Machining Processes

Prepared by Dr. Mohamed Ahmed Awad Machining process Chip Formation 2 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Machining Advantages 3 Prepared by Dr. Mohamed Ahmed Awad

Advantages 1 2 3 High Surface Quality More dimensional accuracy Parts may have external and internal profiles Or special surfaces

Prepared by Dr. Mohamed Ahmed Awad Disadvantages 5 Prepared by Dr. Mohamed Ahmed Awad

Machining Disadvantages Waste material Time consuming Require more energy

Typical Parts Made with These Processes Machine Components Engine Blocks and Heads Parts with Complex Shapes Parts with Close Tolerances Externally and Internally Threaded Parts

Products and Parts Made By These Processes

Prepared by Dr. Mohamed Ahmed Awad Machining Processes Traditional cutting includes Three major types of operations: Turning rotates a cylindrical workpiece, while a single-point tool cuts along its length Drilling rotates a multi-point tool and guides it into the workpiece to create a hole. Shaping moves a single cutting tools along flat surface 9 Prepared by Dr. Mohamed Ahmed Awad

Cutting variables Lab6: Roundness Test 12/7/2018

Lab6: Roundness Test 12/7/2018

Prepared by Dr. Mohamed Ahmed Awad Cutting Angles 12 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Rake angle 13 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Rake angle 14 Prepared by Dr. Mohamed Ahmed Awad

Orthogonal cutting Chip Formation by Shearing The tool has a rake angle of α , and a relief (clearance) angle. The shearing process in chip formation (Fig. 21.4a ) is similar to the motion of cards in a deck sliding against each other. Figure 21.4 (a) Schematic illustration of the basic mechanism of chip formation by shearing. (b) Velocity diagram showing angular relationships among the three speeds in the cutting zone.

Lab6: Roundness Test 12/7/2018

Effect of rake angle on cutting force Lab6: Roundness Test 12/7/2018

Cutting Forces Figure 21.11 (a) Forces acting on a cutting tool during two-dimensional cutting. Note that the resultant force, R, must be collinear to balance the forces. (b) Force circle to determine various forces acting in the cutting zone.

Prepared by Dr. Mohamed Ahmed Awad Chips Metal cutting is essentially a chip formation process. Metal chips are the unwanted bits of material that are removed from the workpiece during the cutting process. 19 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Chip Formation 20 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Chip Formation 21 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Chip Types There are Two types of metal chips created by cutting tools: discontinuous chips continuous chips 22 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Chip types 23 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Continuous chips This type is generally formed when machining ductile metals. Ductile metals include mild steel, copper, and aluminum. The resulting surface finish is generally good, 24 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Discontinuous chips Discontinuous chips consist of distinct chip segments that come off the workpiece in small chunks or particles. Discontinuous chips often form with hard or brittle workpiece material that cannot withstand the high shear stresse involved. 25 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Built-up Edge Continuous chip with a built-up edge (BUE). The built-up edge often forms when cutting soft, ductile metals Small workpiece particles adhere to the tool due to high pressure and heat. By increasing speeds and cutting angle, you can reduce the occurrence of the built-up edge. 26 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Built Up Edge 27 Prepared by Dr. Mohamed Ahmed Awad

Lab6: Roundness Test 12/7/2018

Chips Produced in Turning Figure 21.8 Chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving radially away from workpiece; and (d) chip hits tool shank and breaks off. Source: After G. Boothroyd.

Cutting Tool Materials Strength Hardness Toughness Wear resistance for a long tool life. 30 Prepared by Dr. Mohamed Ahmed Awad

Cutting Tool Material Types Carbon Steel High Speed Steel (HSS) Tungsten Carbide Ceramics Diamond 31 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Carbon Steels It has a limited tool life. Used for small production runs. It has Low cost. It is suited to hand tools, and wood working. Carbon content is about 0.9 to 1.35% with. Maximum cutting speed is about 7 m/min. The hot hardness value is low 32 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad High Speed Steel (HSS) It consists of alloyed steel with tungsten. It can cut materials with tensile strengths up to 105.46 Kg /Sq.mm It has hot hardness value much greater than Carbon Steels. It is used in all type of cutters, single/multiple point tools, and rotary tools. 33 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Tungsten Carbide It is produced by sintering grains of tungsten carbide in a cobalt matrix (it provides toughness). Other materials are often included to increase hardness, such as titanium, chrome, molybdenum, etc. Compressive strength is high compared to tensile strength Speed up to 300 rpm is commonly used on mild steels. Hot hardness properties are very good. 34 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Carbide tools 35 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Ceramics Sintered or cemented ceramic oxides, such as aluminum oxides. Mild steels can be cut at speeds up to 1500 rpm. These tools are the best to be used in continuous cutting operations. There is no occurrence of welding, or built up edges. Coolants are not needed to cool the work piece. Very high hot hardness properties. Often used as inserts in special holders. 36 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Diamond It is very hard material with high resistance to abrasion. Operations must minimize vibration to prolong diamond life. Also used as diamond dust in a metal matrix for grinding and lapping. For example, this is used in the finishing of tungsten carbide tools. 37 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Cutting Fluids 38 Prepared by Dr. Mohamed Ahmed Awad

TEMPERATURE IN CUTTING The main sources of heat generation are the primary shear zone and the tool-chip interface. If the tool is dull or worn, heat is also generated when the tool tip rubs against the machined surface. Cutting temperatures increase with: strength of the workpiece material cutting speed depth of cut Cutting temperatures decrease with increasing specific heat and thermal conductivity of workpiece material.

