Aluminum alloys are readily machined and offer such advantages as almost unlimited cutting speed, good dimensional control, low cutting force, and excellent life. Relative machinability of commonly used alloys are classified as A, B, C, D, or E.
Cutting tool geometry is described by seven elements: top or back rake angle, side rake angle, end relief angle, side relief angle, end cutting edge angle, and nose radius.
The depth of cut may be in the range of 1/i6-1/4 in. for small work up to l/2-\l/2 in. for large work. The feed depends on finish. Rough cuts vary from 0.006 to 0.080 in. and finishing cuts from 0.002 to 0.006 in.
Speed should be as high as possible, up to 15,000 fpm.
Cutting forces for an alloy such as 6061-T651 are 0.30-0.50 hp/in.3/min for a 0° rake angle and 0.25-0.35 hp/in.3/min for a 20° rake angle.
Lubrication such as light mineral or soluble oil is desirable for high production. Alloys with a machinability rating of A or B may not need lubrication.
The main types of cutting tool materials include water-hardening steels, high-speed steels, hardcast alloys, sintered carbides and diamonds:
1. Water-hardening steels (plain carbon or with additions of chromium, vanadium, or tungsten) are lowest in first cost. They soften if cutting edge temperatures exceed 300^0O0F; have low resistance to edge wear; and are suitable for low cutting speeds and limited production runs.
2. High-speed steels are available in a number of forms, are heat treatable, permit machining at rapid rates, allow cutting edge temperatures of over 100O0F, and resist shock better than hard cast or sintered carbides.
3. Hard-cast alloys are cast closely to finish size, are not heat treated, and lie between high speed steels and carbides in terms of heat resistance, wear, and initial cost. They will not take severe shock loads.
4. Sintered carbide tools are available in solid form or as inserts. They permit speeds 10-30 times faster than for high-speed steels. They can be used for most machining operations.
They should be used only when they can be supported rigidly and when there is sufficient power and speed. Many types are available.
5. Mounted diamonds are used for finishing cuts where an extremely high-quality surface is required.