Читать книгу Exploring Advanced Manufacturing Technologies - Steve Krar - Страница 17
Оглавление(Steve Krar, Consultant – Kelmar Associates)
Over the years, many developments helped to improve the metal-removal rate and increase the flexibility of conventional OD cylindrical grinding operations. The development of superabrasive wheels greatly increased metal-removal rates, however the parts produced were limited to the shape of the grinding wheel. Therefore these parts had straight or angular forms and it was not possible to produce contour forms without dressing the wheel to the form required.
Single-point grinding is a process that combines two technologies - superabrasive grinding wheels and high-precision servo control - to provide a contour grinding process that resembles a computer numerical control (CNC) outside diameter (OD) turning operation. It allows one machine to perform several operations such as grinding parallel diameters, tapers, contours, and threads without removing the part from the machine. Performing more operations on a part in one setup reduces the amount of workhandling between operations. For many medium OD grinding applications, it is a means of combining several grinding applications and machines into a single step.
THE GRINDING PROCESS
The basic idea for single-point grinding comes from the modern CNC turning center where a single-point cutting tool can be used to perform various operations. For example, one single-point tool can profile, face, plunge, and cut threads.
A single-point OD grinding machine is similar to a turning center since two axes of movement are generally involved in both metal-removal processes. On turning centers a form tool can be used to cut profiles, or a single-point cutter can be programmed to follow a desired profile through the coordinated movements of the X and Z axis, Fig. 2-2-1.
CONVENTIONAL GRINDING
Production OD grinding traditionally is composed of process-specific steps. For complex workpieces in a medium-sized 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.
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.
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 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
▪Economical for small and large production runs
Conventional Grinding
▪Long cycle times – multiple setups (chucking) required
▪Danger of workpiece thermal damage because of heat buildup
▪High machine and grinding wheel wear
▪Frequent grinding wheel dressing cycles required
▪Inconsistent quality parts due to wheel wear
▪Complex formed wheels minimize the range of part contours that can be ground
SUMMARY
The grinding process has been under fire for some time now. Many shops are looking at alternative methods to reduce or eliminate grinding from their process. Hard turning is one example.
▪Single-point grinding may be a way to use the accuracy and surface finish benefits of the grinding process in a way that has a lower impact on the material flow in the shop.
▪In virtually all metalworking operations including milling and turning, many businesses are looking to perform more operations in a single workpiece handling.
▪For medium volume production grinding of complex workpieces, the single-point grinding process may be a way to accomplish this for shops that rely on grinding for a living.
For more information on SINGLE-POINT OD GRINDING see the Website: www.junker-machinery.com
