SAWING
Mechanical hacksawing of titanium is very common. Coarse saw blades, heavy feed and generous amounts of water soluble oil coolant are recommended. Titanium is also readily friction cut. Such surfaces should be filed to remove about 0.005 inches of contamination. Similarly, abrasive cutting of titanium is satisfactory if coolant is used and contaminated layer is removed by filing.

HAND ABRASIVE GRINDING
A clean wheel, used only on titanium is important. An open type wheel containing large grains has been found to minimize clogging. Excessive buildup of heat should be avoided to minimize metal contamination. Ground surfaces should be filed or mechanically finished to remove abrasive particles and, in particular, any visible metal oxide (burns).

Sandpaper or steel wool should be avoided and wheel type mechanical burrs (rotary files) should be operated at low rpm to avoid burning and maximize tool life.

When grinding is used on titanium, measures must be taken to protect adjacent titanium surfaces and surroundings from the extremely hot grinding sparks.

MACHINING TITANIUM
Machining techniques for titanium are no more difficult than those for other high performance metals; for instance the austenitic stainless steels. Reasonable production rates and excellent surface finish are readily attainable on machined parts, provided some unique characteristics of titanium are taken into account. These characteristics are:

1. The unusual chip-forming tendency and low thermal conductivity of titanium tends to cause a build-up of heat on the edge and face of cutting tools.

2. The reactivity of titanium with cutting tools contributes to seizing, galling, abrasion and pick up on cutting edges and faces.

3. The low elastic modulus of titanium permits greater deflections of workpieces and, therefore, may require proper backup.

Machining conditions can be selected which minimize or circumvent the adverse effects of these characteristics of titanium, thereby allowing good tool life at acceptable production rates.

Observation of the following six guidelines will aid in successfully machining titanium:

1. USE LOW CUTTING SPEEDS. Tool tip temperature is strongly affected by cutting speed. A low cutting speed helps to minimize tool edge temperature and maximize tool life. Lower speeds are required for titanium alloys such as Ti-6Al-4V than for unalloyed titanium.

2. MAINTAIN HIGH FEED RATES. Tool temperature is affected less by feed rate than by speed. Therefore, the highest rate of feed consistent with good practice should be used. The depth of cut should be greater than the work hardened layer resulting from the previous cut.

3. USE A GENEROUS QUANTITY OF CUTTING FLUID. The coolant carries away heat in addition to washing away chips and reducing cutting forces, thereby improving tool life.

4. MAINTAIN SHARP TOOLS. Tool wear results in build-up of metal on cutting edges and causes poor surface finish, tearing and deflection of the workpiece.

5. NEVER STOP FEEDING WHILE TOOL AND WORK ARE IN MOVING CONTACT.
Permitting a tool to dwell in moving contact with titanium causes work hardening and promotes smearing, galling and seizing, which may lead to total tool breakdown.

6. USE RIGID SETUPS. Rigidity of machine tool and workpiece ensures a controlled depth of cut.

TOOL MATERIALS
Cutting tools for titanium require abrasion resistance and adequate hot hardness. Carbide tools (such as Grades C-2 and C- 3), where feasible, will optimize production rates. The general- purpose high speed tool steels (such as Grades M1, M2, M7, and M10) are often suitable for machining titanium. However, best results are generally obtained with more highly alloyed grades (such as T5, T15, M33, or the M40 series).

CUTTING FLUIDS
Correct use of coolants during machining operations on titanium will greatly increase cutting tool life. Chemically active cutting fluids transfer heat efficiently and reduce cutting forces between tool and workpiece. The result is prolonged tool life.

Large quantities of cutting fluid are needed to keep the titanium workpiece and the cutting tool cool during high speed machining operations. Water base fluids are more efficient than oils. A weak solution of rust inhibitor and/ or water soluble oil (5 to 10 percent) is the most practical fluid for high speed cutting operations. Slow speed and complex operations may require chlorinated or sulfurized oils to minimize frictional forces and reduce the galling and seizing tendency of titanium. Best tool life in intermediate speed operations may be achieved by utilizing a good coolant containing a chemically active additive.

If chlorinated cutting fluids are used on alloys which may be subject to stress corrosion cracking, carefully controlled post- machining cleaning operations must be followed.

TURNING OF TITANIUM
Turning is the simplest machining operation for titanium and its alloys. Through proper machine parameters and use of coolant, surface finishes of 20 to 30 microinches RMS are obtainable with + or - 0.001 inch tolerances.

Carbide tools provide highest production rates for continuous turning operations. Interrupted cuts, plunge cuts and grooving are best performed by the softer but tougher high-speed steels or cast alloys. Tools must be resharpened or replaced before final tool failure occurs. An 0.015" wearland for carbide tools and 0.030" wearland for high-speed steel or cast alloy tools can be used as a guide for halting turning operations.

Tool geometry, particularly rake angle, is important. Negative rake angle is recommended for rough turning with carbide tools. Positive rakes are best for finish and semi-finish turning and when high-speed steels or cast alloy tools are used (Figure 6).

Large amounts of water-base soluble oils (5 to 10 percent solution) or chemically active (5 percent sodium nitrate in water) coolants are recommended. Sulfo-chlorinated oils may be used, if necessary, at low cutting speeds.

A summary of recommended tool geometries and machine parameters are given in Table 10.