June 2009 / Volume 61 / Issue 6|
By Edmund Isakov, Ph.D., and David Ohlund, Sandvik Coromant Co.
| 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 difficult-to-machine 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
Titanium exists in two crystallographic forms. At room temperature, pure titanium has a hexagonal, close-packed crystal structure known as alpha (α) phase. At about 1,620° F (880° C), this phase transforms to a body-centered 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 alpha-beta titanium alloys with predetermined properties.
Ti-6Al-4V (6 percent aluminum and 4 percent vanadium) is an alpha-beta 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.Ti-6Al-4V 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 Ti-6Al-4V a popular workpiece material.
The low thermal conductivity of Ti-6Al-4V (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 built-up edge, tool chipping or other types of premature tool failure.Table 1: Mechanical properties of Ti-6Al-4V alloy.
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 metal-removal 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 Ti-6Al-4V are shown in Table 1. Its high strength and low thermal conductivity place Ti-6Al-4V among the most difficult-to-machine materials.
Ti-5Al-5V-5Mo-3Cr and Ti-10V-2Fe-3Al are stronger titanium alloys that are becoming more common, but their machinability rating is 50 percent compared to that of Ti-6Al-4V, meaning the cutting speed has to be reduced.Principal Cutting Geometry
Cutters with positive-negative and double-positive geometries are applied for milling various materials, including titanium alloys. Positive-negative geometry is characterized by positive axial rake angles from 5° to 20° and negative radial rake angles from -2° to -11°. Double-positive geometry is characterized by positive axial rake angles from 5° to 20° and positive radial rake angles from 1° to 14°. Positive-negative geometry milling cutters with an axial rake angle from 5° to 15° are mostly applied for milling Ti-6Al-4V because they generate low cutting forces and consume less power. Most modern milling tools have a positive-negative design. But when considering the insert geometry, it is almost always positive. A cutter is considered positive-positive 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 high-pressure 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 through-coolant 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. Water-based coolants incorporating rust inhibitors or water-soluble oils work best for most facemilling operations. Low-viscosity 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 alpha-beta 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.
The catalog item number of a popular cutter is RA300-102R38-16H:
Cutter diameter, Dc = 3.386 " (D3 = 4.0 ")
Number of inserts, zn = 8
PVD-coated 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 R300-1648E-PL 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 starting-point 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 sfmCalculating 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 Ti-6Al-4V 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, cross-sectional 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 step-by-step 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 Ti-6Al-4V 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, Ti-6Al-4V 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 hpTable 2: The condition and Brinell hardness range when facemilling Ti-6Al-4V and other alpha-beta alloys recommended by Machining Data Handbook.
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 long-edge 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. CTEAbout 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 e-mailed at firstname.lastname@example.org or reached at (561) 369-4063. David Öhlund is a milling development specialist for Sandvik Coromant Co., Fair Lawn, N.J. He can be e-mailed at email@example.com or reached at (201) 794-5037.
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