The grinding of noncircular contours makes particular demands on the machine tool. The manufacture of highly precise components that are also expected to meet certain aesthetic standards calls for special grinding systems.
EMAG: From the simple to the highly complex: modular machine concepts for noncircular grinding must meet diverse market requirements
12.05.2009 - Oliver Hagenlocher
Typical shaft-type components for noncircular machining include camshafts, cam discs and pump shafts (Fig. 1). The various machining tasks also make different demands on the machine concept. Even for the execution of closely defined machining requirements – the grinding of camshafts for passenger cars, for instance – today’s market demands cannot be met with a single, fixed machine concept. The modular machine concept is absolutely essential for out-of-round machining. It may be implemented by employing multiple grinding spindles for pre-grinding and finishing or for complete-machining of the component. Or it may require a machine with two slides that facilitates simultaneous grinding operations.
In order to meet such diverse demands EMAG offers an impressive selection of technology modules for their grinding machines. The grinding systems SN 204/SN208 and SN 310/320 exemplify the successful application of the principle of modularity in the grinding of shaft-type components (Fig. 2). The Series 2 machines are suitable for the grinding of camshafts of up to approx. 800 mm – as used in passenger cars; whereas the Series 3 machines are more suitable for camshafts of up to 2,000 mm grinding length – as used in commercial vehicles. Both series are based on the compound slide principle. There are simpler machines with just one fixed wheel; and then there are machines with B-axis and up to three grinding spindles. The SN 208 can be upgraded to a twin-slide machine (Fig. 3). The most comprehensive configuration is available on the SN 320 and consists of two slides, two B-axes and four grinding spindles. These concepts make simultaneous machining possible. The SN 2 series is suitable for the mass production of camshafts (Fig. 4). Its X-axis features hydrostatic guideways and a linear motor. The CBN grinding spindle can also be used for rough grinding operations that employ electroplated CBN wheels. However, the CBN wheel with its 400 mm diameter does not only allow the grinding of concave or slightly concave contours. In fact, it is suggested that concave contours should be pre-ground with a large wheel and finished with a small wheel (Fig. 5). The B-axis allows for a third spindle to be fitted. This configuration makes it possible to also grind other geometrical elements of the camshaft, such as bearing surfaces. A configuration of two slides and four grinding wheels will complete-machine these shafts (Fig. 6). In an effort to shorten cycle times, bearing surfaces or cams can also be machined simultaneously. The objective of such configurations is to finish grind the shaft in a single set-up.
Direct drives for better surface finishes
The rotary axis (usually the C-axis) is of equal importance in out-of-round machining as the infeed axis. They both must be highly dynamic. When establishing the torque rate, the mass inertia of workpiece and clamping device ought to be taken into consideration. As belt drives tend to create surface shadings, a direct drive with high-resolution rotary encoder is preferred.
What is demanded of the grinding wheel depends, to a large extent, on the task in hand. Out-of-round machining operations are often carried out with CBN grinding wheels. The reason lies in the achievable stock removal rates – which are high – and the contour stability of CBN wheels. During rough grinding, for instance of cast iron camshafts, an allowance of some millimetres is ground away in a few seconds. The relevant metal removal rate can be up to 300 mm³/mms. For such applications the dimensions of the grinding spindle have to be chosen carefully. The finishing operation is subject to tight concentricity limits and requires the use of some kind of balancing system, where this is technically possible. The machining of concave radii dictates that the maximum grinding wheel radius does not exceed 80% of the minimum radius of the contour to be ground. This restriction demands maximum performance within a cutting speed range suitable for CBN machining. As the available space for many applications is limited, these demands lead right up to the very edge of technical feasibility.
To match grinding wheel conditions, the dressing spindles also have to be suitable for CBN, must run at the necessary speeds and should be equipped with contact recognition sensors.
Application example and trends
The machining of camshafts is a typical example of noncircular grinding. In a recently implemented turnkey project four twin-cams on a manually loaded shaft were ground at a cycle time of 90 seconds. The challenge of this application was having to rough-grind an allowance of up to 3 mm and to finish-grind the cams with just a single grinding wheel. These conflicting demands were met with a vitrified bonded grinding wheel. The cutting speed used was 120 m/s and the contouring accuracy an impressive +/- 10 µm.
Another example is the complete-machining of camshafts for commercial vehicles. Twelve control cams, six pump dogs, seven bearing surfaces, the thrust bearing and the ends with their tapers and shoulders were ground in a single set-up. As the grinding of hardened shafts often releases internal stresses and the grinding of the cams has an influence on the concentricity of the bearing surfaces, the components cannot be machined to drawing in a single pass. The frequently best way to achieve the required component quality is therefore to complete-machine the shaft in a number of operations that can be carried out in a single set-up.
Where grinding is concerned, the trend indicates that the use of CBN wheels for noncircular grinding operations is state-of-the-art technology. In future, productivity rates will be raised further with increases in the cutting speed. For less stable components with slight grinding allowances, such as moulded camshafts, the challenge will be to develop grinding systems that generate noticeably lower grinding forces. Future innovations in the area of noncircular grinding technology will be driven by increasing and constantly changing market demands.