INTRODUCTION TO INTRODUCTION TO MACHINING PROCESSESMACHINING PROCESSES

DR. S. BHATTACHARYADR. S. BHATTACHARYA

What is Machining Operation?What is Machining Operation?

• Machining is any of various processes in Machining is any of various processes in which a piece of raw material is cut into a which a piece of raw material is cut into a desired final shape and size by a controlled desired final shape and size by a controlled material-removal process. The many material-removal process. The many processes that have this common theme, processes that have this common theme, controlled material removal, are today controlled material removal, are today collectively known as subtractive collectively known as subtractive manufacturing, in distinction from processes manufacturing, in distinction from processes of controlled material addition, which are of controlled material addition, which are known as additive manufacturing. known as additive manufacturing.

• The three principal machining processes are The three principal machining processes are classified as turning, drilling and milling. Other classified as turning, drilling and milling. Other operations falling into miscellaneous categories operations falling into miscellaneous categories include shaping, planing, boring, broaching and include shaping, planing, boring, broaching and sawing sawing

• Turning operations are operations that rotate the Turning operations are operations that rotate the workpiece as the primary method of moving metal workpiece as the primary method of moving metal against the cutting tool. Lathes are the principal against the cutting tool. Lathes are the principal machine tool used in turning.machine tool used in turning.

• Milling operations are operations in which the Milling operations are operations in which the cutting tool rotates to bring cutting edges to bear cutting tool rotates to bring cutting edges to bear against the workpiece. Milling machines are the against the workpiece. Milling machines are the principal machine tool used in milling.principal machine tool used in milling.

• Drilling operations are operations in which holes Drilling operations are operations in which holes are produced or refined by bringing a rotating are produced or refined by bringing a rotating cutter with cutting edges at the lower extremity cutter with cutting edges at the lower extremity into contact with the workpiece. Drilling operations into contact with the workpiece. Drilling operations are done primarily in drill presses but sometimes are done primarily in drill presses but sometimes on lathes or mills.on lathes or mills.

• Miscellaneous operations are operations that Miscellaneous operations are operations that strictly speaking may not be machining operations strictly speaking may not be machining operations in that they may not be swarf producing in that they may not be swarf producing operations but these operations are performed at a operations but these operations are performed at a typical machine tool. Burnishing is an example of typical machine tool. Burnishing is an example of a miscellaneous operation. Burnishing produces a miscellaneous operation. Burnishing produces no swarf but can be performed at a lathe, mill, or no swarf but can be performed at a lathe, mill, or drill press.drill press.

• Burnishing is the plastic deformation of Burnishing is the plastic deformation of a surface due to sliding contact with a surface due to sliding contact with another object. Visually, burnishing another object. Visually, burnishing smears the texture of a rough surface smears the texture of a rough surface and makes it shinier. Burnishing may and makes it shinier. Burnishing may occur on any sliding surface if the occur on any sliding surface if the contact stress locally exceeds the yield contact stress locally exceeds the yield strength of the material. strength of the material.

Overview of Machining TechnologyOverview of Machining Technology• There are many kinds of machining operations, each of which There are many kinds of machining operations, each of which

is capable of generating a certain part geometry and surface is capable of generating a certain part geometry and surface texture.texture.

• In In turningturning, a cutting tool with a single cutting edge is used to , a cutting tool with a single cutting edge is used to remove material from a rotating workpiece to generate a remove material from a rotating workpiece to generate a cylindrical shape. The speed motion is provided by rotating the cylindrical shape. The speed motion is provided by rotating the workpiece, and the feed motion is achieved by moving the workpiece, and the feed motion is achieved by moving the cutting tool slowly in a direction parallel to the axis of rotation cutting tool slowly in a direction parallel to the axis of rotation of the workpiece.of the workpiece.

• DrillingDrilling is used to create a round hole. It is accomplished by a  is used to create a round hole. It is accomplished by a rotating tool that typically has two or four helical cutting edges. rotating tool that typically has two or four helical cutting edges. The tool is fed in a direction parallel to its axis of rotation into The tool is fed in a direction parallel to its axis of rotation into the workpiece to form the round hole.the workpiece to form the round hole.

• In In boringboring, a tool with a single bent pointed tip is advanced into , a tool with a single bent pointed tip is advanced into a roughly made hole in a spinning workpiece to slightly enlarge a roughly made hole in a spinning workpiece to slightly enlarge the hole and improve its accuracy. It is a fine finishing the hole and improve its accuracy. It is a fine finishing operation used in the final stages of product manufacture.operation used in the final stages of product manufacture.

• In In millingmilling, a rotating tool with multiple cutting edges is moved , a rotating tool with multiple cutting edges is moved slowly relative to the material to generate a plane or straight slowly relative to the material to generate a plane or straight surface. The direction of the feed motion is perpendicular to surface. The direction of the feed motion is perpendicular to the tool's axis of rotation. The speed motion is provided by the the tool's axis of rotation. The speed motion is provided by the rotating milling cutter. rotating milling cutter.

