Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

PowerPoint to accompany

Krar • Gill • Smid

Technology of Machine Tools6th Edition

CNC Turning Center Unit 77

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– Separate sides; each machine work at same time

• Seven-tool upper turret

• Seven-tool lower turret

• Two-axis model has one or two turrets– Will drive only one turret at a time

Type of chucking centerdiscussed in rest of slides

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

TurningCenter Parts

FrameworkComponents

CNCComponents

BedHead-stock

Carriage MCU ServosCross slide

Turret Housing

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 heavy-duty 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 Notes1. Program in absolute mode (G90)2. All programming begins a zero point, centerline and

right-hand face of part3. Carbide tool will be used for all operations4. Use radius programming5. Position established to right front corner for safety6. 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 .

N20 G92 X.690 Z.1

N30 M03 .

N40 G00 X.590 Z.050

N50 G84 X.500 Z-1.250 F.010 H.050

N60 G00 X.500 Z.050

N70 G84 X.400 Z-.750 F.010 H.050

N80 G00 X.400 Z.050::

Number of instruction

Command to MCU

Information neededto carry out command

77-26

Programming Sequence (sample code)

G24 Radius programming

::

M03 Spindle ON clockwise

Full code with descriptions

in text – Followthrough for better

understanding!

% Rewind stop code/parity check.

N10 G24

N20 G92 X.690 Z.1

N30 M03

N40 G00 X.590 Z.050

N50 G84 X.500 Z-1.250 F.010 H.050

N60 G00 X.500 Z.050

N70 G84 X.400 Z-.750 F.010 H.050

N80 G00 X.400 Z.050

G92 Reference point offsetX.690 Tool located .100 in. off the outside finish diameter/.690 in. from part centerline (X0) (point a)Z.1 Tool located .100 in. to right of part face (Z0)

G00 Rapid traverse rate X.590 Tool located .590 in. from part centerline (point b) Z.050 Tool located .050 in. from part face

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