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How to Select the Appropriate Machining Center
Machining Centers, with built-in flexibility of
operation and versatility in tooling, play an
increasingly
important
role in competitive
manufacturing industries. In order to choose the
most appropriate type of machine, whether vertical
or horizontal, and to pick the right optional
features, a thorough study of the production
parameters and product specifications is necessary.
This article outlines the choices available to user
industries.
Before selecting a machining center, we must have a
clear idea of what it can do. It is essentially a milling
machine fitted with a Computer Numerical Control
(CNC) System
and having
independent
axes
movements along all three axes r;x/y /Z) with infinitely
variable feed rates. The spindle has infinitely variable
speeds. These features make the milling machine a CNC
Milling Machine. When an Automatic Tool Changer is
fitted to the CNC Milling Machine, it becomes a
Machining Center.
In its simplest form a Machining Center has three
independent axes movements with infinitely variable
spindle speeds, an Automatic Tool Changer (ATC), the
capabilities for contouring on 2 axes, simultaneous
interpolation on 3 axes and rigid tapping facility.
There are two varieties of Machining Centers - Vertical
Machining Center (VMC) and Horizontal Machining
Center (HMC). When the spindle is mounted along the
vertical axis (perpendicular to the table) it is called VMC
and when the spindle is mounted along the horizontal
axis (parallel to table) it is called HMC. The HMC
.Assembiy oj Machining Centers at Ace Manufacturing Systems
10
Mr. P. Ramadas, Managing Director oj AMS
invariably has an index table or rotary table with pallet
changer as a standard optional feature of the machine.
The VMC is only a 3-axes machine with a 12/20/24/30
tool magazine. An index table, rotary table and pallet
changer are offered as optional features on a VMC.
In today's manufacturing industry, the over-riding factor
is cost per component at the specified quality level.
The developed countries are looking at the developing
countries to supply auto component parts due to the low
cost of manufacturing,
achieved through lower
investments and lower labour cost. This has forced
component manufacturing industries to look for lowcost machines to bring down the cost of production.
By design and construction, the VMC is much lower in
cost compared to the HMC, generally 50%. In
developing countries, industries use VMCs much more
than HMCs. In India the ratio of VMC to HMC
utilization is more than 5:1, whereas in developed
counties itis 2:1.
AMS production is in this
category.
• Machines with BT50
(ISO 50) taper
These
machines
are
generally bigger in SIze
with the spindle power
being more
Machining centersunder various stages of assemblY at AMS
The following varieties
available to users.
• Machines
of
machining
centers
are
than
11kW
They are meant for heavy
machining
of
steel
forgings and big castings.
The population of these
with BT30 (ISO 30) taper
These machines are predominantly used in the twowheeler industry to produce small components
of
aluminium and cast iron material with light machining
operations.
Mr. H. S. Ram Prakash,
GeneralManager, Design and
Development, AMS
machines is less than 10% of the total machining center
population. AMS produces such machines only against
specific orders.
These categories are valid for both VMCs and HMCs.
Milling cutters and boring bars of up to 63mm can be
used for milling operations (up to about 1 to 2 mm depth
of cut). Drilling of holes up to 20mm in aluminium and
12mm in cast iron can be done on these machines.
These machines are much faster in operation with lower
chip-to-chip
time, and use smaller tools weighing
less than 3 kgs. These machines are not generally
recommended for machining steel components.
In this category, AMS offers 3 models of BT30 spindle
VMCs - Spark, DTC-300 & DART, with different rapid
traverse, chip-to-chip time and spindle speeds. In a
machining operation,
the idle (non-machining)
time is
invariably 50% to 60% of the total cycle time. Hence to
reduce the cycle time ( when using more number of
tools), it is always recommended to choose the machine
with lower chip-to-chip time and faster acceleration /
deceleration. The machines with lower chip-to-chip time
and higher rapid traverse are more expensive.
machine
the labour cost is higher, a faster
than the less expensive
recommended.
