The grinding process is unique compared to other machining processes in that there is a very heavy “tool” (wheel) wear and that the process is very sensitive to process parameters like grinding pressure, speed and feed rates and flow of coolant etc. CNC has been successfully applied to all types of grinding machines. The workpiece size control is achieved by some method of automatic wheel wear compensation included in the grinding cycle or through an interactive in-process gauging system which is interfaced with the wheel in-feed drive. CNC can be used either to regulate the machine slides to generate a workpiece or to dress the wheel to the required shape to enable plunge grinding.
In horizontal spindle reciprocating table surface grinding machines, CNC is applied only to the wheel head slide (Y-axis), as the most critical operation in these machines is the depth control. Conventional hydraulic drives are retained for feed and longitudinal traverse. In some machines the cross feed is replaced by an electric drive with a timer circuit or open loop control for feed. A closed loop control on the cross feed axis can be used directly to generate contours in the Y-Z plane while reciprocating the table in the longitudinal axis or to contour-dress the wheel for plunge grinding. In some cases the longitudinal traverse of the table is also provided with an electric drive.
In many cases CNC is provided only for wheel head slide to control diameters. In some cases CNC is provided for longitudinal traverse and wheel head traverse (2-axis) to control length of shoulders and diameter steps. Workpiece size is achieved by dressing the wheel with reference to fixed dressing point or by use of an interactive size control unit. Such machines can generate solids of revolution involving tapers, circular arcs and curved surfaces. Similar controls are available for internal grinders.
Figure 12.18 shows the schematic arrangement of a CNC grinding machine. X-axis is the in-feed direction of the grinding wheel. The longitudinal traverse of the grinding wheel is the Z-axis. The spindle rotation is the S1 axis. If the grinding wheel rpm is programmable, it provides the S2 axis. Swiveling of the wheel head is B-axis. Swiveling axis for dressing the wheel is the A-axis. C-axis is also provided on some machines. Figure shows only X, Z and S1 axes.
Tool and Cutter Grinders
CNC has been applied to complex tool and cutter grinders involving as many as eight
axes. The control system working in an interactive mode with the part program residing
in the memory leads the operator step by step reducing the programming effort to the
CNC has been used to advantage even on optical profile grinders. The control system has
linear, circular and helical interpolation to generate complex contours, and has provision
to dress the wheel and to compensate for wheel wear. Optics serves for tool setting,
positioning the workpiece, checking wheel dressing and inspection of workpiece without
removing it from the fixture.