Introduction to machining process

1.Lathes
Lathes are machine tools designed primarily to do turning, facing and boring. Turning is rarely done on other types of machine tools, and none can do it better than lathes. Because lathes also can do drilling and reaming, and their versatility permits several operations to be done with a single setup of the work piece. Therefore, more lathes of various types are used in manufacturing than any other machine tool.The essential components of a lathe are the bed, headstock assembly, tailstock assembly, the leads crew and feed rod.
The bed is the backbone of a lathe. It usually is made of well normalized or aged gray or nodular cast iron and provides s heavy, rigid frame on which all the other basic components are mounted. Two sets of parallel, longitudinal rails, inner and outer, are contained on the bed, usually on the upper side. Some makers use an inverted V-shape for all four rails, whereas others utilize one inverted V and one flat rail in one or both sets. They are precision-machined to assure accuracy of alignment. On most modern lathes, the rails are surface-hardened to resist wear and abrasion, but precaution should be taken in operating a lathe to assure that the rails are not damaged. Any inaccuracy in them usually means that the accuracy of the entire lathe is destroyed.
The headstock is mounted in a foxed position on the inner rails, usually at the left end of the bed. It provides a powered means of rotating the word at various speeds. Essentially, it consists of a hollow spindle, mounted in accurate bearings, and a set of transmission gears-similar to a truck transmission—through which the spindle can be rotated at different speeds. Most lathes provide 8 to 18 speeds, usually in a geometric ratio, and on modern lathes all the speeds can be obtained by moving from two to four handles. An increasing trend is to provide a continuously variable speed range through electrical or mechanical drives.
Because the accuracy of a lathe is greatly dependent on the spindle, it is of heavy construction and mounted in heavy bearings, usually preloaded tapered roller or ball types. The spindle has a hole extending through its length, through which long bar stock can be fed. The size of spindle hole is very important as the bar stock that can be machined when the material must be fed through spindle.
The tailstock assembly mainly consists of three parts. A base plate fits on the inner rails of the bed and can slide longitudinally, with a device for clamping the entire assembly in any desired location. The tail fits on the base plate and can move transversely upon it to permit aligning the tail with the spindle. The tail sleeve is usually about 51 to 76mm(2to 3 inches) in diameter, that can be moved several inches longitudinally in and out by means of a hand wheel and screw.
The size of a lathe is designated by two dimensions. The first is maximum machining diameter, which is the maximum diameter of a workpiece can be rotated on a lathe. It is approximately twice the distance between the line connecting the lathe centers and the nearest point on the ways. The second is the maximum distance between tips, which indicates the maximum length of a workpiece can be mounted between two tips.

Common lathes are the type most frequently used in manufacturing. They are heavy-duty machine tools with all the components described previously and have power drive for all tool movements except on the compound rest. Their specifications are usually: the maximum machining diameter on the lathe bed is 305~610mm (12~24 inches); however, the maximum machining diameter on the bed is 1270mm (50 inches) and the distance between the two tips is 3658mm. Most of these lathes have a chip tray and an internal coolant circulation system. Small ordinary lathes—the maximum processing diameter of lathe beds is typically no more than 330mm (13 inches) – designed as a bench-top lathe with a bed mounted on a work bench or cabinet.

Although common lathes are versatile and very useful, because of the time required for changing and setting tools and for making measurements on the work piece, thy are not suitable for quantity production. And their actual processing time is less than 30% of their total processing time. In addition, a skilled machinist is required for all the operations, and such persons are costly and often in short supply. However, much of the operator’s time is consumed by simple, repetitious adjustments and in watching chips being made. Consequently, to reduce or eliminate the amount of skilled labor, turret lathes, screw machines, and other types of semiautomatic and automatic lathes have been highly developed and are widely used in manufacturing.

2 Numerical Control
One of the most fundamental concepts in advanced manufacturing technologies is numerical control (NC). Prior to the advent of NC, all machine tools are manually operated and controlled. Among all the limitations associated with manual controled machine tools, perhaps none is more prominent than the limitation of operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical control represents the first major step away from human control of machine tools. 