of machines
• Machines
VMCs with BT40 Taper
The AMS production
slower machine
is
This principle is valid for all categories
range includes various sizes of
machines in this category depending upon axes strokes
and rapid traverse. These include:
• MCV-300 and DTC-300 with X/Y /Z strokes as 350 /
300/300mm
.WI
Finally, it is the cost per component that will decide
whether to go in for a faster machine or a normal
machine. Wherever
Some of the typical components machined on machining centers
ER,WI
ERXLandCHALLE
GERwith
X/Y /Z strokes as 500/ 350/ 350 mm
• MCV-400, MCV-400X, MCV-400S and MCV-400SX
with X/Y /Z strokes as 550/415/ 460mm
• MCV-500, MCV-500S and MCV-500F with X/Y /Z
strokes as 800/500/500mm
• MCV-600 with X/Y /Z strokes as 1000/600/600mm
with BT40 (IS040)
taper.
The maximum number of machines produced
.MCV-600CX
in the
world belong to this category, accounting for about 65%
of the machining center population. About 75% of
with
X/Y/Z
strokes
as
1250/600/600mm
As the machine sizes increase, spindle power also rises.
(Continued onpage 12)
11
How to Select a Machining Centre
(Continued from page 11)
• Axes rapid traverse and chip-to-chip
• Selection of spindle speed and power
This is a very important feature which
considered, depending upon the application.
medium pre-load for such applications with 8,000 rpm.
has to be
Generally, if the machine is meant for aluminium
machining, the spindle speed should be 80-8000 rpm
with direct drive of the spindle. The full spindle power
will be available from 100 rpm to 8000 rpm. This will also
give higher acceleration I deceleration to the spindle.
The spindle run-out will be very good since there is no
side load on the spindle due to the belt drive. For
precision machining on cast iron I steel, direct drive with
higher power spindle motor is recommended. For special
applications, spindle speed of 10000 rpm and 12000 rpm
can be supplied. This will entail longer lead time.
If the machine is for general applications, where the
component material is steel, cast iron or aluminium, then
speed range of 60-6000 rpm is selected. In this range, the
full power is available from 1000 to 6000 rpm, thus
facilitating higher material removal at lower speeds for
rough milling and rough boring operation. If higher
spindle power is selected (which is optional), the material
removal will be still higher at lower spindle speeds.
For example, the use of 801 1001 125 milling cutters and
spindle speeds of less than 1000 rpm, where the spindle
power gets reduced linearly with respect to the spindle
speed. A typical spindle powerl torque diagram is given
with normal power and higher power.
As can be seen, 1 : 1.5 speed ratio between motor and
spindle provides 50% more torque. This will be helpful
for bigger size tapping operation done at lower speeds on
tougher materials like stainless steel and hardened
materiaL Sometimes even higher power spindle motor
may have to be selected with 1 : 1.5 speed ratio to provide
still higher torques and power at lower speeds
understanding of the metal removal requirements is very
important to select the appropriate spindle power and
spindle speed range.
• Selection of spindle bearing and rigidity:
Higher the rapid traverse, lower will be the idle time, and
higher is the cost of the machine. As mentioned earlier,
where labour cost and overheads are higher, it is
preferable to select a machine with higher rapid traverse
with lower chip-to-chip time . Generally, the small-scale
industries select low-speed versions of machines to keep
the initial investment low.
• Size of the spindle
This depends upon the size of the component, number
of setups for each component, number of components
to be loaded for each set-up, type of fixture-plain fixture
or cradle fixture-with index table I rotary table with
tails to ck.
• Increased
Direct drive spindles come with light pre-loaded 60 or 70
dia (optional) bearings.
ormally these are not suitable
for heavy machining on steel components. Very special
executions can be done with spindle cooling and light to
12
daylight area (IDL)
Lower daylight area is better for machining. But when
index tables or rotary tables are used with cradle fixture,
increased
daylight
area
(IDL)of
160mm
is
recommended so that 200mm length of tool can be used
on a component of 150mm height mounted on the
cradle plate. In case of twin-arm tool-changer machines,
IDL of 160mm can help to reduce the tool change time
without taking the table to XY reference points, thus
saving 1see. in chip-to-chip
time. This feature is
preferred whenever many tool changes are required for
producing the same component and is a major cause of
idle time. Sometimes for machining a tall component, a
special daylight area of 320mm could be an advantage.
