Chapter 3 CNC turning and machining centers

Chapter 3
CNC Turning and Machining
Centers
Prepared by: Prof. Rahul Thaker (ACET)

 Turning:-Turning is a machining process carried out in
lathe machine, Where tool moves parallel to axis of lathe
and removes diameter of material. CNC Lathes or Turning
centres are rapidly replacing older producing lathes

TURNING:-
 Basicmovements:–
Mc- Cutting movement (regarding the part)
Ma- Feed movement (regarding the tool)
Mp- Cutting depth movement (regarding the tool)

 Milling:-Milling is a machining operation of
producing flat, helical surfaces by means of
multiple cutting edged rotating tools called
“Milling Cutters”. After the advent of computer
numerical control (CNC), milling machines
evolved into milling machining centers

CNC Machining Centers are classified
THREE types
1.A vertical machining centre
(VMC) :-It has its spindle on a
vertical axis relative to the work table
vertical machining centre is typically
used for flat work that requires tool
access from top. Milling Centre is a
type of vertical machining centre

THREE types
2. Horizontal Machining Centre (HMC):-It is
used for cube shaped parts where tool access can be
best achieved on the sides of the cube. Turning
centre or Lathe is a type of Horizontal machining
centre

THREE types
3. Universal Machining Centre (UMC) :- It
has a work head that swivels its spindle axis
to any angle between horizontal and vertical
making this a very flexible machine tool.
E.g.:Aerofoil shapes, Curvilinear geometries.

CNC machining centers are usually designed with features to
reduce non productive time. The features are:
• Automatic tool changer :-
The tools are contained in a storage unit that is integrated with the
machine tool. When a cutter needs to be changed, the tool drum
rotates to the proper position and an utomatic tool changer (ATC)
operating under program control, exchanges the tool in the spindle
for the tool in the tool storage unit. Capacities of tool storage unit
commonly range from 16 to 80 cutting tools.
• Automatic work part positioner:-
Many horizontal and vertical machining centers have the capability
to orient the work part relative to the spindle. This is accomplished
by means of a rotary table on which work part is fixtured. The table
can be oriented at any angle about a vertical axis to permit the
cutting tool to access almost the entire surface of the part in a
single setup.

• Automatic pallet changer:-
Machining centers are often equipped with two (or more) separate
pallets that can be presented to the cutting tool using an automatic
pallet changer. While machining is performed with one pallet in
position at the machine, the other pallet is in a safe location away
from the spindle. In this location, the operator can unload the
finished part and then fixture the raw work part for next cycle.

CNC Machining Centers
• It is a CNC milling machine equipped by an
automatic tool changer.
Automatic tool changer using tool magazine
or carousel.
Automatic part changer using pallets
(optional).

Types of Machining Centers
• Three types: horizontal, vertical and
universal
Factors to determine type and size
1. Size and weight of largest piece machined
2. Maximum travel of three primary axes
3. Maximum speeds and feeds available
4. Horsepower of spindle
5. Number of tools automatic tool changer
can hold

Two Types of Horizontal
Machining Centers
• Traveling-column
– One or usually two tables where work mounted
• Column and cutter move toward work on one table
while operator changes workpiece on other table
• Fixed-column
– Equipped with pallet (removable table)
• After workpiece machined, pallet and workpiece
moved off receiver onto shuttle; shuttle rotated,
bringing new pallet into position for shuttle and
finished work pallet into position for unloading

Vertical Machining Center
• Saddle-type construction with sliding
bedways that use a sliding vertical head
instead of quill movement
• Generally used to machine flat parts held in
vise or simple fixture
• Versatility increased by addition of rotary
accessories

Universal Machining Center
• Combines features of vertical and horizontal
machining centers
– Spindle can be programmed in both vertical and
horizontal positions
• Allows for machining all side of a part in one setup
• Useful for small and medium batch parts
• Has additional accessories such as indexible
pallets and rotary-tilt tables

Advantages of Universal
Machining Centers
• Eliminate handling and waiting time
between machines
• Reduced number of fixtures and setups
• Reduced programming time
• Improved product quality
• Less work-in-process (WIP) inventory
• Faster product delivery to customers
• Lower manufacturing costs

Column
Saddle
Bed
X axis
Z axis
Y axis
Main Operative Parts
Main operative parts of both
vertical and horizontal
centers basically same.
Position of machining
spindle determines whether
it is classified as vertical
or horizontal.