Numerical control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool, an NC technician writes a program that gives operational instructions to the machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device for accepting and decoding the programmed instructions, known as a reader.

Numerical control was developed to overcome the limitation of human operators, and it has done so. Numerical control machines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology: electrical discharge machining, laser cutting and electron beam welding.

Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce various parts, each involving complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes.

Like many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U.S. air force. In its initial stage, NC machines were able to made straight cuts efficiently and effectively.

However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter the straight lines making up the steps, the smoother is the curve, each line segment in the steps had to be calculated.

This problem led to the development in 1959 of the Automatically Programmed Tools (APT) language. This is a special programming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward in the further development from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development.

A major problem was the fragility of the punched paper tape medium. It was common for the paper tape containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader. If it was necessary to produce 100 copies of a given part, it was also necessary to run the paper tape through the reader 100 separate tines. Fragile paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated use.

This led to the development of a special magnetic plastic tape. Whereas the paper carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper tape, which solved the problem of frequent tearing and breakage. However, it still left two other problems.

The most important of these was that it was difficult or impossible to change the instructions entered on the tape. Even the most minor adjustment in a program of instructions need to be made, it was necessary to interrupt machining operations and make a new tape. It was also still necessary to run the tape through the reader as many times as there were parts to be produced. Fortunately, computer technology became a reality and soon solved the problems of NC associated with punched paper and plastic tape.

The development of a concept known as direct numerical control (DNC) solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed instructions. In direct numerical control, machine tools are tied, via a data transmission link, to a host computer. Programs for operating the machine tools are stored in the host computer and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represented a major step forward over punched tape and plastic tape. However, it is subject to the same limitations as all technologies that depend on a host computer. When the host computer goes down, the machine tools also experience downtime. This problem led to the development of computer numerical control.

3 Turning
The common lathe, one of the oldest metal removal machines, has a number of useful and highly desirable attributes. Today these lathes are used primarily in small shops where smaller quantities rather than large production runs are encountered.

The common lathe has been replaced in today’s production shops by a wide variety of automatic lathes such as automatic of single-point tooling for maximum metal removal, and the use of form tools for finish on a par with the fastest processing equipment on the scene today.

Tolerances for the engine lathe depend primarily on the skill of the operator. The design engineer must be careful in using tolerances of an experimental part that has been produced on the engine lathe by a skilled operator. In redesigning an experimental part for production, economical tolerances should be used.

Turret Lathes Production machining equipment must be evaluated now, more than ever before, this criterion for establishing the production qualification of a specific method, the turret lathe merits a high rating.

In designing for low quantities such as 100 or 200 parts, it is most economical to use the turret lathe. In achieving the optimum tolerances possible on the turrets lathe, the designer should strive for a minimum of operations.

Automatic Screw Machines    Generally, automatic screw machines fall into several categories; single-spindle automatics, multiple-spindle automatics and automatic chucking machines. Originally designed for rapid, automatic production of screws and similar threaded parts, the automatic screw machine has long since exceeded   the confines of this narrow field, and today plays a vital role in the mass production of a variety of precision parts. Quantities play an important part in the economy of the parts machined on the automatic screw machine. It the quantities less than 1000, the cost of the parts machined can be reduced if the minimum economical lot size is calculated and the proper machine is selected for these quantities.
Automatic Tracer Lathes   Since surface roughness depends greatly on material, tooling, and feeds and speeds employed, minimum tolerances that can be held on automatic tracer lathes are not necessarily the most economical tolerances.

In some cases, tolerances of 0.05mm are held in continuous production using one cut. The groove width can be held to 0.125mm on some parts. Bores and single-point finishes can be held to 0.0125mm. On high-production runs where maximum output is desirable, a minimum tolerance of 0.125mm is economical on both diameter and length of turn.

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