Machining

Aluminum exhibits extremely good machinability. Its high thermal conductivity readily conducts heat away from the cutting zone allowing high cutting speeds, usually expressed in surface feet per minute (SFM):

where

D = tool diameter in inches N = tool rotation in revolutions per minute (RPM)

The objective of any machining operation is to maximize the metal removal rate (MRR) in in.3/min which for milling is equal to:

where

RDOC = radial depth of cut in inches ADOC = axial depth of cut in inches F = feed rate in in./min

Another important concept in machining is feed per tooth, also called chip load, which is:

ft = F/nN where ft = feed per tooth in in./tooth F = feed rate in in./min n = number of teeth on cutter N = tool rotation in RPM

Cutting speeds for aluminum alloys are high, with speeds approaching, or exceeding, 1000 sfm common. In fact, as the cutting speed is increased from 100 to 200 sfm, the probability of forming a built-up edge on the cutter is reduced; the chips break more readily and the surface finish on the part is improved. The depth of cut should be as large as possible to minimize the number of cuts required. During roughing, depths of cuts range from 0.250 in. for small parts to as high as 1.5 in. for medium and large parts, while finishing cuts are much lighter with depths of cuts less than 0.025 in. commonplace. Feed rates for roughing cuts are in the range of 0.006-0.080 in./rev., while lighter cuts are used for finishing, usually in the range of 0.002-0.006 in./rev.

Because aluminum alloys have a relatively low modulus of elasticity, they have a tendency to distort during machining. Also, due to aluminum's high coefficient of thermal expansion, dimensional accuracy requires that the part be kept cool during machining; however, the high thermal conductivity of aluminum allows most of the heat to be removed with the chips. The flushing action of a cutting fluid is generally effective in removing the remainder of the heat. The use of stress relieved tempers, such as the TX51 tempers, stress relieved by stretching, also helps to minimize distortion during machining.

Excessive heat during machining can cause a number of problems when machining aluminum alloys. Friction between the cutter and the workpiece can result from dwelling, dull cutting tools, lack of cutting fluid, and heavy end mill plunge cuts rather than ramping the cutter into the workpiece. Inadequate backup fixtures, poor clamping, and part vibration can also create excessive heat. Localized overheating of the high strength grades can even cause soft spots which are essentially small areas that have been overaged due to excessive heat experienced during machining. These often occur at locations where the cutter is allowed to dwell in the work, for example during milling in the corners of pockets.

Standard high speed tool steels, such as M2 and M7 grades, work well when machining aluminum. For higher speed machining operations, conventional C-2 carbides will increase tool life, resulting in less tool changes and allowing higher cutting speeds. For example, typical peripheral end milling parameters for the 2XXX and 7XXX alloys are 400-800 sfm with high speed tool steel and 800-1300 sfm for carbide tools. For higher speed machining operations, large cuttings, such as inserted end mills, should be dynamically balanced. Diamond tools are often used for the extremely abrasive hypereutectic silicon casting alloys.

2.9.1 High Speed Machining

High speed machining is somewhat an arbitrary term. It can be defined for aluminum as: (1) machining conducted at spindle speeds greater than 10000 rpm; (2) machining at 2500 sfm or higher where the cutting force falls to a minimum as shown in Fig. 2.27; and (3) machining at speeds in which the impact frequency of the cutter approaches the natural frequency of the system. In an end milling operation, assume the following roughing parameters:

• A speed of 3600 rpm maximum

Conventional Machining

Conventional Machining

High Speed Machining

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