Part program verification. Operating a CNC machine. CNC planning. Controller alarms. Operator actions. Monitoring and adjusting.
Milling is a machining operation in which a workpart is fed past a rotating cylindrical tool with multiple cutting edges. The axis of rotation of the cutting tool is perpendicular to the direction of feed. The cutting tool in milling is called a milling cutter and the cutting edges are called teeth. The geometric form created by milling is a plane surface.
There are two basic types of milling operations, shown in Figure 7.1: (a) peripheral milling and (b) face milling.
Figure 7.1 Two basic types of milling operations: (a) peripheral or plain millingand (b) face milling.
Peripheral milling. In peripheral milling, also called plain milling, the axis of the tool is parallel to the surface being machined, and the operation is performed by cutting edges on the outside periphery of the cutter. Several types of peripheral milling are shown in Figure 7.2: (a) slab milling, the basic form of peripheral milling in which the cutter width extends beyond the workpiece on both sides; (b) slotting, also called slot milling, in which the width of the cutter is less than the workpiece width, creating a slot in the work—when the cutter is very thin, this operation can be used to mill narrow slots or cut a workpart in two, called saw milling; (c) side milling, in which the cutter machines the side of the workpiece; (d) straddle milling, the same as side milling, only cutting takes place on both sides of the work; and form milling, in which the milling teeth have a special profile that determines the shape of the slot that is cut in the work.
In peripheral milling, the direction of cutter rotation distinguishes two forms of milling: up milling and down milling, illustrated in Figure 7.3. In up milling, also called conventional milling, the direction of motion of the cutter teeth is opposite the feed direction when the teeth cut into the work. It is milling ‘‘against the feed.’’ In down milling, also called climb milling, the direction of cutter motion is the same as the feed direction when the teeth cut the work. It is milling ‘‘with the feed.’’
Figure 7.2 – Peripheral milling: (a) slabmilling, (b) slotting, (c) side milling, (d) straddle milling, and (e) form milling.
Figure 7.3 - Two forms of peripheral milling operation with a 20-teeth cutter: (a) up milling, and (b) down milling.
Face milling. In face milling, the axis of the cutter is perpendicular to the surface being milled, andmachining is performed by cutting edges on both the end and outside periphery ofthe cutter. As in peripheral milling, various forms of face milling exist, several of which are shown in Figure 7.4: (a) conventional face milling, in which the diameter of the cutter is greater than the workpart width, so the cutter overhangs the work on both sides; (b) partial face milling, where the cutter overhangs the work on only one side; (c) end milling, in which the cutter diameter is less than the work width, so a slot is cut into the part; (d) profile milling, a form of end milling in which the outside periphery of a flat part is cut; (e) pocket milling, another form of end milling used to mill shallow pockets into flat parts; and (f) surface contouring, in which a ball-nose cutter (rather than square-end cutter) is fed back and forth across the work along a curvilinear path at close intervals to create a three dimensional surface form.
(a) conventional face milling, (b) partial face milling, (c) end milling, (d) profile milling, (e) pocket milling, and (f) surface contouring.
Milling machines must provide a rotating spindle for the cutter and a table for fastening, positioning, and feeding the workpart. Milling machines can be classified as horizontal or vertical. A horizontal milling machine has a horizontal spindle, and this design is well suited for performing peripheral milling (e.g., slab milling, slotting, side and straddle milling) on workparts that are roughly cube shaped. A vertical milling machine has a vertical spindle, and this orientation is appropriate for face milling, end milling, surface contouring, and diesinking on relatively flat workparts.
Other than spindle orientation, milling machines can be classified into the following types: (1) knee-and-column, (2) bed type, (3) planer type, (4) tracer mills, and (5) CNC milling machines.
A horizontal or a vertical machines illustrated in Figure 7.5. In the horizontal version, an arbor usually supports the cutter. The arbor is basically a shaft that holds the milling cutter and is driven by the spindle. An overarm is provided on horizontal machines to support the arbor. On vertical knee-and-column machines, milling cutters can be mounted directly in the spindle without an arbor.
One of the features of the knee-and-column milling machine that makes it so versatile is its capability for worktable feed movement in any of the x–y–z axes. The worktable can be moved in the x-direction, the saddle can be moved in the y-direction, and the knee can be moved vertically to achieve the z-movement.
Figure 7.6 – Special types of knee-and-column milling machine: (a) universal—overarm, arbor, and cutter omitted for clarity: and (b) ram type.
The universal milling machine, Figure 7.6(a), which has a table that can be swiveled in a horizontal plane (about a vertical axis) to any specified angle. This facilitates the cutting of angular shapes and helixes on workparts. The ram mill, Figure 7.6(b), in which the toolhead containing the spindle is located on the end of a horizontal ram; the ram can be adjusted in and out over the worktable to locate the cutter relative to the work.
Operating of CNC
Compensated program. A program prepared to look for offsets.
Cove. An internal detail that sets the upper limit for cutter radius.
Cutter centerline (cutter path). A less common compensated program, where a given cutter radius has been built into the path but can be compensated with offsets. See minus compensation.
Flank interference. A compensation challenge on lathes whereby the control must keep the sides of the tool from touching the workpiece.
Generating. A curved cutter path that creates a part radius larger than the cutter radius. The motion preferred to forming.
Minus compensation. A negative offset number applied to a cutter centerline program to bring the cutter closer to the part geometry.
Offset. A variable number entered into controller tool memory by the operator. The program refers to the offset by the tool number or from a code word in the program.
Optional stop (opt stop). Given the right halt code in the program, the operator can choose to halt at certain places or to switch them off and run through the halt.
Part path program. A program based on positive compensation offsets away from the shape of the part. The more common program type.
Radius offset. The tangent distance away from the part geometry for lathe or mill cutters.
Remarks. Notes embedded within the program.
Tool bias/approach vector. Defines the lathe tool’s orientation in the setup, thus the way it must move toward the workpiece and away during compensation.
Tool orientation. The direction the lathe tool is pointing.