TEMPERATURE IN CUTTING The mean temperature in turning on a lathe is proportional to the cutting speed and feed: Mean temperature α Va fb (21.19) a and b are constants that depend on tool and workpiece materials, V is the cutting speed, and f is the feed of the tool. Max temperature is about halfway up the face of the tool. As speed increases, the time for heat dissipation decreases and temperature rises Tool Material a B Carbide 0.2 0.125 HSS 0.5 0.375

Cutting Fluids Purposes Cooling Purpose Lubricant Purpose 41 Prepared by Dr. Mohamed Ahmed Awad

Purposes of cutting fluids Cool and lubrication of the tool bit and work piece that are being machined, increase the life of the cutting tool, make a smoother surface finish, deter rust, and wash away chips. Cutting fluids can be sprayed, dripped, wiped, or flooded onto the point where the cutting action is taking place. 42 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Tool Wear 43 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Lubrication Process 44 Prepared by Dr. Mohamed Ahmed Awad

TOOL LIFE: WEAR AND FAILURE Conditions that would cause tool wear: High localized stresses High temp Sliding of chip along the rack face Sliding of the tool along the machined surface The rate of wear depends on: Tool and workpiece materials Tool shape Cutting fluids Process parameters Machine tool characteristics

TOOL LIFE: WEAR AND FAILURE The Cutting tool should be replaced when: Surface finish of the workpiece is poor Cutting force increases significantly Temperature increases significantly Poor dimensional accuracies

TOOL LIFE: WEAR AND FAILURE

Prepared by Dr. Mohamed Ahmed Awad Tool Sharpening 48 Prepared by Dr. Mohamed Ahmed Awad

Tool-life Curves Figure 21.17 Tool-life curves for a variety of cutting-tool materials. The negative inverse of the slope of these curves is the exponent n in the Taylor tool-life equation and C is the cutting speed at T = 1 min, ranging from about 200 to 10,000 ft./min in this figure.

TOOL LIFE: WEAR AND FAILURE Allowable Wear Land

Feed Marks on a Turned Surface Surface roughness: the higher the feed, and the smaller the tool-nose radius, R, the more prominent these marks will be. Vibration and chatter adversely affect surface finish because a vibrating tool periodically changes the dimensions of the cut. Figure 21.23 Schematic illustration of feed marks on a surface being turned (exaggerated).

Lab6: Roundness Test 12/7/2018

Lab6: Roundness Test 12/7/2018

Lab6: Roundness Test 12/7/2018

Lab6: Roundness Test 12/7/2018

Lab6: Roundness Test 12/7/2018

Prepared by Dr. Mohamed Ahmed Awad Cutting Conditions Main cutting motion (R) This is represented in either Linear cutting speed in m/min or in rotational cutting speed in rpm Feed motion (S) This is represented in either mm/min or in mm/rev. Depth of Cut (a) This is represented in mm 57 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Cutting Action 58 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Cutting Speed 59 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Depth of Cut 60 Prepared by Dr. Mohamed Ahmed Awad

Cutting variables in Turning point 61 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Cutting Action 62 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Feed Motion 63 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Feed motion 64 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Feed Motion 65 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Surface Roughness 66 Prepared by Dr. Mohamed Ahmed Awad

Cutting variables in Turning point 67 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Cutting Tool Single point Cutting tool 68 Prepared by Dr. Mohamed Ahmed Awad

The factors that influence the feeds and speeds are:- The maximum and minimum speeds available on the machine The type of coolant used Type of cutting tool used The condition of the machine The type of material being machined The clamping method used to secure the component Diameter of the cutting tool The type of material the cutting tool is made of The condition of the tool Lab6: Roundness Test 12/7/2018

Cutting variables in Turning point 70 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Milling 71 Prepared by Dr. Mohamed Ahmed Awad

Cutting Variables in Milling 72 Prepared by Dr. Mohamed Ahmed Awad

Shaper and Planner variables Lab6: Roundness Test 12/7/2018

Prepared by Dr. Mohamed Ahmed Awad Drilling Process 74 Prepared by Dr. Mohamed Ahmed Awad

Cutting Variables in Drilling 75 Prepared by Dr. Mohamed Ahmed Awad

Prepared by Dr. Mohamed Ahmed Awad Grinding 76 Prepared by Dr. Mohamed Ahmed Awad

Lab6: Roundness Test 12/7/2018

Cutting Variable in Grinding 78 Prepared by Dr. Mohamed Ahmed Awad