Chapter Six Materials Removal Processes (Machining) 6.1 Theory of Material Removal Processes 6.1.1 Machining Definition Machining is a manufacturing process in which a cutting tool is used to remove excess material from a work part so that the remaining material is the desired part shape. 6.1.2 When To Consider Machining Reasons why machining is important commercially and technologically include the following: 1. Machining can be applied to a wide variety of work materials. 2. Machining can be used to generate any regular geometry. 3. Machining can produce dimensions to very close tolerances of less than (0.025mm). 4. Machining is capable of creating very smooth surface finishes. 5. More economical if the number of parts is small. 6.1.3 Limitations and Disadvantages Machining has some limitations or disadvantages: 1. It produces waste material. 2. It takes longer time compared to other methods. 3. Depends on the cutting conditions, it can have adverse effect on surface quality. 6.1.4 Types of Machining and Machining Operations 1. Cutting (Single and multiple points). 2. Abrasive (Grinding). 3. Non-traditional (Ultrasonic, Laser, EDM, ECM). 6.1.5 Machining Operations There are many kinds of machining operations, each of which is capable of generating a certain part geometry and surface texture. The differences are based on the shape formed, cutting tool, cutting conditions, tolerances, and surface roughness produced. The most common machining operations are the followings: Turning Shaping Drilling Planning Reaming Breaching Boring Sawing Milling Parting Facing Threading See Fig. (6.1) Fig. (6.1) 6.1.6 Machining Variables There are two kinds of machining variables: 1. Independent Variables: These include the tool material, tool shape, work part material, cutting conditions, machine tool, and fixture. 2. Dependent Variables: These include the type of chip formed, force and energy consumed, wear, temperature, surface finish, and use of cutting fluid. 6.1.7 Types of Chips 1. Continuous chip: when relatively brittle materials are machined at low cutting speed, the chips often from into separated segments. (See Fig. (6.2.a)). 2. Continuous chip: when ductile materials are cut at high speeds, long continuous chips are formed. (See Fig. (6.2.b)) 3. Continuous chip with built-up-edge: when machining ductile materials at low to medium cutting speeds, adherence of work material to the tool will occur. (See Fig. (6.2.c)). Fig. (6.2) 6.1.8 Cutting Tool & Temperatures Of the total energy consumed in machining, nearly all of it (approximately 98%) is converted into heat; this will raise the temperature of cutting tool, which will have the following effects: 1. Strength, hardness, and wear resistance of cutting tool will decrease. 2. Quality (toughness and surface finish) will decrease. 3. Internal thermal stresses will increase causing damages. The sources of heat are: 1. Plastic deformation in the shearing process. 2. The friction between the tool and the chips. Effect of the cutting conditions on the temperature: The cutting temperature will increase as the strength of the work piece, cutting speed, depth of cut, feed rate, coefficient of friction increase. 6.1.9 Tool Wear There are three possible modes by which a cutting tool can fail in machining: 1. Fracture failure: when excessive cutting force. 2. Temperature failure: when cutting temp- is too high. 3. Gradual wears: loss of tool shape and cutting efficiency. Variations in the cutting conditions such as force, temperature, tool and work piece material, tool geometry, and cutting fluid will cause tool wear. Types of tool wear See Fig. (6.3) 1. Crater wears: concave section, by the action of the chip sliding against the surface. 