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Fig. 2-2-1 Single-point grinding combines two technologies to grind parallel diameters, tapers, contours, threads, etc. (Junker Machinery Inc.)
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.
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.
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.
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.)
The Grinding Wheel
Most applications use a cubic boron nitride (CBN) vitrified-bond 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 single-point. The grinding wheel’s angle of attack presents an edge of the wheel that makes the contact between the wheel and work-piece 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.
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.
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