CONVENTIONAL GRINDING

Production OD grinding traditionally is composed of process-specific steps. For complex workpieces in a mediumsized batch, these steps are often sequential. The work moves from one process-specific machine to the next. For example, a plunge or step grinding machine will finish bearing races and shoulders, a form grinding machine will clean up tapers and profiles, a thread grinding machine will cut threads, and so on.

Individually, each process step is performed very quickly. An analysis of the total throughput time, however, reveals that significant savings could be made if work handling between operations could be reduced or eliminated. Additionally, keeping a workpiece on a single machine provides better workpiece accuracy because concentricity (dimensional relationships) between workpiece features can be maintained.

Many conventional OD grinders use a wheel with a desired geometric shape dressed into the wheel face. Once a dressing or truing unit shapes the wheel, that shape is then transferred to the workpiece by movement of one or both of the machine’s slides. Fig. 2-2-2.

Fig. 2-2-2 On conventional grinders the part profile is dressed into the wheel face and then transferred to the part. (Junker Machinery Inc.)

On conventional OD grinders, the wheel/workpiece interface forms a line of contact between the face of the wheel and the work. For example, if two pencils are laid side-by-side, with one representing the workpiece and the second the grinding wheel, contact between them forms a line; a wider wheel contacts more of the workpiece.

SINGLE-POINT GRINDING

Single-point grinding uses a process that imitates single-point turning; a single grinding wheel is used to perform a variety of operations. Profiling, plunging, and thread cutting are accomplished by precise CNC control of the X and Z axes through servomotor and ballscrew actuation, Fig. 2-2-3. That control is the key to single-point grinding because the workpiece shape is ground by the coordinated movement of the  machine axes and not by the shape that is dressed into the grinding wheel.

Fig. 2-2-3 A variety of grinding operations can be performed on a part using single-point OD grinding. (Junker Machinery Inc.)

The single-point CNC controlled grinding technology allows an operator to completely finish straight sections, shoulders, contours, tapered contours, slots, etc., on a workpiece in a single setup using single or multiple wheels. Single-point grinding produces high accuracy parts, increases productivity, and reduces grinding costs.

The Grinding Wheel

Most applications use a cubic boron nitride (CBN) vitrifiedbond superabrasive wheel rather than metal-bond wheels that are very time consuming and expensive to dress. Advances in vitrified superabrasive bonds now make them practical for single-point grinding, however these wheels tend to be rather coarse. Advantages include aggressive cutting action and reduced frequency of wheel dressing.

In single-point grinding, a narrow .156 to .236 in. (4 to 6 mm) CBN grinding wheel, dressed flat across its face, is used. When the grinding wheel is swiveled one-half of a degree, the contact area between the wheel and work becomes a singlepoint. The grinding wheel’s angle of attack presents an edge of the wheel that makes the contact between the wheel and workpiece tangential.

Grinding Wheel Setup

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.

Fig. 2-2-4 The one-half degree vertical tilt allows the wheel to perform edge and side grinding operations. (Junker Machinery Inc.)

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.