Between Centers And Centerless Grinding In One Setup
It sounds like a contradiction in terms-between centers and centerless grinding on one machine. But for some categories of workpieces, it's a viable production process that can yield machining time reductions of 45 percent over separate grinding operations.
It sounds like a contradiction in terms—between centers and centerless grinding on one machine. But for some categories of workpieces, it's a viable production process that can yield machining time reductions of 45 percent over separate grinding operations.
In general, OD grinding is performed either between centers or on a centerless grinding machine. When the features of the workpiece, such as crankshafts, camshafts, armatures or gear shafts require both grinding operations, they are usually done sequentially on purpose-built machine tools.
Primarily a shop uses these grinding operations because of specific strengths inherent in each. For between center work, a workpiece that has a critical concentric feature such as a cam lobe or gear or crank pin is processed between centers because the relative positions to the shaft diameter can be maintained.
Centerless grinding, on the other hand, is best applied for creating roundness. The process is self-correcting for most out of round conditions. It is limited, however, in its ability to maintain concentricity between the shaft diameter and a lobe, gear or pin.
All across metalworking, shops are combining what were once independent production steps into single setup, multi-process operations that can machine a workpiece complete from a blank. In the case of the Kronos L dual from Shaudt, Mikrosa, BWF, two traditionally distinct OD grinding operations are combined on a single machine tool frame. It's basically a centerless grinder that can perform between center OD grinding.
In order to achieve a "2-in-1" process, a workhead and tailstock are installed at either end of the centerless grinder workrest blade. The workhead is equipped with a driver and programmable rotational speed.
Both workhead and tailstock centers are retractable and adjustable to the workpiece length and diameter. A recirculating ball spindle, programmable from the CNC, can position the work spindle axially. On the tailstock side, its center support (MT 4) is arranged with a sleeve that is hydraulically actuated.
In operation, the grinding wheel and regulating wheels are moved back for loading clearance. Once loaded, the shaft is clamped between centers. The wheel head slowly brings the workpiece up to its programmed rotational speed.
Next the grinding wheel is infed to the workpiece. In this operation the regulating wheel is not involved. The workpiece is ground between centers. This step grinds the desired diameters of the workpiece concentric with the centers at a cutting rate from 0.05 mm/min. to 0.10 mm/min. (0.002 ipm to 0.004 ipm).
Following sparkout, the grinding wheel retracts, and the regulating wheel is infed. It forms a prism relative to the workrest blade. With the regulating wheel in position, the work driver and tailstock retract, causing the workpiece to contact the workrest blade.
The Kronos is now in centerless grinding mode. The grinding wheel feeds in, and the workpiece is finish ground using the centerless method. When size is achieved, the wheels retract, the workpiece is unloaded and the process begins again.
For workpieces with features that are concentrically critical yet need the roundness tolerances that centerless grinders provide, this "2-in-1" process can save production time and cost.
Two enabling technologies -- superabrasive wheels and high precision servo control -- come together to provide a contour grinding process that resembles an OD turning operation. For many medium volume OD grinding applications, this method may be a means to consolidate several manufacturing steps into a single setup.
Roughing and finishing on a single machine, using a single setup, has appeal for most shops. The advantages in time savings and accuracy are obvious. Eliminating the transport of workpieces between machines, as well as the setup for those secondary operations, is a boon for throughput. Critical features that need to maintain dimensional relationships can be much more reliably produced if machined complete in one clamping.
Optimizing a camshaft lobe grinding cycle has traditionally been based less on science and more on educated guesswork and numerous test grinds. Now, computer thermal modeling software can predict areas where lobe burning is likely to occur, in order to determine the fastest possible work speed that won't thermally damage lobes and greatly reduce the number of requisite test grinds.