On various models of single-point grinder, the grinding wheelhead can be programmed to swivel from zero to 30° perpendicular to the workpiece. The single-point process compounds the angularity of the wheel by tilting it 0.5° in the vertical plane. The tilting of the wheel is critical to getting free cutting action from the single-point process. It brings more of the side of the wheel into the cut, which in turn brings more cutting grit into the grind, Fig. 2-2-4. Most of the cutting is done with the side of the wheel. This slightly skewed contact also reduces the severity of spiral cut lines that are found on single-point turned parts.
A hydraulic cylinder actuates the grinding wheel tilt to +0.5° by a M code command from the CNC machine control unit. Cutting forces are greatly reduced due to the combination of the swivel angle and wheel tilt. The area of contact is much less than a conventional OD grinding wheel. Reduced cutting forces lessen the heat buildup on the workpiece, reducing the possibility of thermal damage to the part being ground. Coolant application is more effective in single-point grinding because of the relatively small area of contact between the wheel and work.
Quick-Change Wheel
Critical to optimum operation of superabrasive grinding wheels is good balance and virtually no runout. For balance, the grinding wheel spindle uses an electronic automatic balancing system built into the spindle that keeps wheel runout to 50 millionths (.000050 in. or 0.00127 mm).
A three-point centering system is built into the wheel and flange, Fig. 2-2-5. Three cam followers are installed on the periphery of the wheel core. A three-lobed cam is machined into the flange plate. When the wheel and flange are mated, a partial turn of the flange engages the cam followers that self-center the wheel on the flange. Once the wheel is properly located, it is secured by bolts.
Fig. 2-2-3 A variety of grinding operations can be performed on a part using single-point OD grinding. (Junker Machinery Inc.)
Fig. 2-2-4 The one-half degree vertical tilt allows the wheel to perform edge and side grinding operations. (Junker Machinery Inc.)
Fig. 2-2-5 A three-point cam and follower system, built into the wheel flange, keeps the wheel runout to within .000,050 in. (0.00127 mm) (Junker Machinery Inc.)
A scaled version of this centering system is used for both centers. In addition to quick changeover considerations for setup and tear down of jobs, the self-center system provides accurate positioning of the workpiece.
HIGH SPEED GRINDING
The relatively small contact area between the single-point superabrasive grinding wheel and the workpiece reduces the cutting forces generated by the metal-removal process. Reduced cutting forces produce less heat so the single-point grinder can be run at higher cutting speeds without causing surface (thermal) damage to the workpiece.
Cutting speeds of 27,600 sf/min (8412.48 m/min) are possible with superabrasive (CBN or diamond) grinding wheels, Fig. 2-2-6. These speeds take full advantage of the aggressive cutting capability of the wheels and most applications that have the single-point grind can be completed in one pass.
Because grinding wheel rotation is limited by centrifugal force, the wheel alone does not achieve these high speeds. To achieve high metal-removal rates, a combination of wheel surface speed and workpiece surface speed is necessary because the wheel’s speed is limited by safety concerns. In order to increase the speed, workpieces can be rotated up to 12,000 r/min. The high surface speed achieved with single-point grinding is the combined speed of the workpiece rotation plus the rotation of the grinding wheel.
Fig. 2-2-6 Superabrasive grinding wheels are capable of high surface speeds and high metal-removal rates. (Junker Machinery Inc.)
Rotating a workpiece at these speeds requires a balanced setup. The low cutting forces generated by single-point grinding simplify the workpiece drive mechanism. In most grinding applications, drive dogs or other workpiece drivers are not required since the frictional pressure of the center is enough to keep the part rotating. This definitely helps contribute to a balanced setup.
Feed lines also impact how fine a finish can be ground with the single-point process, Fig. 2-2-7. These lines border on microscopic but are imparted onto the workpiece. Surface finishes of 7 rms. are about as good as can be achieved. If a finer surface finish is required, a further superfinishing operation may be required.
Single-point grinding is a unidirectional process either from right to left or vice versa and wheel wear moves across the face of the grinding wheel. Because the wheel cuts on one edge, its opposite edge, which is dressed square, can be used to cut shoulders. It can only cut one side of a shoulder, for right and left shoulders, the wheelhead must be indexed 180° to allow the same side of the wheel to cut both shoulders. The indexing takes only a few seconds; some machines may be equipped with a second wheel for parts with numerous shoulders.
A G-ratio of 60,000, the ratio of the volume of workpiece material removed divided by the amount of grinding wheel used, is achievable with single-point grinding using superabrasive wheels. The higher the grinding ratio, the longer the life of the grinding wheel. For example, a G-ratio of 60,000 means 60,000 cubic inches of metal will be removed from the work-piece with relatively little wheel wear.
ACCURACY
Grinding accuracy of the single-point process is a function of the machine tool’s accuracy. It relies only on the positioning and repeatability capabilities of the grinder. Single-point grinding eliminates one source of potential grinding inaccuracy. The wheel dresser or truing unit has a tolerance band while a grinding machine has a positioning and repeatability tolerance. If the profile dressing can be replaced with the servo control, then only the machine’s tolerance band is involved in generating workpiece accuracy.
Single-point grinding allows the shop to process cylindrical parts, tapered contours, plunge cuts, shoulders, and slots in a single chucking. Instead of dressing a specific taper or contour into the grinding wheel, workpiece features are ground by simultaneous control of the X axis infeed and Z axis table positioning.
Fig. 2-2-7 The single-point grinding process can produce surface finishes up to 7 rms. (Norton Co)
Both axes use closed-loop digital servo feedback through the CNC control unit and are actuated by precision, anti-backlash ballscrews, Fig. 2-2-8. Positioning scales are used on both the X and Z axes. A CNC program, which can be input at the machine or downloaded from a remote location using a variety of network protocols, directs the wheel and table to follow simple or complex geometry with accuracy within two microns. Where this process is most advantageous is medium runs and families of parts where grinding methods require long wheel-dressing cycles or transfer between several machines. Single-point grinding eliminates profile dressing entirely and cuts most workpiece geometry in a single chucking. Only periodic cleanup of the wheel is needed.
APPLICATIONS
Implementing single-point grinding could be the solution for many production grinding shops because this process is well suited for grinding complex and varied workpiece shapes. The high surface speeds made possible by combining the wheel rotation and workpiece rotation, allow