work almost any contour
automatically and efficiently
77-4
Three Main Types of
Turning Centers
1. CNC chucking center
•
•
Holds part in some form of jaw chuck
Some have dual spindles (work both ends)
2. CNC universal turning center
•
•
Can use continuous bar feed system to
machine and cut off parts from bar
Some have dual tool turrets
3. Combination turning/milling center
•
Uses combination of turning tools
77-5
CNC Chucking Center
• Designed to machine work held in chuck
• Variety of sizes from 8 to 36 in. in diameter
• Four-axis chucking center has two turrets
Type
of chucking
center
– Separate
sides;
each machine work at same time
discussed in rest of slides
• Seven-tool upper turret
• Seven-tool lower turret
• Two-axis model has one or two turrets
– Will drive only one turret at a time
77-6
Construction
• Main operative parts of all turning centers
basically same
– Framework components and CNC components
• Bed and machine frame must be rugged
– Heavy, one-piece cast-iron casting or polymer
cast base
– Slanted 40º from vertical plane
77-7
Turning
Center Parts
Framework
Components
Bed
Headstock
Carriage
Cross slide
Turret Housing
CNC
Components
MCU
Servos
77-8
Tooling
• Toolholders for machining
– Outside diameters located in lower turret and
are preset
– Inside diameter mounted in dovetailed block
and preset off machine by tool-setting gage
• Mounted on upper turret
• Automatic tool-setting probe used for
presetting tools
– Available on some machines
77-9
Computer Numerical Control
• Microprocessor
– controls logic calculations, mechanism control
and input-output control
• Video display
– Visual output of data, processes, and diagnostics
• Input unit
– Keyboard and/or diskettes used to communicate
with system, enter setup and data
• Program storage
77-10
CNC Turning Center
• Designed mainly for machining shaft-type
workpieces supported by chuck and heavyduty tailstock center
• On four-axis machines, two opposed turrets,
mounted on separate cross-slides
– One above and one below center line of work
– Balance cutting forces so extremely heavy cuts
can be taken on workpiece
77-11
Other Operations Can Be
Performed by Dual Turrets
• Roughing and finishing cuts in one pass
• Machining different diameters on shaft
simultaneously
• Finish-turning and threading simultaneously
• Cutting two different sections of shaft at
same time
77-12
Other Turning Center Equipment
• Steadyrest
– Allow facing and threading on end of shafts
• Follower rest
– Used to support long, thin shafts
• Bar-feeding mechanism
– Permits machining of shafts and parts from bar
stock smaller than spindle through-hole
• Production part loader
– Can complete part changeover when individual
precut shafts machined
77-13
Combination Turning/Milling
Center
• Allows operations such as drilling, milling,
and tapping to be performed on part while
still in machine
• Special tool turret contains pockets that
have own drive for live tools
• Drilling and tapping can be performed if
machine has contouring spindle
– Can be indexed to exact locations around
circumference of workpiece
77-14
Programming Considerations
• Programmer must be able to analyze part
print and decide on sequence of machining
operations
• Good practice to develop habit of labeling
start and end points for both roughing and
finishing operations
• Be certain the programming format suits
your equipment before machining parts
77-15
Typical Tooling System
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
77-16
Inserts
• Made from many types of material
• Great variety to suit any workpiece material
or machining operation
– Include carbide, coated carbide, ceramic,
cermet, cubic boron nitride, and diamond
coating
• Standardized so most inserts fit in same
holders
77-17
Tool Nose Radius Compensation
• Wide variety of tool nose radii
– Starting with sharp point and increase in
1/64 in. increments from 1/64 to 1/8 in.
