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 Tools
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.