Page 298 - Cam Design Handbook

THB10 9/19/03 7:28 PM Page 286

286 CAM DESIGN HANDBOOK

Let us consider the overall machine tool system next. The kinds and ages of the machine
tools are important factors. New equipment always functions with greater precision than
old equipment. Also, in cutting cams, the cams’ surface and accuracy are to be controlled.
Another factor is the cam pressure angle effect on the machine tool-cutting accuracy due
to the elastic deflection of the parts during machining. Furthermore, the cutting increments
vary according to the pressure angle of the cam being machined. In addition, the tool cutter
material and hardness should be selected for the cam material being produced, and the
tool cutter life and wear should be known at all times. Many other factors are not included
here. One might say that the making of a cam approaches that of an art. In somewhat tech-
nical abstraction, one could relate that the cam fabricating machine tool cutting action is
a dynamic elastic system performing in a fundamentally similar manner to the operation
of the cam-follower system discussed in this book. In both cases, experience is necessary
to understand and control performance.
The main areas of concern in this chapter are:
• manufacturing
• inspection
• assembly

Inspection and assembly are discussed in Sec. 10.3, CNC cams.
The method employed to produce a particular cam is a function of the cam’s contour,
application, and quantity of production. One would certainly not manufacture a cam for
a simple clamping fixture using the same degree of precision and wear characteristics as
those used for a high-speed textile machine cam. Present-day common cam manufactur-
ing methods can be categorized as:
• manual or numerical control (NC) machining to cam coordinates
• analog duplication of a hand-dressed master cam
• computer numerical control (CNC) with linear, circular, spherical, or Bezier curve
interpolation (Special computer software is usually developed for this purpose.)
• electrodischarge machining (EDM)
• miscellaneous methods, such as flame cutting, die casting, die forging, stamping, and
powder metallurgy

In the first three methods, the cam profiles are formed on prepared cam blanks by the
appropriate machine tool, such as milling and grinding machines. Heat-treated steel cams
usually require a grinding operation to correct geometric distortions. Figure 10.1 shows
milled and ground cam surfaces at 100¥ magnification. Standard milling machines are
accurate to ±0.001in tolerance range. High-quality tooling, expensive jig borers, and
grinders are accurate from 0.001 to 0.0005in. Sometimes, the final stage of a cam-making
operation consists of the machinist lightly “touching up” the cam and removing the burrs
from the edges.
In manual or numerical control (NC), the cam is mounted on a milling machine or pre-
cision jig borer for the cutter to plunge cut the cam from tabulated data. This step-by-step
process creates finely scalloped surfaces and hand finishing is necessary to remove the
asperities.
Electrodischarge machining (EDM) utilizes a hot wire to electrically cut the cam profile
while the cam plank is fixed on the CNC table. Electrodischarge machining is a process
similar to CNC and allows a prehardened cam blank to be machined to size without sig-
nificant grinding. The practice is to EDM leaving approximately 0.003in grinding stock.
The local hardening effect of the hot wire and the fact that the wire can have a tendency