This is generally selected on the basis of component and
fixturing.
• Coolant through spindle(CTS)
This feature is given in the machine to do deep-hole
drilling (up to LID ratio of 10) and provide coolant to
the cutting point to improve cutting parameters and tool
life. There are two systems - one with 20 bar coolant
SPINDLE SPEED AND POWER
1Smin
Operating Zone
output
(kW)
10
, .••... .••••..•....6.
B
Output
(kW)
10
B
,=
6
For exclusively machining steel components,
it is
recommended
to select 70 dia bearing spindle with
medium pre-load. However, medium pre-loaded 70 dia
bearing spindle cannot run beyond 4000 rpm.
time
,
21/
0
o
6
35kW
4
I
, -"'= rJ---
.••••..•....4.
4
/,IJJ :
23kW
15 min
Operating Zone
Co
Open . gZo e
ti;~OUS
,,
'
15J
4500
2000 4000 6000 BOOO
Spindle speed (RPM)
Direct Drive/Tndirect Drive (1:1)
Power torque dia for standard power
of 5.5kW
Power torque dia. for higher power
of 7.5kW
0
J:
-
Con nuous
Opera ngZon
I',oAt
o
1333 3000 6000
Spindle speed (RPM)
Indirect Drive (1 : 1.5)
With 1:1.5 ratio between spindle
motor and spindle, spindle torque
gets increased by 50%
pressure for holes of more than 8 and L/D ratio up to 5
and the second with 50 bar coolant pressure for holes of
6 to 8 and L/D ratio of more than 5 and less than 10.
Though this feature is expensive, it will pay for itself
within 6 to 12 months, depending upon the application,
with reduced tooling cost and higher productivity.
• Automatic
Pallet Changer
(APC)
This is done sideways since the pallets are rectangular in
size and to maintain the minimum reach to the spindle.
Hydraulic component clamping feature is not on this
machine since the hoses cannot be carried to the
machine along with the pallet. This feature overlaps the
component
loading and unloading time with the
machining time and would be useful only when the
machining time is more than the load and unload time.
When the machining time is very high compared to the
load and unload time, the feature may not be very cost
effective. With this feature, the productivity of the
machine is not operator-dependent
and can be predetermined. The operator will have enough time to clean
the rest of the pads before loading the new part. The
parts can be inspected when the machining is going on.
Another advantage with this feature would be to load the
first and second set-up fixture on one pallet, and the
third and fourth set-up on the second pallet, thus
eliminating the need to change the fixture and hold the
part inventory for the third and fourth set-up. A quarter
of the machines produced by AMS are with APe.
• Number
of tools on the ATC
This is decided on the number of set-ups and number of
tools required for all the set-ups on the machine. If
empty pockets are available on the magazine, sister tools
(duplicate tools) of low tool life can be loaded in the
empty pockets, eliminating the tool set-up time. AMS
machines come with 6 / 12 / 20/ 24 and 30 tool
magazmes.
• Index Table / Rotary Table
Whenever operations are to be done at intervals of 30 /
SPINDLE SPEED AND TORQUE
Torque 80
(Nm)
Torque
50
40
30
OperatingZone
--n/
-,\
-t
20
10
00
70
15 min
(Nm)
60
72
,
FIiI'I-,
+--+----1
o
6
60 in
Op rating one
'\
I \
:~\"
o
"\
o
-..::::::
uous
Opera ing Zon
I ('I\nti,
"
7.5
5.2
1500
2000 4000 6000 8000
Spindle speed (RPM)
Direct Drive /Lndirecr Drive (1:1)
Power torgue dia for standard power
of 5.5kW
Power torque dia. for higher power
of 7.5kW
• Chip conveyor
This is a very important feature. All chip conveyors
supplied to AMS have 100 microns drum filtration
facility to keep the coolant tank cleaner and to prevent
the blocking of the coolant passage and pipe.
