15.14.1 Hydraulic Spindle Drives
Although much of the interest is in new and emerging technologies, many machines remain in the field built 30 or more years ago. Very often, aspects of an older technology will linger because it has proved such a practical or reliable method. A case in point is the use of gerotor hydraulic motors for dressing spindles discussed in the previous chapter. Hydraulic motors were common wheel drives for applications such as centerless grinding. The motors were compact and had good torque at low rpm. Lidkoping [1998] still offered a state-of-the-art grinder suitable for vitrified CBN wheels with hydraulic piston pump drives on both its grinding and regulating wheels. The hydraulic drive was claimed to give an exact control of wheel velocity.
15.14.2 Air Motors and Bearings
The other drive still seen is based on air motors and bearings. Air bearings have very low friction and are, therefore, used for very high speeds. The limitation with air is the load-carrying capacity of the air film. The supply pressures that can be employed are much lower than can be employed with oil. This limits the bearing pressures that can be generated [Rowe 1967].
The most common use for air bearings is in jig grinding. The process is a very low stock — removal process, often carried out dry, with small superabrasive wheels. The workpieces can have deep bores to be ground leading to long, flexible wheel shanks. The torque requirements for the motor are, therefore, low. Moore jig grinders are available with a complete range of air turbine and vane-driven spindles from 9,000 to 175,000 rpm. Similarly, Hauser provides an air spindle for operations in the range of 80,000 to 160,000 rpm although it also provides direct-drive electric options for slower speeds.
Air bearings in spindles are seen in ultraprecision grinders for specialist applications such as carbide grinding for the die industries with tolerances of <0.2 pm and surface finishes of 0.025 Ra [Pride n. d.]. Many of the grinder manufacturers are focused on the electro-optical and semiconductor businesses and as such are outside the scope of this book but, on occasion, this type of machine is found suitable for ultrahigh precision prototype work.
A case in point is Precitech who manufactures machines for single-point diamond turning or grinding of aspheric surfaces for lenses. The machines are equipped with aerostatic bearing, air- driven spindles for speeds up to 60,000 rpm, although the motor is replaced with a dc servomotor for speeds around 10,000 rpm. With laser interferometer feedback, the machine positional resolution is <10 nm, indicative of the type of low stock removal, high-precision applications where air spindles and bearings are found.
Rotary dressing heads and spindles have become a key feature of modern grinding systems particularly for application with superabrasive wheels. Rotary dressing units are often purchased separately from the basic machine tool. In many cases, dressing spindles are merely an option when buying a new grinder and are only fitted at a later date by end users seeking process improvements. Many older machines were never designed for use with dressing spindles and may need a complete retrofit including modifications to slides, guarding, and machine control programming. Spindle design for these applications is very dependent on available space within the machine as usually it will be replacing a small, simple stationary tool post. In the newest grinders capable of using CBN, the demand becomes more a factor of motor power density and rotational speed to match the higher wheel speeds and dressing forces, and positive crush ratios. Dresser spindle design is a subject that has previously been rather overlooked in books on grinding technology. The following section, therefore, attempts to provide a picture of the overall range of provision and application.
Rotary dressing drives can be basically categorized by their source of power.
15.15.1 Pneumatic Drives
Pneumatic spindles are relatively inexpensive, can run on shop air of 6 to 7 bar (90 psi), and do not suffer from heat problems that would cause significant thermal movement. However, they have low torque or power (or more specifically a low-power density) and are noisy. Larger motors suitable for form roll dressing can require 3 m3/min (100 ft3/min) of air. In general, their use is limited to internal grinding often in cross-axis dress mode, although even here they are being superceded by electric motors. Where used in uniaxial dress mode (as opposed to a cross-axis configuration) they tend to be equipped with small-diameter cup dressers to limit torque requirements. Speed selection is made by adjustment of airflow.