Machine tool basics: Part 2

This second installment of our series looks at key machine-tool elements and addresses the tool, the toolholder, and machine control.

The cutting action in a machine tool when milling and drilling involves the spindle, toolholder, and tools.

Spindle Design

Spindles, which secure the tool and its holder, are key in determining machine tool accuracy. In early machine tools, the spindles were simple bearing-- mounted shafts driven by a constant-speed electric motor, achieving different speeds through belts and gears. Operators changed spindle speed by shifting gears or moving belts to and from various pulleys.

As drive motors achieved higher torque and were designed to operate at variable speeds, belt and gear-driven systems began to wane in popularity, but both are still used. Stronger, longer-wearing, quieter belt and gear-- drive designs have been developed. Variable-speed direct drive, or integral-- motor spindles have replaced geared spindles for high-speed applications. At the same time, spindles with planetary gear systems, much like a car's automatic transmission, are now used to provide a wide torque output.

Three important variables in spindle design are the type of bearing, bearing placement, and drive motor.
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Spindle bearings. Spindles have from one to four sets of bearings. The most common bearing configuration on a spindle is one pair back to back in front, and a floating pair or single unit in back. The configuration depends largely upon the balance between axial and radial load, or whether there is an integral, belt, or gear drive.

Although most bearings use steel balls, hybrid bearings with ceramic balls and steel races are gaining favor. Ceramic balls are about 60% lighter than steel units, and so have less inertia and cause less wear. The lighter weight allows spindle shafts to turn from 30 to 50% faster. Ceramic balls are also significantly more expensive than steel.

There are other alternatives to ball bearings depending on the application, performance specifications, and the allowable cost.

Air bearings are used for light, very high-- speed loads such as small drills and high-precision polishing. Air spindles, which operate at 250,000 rpm, carry small tools, just several millimeters in diameter, and do light cuts. They can be used on hardened steels and can produce a mirror finish, eliminating EDM in some cases.

Magnetic bearings suspend the spindle shaft in a frictionless magnetic field, and are used at speeds of more than 40,000 rpm. The main problems are control, complexity, and cost. In the more distant future, superconducting bearings may be practical.

For slower, high-precision operations, hydrodynamic and hydrostatic bearings, can be used. Speed is limited due to fluid shear to slower speeds, such as are employed in some precision grinding operations.

Bearing placement. Most bearings used in spindles employ angular contact, which is the angle between the ball-to-face contact line and a plane through the ball centers perpendicular to the bearing axis. It is usually 12 to 25 deg. The smaller the contact angle, the greater the radial load-carrying ability. The greater the contact angle, the greater the axial load. Thus, choosing the correct bearing is a compromise. For example, in drilling, a very high vertical load operation might require an angle of 25 deg, while milling can be carried out at 15 deg.

The amount of preload placed on the bearing during assembly is important, particularly at speeds over 3000 rpm. At these speeds, temperature becomes an issue, and it is important that there is enough preload to compensate for thermal expansion. Preload settings are based on a combination of maximum speed and maximum cutting forces, plus the type of bearing.

Proper sealing of the bearing is essential when reliability and maintainability are important. Many spindle failures are the result of coolant, moisture, or other materials getting into the bearing. Foreign fluids degrade the lubricant, and solids can spall the raceway and bearings. Sealing problems reportedly cause more than half of the field problems encountered. Machine wrecks are the other major cause of spindles not getting to their design life.

Motor evolution. Motor technology has come a long way, particularly in the ability to get more horsepower from a smaller package. Initially, spindle motors were simple induction motors used with gearboxes and drive belts. Later there was a shift to dc units that delivered more torque. Then, in the 1980s, there was a transition to ac for many applications. The ac motors offered higher performance, higher speed, and fewer wearing parts, and don't employ electromechanical commutation. Ratings range from 5 to 15 hp. These motors can run at a fixed torque with a wide constant-horsepower range.