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

* Thermal partition constant--This is a percentage of the amount of heat energy that will go into the part versus heat removed by the coolant. The thermal partition constant differs for oil- and water-based coolants.

* Feed increment--This is the depth that the wheel plunges into the lobe for each new pass (usually at the nose where wheel/lobe contact area is smallest), and it will differ for roughing and finishing passes. The wheel does not gradually feed inward in a spiral fashion down to that increment depth, but rather it immediately feeds in that increment amount and will maintain that depth around the entire lobe. This feed increment typically is larger for roughing cycles than finish cycles, and it is directly related to the material removal rate.

Telling Temperatures

There are two types of color plots that the thermal model generates. One depicts the actual lobe shape, with colored temperature bands that show the thermal distribution under the surface of the lobe during one camshaft revolution (shown in Figure 1 on page 66). Temperature spikes toward the center of a lobe show where problematic areas are located.

Another plot illustrates temperature distribution in terms of a graph of camshaft rotation in degrees versus surface depth. This graph shows to what depth various temperatures reach into the lobe at a particular point around the perimeter of the lobe (Figure 2 on page 68 is an example of this graph for one camshaft revolution).

Typically the entire grinding cycle is plotted for analysis, including roughing and finishing passes (the software is capable of modeling up to 20 camshaft revolutions). Benchmark data dictates at what temperature levels and penetration depths the process should be held, depending on camshaft material. After evaluating the heat values and penetration depth, parameters can be adjusted and the grinding finish cycle can be determined to ensure that any previously thermally damaged layers are removed.

For camshafts that have re-entrant profiles, two grinding cycles would be modeled. One model would be generated for roughing the overall lobe shape with a large-diameter wheel, and another for a small-diameter wheel (typically 80 percent of the re-entrant diameter) for finishing and grinding the re-entrant profile, in order to complete both roughing and finishing on one grinder requires a machine with a subspindle for the small wheel.

Bridging Engineering Disciplines

Thermal modeling for camshaft grinding helps join design and manufacturing engineers to deliver optimal camshaft design and manufacturing processes. Manufacturing engineers' prime concerns revolve around throughput, productivity and quality essentially how to make a good part as quickly as possible. The camshaft design engineer must make decisions about material type and lobe profile based on camshaft loading conditions. Metallurgists may also enter into the picture, having concerns about residual stresses and the amount of heat that occurs during grinding. Thermal modeling allows the manufacturing engineer to model a new camshaft design and report back to the designer and metallurgist what the model predicts will happen to the camshaft during grinding, and whether this is acceptable based on material and design.