Maximizing productivity and calculating required machining power when facemilling titanium alloys.
Titanium is a reactive metal used for making parts. Other reactive metals include magnesium, hafnium, zirconium and depleted uranium. The reactive metals are pyrophoric, creating potential fire hazards. Special precautionary measures allow managing this problem, such as applying large quantities of cutting fluids, having an argon or nitrogen atmosphere and maintaining a Class D fire extinguisher for emergencies.
Titanium alloys are considered difficulttomachine metals, especially when facemilling. To manage this problem, the following data should be considered:
• Titanium alloy properties,
• Principal geometry of facemills,
• Suitable cutting tool materials,
• Machining parameters, and
• Coolant application.
Courtesy of Sandvik Coromant A Sandvik Coromant CoroMill 300 cutter tooled with round inserts facemills titanium. A Sandvik Coromant round insert with a sharp edge (above). Titanium exists in two crystallographic forms. At room temperature, pure titanium has a hexagonal, closepacked crystal structure known as alpha (α) phase. At about 1,620° F (880° C), this phase transforms to a bodycentered cubic structure known as beta (β) phase. The combination of these crystallographic phases through addition of alloying elements and thermomechanical processing is the basis for development of various alphabeta titanium alloys with predetermined properties.
Ti6Al4V (6 percent aluminum and 4 percent vanadium) is an alphabeta alloy. It is used for making aircraft gas turbine discs and blades and other components where strength at temperatures up to 600° F (315° C) is required.
Ti6Al4V Properties
High tensile strength at annealed condition (130,000 psi, or 900 MPa) and low density (4.42 g/cm3, or 0.160 lb/in.3) make Ti6Al4V a popular workpiece material.
The low thermal conductivity of Ti6Al4V (6.6 W/m×K), which is about 15 percent of AISI 4340 alloy steel’s 44.6 W/m×K, results in high cutting temperatures. About 80 percent of the generated heat goes into the cutting tool rather than dissipating with the evacuated chips. High heat may cause builtup edge, tool chipping or other types of premature tool failure.
Table 1: Mechanical properties of Ti6Al4V alloy.
Ultimate tensile strength