Primary Components of a
Machining Center

Machining Center Accessories
• Number of accessories available
• Two types
– Those that improve efficiency or operation of
machine tool
– Those that involve holding or machining
workpiece

Torque Control Machining
• Calculates torque from measurements at
spindle drive motor
• Increases productivity by preventing and
sensing damage to cutting tool
– Torque measured when machine turning, not
cutting and value stored in memory
– As cutting begins, stored value subtracted from
reading at motor giving net cutting torque
• Goes higher, computer reduces feedrate, turns on
coolant or even stops cycle

Automatic Tool Changers:
Large Capacity Horizontal-Type
• Hold up to 200 tools
– Identified by either tool number or storage pocket
number
– Held in storage chain
• Process: (~ 11 seconds)
– When one operation being performed, tool required for
next moved to pick-up position
– Tool change arm removes and holds it; exchanges when
operation complete; returns tool to storage

Automatic Tool Changers:
Smaller-Capacity Vertical, Disk-Type
• Holds from 12 to 24 tools
• Next tool selected upon completion of
machining operation (~ 2.5 to 6 seconds)
– Tool carriage mounted on shuttle that slides
carriage next to tool spindle
– Tool pocket aligned, spindle orients toolholder
and tool lock releases
– Tool changer rotates to number called, tool lock
energized and carriage slides out of way

Tools and Toolholders
• Wide variety of cutting tools
• Conventional milling machines, cutting tool
cuts 20% of time
• Studies show machining center time
– 20% milling, 10% boring, and 70% hole-making
in average machine cycle
– Cutting time can be as high as 75%
• Large consumption of disposable tools
caused by increased tool use

face milling cutters
two-flute end mill
four-flute end mill

Stub Drills
high-helix drill
core drill
oil hole drill

Taps
gun
stub flute
spiral flute
fluteless

rose reamer
fluted reamer
carbide-tipped reamer
Single-point boring tools
are used to enlarge a hole
and bring it to location.

Combination Tools
• If machining center has helical
interpolation capability, one
tool can perform drilling,
chamfering, and threading
operations in one cycle
Solid-carbide combination drill/thread tool with drill
tip on end, chamfer located at correct length for
selected
application

1 2 3 4
Sequence of operations for
combination tool, the Thriller®
1. Drill point can produce through hole or a blind
hole no deeper than two times tool diameter
2. Chamfer is cut, and tool is retracted approximately
2 ½ thread pitches from the bottom of the hole
Tool fed radially into wall of hole to full thread depth
during ½ of a turn (180º, while moving ½ of thread
pitch in –Z axis
3. Next, thread is formed by helical interpolation
cycle during one full turn (360º), while moving
one thread pitch in –Z axis.
Tool is brought out radially from wall, to center of
hole during ½ of a turn (180º) while moving ½ of a
thread pitch in the –Z axis.
4. On completion of the cycle, the tool is retracted
out of the hole

Toolholders
• Must have compatibility in toolholders in
order for wide variety of cutting tools to be
inserted into machine spindle quickly and
accurately
• Most common toolholder has V-flange and
self-releasing taper shank
– Size (range from No. 30 to 60) determined by
machine capacity and designed horsepower

Features of a work holding device:
Proper part immobilization.
Minimum part deformation.
Short part clamping & unclamping time.
Accessibility to the machining area.
Ability to absorb machining efforts.

Work-Holding Devices
• Standard step clamp
– Used to hold down flat, large parts
– Quick-release clamp good when clamps have to
be temporarily moved to machine edge
• Table plate
– Flat aluminum plate bolted to machine table
– Dowel pin and tapped holes machined into plate
to permit fastening vises or clamps
• More flexible than limit of table T-slots

• Plain-style precision vises
– Keyed directly to table slots
• Make positioning and clamping accurate and simple
– When machining multiple identical parts,
matched set of qualified vises can be used
• Qualified vises used when long part requires support
on both end to maintain parallelism
– When using double-station cluster vises; total of
up to 20 parts held for machining operation

• Vise jaw systems
– Set of master jaws placed in vise and items
snapped into position
• Parallels, modular workstops, angle plates, V-jaws,
and machinable soft jaws
– Add versatility and increase flexibility of a
precision vise
– Can be used in both single-station and double-
station vises
• CNC fixtures
– Used to accurately locate many similar parts and
hold them securely for machining

Chapter 3 CNC turning and machining centers

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Chapter 3 CNC turning and machining centers