Page 305 - Cam Design Handbook

THB10 9/19/03 7:28 PM Page 293

CAM MANUFACTURING 293

• variable backlash errors
• miscellaneous errors

10.4.1 Structural Errors
Structural errors are mistakes in the dimension of the part or the utilization of wrong parts
in building the machine. Another error of this type is one in the computer programming
data, for example, in the case of a cam driving an oscillating follower. If the lever were
the wrong length it would give the follower the wrong information because it would rest
on the improper part of the cam curve.

10.4.2 Fixed Backlash Errors
Most mechanisms have clearances necessary for assembly and operation, i.e., clearance
between gear teeth and clearance between the roller follower and the cam groove. The
smallest necessary clearance will result in a minimum backlash error. For cam-follower
systems, backlash is a major source of error and the designs should be established to reduce
it as much as possible.

10.4.3 Variable Backlash Errors

Variable backlash errors are due to the additional clearance caused by tolerances. Toler-
ances are the differences between the limits of a dimension. Tolerances are assigned so
that even when not made to ideal dimensions parts still assemble and operate satisfacto-
rily. Statistically, if the tolerances controlling the amount of this additional clearance are
normally distributed, then the amount of additional backlash will also be normally dis-
tributed. In this case the total backlash error is a combination of fixed and probable values.

10.4.4 Miscellaneous Errors
Cyclic errors are associated with the eccentricity of rotating elements, the wobble of the
cam faces, etc. The amplitude of cyclic errors is normally distributed.
Cam profile errors are deviations from the theoretical cam curve. Depending on the
installation, speeds, and load, this surface error may produce operational difficulties of
excessive noise and wear of the parts. These errors can be produced by an incorrect setting
of the milling cutter, by a file scratch, or by holding the grinding wheel at a point on the
cam for too long a time. Errors on the cam are often observed at points where the milling
cutter or grinding wheel starts or stops as it travels around the cam. Also, the grinding
machine may chatter in grinding which may affect the cam. In addition, the start and stop
of the cutter or grinder as it sweeps in cutting around the cam contour sometimes leaves
an error on the cam. This is often seen on acceleration tests of the cam profile as a “blip.”
Most cams require no operation after the jig borer, milling, or grinding machine cutting
stages. Others may occasionally need hand filing or stoning of scallops or flats, especially
for the master NC cams. Any hand operation is subject to error and is completely depend-
ent on the operator’s skill and experience.
Waviness in the cam surface is a periodic imperfection or error. It is a uniform distri-
bution of high and low points of longer duration than roughness. Waviness of a surface is
the vertical distance between peaks and valleys of relatively long wavelengths. It may be