Machining Data Speeds & Feeds
Cutting speed and feed are two of the most important parameters for all types of machining operations. Extensive testing has developed the tool-life data and tool life charts are available now available. One Manufacturer offers the following general guidelines for typical machining operations.
Although the basic machining properties of titanium metal cannot be altered significantly, their effects can be greatly minimized by decreasing temperatures generated at the tool face and cutting edge. Economical production techniques have been developed through application of these basic rules in machining titanium:
- Use low cutting speeds. Tool tip temperatures are affected more by cutting speed than by any other single variable. A change from 6 to 46 meters per min (20 to 150 sfm) with carbide tools results in a temperature change from 427ºC to 927ºC (800ºF to 1700ºF).
- Maintain high feed rates. Temperature is not affected by feed rate so much as by speed, and the highest feed rates consistent with good machining practice should be used. A change from 0.05 to 0.51 mm (0.002 in. to 0.020 in.) per revolution results in a temperature increase of only 149ºC (300ºF).
- Use generous amounts of cutting fluid. Coolant carries away heat, washes away chips, and reduces cutting forces.
- Use sharp tools and replace them at the first sign of wear, or as determined by production/cost considerations. Tool wear is not linear when cutting titanium. Complete tool failure occurs rather quickly after small initial amount of wear takes place.
- Never stop feeding while a tool and a workpiece are in moving contact. Permitting a tool to dwell in moving contact causes work hardening and promotes smearing, galling, seizing, and total tool breakdown.
Machining recommendations, such as noted above, may require modification to fit particular circumstances in a given shop. For example, cost, storage, or requirements may make it impractical to accommodate a very large number of different cutting fluids. Savings achieved by making a change in cutting fluid may be offset by the cost of changing fluids. Likewise, it may be uneconomical to inventory cutting tools which may have only infrequent use. Also, the design of parts may limit the rate of metal removal in order to minimize distortion (of thin flanges, for example) and to corner without excessive inertia effects.
An example of typical machining parameters currently used to machine Ti-6Al-4V bulkheads containing deep pockets, thin flanges, and floors at an important United States airframe manufacturer are shown in Table 6.2. A bulkhead frequently contains numerous pockets and some flanges as thin as 0.76 mm (0.030 in.). Typical example bulkhead rough forgings weigh in excess of 450 kg (1000 lb), but the finished part is less than 67.5 kg (150 lb) after machining. Extensive machining is done on gas turbine engine components, just as is done on the larger airframe components. Table 6.3 lists typical parameters for machining Ti-6Al-4V jet engine components such as fan disks, spacers, shafts, and rotating seals.