Grinding concave cam curves.
Two new CNC camshaft grinders at Navistar International Corp, Melrose Park, IL, are doing a job none of the previous grinders could do: finish grind cam lobes with negative curvature. A new camshaft design was necessary for new six-cylinder, 466-cu-in diesel engines to meet 1988 emission requirements.
The newly configured cam lobes have a base circle, flank, and nose. The flanks are now concave, not straight or convex as before. Cam shape is critical because it controls valve-lifter speed and regulates the amount of pollutants dispersed into the atmosphere. This negative curvature cannot be done with conventional grinders without extensive modifications. It required the design of a new line of 20-hp Landis 3LR CNC grinders with re-entrant grinding capabilities. This is the first installation of these machines produced by the Grinding Machines Unit of Litton Industrial Automation Systems Inc, Waynesboro, PA.
There are 12 cams on each 34.4" shaft, each measuring 1.5" from base circle to nose. Material is 1046 steel forgings induction hardened to between 57 and 63 Rockwell C and weight is 25 lb each. Cycle time is 8.5 min or 42 sec per lobe. Running two shifts, the two machines grind 112 shafts per day, with one operator loading/unloading both machines.
Rough grinding on an existing grinder takes off 0.025" stock before the camshaft is transferred to one of the new machines. There, a semi-finish pass takes off 0.018", followed by a 0.002" finish pass for each cam, after a 3-sec automatic dress by a diamond dressing tool. Cam surface finish is 17-20 RMS.
Wheel speed and work speed are variable, fastest around the base circle, slower around the re-entrant flank profile where considerable stock is removed, and then faster around the nose where contact is minimal. Seven work-speed changes occur for each cam revolution, the fastest being 98 rpm. Wheel speed automatically adjusts for wheel diameter (from 16" to 9.5" dia) to maintain constant sfm. Wheel changes are about every 700 camshafts, with the machine alerting the operator on the cycles remaining before the wheel needs changing.
Randomly selected camshafts--three per shift--are audited on a cylindrical coordinate measuring machine, inspecting base-circle radius, base-circle runout, cam-lobe taper, timing, and lift error. Surface-finish checks are done on other equipment. Cams are acid-etched to check for burn.
Some of design changes required of the Landis grinders: * Downsizing the wheel from 24" to 16" required a new grinding head. * Traditional hard master cam was replaced with servo drives, a ball screw under the wheel head, and a very sophisticated CNC. * A custom-designed four-axis control to monitor and control the more complex motions, speeds, and feeds, including wheel-head movement in increments of 0.000 025". Software allows camshafts, model cams, and master cams to be generated direct from component lift figures.
"Flexibility is one of the main reasons the Landis 3LR grinders were chosen," explains Sylvester Sample, Navistar process engineer. "Although finish grinding is their primary function, they also are programmed to both rough and finish grind camshafts if one of the existing rough grinders is down for maintenance." The new machines are also programmed to grind all of Navistar's camshafts, not just those with the re-entrant profile.
Before shipment, each grinder had to pass a consecutive 30-piece six-sigma SPC runoff with 100-percent inspection, plus a stringent 50-piece SPC runoff upon arrival. A [C.sub.pk] index of 1.33 or greater was required--and easily met--with some achieving a [C.sub.pk] of 5.39.
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|Publication:||Tooling & Production|
|Date:||Oct 1, 1989|
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