Machine Tool Design for Titanium Machining San Diego, Titanium 2011 conference
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 1
Introduction
Titanium has the best strength to weight ratio of all metals.
It is together with CFK is material of choice for today‘s aerospace industry.
Examples
Turbine blades, blisks and impellers
Structural parts and turbine casings
In comparison to other materials, its toughness and low thermal conductivity makes it
hard to cut
Material removal rate Aluminum 900 in3/min
Material removal rate Titanium 30 in3/min
Tough material and long operating times require strong and stable machines
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 2
Example workpieces
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 3
Example machine: Starrag BTP 5000/2
Many of the parts are very big and therefore require
large machines
Since the machine times are often in the region
between 10 and > 100h the machines must be very
stable to keep thermal drift to a minimum
– requires homogeneous design
– is improved by using thermal compensation
Achievable stability: Thermal drift within 10 hours over
full spindle range (up to 8‘000 rpm) in the area of 1
micro inch)
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 4 4
Titanium machining
The technology of Titanium cutting has
changed tremendously with the developments
of new tools
Cutting speeds
– for roughing of about 250 ft/min (old
profilers 50 ft/min)
– for finishing of about 500 ft/min
Radial and axial immersions are much lower,
but the material removal rate has multiplied
(24 to 48 in3/min)
Consequences for machine tools
Machines with high dynamic capabilities (e.
g. acceleration, feed rates) are required
High spindle speeds > 5‘000 rpm are
required for efficient finishing
Spindle torque can remain within comparable
low boundaries (~800 lbs ft)
Process stability must be extremely high
Current developments in Titanium machining
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 5
Machine tool dynamics
Dynamic cuts
Spindle speed 5‘500 rpm
Feed rate 150 in/min
Movement of the tool center point requires rapid motions of the axes
High velocities and acceleration of the axes
Short distance from tool tip to rotary axis
Conventional milling head Starrag milling head
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 6
Ideal Main Spindle for Titanium Machining
0
200
400
600
800
1000
1200
1400
1600
1800
2000
10 100 1000 10000
Spindle Speed [rpm]
To
rqu
e [
Nm
]
375
1300
56
00
8000
a) Face mill 45° Dia 160 mm
b) Porcupine Dia. 80 mm
c) End mill Dia. 20 mm
d) Conical Ball end mill Dia. 6 mm
a)
b)
d) c)
Face milling
Full cut, DoC = 6 mm, Vc = 80, feed/tooth= 0.14 mm
Contouring
DoC = 1 x Dia., WoC = ½ x Dia., Vc = 80,
feed/tooth= 0.13 mm
Finishing, Vc = 500 ft/min
Dia 6 mm: 7960 rpm
Dia 8 mm: 5970 rpm
Dia 10 mm: 4780 rpm
5‘600 rpm / 959 ft lbs 100% duty
8‘000 rpm / 693 ft lbs 100% duty
Trends due to
-higher machine stabilty
-improved tools
-improved cooling
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 7 7
Spindle requirements
High cutting speeds and high material
removal rates result in
High spindle speeds
High heat generation and therefore
the need of an efficient cooling
system (Starrag: internal cooling
with 1450 psi)
High damping and stiffness of the
machine as well as the spindle
system
Possible with excellent robustness of
a geared spindle
Shaft
diameter
Bearing dist.
Bearing distance
Shaft
diameter
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 8 8
Stiffness and Damping
Machine design must be balanced since achievable cutting depths are limited by the
weak spot of the system, e. g.
spindle rotor
spindle support
structural vibration of the machine
Instable cuts reduce tool life and destroy the workpiece surface
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 9
Stiffness and Damping
a
b
F
M2
M1
MA1
MA2
Short distance between tool tip and A-axis
Minimal load during roughing operations
With a tool length of 180 mm there is up to a 50 % reduction in effect torque acting on
the A-axis!
High stiffness on the tool tip resulting in the highest machining quality and tool life.
Conventional milling
head
Starrag milling head
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 10
Stability and Damping
Unstable cuts are sometimes unavoidable because the
weak spot of the system are
vibration of the fixture
vibration of the tool
vibration of the workpiece
Chatter analysis on machine to support finding stable
conditions: Sensor support integrated on machine
0 50 100 150 200
Frequenz / Hz
0
10
20
30
40
50
Weg / µm
Messung 2_Clip_FFT2_DimConv/K:1/Sensorposition:/Messrichtung:+X/KnotenNr.:1/FFT
Time signal
Frequency spectrum
Chatter freq. = 46.8 Hz
Tool freq. = 15.9 Hz
m/s
^2
10
-10
µm
0
50
Dörries Scharmann SIP Droop + Rein TTL Berthiez WMW Ecospeed Heckert Starrag 11
Conclusion
While Titanium is still a hard to cut material, new tools and process strategies allow much
higher productivity.
For achieving highest efficencies and accuracies in Titanium cutting, the machine tools
must have
highest thermal stability
much higher dynamic capabilities
a balanced machine design for process stability without dominant weak spots
analysis capabilities for supporting the process engineer to optimize the strategy