2. Flank wears: from rubbing between newly work surface and the land face. 3. Chipping (catastrophic) Fig. (6.3) 6.2 Machining Operations 6.2.1 Cutting Processes & Machine Tools For Producing Round Shapes There is a set of operations in which the part is rotating while it is being machined to produce part that is round in shape. A variety of other machining operations can produce a round shape part. See Fig. (6.4). • Turning: a single-point tool removes material from the surface of a rotating cylindrical work piece, (e.g. straight, taper, profiling, and grooving.). • Facing: produce flat surface at the end of the part. • Taper turning: creating a conical geometry. • Threading: creating threads in the cylinder. • Drilling: producing holes in a cylinder shape. • Cut off/parting: cutting the end of the part. • Knurling: producing a regular crosshatched pattern in the part surface. Fig. (6.4) 6.2.2 Turning & Turning Machine The tool is fed linearly in a direction parallel to the axis of rotation. See Fig. (6.5). Turning is traditionally carried out on a machine tool called a lathe, which provides power to turn the part at a given rotational speed, and to feed the tool at a specified rate and depth of cut. Fig. (6.5) 188.8.131.52 Cutting Parameters in Turning N: The rotational speed of the work part (rev/min) f : Feed (mm/rev) on (in./rev). v: Feed rate or linear speed (mm/mm) or in/mm). DOC: Depth of cut= (D0-Df)/2 -d. D0: initial diameter. Df: final diameter. Work holding methods are illustrated in Fig. (6.6). Fig. (6.6) 184.108.40.206 Turning machine (The Engine Lathe) Fig. (6.7) is a sketch of an Engine Lathe showing its principal components. • The headstock: contains the drive unit. • The tailstock: to support the other end of the work part. • The tool post: fastened to the cross-slide. • The carriage: to feed the tool parallel to the axis of rotation. • Bed - Bead screw- Chucks - Feed rod. Fig. (6.7) 220.127.116.11 The Cutting Tool Tool selection and turning parameters are selected depending on the inputs. Inputs Work piece material Toughness Work piece geometry Hot hardness Lot size Wear resistance Quality required Chemical Stability. 6.2.3 Drilling Drilling is usually performed with a rotating cylindrical tool that has two cutting edges on its working end. The tool is called twist drill, or drill bit. See Fig. (6.8). The rotating drill feeds into the stationary work part to form a hole whose diameter is determined by the drill diameter. Drilled holes are either through holes, or blind holes. See Fig. (6.9). Fig. (6.8) Fig. (6.9) 18.104.22.168 Operations Related to Drilling • Reaming: to slightly enlarge a hole. (Fig (6.10.a)). • Tapping: to provide internal screw thread. (Fig. (6.10.b)). • Counter boring: to provide a stepped hole. (Fig (6.10.c)). • Countersinking: to provide a cone shaped stepped hole (Fig (6.10.d)). • Centering: to drill a starting hole. (Fig. (6.10.e)). • Spool facing: to provide a flat-machined surface (Fig. (6.10.f)). Fig. (6.10) 22.214.171.124 Drilling Machine (Drill Presses) The drill press is the standard machine tool for drilling; the most basic type is the upright drill. See Fig. (6.11). Fig. (6.11) 6.2.4 Milling & Milling Machines Milling is a machining operation in which a work part is fed past a rotating cylindrical tool with multiple cutting edges. Milling is compared to turning and drilling by the following table: Turning Milling` Drilling Cutting tool Single-cutting edge Multi-cutting edge Double-cutting edge. Rotation W.P is rotating cutting tool is rotating cutting tool is rotating Axis of rotation Parallel to feed Perpendicular to feed Parallel to feed 126.96.36.199 Milling Cutting Conditions See Fig. (6.12). 1. Cutting speed, N (RPM) 2. Feed rate, f (mm/min) 3. Depth of cut, d (mm) Fig. (6.12) Fig. (6.13) shows a cycle of impact force and thermal shock on every rotation. Fig. (6.13) Milling cutter and tool geometry is shown in figure (6-14). Fig. (6.14) 188.8.131.52 Types of Milling Operations 184.108.40.206.1 Peripheral Milling (Plain Milling) The axis of the tool is parallel to the surface being machined. (See Fig. (6.15a)). Fig. (6.15) Types of Peripheral Milling: • Slab milling: the cutter width extends beyond the work piece on both sides. Fig. 6.16a)). • Slotting: the width of the cutter is less than the work piece width. Fig. (6.16b)). • Side milling: the cutter machines the side of the work piece. Fig. (6.16c)). • Straddle milling: the cutter machines the both sides of the work piece. Fig. (6.16d)). Fig. (6.16) Directions of rotations (up and down milling): 1. Up milling: (Conventional Milling), the direction of motion of the cutter teeth is opposite the fed direction. Fig. (6.17.a). 