The term "eccentric" is given to a rotating machine part which is used to "throw" a mechanism eccentric with its main center line. Eccentrics may be said to include all crank motions, also many cam motions. In general shop terms, however, an eccentric is a machine part having an outer circle which is off center or eccentric with its shaft.

In construction it may be machined as a part of its own shaft, or it may be so made as to slip onto a shaft in which case provision must be made for keying it to the shaft.

Throw of Eccentric

The throw of an eccentric may be taken as the radius of eccentricity or it may He taken to mean twice the radial eccentricity.

Machining Eccentrics

While eccentrics may be machined in a variety of ways, the accompanying text will consider the lathe only. If the eccentric is of the simple form of two circles with their centers offset in relation to each other, the work must be done on a mandrel provided with two sets of centers, one pair for each circle, Fig. 149.

Fig. 148. Machining Crank Shaft.

Eccentric Solid with Shaft

In this case if the throw of the eccentric is less than the radius of the shaft, both sets of work centers may be made in the shaft ends. Where the throw is too great to allow this, some provision must be made for the second set of centers.

Two methods for doing this are in common use, (a) casting or forging lugs upon each end of the shaft sufficiently large to include the needed centers, (b) use of attachments for the shaft ends, the attachments themselves being provided with the desired centers. Eccentrics Not Solid with Shaft. Eccentrics of this sort are usually those which have a hole chucked through their center of throw. Such eccentrics are usually finished upon mandrels having two sets of centers. Fig. 150 shows such a mandrel. Work centers

Fig. 149. General Shape of Eccentric.

A and A' are those to be used while the throw surfaces are being machined. B and B' the centers used while constructing the mandrel. With such a mandrel as this driven into the provided hole, work can be done upon surfaces which are concentric to the axis of the mandrel or which are eccentric with it.

Fig. 150. Mandrel for Holding an Eccentric.

Using a Faceplate or Work Chuck, Eccentrics can and often are machined by mounting them upon a suitable faceplate or by holding the work in a suitable chuck. Previous to mounting the work upon the faceplate for eccentric turning, it is usual to face off a surface to set squarely on the front face of the plate, as in Fig. 151.