Treatment

Rockwell hardness, HRC

ksi

MPa

120 to 130

830 to 900

Annealed

35

130 to 144

900 to 990

Annealed

36

170

1,170

Aged

41

Cutting speed is the most influential parameter that increases the cutting temperature. Increasing the feed per tooth, or chip load, produces thicker chips, which dissipate more heat than thin chips. That reduces the cutting temperature. A higher metalremoval rate can also be obtained by increasing the feed per tooth when the axial DOC and the radial WOC, or radial engagement, are optimal.
Strength and hardness data of Ti6Al4V are shown in Table 1. Its high strength and low thermal conductivity place Ti6Al4V among the most difficulttomachine materials.
Ti5Al5V5Mo3Cr and Ti10V2Fe3Al are stronger titanium alloys that are becoming more common, but their machinability rating is 50 percent compared to that of Ti6Al4V, meaning the cutting speed has to be reduced.
Principal Cutting Geometry
Cutters with positivenegative and doublepositive geometries are applied for milling various materials, including titanium alloys. Positivenegative geometry is characterized by positive axial rake angles from 5° to 20° and negative radial rake angles from 2° to 11°. Doublepositive geometry is characterized by positive axial rake angles from 5° to 20° and positive radial rake angles from 1° to 14°. Positivenegative geometry milling cutters with an axial rake angle from 5° to 15° are mostly applied for milling Ti6Al4V because they generate low cutting forces and consume less power. Most modern milling tools have a positivenegative design. But when considering the insert geometry, it is almost always positive. A cutter is considered positivepositive if its true rake angle is positive.
Climb milling is a common practice when milling titanium alloys. Because the insert enters the workpiece with the maximum chip thickness and exits the cut with a zero chip thickness, friction and rubbing between the insert and workpiece is minimal. Climb milling reduces heat by dissipating it partly into the chip and partly into the insert. With less heat entering the workpiece, workhardening is minimized.
To increase tool life, it’s important to program toolpaths that minimize chip thickness when inserts exit the cut.
Coolant, coolant delivery and coolant pressure influence tool life. By applying highpressure coolant (1,000 psi) through a tool and directing it at the cutting zone, tool life can be increased up to 100 percent. Using a throughcoolant tool minimizes the risk of recutting chips, which also helps increase tool life and process security. Chips often weld onto an insert’s rake face, which puts extra stress on the insert and may cause premature failure. Accurately delivering coolant to the cutting edge ensures chips are flushed away from the insert.
Chemically active fluids transfer heat from the cutting zone and reduce frictional forces between the tool and workpiece. Large quantities of fluid are needed to keep the workpiece and cutting tool cool during machining operations. Waterbased coolants incorporating rust inhibitors or watersoluble oils work best for most facemilling operations. Lowviscosity sulfurized and chlorinated oils may be applied when cutting speeds are low. The coolant concentration should be 8 to 12 percent.
Machining parameters recommended by Machining Data Handbook (the traditional method from about 30 years ago) for facemilling alphabeta titanium alloys are summarized in Table 2.
Milling Cutters, Parameters
Sandvik Coromant Co., Fair Lawn, N.J., offers the CoroMill 300 series of milling cutters with round inserts for roughing and semifinishing titanium alloys. Round inserts are recommended because they provide chip thinning similar to a cutter with a 45° lead angle, which increases the mrr. In addition, as shown in Figure 1, the length of a round insert’s cutting edge (1a) is 75 percent longer than an insert with a 0° lead angle and 24 percent longer than one with a 45° lead angle, so the wear on the round insert spreads out over a longer length. This extends tool life.
Figure 1: The length of a round insert’s cutting edge (bottom) is 75 percent longer than an insert with a 0° lead angle (top) and 24 percent longer than one with a 45° lead angle (middle), so the wear on the round insert spreads out over a longer length. This extends tool life. The catalog item number of a popular cutter is RA300102R3816H:
Cutter diameter, Dc = 3.386 " (D3 = 4.0 ")
Number of inserts, zn = 8
PVDcoated carbide grade is GC1030 (ISO P30, S15)
Effective (true) rake angle, γoe = 20°
The catalog item number of the round insert for the cutter is R3001648EPL 1030:
R300 = product family
16 = insert size, IC (diameter of inscribed circle) = 16mm (0.63 ")
48 = insert thickness, 4.8mm (0.189 ")