2. Down milling: (climb milling), the direction of motion is the same as feed direction. Fig (6.17.b). Fig. (6.17) 220.127.116.11.2 Face Milling The axis of the cutter is perpendicular to the surface being milled. (See Fig (6.15.b)). Types of Face Milling: • Conventional: the diameter of the cutter is greater than the work part width. (See Fig. (6.18a)). • Partial: the cutter overhangs the work part on only one side. (See Fig. (6.18b). • End milling: the cutter diameter is less than the work piece width. (See Fig. (6.18c). • Profile: the outside periphery of a flat part is cut. (See Fig (6.18d)). • Pocket Milling. (See Fig. (6.18e)). • Surface contouring: a ball-nose cutter creates a three-dimensional surface. (See Fig. (6.18f)). Fig. (6.18) 18.104.22.168 Milling Machines Milling machines are classified into two main categories; horizontal and vertical. Fig. (6.19) 22.214.171.124.1 Horizontal Milling Machines (See Fig. (6.19a)). Characteristics of horizontal milling machines: 1. Horizontal spindle. 2. Suitable for peripheral milling. 3. Use stud arbor to mount milling cutters. 4. Use V-block, table and vice to hold a work piece. See Fig. (6.20). 126.96.36.199.2 Vertical Milling Machines (See Fig. (6.19b)). Characteristics of vertical milling machines: 1. Vertical spindle. 2. Suitable for face milling. 3. Use cullet chuck to hold a cutting tool. 4. Use V-blocks, table and vice to hold the work piece. (See Fig. (6.21)). (Drawing from papers (Collet, table, V-block) Vertical milling, fig. (6-21). 6.2.5 Shaping & Planning 188.8.131.52 Shaping Shaping is one of the metal removal processes in which slots, notches, keyways are deformed. The speed motion is accomplished by moving the cutting tool along the work piece in linear motion. (See Fig. (6.22a)). 184.108.40.206 Planning The same as shaping, but the speed motion is accomplished by moving the work part. (See Fig (6.22b)). Fig. (6.22) Fig. (6.23) Fig. (6.24) 6.2.6 Grinding & Grinding Machines Grinding is the most important of the abrasive processes, which involves material removal by the action of hard, abrasive particles that are usually in the form of a bonded wheel. Grinding is considered as finishing operation. 220.127.116.11 Grinding Elements 1. The grinding wheel: consists of abrasive particles and bonding material, which is usually made of carbide, oxide, or nitride-based material and rotates in a high speed. (See Fig. (6.25)). 2. Work piece: which is usually semi- finished product fixed on grinding machine table. (See Fig. (6.25)). 18.104.22.168 Temperature at The Work Surface Because of the size effect, plowing and rubbing (See Fig. (6.26)) of the abrasive grits against the work surface, the grinding process is characterized by high temperatures and high friction, which cause in surface burns and cracks. Fig. (6.26) 22.214.171.124 Grinding Operations & Grinding Machines • Surface grinding: is normally used to grind plain flat surfaces. (See Fig. (6.27)). • Cylindrical grinding: is used for rotational parts, external and internal. (See Fig. (6.28)). • Center less grinding: the work piece is not held between centers, and is often used for high production work. (See Fig (6.29), (6-30)). Fig. (6.27) Fig. (6.28) Fig. (6.29) Fig. (6.30) A surface grinding machine with horizontal spindle and reciprocating worktable is shown in Fig. (6.27). Notes 1. The chip deformed is relatively small because of high cutting speed and small depth of cut. 2. The sparks observed in metal grinding are actually glowing chips because of the exothermic reaction of the hot chips with oxygen in the atmosphere.
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