Civil Engineering Reference
In-Depth Information
FIGURE 3.1
Typical grinding cycle time.
required to reduce the force is at the end of a grinding cycle. A typical grinding cycle is shown in
Fig. 3.1
.
There is a trade-off between spark-out time and residual error in the ground part. Under force control a
force trajectory can be followed to reduce the spark-out time, and the positional error can be reduced by
providing a constant and repeatable force. The accuracy and repeatability of the imposed grinding forces
translates to dimensional accuracy and repeatability from part to part.
Unfortunately, the grinding process is time-varying with changing properties and conditions in the
grinding wheel, material, and existing surface profile. Therefore, a simple fixed-parameter model of the
grinding process is insufficient for certain high precision applications. Thus, a controller that adapts to
variations in the grinding process parameters must be developed as well as an observer (or estimator)
to identify these parameters in real-time. The observer provides the controller with updated process
parameters, thereby permitting the controller to adapt to process variations. This is an effective means
to achieve a desired profile and surface finish with minimal damage.
3.2
Grinding Process
The tool in the grinding process is the grinding wheel, which is generally composed of two materials: grits
and bonding agent. The hard abrasive grits (small hard particles) are affixed to rotating hub or axis in the
general configuration of a wheel to erode material away from a workpiece as the wheel spins. Grinding
wheels are formed into shape by casting and curing a slurry of bonding material with the grits. The cured
wheel forms a three-dimensional matrix of grits held in place by the bonding agent. This yields a complex
operation by nature, as the uneven and truly random distribution of grit surfaces of the wheel and the
contact they make with the ground part are difficult at best to systematically model.
History and Perspective
Abrasive material removal (grinding) is one of the oldest machining technologies employed today, and
has been utilized by people in the manufacturing of parts since the Stone Age (Malkin, 1989). A simplified
grinding process can be thought of as milling using a “cutter” with a large number of teeth of irregular
shape, size, and spacing (
Fig. 3.2.
)
. Each grit can be seen as a cutting tooth with varying orientation and
sharpness. These grits are suspended in a bonding agent that holds the three-dimensional matrix of grits
together in a form. The grinding process can vary for many reasons including: wheel sharpness, wheel
microstructure, workpiece material variation, loading of workpiece material on the wheel, and other
phenomena that contribute to the changing nature of the grinding process. (Loading is the phenomena
of the ground material becoming attached to or embedded onto the surface of the grinding wheel.