E = insert tolerance class, ground edge
PL = positive insert geometry
1030 = carbide grade
Sandvik Coromant recommends the following startingpoint cutting parameters when applying the cutter:
Maximum DOC, ap = 0.236 "
WOC, ae = 2.8 "
Feed per tooth, fz = 0.0098 "
Cutting speed, Vc = 210 sfm
Calculating Machining Power
The traditional method of calculating required machining power (Pm) is based on the mrr (Q) and the unit power value for milling (P) provided by Machining Data Handbook. The mrr is calculated by the formula:
Q = ae × ap × fz × zn × n (in.3/min.)
Where ap = 0.300 " and fz = 0.008 " are the values shown in Table 2,
ae = 2.8 " and zn = 0.0098 " are the values described earlier, and
n is a spindle speed calculated by the commonly used formula:
n = 12 × Vc ÷ (π × D3)
Substitution of n in the mrr formula by n from the second formula gives:
Q = 12 × Vc × ae × ap × fz × zn ÷ (π × D3)
Where Vc = 115 sfm is a recommended cutting speed (Table 2), and
D3 = 4.0 " is the diameter of the milling cutter from Sandvik Coromant.
Q = 12 × 115 × 2.8 × 0.3 × 0.008 × 8 ÷ (π × 4.0) = 5.9 in.3/min.
The required machining power calculated by the traditional method is shown in the following formula:
Pm = Q × P (hp)
Where P is the unit power. According to Machining Data Handbook, when milling titanium alloys with a hardness of 250 to 375 HB with sharp inserts at feed rates from 0.005 to 0.012 ipt, the unit power (P) is 1.1 hp/in.3/min. When milling titanium alloys with dull inserts, P is 1.4 hp/in.3/min. (A dull edge usually means an insert’s flank wear is 0.012 ", indicating the inserts need to be indexed or replaced.) The unit power values represent an 80 percent machine efficiency factor (η = 0.8).
Assuming that the hardness of Ti6Al4V at annealed condition is 35 HRC (Table 1), which is equivalent to 327 HB, the required machining power when milling with sharp cutting inserts would be:
Pm = 5.9 × 1.1 = 6.5 hp
The required machining power when milling with dull cutting inserts (before indexing) would be:
Pm = 5.9 × 1.4 = 8.3 hp
Dr. Edmund Isakov’s method of calculating required machining power is based on the cutting force and the cutting speed. The cutting force depends on the ultimate tensile strength of the workpiece material, crosssectional area of the uncut chip, the number of inserts (teeth) in the cut and the engagement factor. The engagement factor depends on the ratio of the cutter diameter to radial WOC and the type of workpiece material. This method is described in Engineering Formulas for Metalcutting, written by Dr. Isakov and published by Industrial Press Inc.
All stepbystep calculations were performed on Dr. Isakov’s Advanced Milling Calculator, but are omitted here due to space limitations. Therefore, only the final results are provided. Calculations are based on the ultimate tensile strength of Ti6Al4V at the annealed condition (130,000 psi), and the milling cutter and the cutting parameters described earlier.
Tangential cutting force (Fc), torque (T) and net power (Pn), or power at the cutter, when milling with new or indexed inserts are:
Fc = 1,138 lbs., T = 190 ft.lbs. and Pn = 7.2 hp
The same parameters when milling with inserts prior to indexing or changing them (dull inserts) are:
Fc = 1,480 lbs., T = 247 ft.lbs. and Pn = 9.4 hp
The required machining power (Pm), or power at the motor, when milling with sharp indexable inserts (machine efficiency factor is 80 percent as mentioned earlier) is:
Pm = 9.0 hp.
When milling with inserts prior to indexing or changing them (dull inserts), the required machining power is Pm = 11.8 hp.
The Sandvik Coromant method of calculating the net power (Pn) is based on the following formula:
Pn = ae × ap × fz × n × zn × kc × Mγ ÷ 396,000 (hp)
where ae, ap, fz and zn have been described earlier and n = spindle speed in rpm; n = 12 × Vc ÷ (π × D3) = 12 × 210 ÷ (π × 4.0) = 201 rpm
kc = specific cutting force, lb./in.2
Mγ = correction factor based on effective rake angle (Mγ = 1 for 0° rake angle)
According to Coromant Material Classification, Ti6Al4V is classified as a CMC23.22 material. The kc value for this material group is 253,000 psi.
Because the effective rake angle is 20°, the value of the correction factor Mγ is set to 0.80.
Pn = 2.8 × 0.236 × 0.0098 × 201 × 8 × 253,000 × 0.80 ÷ 396,000 = 5.3 hp
The machine efficiency factor (η) must also be considered. The required machining power (Pm) is calculated using the following formula:
Pm = Pn ÷ η
Assuming an efficiency factor of 0.8, the required machining power will be:
Pm = 5.3 ÷ 0.8 = 6.6 hp
Table 2: The condition and Brinell hardness range when facemilling Ti6Al4V and other alphabeta alloys recommended by Machining Data Handbook.
Condition and Brinell hardness range

