Design And Use Of Cutting Tools By Leo St Clair

Basic machine tools

Hundreds of varieties of metal machine tools, ranging in size from small machines mounted on workbenches to huge production machines weighing several hundred tons, are used in modern industry. They retain the basic characteristics of their 19th- and early 20th-century ancestors and are still classed as one of the following: (1) turning machines (lathes and boring mills), (2) shapers and planers, (3) drilling machines, (4) milling machines, (5) grinding machines, (6) power saws, and (7) presses.

Turning machines

The engine lathe, as the horizontal metal-turning machine is commonly called, is the most important of all the machine tools. It is usually considered the father of all other machine tools because many of its fundamental mechanical elements are incorporated into the design of other machine tools.

The engine lathe is a basic machine tool that can be used for a variety of turning, facing, and drilling operations. It uses a single-point cutting tool for turning and boring. Turning operations involve cutting excess metal, in the form of chips, from the external diameter of a workpiece and include turning straight or tapered cylindrical shapes, grooves, shoulders, and screw threads and facing flat surfaces on the ends of cylindrical parts. Internal cylindrical operations include most of the common hole-machining operations, such as drilling, boring, reaming, counterboring, countersinking, and threading with a single-point tool or tap.

Boring involves enlarging and finishing a hole that has been cored or drilled. Bored holes are more accurate in roundness, concentricity, and parallelism than drilled holes. A hole is bored with a single-point cutting tool that feeds along the inside of the workpiece. Boring mills have circular horizontal tables that rotate about a vertical axis, and they are designed for boring and turning operations on parts that are too large to be mounted on a lathe.

Shapers and planers

Shaping and planing operations involve the machining of flat surfaces, grooves, shoulders, T-slots, and angular surfaces with single-point tools. The largest shapers have a 36-inch cutting stroke and can machine parts up to 36 inches long. The cutting tool on the shaper oscillates, cutting on the forward stroke, with the workpiece feeding automatically toward the tool during each return stroke.

Planing machines perform the same operations as shapers but can machine longer workpieces. Some planers can machine parts up to 50 feet long. The workpiece is mounted on a reciprocating table that moves the workpiece beneath a cutting tool. This tool, which remains stationary during the cutting stroke, automatically feeds into the workpiece after each cutting stroke.

Drilling machines

Drilling machines, also called drill presses, cut holes in metal with a twist drill. They also use a variety of other cutting tools to perform the following basic hole-machining operations: (1) reaming, (2) boring, (3) counterboring, (4) countersinking, and (5) tapping internal threads with the use of a tapping attachment.

Milling machines

A milling machine cuts metal as the workpiece is fed against a rotating cutting tool called a milling cutter. Cutters of many shapes and sizes are available for a wide variety of milling operations. Milling machines cut flat surfaces, grooves, shoulders, inclined surfaces, dovetails, and T-slots. Various form-tooth cutters are used for cutting concave forms and convex grooves, for rounding corners, and for cutting gear teeth.

Milling machines are available in a variety of designs that can be classified as the following: (1) standard knee-and-column machines, including the horizontal and the vertical types; (2) bed-type or manufacturing machines; and (3) machines designed for special milling jobs.

Grinding machines

Grinding machines remove small chips from metal parts that are brought into contact with a rotating abrasive wheel called a grinding wheel or an abrasive belt. Grinding is the most accurate of all of the basic machining processes. Modern grinding machines grind hard or soft parts to tolerances of plus or minus 0.0001 inch (0.0025 millimetre).

The common types of grinding machines include the following: (1) plain cylindrical, (2) internal cylindrical, (3) centreless, (4) surface, (5) off-hand, (6) special, and (7) abrasive-belt.

Power saws

Metal-cutting power saws are of three basic types: (1) power hacksaws, (2) band saws, and (3) circular disk saws. Vertical band saws are used for cutting shapes in metal plate, for internal and external contours, and for angular cuts.

Presses

This large class of machines includes equipment used for forming metal parts by applying the following processes: shearing, blanking, forming, drawing, bending, forging, coining, upsetting, flanging, squeezing, and hammering. All of these processes require presses with a movable ram that can be pressed against an anvil or base. The movable ram may be powered by gravity, mechanical linkages, or hydraulic or pneumatic systems.

Appropriate die sets, with one part mounted on the movable ram and the matching part mounted on the fixed bed or platen, are an integral part of the machine. Punch presses stamp out metal parts from sheet metal and form the parts to the desired shape. Dies with cavities having a variety of shapes are used on forging presses that form white-hot metal blanks to the desired shapes. Power presses are also used for shearing, bending, flanging, and shaping sheet metal parts of all sizes. Power presses are made in various sizes, ranging from small presses that can be mounted on a workbench to machines weighing more than 1,000,000 pounds (450,000 kilograms).