Milling machines
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.
Figure 7.5 – Two basic types of knee-and-column milling machine: (a) horizontal and (b) vertical.
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.
References: Gen [1-3].
Assessing questions:
What kind of process is called milling?
What types of milling operations do you know?
What types of milling machines do you know?
How can be operated CNC machines?
Tool orientation.
Lecture 8. CAM software system, objectives and structure of the software, characteristics of operating systems, algorithms and program management functions of CNC machines, post processing
Machining centers and turning centers
A machining center is a highly automated machine tool capable of performing multiple machining operations under CNC in one setup with minimal human attention. Typical operations performed on a machining center are milling and drilling, which use rotating cutting tools.
The typical features that distinguish a machining center from conventional machine tools and make it so productive include:
1. Multiple operations in one setup;
2. Automatic tool changing;
3. Pallet shuttles;
4. Automatic workpart positioning.
Machining centers are classified as horizontal, vertical, or universal.
A modern CNC turning center, is capable of performing various turning and related operations, contour turning, and automatic tool indexing, all under computer control.
In addition, the most sophisticated turning centers can accomplish (1) workpart gaging (checking key dimensions after machining), (2) tool monitoring (sensors to indicate when the tools are worn), (3) automatic tool changing when tools become worn, and even (4) automatic workpart changing at the completion of the work cycle.
Several other machining operations should be included in our survey: (1) shaping and planing, (2) broaching, and (3) sawing.
(a) example part with turned, milled, and drilled surfaces; and (b) sequence of operations on a mill-turn center: (1) turn second diameter, (2) mill flat with part in programmed angular position, (3) drill hole with part in same programmed position, and (4) cutoff.
Figure 8.1 – Operation of a mill-turn center:
Shaping and planing are similar operations, both involving the use of a single-point cutting tool moved linearly relative to the workpart. In conventional shaping and planing, a straight, flat surface is created by this action. The difference between the two operations is illustrated in Figure 8.2. In shaping, the speed motion is accomplished by moving the cutting tool; while in planing, the speed motion is accomplished by moving the workpart.
Figure 8.2 - (a) Shaping, and (b) planing
Shaping is performed on a machine tool called a shaper. The components of the shaper include a ram, which moves relative to a column to provide the cutting motion, and a worktable that holds the part and accomplishes the feed motion.
The machine tool for planing is a planer. Cutting speed is achieved by a reciprocating worktable that moves the part past the single-point cutting tool. Planers can be classified as open side planers or double-column planers. The open-side planer, also known as a single-column planer.
Broaching is performed using a multiple-teeth cutting tool by moving the tool linearly relative to the work in the direction of the tool axis, as in Figure 8.3. The machine tool is called a broaching machine, and the cutting tool is called a broach
Figure 8.3 – The broaching operation
Sawing is a process in which a narrow slit is cut into the work by a tool consisting of a series of narrowly spaced teeth. Sawing is normally used to separate a workpart into two pieces, or to cut off an unwanted portion of a part. These operations are often referred to as cutoff operations.
In most sawing operations, the work is held stationary and the saw blade is moved relative to it. According to the type of blade motion involved: (a) hacksawing, (b) bandsawing, and (c) circular sawing (Figure 8.4).
Figure 8.4 – Three types of sawing operations: (a) power hacksaw, (b) bandsaw (vertical), and (c) circular saw.