It is recommended to use scraper conveyor to handle
aluminium and cast iron chips and duplex conveyor(slatcum-scraper) to handle long chips of steel, cast iron and
aluminium chips. Screw conveyors to transfer the long
and heavy chips of steel and cast iron are recommended
when chips produced are more. This is an optional
feature on the machine.
• Hydraulic
power pack
Hydraulic power pack is required for machines with
APC for pallet transfer and component clamping.
Generally power packs work at 35/40 bar. In exceptional
cases, power packs with 70/100 bar pressure are
supplied. The power packs are provided with drain
cooling systems to control oil temperatures. Standard
AMS machines are not supplied with hydraulic power
packs.
• Coolant systems
Coolant through flexinox is supplied as a standard
feature. To provide more coolant to flush chips on the
fixture, flood coolant system can be selected. A separate
pump facilitates chip flushing on the base. The coolant
gun can be provided to clean the rest pad or component
on the machine. Whenever chip conveyor is selected,
there is a continuous back washing of the drum filter to
remove chips from the periphery of the drum filter.
• Voltage stabilzer
60 in
Ooer tin Z e
ot'
Co tinuou
Op rating one
: I'" ~
15 min OperatingZone
j
45, then 12 / 8 position index tables are selected. There
are various types and sizes of index tables -ltll0 / 200,
BTP 80 / 100, depending upon the application. Index
tables use Hirth coupling to provide the final position,
thus giving very good repeatability of the position within 3 sees to arc. Whenever the positions required are
in increments of 0.001, CNC Rotary tables are used.
CNC Rotary tables of 180 / 202 / 250 are provided
depending upon the size of the machine and the inertia
of the fixture. These tables are also provided with helical
interpolation feature, using it as the fourth axis of the
machine (cam cutting).
These are supplied with various capacities of 10 kVA to
40 kVA, depending upon the size of the machine. It can
either be directly procured by the customer or supplied
byAMS.
• False plate
o
2000 4000 6000
Spindle speed (RPM)
Indirect Drive (1 : 1.5)
With 1:1.5 ratio between spindle
motor and spindle, spindle torque
gets increased by 50%
This is provided over the table/pallet.
Whenever
frequent fixture change is envisaged on the machine, this
feature is selected to prevent any damage to the machine
table.
(Continued onpage 14)
13
How to Select a Machining Centre
(Continued from page 13)
It is the experience of using machining centers that will
finally help the customer
to select the right type of
machine and features. The machining
centers can be
Choice of VMC versus HMC
used either for cell manufacturing
By the virtue of the design, HMCs are more than double
alone machines. HMCs are provided with multi-pallet
the cost of VMCs. In the Indian manufacturing industry,
systems to integrate them with FMS. However, these are
VMCs using index / rotary tables are more cost-effective
not very common in Indian industry.
in the manufacture
of
components
for both
systems or as stand-
the
domestic and export market. For precision applications,
Of 'l~te,
HMCs are generally selected. The APC design in HMC
components
provides
stand-alone robots and interconnection
the
feature
to
clamp
the
component
automatic
loading
and
unloading
of the machine
hydraulically. But this feature is very expensive. Most of
with a conveyor system is picking up. This may become a
the features relevant to VMCs are applicable' for HMCs
common requirement within the next 5 years time, as the
also.
trend is towards unmanned machining.
Grinding Applications
External CNC Cylindrical
Grinder eGRIND 200
Component: Spring seat needle lift sensor
Material
16MnCr55
Application
OD & Face Grinding
Hardness
700Hv
Work holding
Diaphragm
Specials
In process gauging
Accuracies
Dia. 3.932
Chuck
+ 0.004.
Cpk achieved 3.27
Length 46.8
+ 0.015mm.
Cpk achieved 2.77
Cycle time
48 Seconds
Component: Gear fuel pump
Application
Hardness
Work holding
: OD & Face Grinding
: 34-56HRC
: Expanding Mandrel
Setup 1 for OD & Setup
2 for face grinding
14
of
on the machine through gantry robots or
Specials
: In process gauge for
OD and face
Accuracies
: Dia. 29.616 + 0.0064.
Cpk achieved 3.27
Length 31.714 - 0.008mm.
Cpk achieved 2.77
Cycle time
: 56 Seconds