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CHAPTER 4 MACHINING PROCESSES. 4.1 Basic principles. The process of metal removal goes back many years. It is a process in which a wedge-shaped tool engages a work piece to remove a layer of material in the form of a chip .
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4.1 Basic principles • The process of metal removal goes back many years. It is a process in which a wedge-shaped tool engages a work piece to remove a layer of material in the form of a chip . • Even with all of the sophisticated equipment and techniques used in today’s modern industry, the basic mechanics of forming a chip remain the same. • As the cutting tool engages the work piece, the material is sheared and deformed under tremendous pressure. The deformed material then seeks to relieve its stressed condition by fracturing and flowing into the space above the tool in the form of a chip. Machining is the broad term which describes the removal of material from a work piece.
4.1.1 Advantages of machining • More dimensional accuracy may be required than the accuracy provided by casting, shaping, or in some forming process alone. • Parts may have external and internal profiles that cannot be produced by forming or shaping processes. • Heat treated parts may undergo distortion and require additional finishing operations such as grinding. • Special surfaces or textures may be needed.
4.1.2 Disadvantages of Machining • Removal processes waste material. • Require more energy and labor than forming and shaping operations. • Removing a volume of material is time consuming. • Improper techniques can affect the surface quality or properties of the product.
4.1.3 Variables in the Cutting Process • The quality resulted from any machining operation depends on: • Tool material and its condition. • Tool shape, surface finish, and sharpness. • Work piece material, condition, and temperature. • Cutting conditions (such as speed, feed and depth of cut). • Type of the cutting fluid.
Changing any of the above variables will affect: • Type of chip produced. • Force and energy dissipated in the cutting process. • Temperature rise in the work piece. • Wear and failure of the tool. • Surface finish of the produced part.
4.1.4 Cutting Conditions ظروف القطع • The main Cutting Motions for Metal Cutting Processes are: • Main cutting motion (R) سرعة القطع الرئيسية • This is represented in either Linear cutting speed in m/min~سرعة خطية or in rotational cutting speed in rpm سرعة دورانية • Feed motion (S)تغذية • This is represented in either mm/min or in mm/rev. • Depth of Cut (a)عمق القطع • This is represented in mm.
4.1.5 Cutting Tool Materials Cutting tools must have the following characteristics in order to produce the required quality and economical parts: • Hardness (particularly at elevated temperatures). • Toughness, so that impact forces will not chip the tool. • Wear resistance for a long tool life. • Chemical stability and inertness.
Some types of the Tool Materials • Carbon Steels الصلب الكربونى • High Speed Steel (HSS) صلب عالى السرعات • Tungsten Carbide اللقم الكربيدية • Ceramics سيراميك • Diamonds الألماظ • Cemented Oxides الأكسيدات
Drilling • One of the most important and essential tools in any metalworking shop is the drilling machine or drill press • it is often used for reaming, boring, tapping, counter boring, countersinking, and spot facing.
Drilling • All drilling machines operate on the same basic principle. The spindle turns the cutting tool, which is advanced either by hand or automatically into a work piece that is mounted on the table or held in a drill press vise.
Types of drill presses 1- General types of drill presses
Types of drill presses 2- Vertical or Pillar 3- Radial Arm
Types of drill presses 4- Gang Type
Types of drill presses 5-Multi Spindle 6- Numerical Control Type
Drill Nomenclature • The most important type of drill is the twist drill. The important nomenclature listed below twist drill parts.
Milling • Milling is the process of machining flat, curved, or irregular surfaces by feeding the work piece against a rotating cutter containing a number of cutting edges. • The milling machine consists basically of a motor driven spindle, which mounts and revolves the milling cutter, and a reciprocating adjustable worktable, which mounts and feeds the work piece.
Milling machines • vertical or horizontal.
Types of milling machines • Knee and Colum • Vertical 2. Horizontal • Horizontal with vertical head attachmen • Universal (table rotates in plan view) and is used for helical milling • Ram & Turret - Light weight machine tool with slotter on one end of turret. No power feeds. • Special Purpose - For production usage. Usually more rigid construction
Milling Cutters • Milling cutters are usually made of high-speed steel and are available in a great variety of shapes and sizes for various purposes. • The basic types include • End Mills - The face and sides at the bottom end of this tool are used for plunge cutting (two flutes) and side and end cuts (four flute). • Plain - These mills are used to cut with the sides only. They are generally mounted on an arbor. Side or Side & Face
Milling Cutters • Face - This cutter is held on a spindle nose.
Up-Cut Milling & Down - Cut Milling • Up-Cut Milling • Down - Cut Milling
Down - Cut Milling • This method has advantages, • The cutter forces are directed into the table, which reduces fixture forces, and allows thinner work pieces • There is less radial pressure on the arbor • Better surface finishes obtained because there is no “dig-in”
4.5 Shaping & planning • Both the shaper and the planer are single point tools that cut only in straight lines. They both make the same types of cuts. The shaper handles relatively small work. The planer handles work weighing up to several tons. The cutting stroke of the shaper is made by moving the tool bit attached to the ram. The cutting stroke of the planer is achieved by moving the work past a stationary tool bit.
4.6 GRINDING • Grinding has two main uses, • • As a surface finishing process for flat or cylindrical features that have already been cut. • • To cut complex surfaces with high tolerances or hard materials. • The basic process involves an abrasive wheel spinning at high speed that is brought into contact with the work surface, thus giving a smooth finish.
4.6.1 Machine Types • Typical categories of grinders include, • Surface • Cylindrical • Internal • Center • Center less
