Tech Talk
What is “Cam Grinding”?
Cam grinding is a very special form of plunge grinding. It is very complex and most accurate process. The cam to be ground is usually held between centers, or in a hydraulically operated chuck, with special steady-rests located on the bearing diameters of the cam shaft. It is the latter system which achieves the most accurate chucking of the cam. Chucking on the bearing diameters simulates the mounting of the cam shaft in the engine, and therefore guarantees higher concentricity of the base circle in relation to the bearings. The workpiece is rigidly held and mounted on a torsionally stiff, rocking table. The rocking motion of the table creates the cam profile by moving the workpiece in an dout of the grinding wheel. The rocking table is positioned by a series of master cams preset and spring-loaded onto a rotating drive. It is important that the grinding wheel maintains the desired velocity profile relative to the angle of rotation of the cam. As the cam follower, in the engine, changes velocity, the grinding wheel must effectively take the same path, although it is the cam shaft which is moving during the machining operation. In a modern cam grinder, a microprocessor receives a signal relative to the angular position of the cam as it is rotating and determines the correct velocity profile around the cam being machined. The sensor feedback to the microprocessor control not only improves the accuracy and quality of the cam, but also significantly reduces the grinding time, particularly in the spark-out phase. It is important to control the stock removal rate of the grinding wheel. As the rough cam rotates, the arc of contact will vary with the angular position of the cam with respect to the grinding wheel. If the RPM of the cam shaft were kept constant, the lobed shape of the cam would significantly change the stock removal rate as the cam shaft rotated. On a high-velocity rising section, the effective depth of cut of the grinding wheel may cause thermal damage, softening the cam surface due to excessive heat input from the overload condition. Cam grinding is a complex process to control and although, historically, success has been enjoyed with semi-manual systems, the use of a superabrasive CNC cam grinding machine is by far the best method, achieving the most accurate and consistent cams.
Cam grinding in the automotive industry is one of the most complex processes, yet it enjoys the benefits of high automation. Cam shafts are typically machined on a transfer line, which takes the rough forged or cast cam-shaft and not only completes the machining of the rotary features, cams, and bearing diameters but also inspects and records the dimensions of each workpiece automatically.
New and more efficient internal combustion engines are being designed with a reentrant cam profile. This is a profile which has concave features. Such cam profiles require the use of much smaller grinding wheel diameters to accommodate the reentrant features and are spawning a new era in cam grinders, which have been specifically designed to use small diameter superabrasive wheels.
