A dominant Mid-Western manufacturer of cam and crankshafts for large diesel and locomotive engines was faced with a challenge from one of their major customers. The requirement was for a new camshaft design and size that could not be ground on their existing equipment. They decided to look to machine tool rebuilders for someone that could rebuild and modify an existing manual cam grinder, to accommodate the larger diameter camshaft. As part of the rebuild, they also wished to have the machine converted to CNC control. Traditional cam grinders employ a master cam and rocking table to grind a cam shaft. A master cam is a pattern which causes the table holding the workpiece to rock into and away from the grinding wheel, creating the lobeshape. A master cam must be created for each different cam design, at a substantial cost. The CNC retrofit was intended to automate the wheel infeed, table traverse and wheeldressing axes, while leaving the mechanical rocking mechanism in place.
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As electrical integrator for this project, DRV proposed a control system with servo motors to automate these axes, and also to recreate the cam profile electronically, eliminating the mechanical rocker and master cam. The proposed control system was based on a Giddings & Lewis motion control system, with a touch screen operator interface and software designed specifically for a grinding operation.
After the contract was awarded, DRV began engineering the control system. Servo motors were sized based on leadscrew, gearing and mechanical data. A variable speed AC motor and drive combination was selected to enable the grinding wheel speed to be controlled for constant surface speed. The variation in wheel sizes required a turndown of roughly 1.5:1. The Giddings & Lewis system would control all five axes of motion as well as the wheel speed. The mechanical rocker was to be replaced by using a standard cam-follower function. The cam follower would electronically slave the wheel infeed to the rotation of the camshaft, allowing incremental lift distances to be programmed for every 1/2 degree of shaft rotation.
The key to the electronic recreation of the mechanical motion is to understand and compensate for the actual point at which the grinding wheel is touching the workpiece during rotation. Lift data from blueprints had to be translated into incremental moves of the wheel infeed axis. The large difference in diameters between the grinding wheel and the cam lobe creates a special problem of its own. Since the lobe is not circular, the point of contact is not always on center line of the wheel and workpiece. The theoretical surface contact points had to be calculated. In determining these lift points, the G&L system performs no less than 45 million individual calculations for each profile. The phenomenon of variable surface speed passing the wheel as the workpiece moves off base circle on to the ramp of the lobe also had to be examined. This variation can be quite considerable and must be compensated for so as not to create burning or chatter on the lobe.
In order to make the machine suitable for a production operation, the cam profiles for the cam follower would be downloaded from recipes stored in the control. Initial data from blueprints is input to a standard spreadsheet program on a PC. This data includes profile data, lobe spacing, phase angle and base radius of the lobes. Additionally, the number of lobes, what type (injector, exhaust etc.) and the order to be ground is entered. Up to six different profiles can exist for anyone recipe, and a shaft may have up to 18 individual lobes. Once all data has been entered into the spreadsheet, it is run through a small program to convert it to a file format for transfer to the control. This file is then downloaded to the control via a serial connection.
The control system can store up to 10 recipes on an internal Ram-Disk. The recipe files can be viewed, deleted and loaded from the File Manager screen. Once loaded into memory, the profiles are recalculated based on the current grinding wheel size. Any time a wheel is dressed, the profiles are automatically recalculated. Manual modes of operation are provided, for machine setup, and axis homing. A Setup Screen is provided for entering grind parameters such as roughing and finishing increments, dwell time, and wheel and workpiece speeds.
Once a shaft has been placed on the machine and referenced, the operator may go to the Automatic Screen. Here, one may choose to single step through the grinding cycle, or go to full automatic operation. The Table will index to each successive lobe, and the wheel will infeed to within a specified distance of the maximum lift for that lobe. The cam profile will then begin to execute, and infeeding will commence. Under full automatic operation, each lobe is ground to the designated profile, in the selected order.
