Machinedesign 2797 Lowdown On Machine Tools 0

Designer’s Guide to Metalcutting Machinery

June 8, 2010
 A designer’s guide to metalcutting machinery.
Authored by:
Leslie Gordon
Senior Editor
[email protected]

Grasping the mysteries of metalcutting machinery eases once you understand the terminology that tends to mislead. For example, industry often refers to the entire machine itself as the “machine tool.” This can be confusing because all metalcutting machines use cutting “tools” made from hard carbide, hard steel, or diamonds. In addition, machinists sometimes refer to jigs and fixtures as “tooling.”

Some shops and machine builders make a distinction between machine tools for mass production and those for support activities such as cutting prototypes, repair parts, dies and molds, or jigs and fixtures. They often call this kind of equipment tool-room machinery.

Mass-production operations such as casting and forging are limited as far as dimensional precision and surface smoothness. Thus, the fit and finish needed for highly engineered products usually requires some sort of stock removal by machine tools.

Exceptionally rugged to endure the stress of high-volume runs, production machinery also emphasizes fast output. Parts for this kind of machining often come preshaped to their approximate final dimensions by such operations as casting or forging. A forging, for example, might just need one surface milled and a few holes drilled and tapped.

Other parts don’t come preshaped. Instead, the machine tool cuts gross amounts of metal from plate, bar stock, or weldments. These parts often have intricate geometries needing extreme precision, such as those for aerospace. In other cases, stock such as rod or bar closely approximates the final shape. Machine tools can make small circular or tubular parts in high volumes using relatively light cuts.

Industries such as automotive, consumer products, and farm equipment have large, special-purpose machines built to work in assembly-line fashion. The machines drill, ream, tap, and mill parts as they pass from station to station. Called transfer lines, the machines are often what is meant when automotive companies are said to be purchasing “tooling” for a new model.

Metal cutting can be thought of as the art and science of “chipmaking.” Knowledgeable machinists run jobs to produce the optimal chip, usually helical or comma shaped. Controlling chip size, shape, and color help machinists keep jobs running correctly. These chip parameters depend on the material being cut, the machine tool’s feed rate and depth-of-cut, and the rake angle (the angle between the chip face and a normal to the workpiece surface) of the tool bit, among other variables.

The cutting tool actually deforms some of the workpiece material plastically and then pushes the chip off. Removing unwanted material from the workpiece in this fashion eventually shapes the part.

In all conventional metalcutting (not, for example, electrical-discharge machining), either the workpiece rotates; the cutting tool rotates; or the machine tool or workpiece “translates” (moves in a flat plane). The workpiece rotates in turning and boring; the cutting tool rotates in drilling and milling; and either the cutting tool or the workpiece translates in shaping, planing, and broaching.

Workpiece rotation
Machine tools called lathes perform turning work. Lathes hold the workpiece in a gripping device in the spindle called a chuck. The workpiece rotates; the cutting tool does not.

Lathes are classified as either horizontal or vertical, depending on the position of the axis of rotation of the workpiece. One type of horizontal machine: general-purpose lathes. They run manually or via CNC to make cuts of almost any configuration. These machines further fall into the smaller classes of engine lathes or turret lathes.

Engine lathes use a cross slide to bring the cutting tool towards and away from the part and normally cut on the outside diameter of the workpiece. Turret lathes have a number of tools in an indexable mount, usually positioned to face-cut parts. The machines often have a second, smaller turret on a cross slide for OD cuts.

Some engine and turret lathes also have a support called a tailstock, which holds the workpiece at the end opposite the chuck. This allows the machining of long, slender parts such as shafts or axles.

Another kind of horizontal machine: special-purpose lathes. Cams on these machines produce specific shapes. Machines equipped with chucks are referred to as automatic chuckers, while lathes equipped with collets instead of chucks are often called screw machines or bar automatics. They shape the bar workpiece while cutting it to the correct length.

Vertical lathes differ significantly from horizontal lathes. In a vertical lathe, the workpiece rests on a horizontal table that rotates about a vertical axis. Cutting tools, often on a turret, move in from the side for turning, and down from the top for facing or boring. Vertical lathes suit large, heavy parts because the part weight helps secure it to the table.

Both horizontal and vertical lathes can have turrets holding a variety of tools which cut the workpiece in sequence via CNC. Such lathes are often called turning centers.

Tool rotation
Machines with rotating tools are also either vertical or horizontal, depending on the alignment of the spindle. A drill press, for instance, vertically plunges the rotating drill to produce holes.

Milling involves specialized tools with more than one cutting edge that moves over the workpiece. The side or the face of the milling cutter, or both, perform the major cutting action.

Bed-type milling machines have worktables that move back and forth in one linear direction. The cutting spindle mounts on a carriage that moves both horizontally and vertically. Knee-type machines, on the other hand, have worktables that move in all three axes. Bed-type machines are the most rigid.

Horizontal boring mills are similar to horizontal milling machines except the tool spindle in a boring mill mounts on an extendable quill for a deeper tool reach.

In shaping, a cutting tool translates over a stationary workpiece, producing slots or flat surfaces to close tolerances. Shaping machines are relatively small. The process suits one-of-a-kind production and tool-room work. Milling provides a better option for high-production volumes.

With planing, on the other hand, the workpiece translates under a stationary tool. Planing usually makes flat surfaces on large workpieces. Planers are less expensive than milling machines and they accurately cut shapes such as dovetails. However, milling makes for a better method when cutting a fairly large quantity of identical parts.

In broaching, the machine pushes or pulls the broach, a cutting tool with many teeth positioned along a bar, through a hole or over a surface. Broaching resembles filing except the broach’s teeth cut much more aggressively. Broaching works similar to milling or reaming, but at much greater speed. However, except for simple round through-holes or keyways, it makes sense to justify the high cost of the broaching tool by high production volumes.

Metalcutting lingo
There are numerous terms associated with machine tools. Here are a few of the most basic.

Boring enlarges a hole that has already been drilled (or cast), by means of a single-point cutting tool (or of a boring head containing several such tools).

Casting involves pouring a liquid metal into a mold that contains a hollow cavity of the needed shape and is allowed to solidify.

Facing or face cutting involves moving the cutting tool at right angles to the axis of rotation of the rotating workpiece.

Fixtures hold or support workpieces on a machine tool.

Forging shapes hot metal using localized compressive forces.

Jigs guide cutting tools to the workpiece, for instance, a jig might locate and space drilled holes.

Reaming makes existing holes dimensionally more accurate and improves their surface finish.

Single-point cutting tool or tool bit refers to a nonrotary cutting tool used in metal lathes, shapers, and planers. The tool bit is held rigidly by a toolholder while it is cutting.

In turning, a single point cutting tool parallel to the workpiece surface mounts to a toolholder on a movable cross slide.

Copyright 2010, Penton Media Inc. All rights reserved.

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