Axial DOC (ap), in.

Feed per tooth (fz), in.

Cutting speed (Vc), sfm

Uncoated carbide inserts,
Cgrade/ISO

Annealed, (310 to 350) HB

0.300

0.008

115

C2/M20, K10

0.150

0.006

145

C2/M30, K20

0.040

0.004

185

C2/M30, K20

Solution treated and aged, (320 to 380) HB

0.300

0.008

95

C2/M20, K10

0.150

0.006

120

C2/M30, K20

0.040

0.004

160

C2/M30, K20

Table 3: A comparison of calculation methods.
Nomenclature

Machining Data Handbook

Dr. Isakov

Sandvik Coromant

Entering cutting data for Ti6Al4V

Cutter diameter, in.

4.0

4.0

4.0

Number of inserts in cutter

8

8

8

Cutting speed, sfm

115

210

210

Feed per tooth, in.

0.008

0.0098

0.0098

Axial DOC, in.

0.300

0.236

0.236

Radial WOC, in.

2.8

2.8

2.8

Ultimate tensile strength, psi

n/a

130,000

n/a

Specific cutting force, lbs/in.2

n/a

n/a

253,000

Unit power, hp/in.3/min. (sharp cutting edges)

1.1

n/a

n/a

Unit power, hp/in.3/min. (dull cutting edges)

1.4

n/a

n/a

Machine efficiency factor

0.8

0.8

0.8

Calculated cutting data for Ti6Al4V

Metalremoval rate, in.3/min.

5.9

10.4

10.4

Number of inserts in the cutter

n/a

3

n/a

Cutting force, lbs. (sharp tool)

n/a

1,138

n/a

Torque, ft.lbs. (sharp tool)

n/a

190

n/a

Net power, hp (sharp tool)

n/a

7.2

5.3

Required machining power, hp (sharp tool)

6.5

9.0

6.6

Cutting force, lbs. (dull tool)

n/a

1,480

n/a

Torque, ft.lbs. (dull tool)

n/a

247

n/a

Net power, hp (dull tool)

n/a

9.4

n/a

Required machining power, hp (dull tool)

8.3

11.8

n/a

Facemilling Productivity
Rough facemilling generates a higher mrr than any other type of milling, including endmilling, plunge milling and helical interpolation. The mrr indicates cutting productivity. The maximum mrr is limited to the nominal machining power and available torque value (at selected spindle speed) of a given machine tool. Therefore, it is important to calculate how much power and torque are required to achieve the desired mrr. (The calculated torque should not exceed the allowed torque applied to the arbor mounting, and the calculated cutting force should not exceed the force that the inserts’ cutting edges can withstand.)
Knowledge of the required machining power and torque values at given cutting conditions help to select the appropriate milling machine.
Depending on the component, of course, there are other operations that will actually set the demands on the machine tool. For example, milling aerospace frame components often requires longedge cutters and those operations also draw a lot of power and torque.
As is shown in Table 3, the starting cutting speed recommended by Sandvik Coromant today is 83 percent higher than the recommendation in Machining Data Handbook. A higher cutting speed allows for a 75 percent increase in the mrr.
The Sandvik Coromant method is more accurate than the traditional method and shows that actual machining power is significantly lower than that obtained through the Machining Data Handbook.
Dr. Isakov’s method allows for calculating not only the machining power, but also the cutting force and the torque applied to the milling cutter. His method for calculating machining power provides significantly lower values than those obtained through the Machining Data Handbook. CTE
About the Authors: Edmund Isakov, Ph.D., (left) is a consultant and writer. He is the author of several books, including Engineering Formulas for Metalcutting (Industrial Press, 2004) and Cutting Data for Turning of Steel (Industrial Press, 2009). He can be emailed at edmundisakov@bellsouth.net or reached at (561) 3694063. David Öhlund is a milling development specialist for Sandvik Coromant Co., Fair Lawn, N.J. He can be emailed at david.ohlund@sandvik.com or reached at (201) 7945037.
