ALUMINUM MACHINING CUTTING TOOLS BASICS

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.

GEAR LUBRICATION BASICS

The motion between gear teeth as they go through mesh is a combination of sliding and rolling. The type of gear, the operating load, speed, temperature, method of application of the lubricant, and metallurgy of the gears are all important considerations in the selection of a lubricant.

Industrial gearing may be enclosed, where the gears and the bearings that support them are operated off the same lubricant system; or open, where the bearings are lubricated separately from the gears themselves.

Due to the high sliding contact encountered in enclosed worm and hypoid gears, lubricant selection for these should be considered separately from lubrication of other types of enclosed gears.

As with all equipment, the first rule in selecting a gear lubricant is to follow the manufacturer’s recommendation, if at all possible. In general, one of the following types of oils is used:

Rust- and Oxidation-Inhibited (R & O) Oils.

R & O oils are high-quality petroleum-based oils containing rust and oxidation inhibitors. These oils provide satisfactory protection for most lightly to moderately loaded enclosed gears.

Extreme-Pressure (EP) Oils.

EP oils are usually high-quality petroleum-based oils containing sulfur- and phosphorus-based extreme-pressure additives. These products are especially helpful when high-load conditions exist and are a must in the lubrication of enclosed hypoid gears.

Compounded Oils.

These are usually petroleum-based oils containing 3 to 5 percent fatty or synthetic fatty oils (usually animal fat or acid less tallow).They are usually used for worm gear lubrication, where the fatty content helps reduce the friction generated under high sliding conditions.

Heavy Open-Gear Compounds.

These are very heavy bodied tarlike substances designed to stick tenaciously to the metal surfaces. Some are so thick they must be heated or diluted with a solvent to soften them for application. These products are used in cases where the lubricant application is intermittent.

A number of gear lubrication models and viscosity selection guides exist. In the United States, the most widely used selection method employs the American Gear Manufacturers Association (AGMA) standards. Under its specifications for enclosed industrial gear drives, the AGMA has defined lubricant numbers, which designate viscosity grades for gear oils.

Open gears operate under conditions of boundary lubrication. The lubricant can be applied by hand or via drip-feed cups, mechanical force-feed lubricators, or sprays.

Heavy bodied residual oils with good adhesive and film-strength properties are required to survive the relatively long, slow, heavy tooth pressure while maintaining some film between applications of lubricant. Several PC software programs exist to aid in lubricant selection to reduce wear, scuffing, and pitting of gear-tooth surfaces.