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 high traverse rates that make the process competitive with straight, plunge, and other conventional grinding operations for producing relatively simple geometries.
Fig. 2-2-8 The CNC controller and servodrive system guides the wheel and table to follow simple or complex forms to within two microns of accuracy. (Junker Machinery Inc.)
In applications where there is complexity, single-point grinding will do a better job because the narrow grinding wheel allows access to features that a conventional wheel cannot get to without significant wheel dressing. Varying widths of flat and tapered surfaces, crowned surfaces, slots, and undercuts can all be produced by the single-point process.
The use of superabrasives also enhances the flexibility of the single-point process for use in a variety of materials.
In one application, an electric motor rotor composed of three distinct materials was ground in one setup.
▪The rotor had a soft steel shaft with an aluminum armature pressed on the shaft.
▪Tungsten carbide was used on the shaft as contacts for the bearings.
▪All three materials were ground using two wheels, a diamond wheel for the tungsten carbide and a CBN (cubic boron nitride) wheel for the soft steel shaft and aluminum.
▪Concentricity was maintained because the workpiece was ground complete in one chucking.
ADVANTAGES OF SINGLE-POINT GRINDING
The single-point grinding process using CBN grinding wheels can benefit many grinding shops, especially those that can only afford one cylindrical grinder.
▪The single-point process can be up to six times faster than conventional OD grinding.
▪Due to low grinding forces, workpiece drivers are not required since in most cases the frictional pressure for the centers is enough to rotate the part.
▪Complete grinding of almost all possible contours in one setup is possible.
•The work done on this type of machine normally requires two or three specialized grinders to produce the same lot quantities.
▪It is ideal for low-volume shops that cannot afford to buy specialized grinders to perform specific operations.
▪Indirect cost savings result from fewer operators required, lower maintenance, and less consumables.
▪Better workpiece accuracy is maintained because all operations are done in one setup.
▪Contours are easily ground by CNC programming of the axes and form requirements.
▪The workhandling time normally done on conventional grinders is reduced.
▪Long CBN wheel life between dressing cycles result in consistently producing high-quality parts.
▪There is little or no thermal damage to the parts being ground because less heat is generated during single-point grinding with CBN wheels.
▪The process is competitive with straight plunge and other conventional grinding operations for producing relatively simple geometries.
SINGLE-POINT VS CONVENTIONAL GRINDING
The following is a comparison of some of the major points between single-point and conventional grinding.
Single-Point Grinding
▪High stock removal with little or no thermal damage to the part
▪Minor production related forces and largely driver-free operation
▪Consistent part accuracy for long production runs
▪Long CBN grinding wheel life with few dressing cycles
▪Higher initial CBN grinding wheel cost but lower cost per part produced
▪Complete grinding of a part in one setup (chucking) replacing the need for multiple machines
