Use low cutting speeds.. . . maintain high feed rates.. . . use copious amounts of cutting fluid.. . . use sharp tools and replace them at the first signs of wear.. . . moving parts of the machine tool should be free from backlash or torsional vibrations.. . . Titanium Machining Techniques, Timet Titanium Engineering Bulletin No. 7, 1960s.

Unfortunately, not much has changed in machining titanium since the mid-1960s. Titanium was a difficult-to-machine metal then and remains a difficult-to-machine metal. Titanium is difficult to machine for several reasons:

1. Titanium is very reactive and the chips tend to weld to the tool tip leading to premature tool failure due to edge chipping. Almost all tool materials tend to react chemically with titanium when the temperature exceeds 950° F.27

2. Titanium's low thermal conductivity causes heat to build-up at the tool-workpiece interface. High temperatures at the cutting edge is the principal reason for rapid tool wear. When machining Ti-6-4, about 80% of the heat generated is conducted into the tool due to titanium's low thermal conductivity. The thermal conductivity of titanium is about 1/6 that of steel.27 This should be contrasted with high speed machining of aluminum in which almost all of the heat of machining is ejected with the chip.

3. Titanium's relatively low modulus causes excessive workpiece deflection when machining thin walls, i.e. there is a bouncing action as the cutting edge enters the cut. The low modulus of titanium is a principal cause of chatter during machining operations.

4. Titanium maintains its strength and hardness at elevated temperatures, contributing to cutting tool wear. Very high mechanical stresses occur in the immediate vicinity of the cutting edge when machining titanium. This is another difference between titanium and the high speed machining of aluminum; aluminum becomes very soft under high speed machining conditions.

In addition, improper machining procedures, especially grinding operations, can cause surface damage to the workpiece that will dramatically reduce fatigue life.

The following guidelines are well established for the successful machining of titanium:

1. Use slow cutting speeds. A slow cutting speed minimizes tool edge temperature and prolongs tool life. Tool life is extremely short at high cutting speeds. As speed is reduced, tool life increases.

2. Maintain high feed rates. The depth of cut should be greater than the work hardened layer resulting from the previous cut.

3. Use generous quantities of cutting fluid. Coolant helps in heat transfer, reduces cutting forces, and helps to wash chips away.

4. Maintain sharp tools. As the tool wears, metal builds-up on the cutting edge resulting in a poor surface finish and excessive workpiece deflection.

5. Never stop feeding while the tool and workpiece are in moving contact. Tool dwell causes rapid work hardening and promotes smearing, galling, and seizing.

6. Use rigid setups. Rigidity insures a controlled depth of cut and minimizes part deflection.

Rigid machine tools are required for machining of titanium. Sufficient horsepower must be available to insure that the desired speed can be maintained for given feed rate and depth of cut. Titanium requires about 0.8 horsepower per cubic inch of material removed per minute.26 The base and frame should be massive enough to resist deflections, and the shafts, gears, bearings, and other moving parts should run smoothly with no backlash, unbalance, or torsional vibrations. Rigid spindles with larger taper holders are recommended; a Number 50 taper or equivalent provides stability and the mass to counter the axial and radial loads encountered when machining titanium.

Cutting tools used for machining titanium include cobalt-containing high speed tool steels, such as M33, M40, and M42, and the straight tungsten carbide grade C-2 (ISO K20). While carbides are more susceptible to chipping during interrupted cutting operations, they can achieve about a 60% improvement in metal removal rates compared to HSS. Ceramic cutting tools have not made inroads in titanium machining due to their reactivity with titanium, low fracture toughness, and poor thermal conductivity. It should be noted that although improvements in cutting tool materials and coatings have resulted in tremendous productivity improvements in machining for a number of materials (e.g., steels), none of these improvements have been successful with titanium.27

Cutting fluids are required to achieve adequate cutting tool life in most machining operations. Flood cooling is recommended to help remove heat and act as a lubricant to reduce the cutting forces between the tool and workpiece. A dilute solution of rust inhibitor and/or water soluble oil at 5-10% concentration can be used for higher speed cutting operations, while chlorinated or sulfurized oils can be used for slower speeds and heavier cuts to minimize frictional forces that cause galling and seizing. The use of chlorinated oils requires careful cleaning after machining to remove the possibility of stress corrosion cracking.

For the production of airframe parts, end milling and drilling are the two most important machining processes, while turning and drilling are the most important for jet engine components.27 In turning operations, carbide tools are recommended for continuous cuts to increase productivity, but for heavy interrupted cutting operations, high speed tool steel tools are needed to resist edge chipping. Tools need to be kept sharp and should be replaced at a wearland of about 0.015 in. for carbide and 0.030in. for HSS. Tool geometry is important, especially the rake angle. Negative rake angles should be used for rough turning with carbide, while positive rakes are best for semi-finishing and finishing cuts and for all operations using HSS. Typical end mill configurations are shown in Fig. 4.29. Note the improvements in metal removal rates for some of the newer configurations, as compared to standard HSS four flute end mill.

When milling titanium, climb milling, as shown in Fig. 4.30, rather than conventional milling, is recommended to minimize tool chipping, the predominate failure mode in interrupting cutting. In climb milling, the tooth cuts a minimum thickness of chip, minimizing the tendency of the chip to adhere to the tool as it leaves the workpiece. Slow speeds and uniform positive feeds help to reduce tool temperature and wear. Tools should not be allowed to dwell in the cut or rub across the workpiece. This will result in rapid work hardening of the titanium making it even more difficult to cut. Both carbide and HSS cutting tools can be used; however, carbide tools are more susceptible to chipping and may not perform as well in heavy interrupted cutting operations. Increased relief angles

Standard Four Flute High Speed Steel End Mill

Solid Carbide Finishing Cutter

Plunge Roughing Cutter with Carbide Inserts

Cress-Cut High Speed Steel Roughing Cutter

Standard Four Flute High Speed Steel End Mill

Solid Carbide Finishing Cutter

Plunge Roughing Cutter with Carbide Inserts

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