The Cutting ToolThe Cutting Tool

• A cutting tool has one or more sharp cutting edges A cutting tool has one or more sharp cutting edges and is made of a material that is harder than the and is made of a material that is harder than the work material. The cutting edge serves to separate work material. The cutting edge serves to separate chip from the parent work material. Connected to the chip from the parent work material. Connected to the cutting edge are the two surfaces of the tool:cutting edge are the two surfaces of the tool:

• The rake face; andThe rake face; and• The flank.The flank.• The rake face which directs the flow of newly formed The rake face which directs the flow of newly formed

chip, is oriented at a certain angle is called the rake chip, is oriented at a certain angle is called the rake angle "α". It is measured relative to the plane angle "α". It is measured relative to the plane perpendicular to the work surface. The rake angle perpendicular to the work surface. The rake angle can be positive or negative. The flank of the tool can be positive or negative. The flank of the tool provides a clearance between the tool and the newly provides a clearance between the tool and the newly formed work surface, thus protecting the surface formed work surface, thus protecting the surface from abrasion, which would degrade the finish. This from abrasion, which would degrade the finish. This angle between the work surface and the flank angle between the work surface and the flank surface is called the relief angle.surface is called the relief angle.

• There are two basic types of cutting tools:There are two basic types of cutting tools:• Single point tool; andSingle point tool; and• Multiple-cutting-edge toolMultiple-cutting-edge tool• A single point tool has one cutting edge and is A single point tool has one cutting edge and is

used for turning, boring and planing. During used for turning, boring and planing. During machining, the point of the tool penetrates below machining, the point of the tool penetrates below the original work surface of the workpart. The the original work surface of the workpart. The point is sometimes rounded to a certain radius, point is sometimes rounded to a certain radius, called the nose radius.called the nose radius.

• Multiple-cutting-edge tools have more than one Multiple-cutting-edge tools have more than one cutting edge and usually achieve their motion cutting edge and usually achieve their motion relative to the workpart by rotating. Drilling and relative to the workpart by rotating. Drilling and milling uses rotating multiple-cutting-edge tools. milling uses rotating multiple-cutting-edge tools. Although the shapes of these tools are different Although the shapes of these tools are different from a single-point tool, many elements of tool from a single-point tool, many elements of tool geometry are similar.geometry are similar.

Stages in Metal CuttingStages in Metal Cutting• Machining operations usually divide into two Machining operations usually divide into two

categories, distinguished by purpose and cutting categories, distinguished by purpose and cutting conditions:conditions:

• Roughing cuts, andRoughing cuts, and• Finishing cutsFinishing cuts• Roughing cuts are used to remove large amount of Roughing cuts are used to remove large amount of

material from the starting workpart as rapidly as material from the starting workpart as rapidly as possible, in order to produce a shape close to the possible, in order to produce a shape close to the desired form, but leaving some material on the piece desired form, but leaving some material on the piece for a subsequent finishing operation. for a subsequent finishing operation.

• Finishing cuts are used to complete the part and Finishing cuts are used to complete the part and achieve the final dimension, tolerances, and surface achieve the final dimension, tolerances, and surface finish. In production machining jobs, one or more finish. In production machining jobs, one or more roughing cuts are usually performed on the work, roughing cuts are usually performed on the work, followed by one or two finishing cuts. followed by one or two finishing cuts.

• Roughing operations are done at high feeds and depths – Roughing operations are done at high feeds and depths – feeds of 0.4-1.25 mm/rev (0.015-0.050 in/rev) and depths feeds of 0.4-1.25 mm/rev (0.015-0.050 in/rev) and depths of 2.5-20 mm (0.100-0.750 in) are typical. Finishing of 2.5-20 mm (0.100-0.750 in) are typical. Finishing operations are carried out at low feeds and depths - feeds operations are carried out at low feeds and depths - feeds of 0.0125-0.04 mm/rev (0.0005-0.0015 in/rev) and depths of of 0.0125-0.04 mm/rev (0.0005-0.0015 in/rev) and depths of 0.75-2.0 mm (0.030-0.075 in) are typical. Cutting speeds 0.75-2.0 mm (0.030-0.075 in) are typical. Cutting speeds are lower in roughing than in finishing.are lower in roughing than in finishing.

• A cutting fluid is often applied to the machining operation A cutting fluid is often applied to the machining operation to cool and lubricate the cutting tool. Determining to cool and lubricate the cutting tool. Determining whether a cutting fluid should be used, and, if so, whether a cutting fluid should be used, and, if so, choosing the proper cutting fluid, is usually included choosing the proper cutting fluid, is usually included within the scope of cutting condition.within the scope of cutting condition.

• Today other forms of metal cutting are becoming Today other forms of metal cutting are becoming increasingly popular. An example of this is water jet increasingly popular. An example of this is water jet cutting. Water jet cutting involves pressurized water in cutting. Water jet cutting involves pressurized water in excess of 620 MPa (90 000 psi) and is able to cut metal excess of 620 MPa (90 000 psi) and is able to cut metal and have a finished product. This process is called cold and have a finished product. This process is called cold cutting, and it increases efficiency as opposed to laser cutting, and it increases efficiency as opposed to laser and plasma cutting.and plasma cutting.