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302 CAM DESIGN HANDBOOK

stress raisers. Incipient cracks emanate from these inclusions and enlarge and propagate
under repeated stresses, forming a network of cracks that develops into a fatigue spall or
pit. Inclusion location with respect to the maximum shear stress is of primary importance,
as are size and orientation.

10.7.1 Melting Practice
Both sufficient data and practical experience exist which indicate that the use of vacuum-
melted materials, and, specifically, CVM, can increase cam surface-pitting fatigue life
beyond that obtainable by air-melted materials. However, it is recommended that conser-
vative estimates for life improvements be considered, such as a factor of 3 for CVM pro-
cessing and a factor of 10 for VIM-VAR processing.
Data available on other melting techniques such as a vacuum induction, vacuum
degassing, and electroslag remelting indicate that the life improvement approaches or
equals that of the CVM process. However, it is also important to differentiate between
CVM and CVD (carbon vacuum degassing). The CVM process yields cleaner and more
homogenous steels than CVD does.

10.7.2 Heat Treatment
Heat treatment of cams is employed to improve cam surface-fatigue life. At present, no
controls exist over heat treatment to produce a consistent, finished product. The thermal
cycle is left to the individual producer for grain size and hardness ranges. Hardness is dis-
cussed, but in the case of grain size, lack of definitive data precludes discussing here.
Heat treatment is accomplished by furnace hardening, carburizing, induction harden-
ing, or flame hardening. Cams are either case-hardened, through-hardened, nitrided, or
precipitation-hardened for the proper combination of roughness and hardness.
Heat-treatment distortion must be minimized if the cam is to have increased service
life. Several hardening techniques have proved useful. For moderate service life increases,
the cams are hardened but kept within the range of machinability so that distortion pro-
duced by heat treatment can be machined away.
Where maximum durability is required, surface hardening is necessary. Carburizing,
nitriding, and induction hardening are generally used. However, precision cams can be
ensured only by finishing after hardening. Full-contour induction hardening is an eco-
nomical and effective method for surface-hardening cams. The extremely high but local-
ized heat allows small sections to come to hardening temperatures while the balance of
the gear dissipates heat. Thus, major distortions are eliminated. Conventional methods
such as flame hardening can be employed.
Nitriding is a satisfactory method of hardening small-and medium-size cams. Distor-
tion is minimal because furnace temperatures are comparatively low. The hardening pattern
is uniform but depth of hardness is limited. Best results are achieved when special mate-
rials, suited to nitriding, are specified. Also, cam contour cross-sections should be thick
enough to withstand distortion from heat treatment. Figure 10.14 shows the plasma/ion
nitriding of a cast iron cam drive for the graphic arts industry. The glow is a result of the
iron nitriding process. The thin dark line is a thermocouple wire utilized in the process.

10.7.3 Metalworking

Proper grain flow or fiber orientation can significantly affect pitting-fatigue life. Proper
fiber orientation can be defined as grain flow parallel to the cam shape. Standard forging
of cams as opposed to machining a forged disk is one way of obtaining proper fiber