Machined parts can be classified as rotational or nonrotational (Figure 3.1). A rotational workparth as a cylindrical or disk-like shape. The characteristic operation that produces this geometry is one in which a cutting tool removes material from a rotating workpart. Examples include turning and boring. A nonrotational (also called prismatic) workpart is block-like or plate-like, as in Figure 3.1(b). This geometry is achieved by linear motions of the workpart, combined with either rotating or linear tool motions. Operations in this category include milling, shaping, planing, and sawing.
Figure 3.1 – Machined parts are classified as (a) rotational, or (b) nonrotational, shown here by block and flat parts.
A part shape is created as generating and forming. In generating, the geometry of the workpart is determined by the feed trajectory of the cutting tool. Examples of generating the work shape in machining include straight turning, taper turning, contour turning, peripheral milling, and profile milling, all illustrated in Figure 3.2.
Figure 3.2 – Generating shape in machining: (a) straight turning, (b) taper turning, (c) contour turning, (d) plain milling, and (e) profile milling.
In forming, the shape of the part is created by the geometry of the cutting tool. Form turning, drilling, and broaching are examples of this case. In these operations, illustrated in Figure 3.3, the shape of the cutting tool is imparted to the work in order to create part geometry.
Forming and generating are sometimes combined in one operation, as illustrated in Figure 3.4 for thread cutting on a lathe and slotting on a milling machine. In thread cutting, the pointed shape of the cutting tool determines the form of the threads, but the large feed rate generates the threads. In slotting (also called slotmilling), the width of the cutter determines the width of the slot, but the feed motion creates the slot.
Machining is classified as a secondary process. In general, secondary processes follow basic processes, whose purpose is to establish the initial shape of a workpiece. Examples of basic processes include casting, forging, and bar rolling (to produce rod and bar stock). The shapes produced by these processes usually require refinement by secondary processes. Machining operations serve to transform the starting shapes into the final geometries specified by the part designer. For example, bar stock is the initial shape, but the final geometry after a series of machining operations is a shaft.
Figure 3.3 – Forming to create shape in machining: (a) form turning, (b) drilling, and (c) broaching.
Figure 3.4 – Combination of forming and generating to create shape: (a) thread cutting on a lathe, and (b) slot milling.