• Theoretical sharp point of tool is
programmed
– Does not position tool at correct location
• G41 or G42 turns on tool nose radius for
finish cuts
• Radius of each insert stored in numbered
tool list of control tool management system
77-18
Tool Offsets
• Programmer must provide tool setup sheet
for setup operator
• MCU will calculate correct position at
which tool should be located to accurately
machine part
77-19
Diameter Versus Radius
Programming
• Method used determined by preset
parameters within machine control unit or
by correct G-code
• Diameter (default)
– Part print drawn complete with both sides of
centerline and full diameter dimensions
• Radius
– Part print drawn on just one side of centerline
77-20
Establish Part Zero
• Programmer's choice to place part zero at
most convenient location
• Location of X axis usually centerline of part
• Z axis either:
– Right-hand (tailstock) end of part
• Movements into part will be negative numbers (-Z)
– Left-hand (chuck) end of part
• Movements into part will be positive numbers (Z)
77-21
Codes
• Function of some G-codes and M-codes
may differ from function of those on
machining center
• Many of common turning center G-codes
and M-codes that conform to EIA standards
shown in tables 77.1 and 77.2 in textbook
77-22
Programming Procedures
• CNC control units can vary from
manufacturer to manufacturer
– Important to follow programming manual
supplied for each machine
• This textbook concentrates on two classes of
CNC machines:
– Bench-top teaching model
– Standard turning center
77-23
Bench-Top Teaching Machines
• Very easy to program and ideal for teaching
• Perform turning operations similar to larger
machines
– Except smaller workpieces and lighter cuts
• Relatively inexpensive
• Most of B- and M- codes apply to both
bench-top CNC turning lathes and
standard-size turning centers
– Few variations
77-24
Simple Programming
•
Example of notes and code to machine a sample
part (radius programming)
Program Notes
1.
2.
3.
4.
5.
6.
Program in absolute mode (G90)
All programming begins a zero point, centerline and
right-hand face of part
Carbide tool will be used for all operations
Use radius programming
Position established to right front corner for safety
Material aluminum; cutting speed 600 sf/min, feed
rate at .010 in.
77-25
Programming Sequence (sample code)
%
Rewind stop code/parity check.
N10 G24
.
Number of instruction
N20 G92 X.690 Z.1
N30 M03
.
Command to MCU
N40 G00 X.590 Z.050
Information needed
N50 G84 X.500 Z-1.250 F.010 H.050 to carry out
command
N60 G00 X.500 Z.050
N70 G84 X.400 Z-.750 F.010 H.050
N80 G00 X.400 Z.050
:
:
77-26
Programming Sequence (sample code)
%
Rewind stop code/parity check.
G92 G24Reference
point
offset
Radius
programming
N10 G24
N20 G92 X.690 Z.1 X.690 Tool located .100 in. off the
outside finish diameter/.690 in
N30 M03
M03
Spindle
ON clockwise
from
part centerline
(X0)
N40 G00 X.590 Z.050 G00 (point
Rapid
a) traverse rate
Full.100
code
with
X.590
Tool
located
.590
in.
Z.1
Tool
located
in.
to
N50 G84 X.500 Z-1.250 F.010 H.050
descriptions
from
centerline
(point b)
rightpart
of part
face (Z0)
N60 G00 X.500 Z.050 Z.050 Tool located
in text –.050
Follow
in.
forface
better
N70 G84 X.400 Z-.750 F.010 H.050 through
from part
understanding!
N80 G00 X.400 Z.050
:
:
77-27
Standard-Size Turning Center
• To introduce additional machining and use
of diameter programming, a full sample
program is given in the text
• Complete with program notes and code with
explanations
– Similar to previous example
77-28
Turning Center Setup
• Before setup, become familiar with control
panel and operational procedures
• Power on to machine:
– Turn on servos and zero out/align all axes so
control knows location of machine home
position
– Load program if not already in memory
– Check manuscript, and prepare tools listed by
programmer
77-29
Program Test Run
• Part never machined without test running
program first
– Some controls have control screen which allows
visual progression through program without
cutting part
– Dry run program without part
• Use step/single block mode and feedrate override to
slow programmed rate
• Finger on hold button in case of error in program
– Good idea to know where emergency stop button located