A model camshaft grinding process: camshaft lobe grinding is tricky business. A new computer modeling technique aims to make it more predictable by identifying potential areas of thermal damage before wheel meets lobe in order to generate the fastest possible work speed and throughput without burning parts

Modern Machine Shop, Dec, 2004 by Derek Korn

Process optimization is an essential exercise for today's evolving and adapting machine shop. Increasing international competition, short part runs and need-it-yesterday delivery requirements demand it.

Unfortunately, optimizing a camshaft lobe grinding process has never been cut and dry.

Its degree of success has largely depended upon operator experience and gut instincts. Computer programs, which take into account known machine dynamic constraints and lobe profile to suggest an "in-the-ballpark" work speed, do exist. Still, many test grinding iterations, paired with opining skilled operators, have been required to dial-in the process. And in cases where lobe burning occurred, some manufacturers chose to decrease the wheel feed increment, while others slowed the work speed. These sort of "seat-of-the-pants" changes usually eliminated grinder burn, but didn't necessarily yield an optimized process.

Digital modeling is an optimization tool that product designers have relied on for years to refine new designs and assemblies. The technique is increasingly finding favor among manufacturers hoping to fine-tune their metalworking processes. Such a predictive computer software tool gives them the opportunity to immediately see the results after adjusting process variables (playing out "what if" scenarios) while removing some process guesswork and trial and error before any material is removed from a part.

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A digital modeling tool for camshaft lobe grinding is now available. New thermal modeling software takes existing computer work speed generation programs for camshaft grinding a step further, actually predicting heat amount, location on lobe perimeter and depth reached under the lobe surface during grinding. Likely problem areas can immediately be identified from simple colored graphs, and process variables can be massaged to determine the fastest possible work speed that won't thermally damage the lobe. Landis Grinding Systems (Waynesboro, Pennsylvania) has added such a thermal modeling module to its Tetra4000 camshaft grinding analysis program. The program is available for use with the company's 3L CNC camshaft lobe grinders that have linear motor wheel feed drive.

This timely modeling tool comes at a critical juncture, as increasing numbers of skilled workers are reaching retirement age. Not only does thermal modeling optimize the camshaft grinding process, but it also serves as an educational tool for, and bridge between, camshaft designers and manufacturers.

Problematic Profiles

Camshaft lobe grinding presents processing difficulties not found in concentric grinding operations. The contact area between the wheel and lobe (known as arc of contact) is continuously changing as the wheel passes around the perimeter of the lobe. Contact area is greatest in the relatively flat flank area, versus a camshaft's more rounded base circle and nose. For that reason, it is in the flank areas that burning is most likely to occur, and where many manufacturers slow the work speed to prevent it from happening. However, educated guesswork has typically dictated how much to reduce the work speed.

Lobe geometries are also becoming more complex. Many of today's roller camshafts have a re-entrant (concave) profile in the flank areas. This feature, also referred to as a negative radius of curvature (NROC), is designed to optimize valve opening and closing for greater engine power and reduced emissions. However, it introduces additional changing contact areas, making an already difficult grinding process even more hairy. In addition, new roller camshafts experience higher contact stresses than previous designs, which means thermal damage must be closely watched.

Lobe grinding is typically divided into roughing and finishing stages, even though they occur during one processing cycle. The purpose of roughing is to remove as much material as possible. Here, the concern about thermal damage isn't as great, because successive roughing passes are taken at a wheel infeed depth that is sufficiently deep enough to remove any previously damaged material layers. However, in the final roughing passes, thermal damage must not be so deep that finishing passes at a smaller wheel infeed can't remove it.

Parameter Input

On the surface, the exercise of modeling a manufacturing process as intricate as camshaft grinding might not seem to be intuitive. However, it is a relatively simple process of entering known and published values for machine, wheel and coolant.

The thermal modeling module piggybacks onto an existing work speed optimization and acceleration smoothing program, which considers traditional machine performance variables, in addition to material removal rate and lobe lift profile. Camshaft designers provide lift profiles in terms of the amount of lift per degree around the perimeter of the lobe. Grinder manufacturers provide data about machine dynamic limits such as wheel feed acceleration and jerk, and headstock velocity and jerk.