With the next lectures we will study two types of mechanical operations with solid state manufacturing, (Machining and forming operations). Machining process can be divided into two groups; conventional and non-conventional machining, conventional machining involves changing the shape of work piece by using device or implement mad of a harder material is name cutting tool with conventional methods to machine the materials. Conventional machining as process using mechanical energy but non-conventional machining using thermal or chemical or any source of energy.
Machining processes produce the required shape with new dimension and change of material volume by removal of selected areas of the workpiece through a machining process. Or is any process in a piece of raw material with cutting to desired final shape and dimension by removing or separate a layer of materials from work piece is name a chip. This work is achieved by cutting tools and controlled material removal process. The machining processes are classified as turning, milling, drilling, shaping, etc.
Deformation processes; produce the required shape with the necessary mechanical properties by plastic deformation in which the material is moving and its volume is conserved. In other words, the primary difference between the machining processes and deformation processes is the removal a part of the article with machining but without removal part of article with forming which will clarify by detail in the next lectures.
Mechanism of Machining
The purpose of any metal cutting operation commonly called machining, in other world to produce a desired shape, size and finish of a component by removing the excess metal in the form of chips from a rough block of material is termed machining, the metal cutting is depend on some factors as;
1-Properties of workpiece.
2- The properties and geometry of the cutting tool.
3-The interaction between the tool and the workpiece during cutting processes.
The exact mechanism of metal cutting briefly is that a cutting tool exerts a compressive force on the workpiece. Under this compressive force the material of the workpiece is stressed beyond its yield point causing the material deform plastically and shear off. The plastic flow takes place in a localised region called shear plane which extends from the cutting obliquely up to the uncut surface ahead of tool. The sheared material begins to flow along the cutting tool face in the form of small pieces called chips. The compressive force applied to form the chip is called cutting force.
Machining operations usually classified according to purpose and cutting conditions divide into two types: Roughing cut & Finishing cut.
Roughing cutting are used to remove large amount of material from the workpiece as rapidly as possible, i.e. with a large Material Removal Rate (MRR), in order to produce a shape close to the desired form, but leaving some material on the piece for a subsequent finishing operation. Finishing cut are used to complete the part and achieve the final dimension, tolerances, and surface finish. In production machining jobs, one or more roughing cuts are usually performed on the work, followed by one or two finishing cut. Roughing operations are done at high feeds and depth.
A cutting tool has one or more sharp cutting edges and is made of a material that is harder than the work material. The cutting edge serves to separate chip from the parent work material. Connected to the cutting edge are two surfaces of the tool: Rake face & Flank.
The rake face which directs the flow of newly formed chip or to guide the direction of the chip flow, is oriented at a certain angle is name the rake angle "α". The purpose of this angle is to guide the chip and to protect the point of the cutting tool. It is measured relative to the plane perpendicular to the work surface or it is the downward slope of the top surface of the tool from the nose to the rear along the longitudinal axis. The rake angle can be positive with softer materials and negative with brittle and with heavy feed of materials.
The flank of the tool provides a clearance between the tool and the newly formed work surface, thus protecting the surface from abrasion. This angle between the work surface and the flank surface is name the relief angle or tolerance angle.
There are two basic types of cutting tools: Single point tool & Multiple-cutting-edge tool. A single point tool has one cutting edge and is used for turning, during machining the point of the tool penetrates below the original work surface of the workpiece. The point is sometimes rounded to a certain radius, called the nose radius.
Multiple-cutting-edge tools have more than one cutting edge and usually achieve their motion relative to the workpiece by rotating. Drilling and milling uses rotating multiple-cutting-edge tools. Although the shapes of these tools are different from a single-point tool, many elements of tool geometry are similar.
The relationship between thickness of chip before cutting (to) and after cutting (tc) is called; chip thickness ratio(R).
L1; length of chip before cutting.
L2; length of chip after cutting.
D; diameter of workpiece.
N; revolution per minute of workpiece.
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