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BORING AND BORING MACHINES
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(一)BORING AND BORING MACHINES

As carried out on a lathe, boring produces circular internal profiles in hollow work-pieces or on a hole made by drilling or another process, Boring is done with cutting tools that are similar to those used in turning. Because the boring bar has to reach the full length of the bore, tool deflection and, therefore, maintainance of dimensional accuracy can be a significant problem.

The boring bar must be sufficiently stiff—that is, made of a material with high elastic modulus, such as tungsten carbide –to minimize deflection and avoid vibration and chatter. Boring bars have been designed with capabilities for damping vibration.

Although boring operations on relatively small work-pieces. Can be carried out on a lathe, boring mills are used for large work-pieces. These machines are either vertical or horizontal, and are capable of performing operations such as turning, facing, grooving, and chamfering. A vertical boring machine is similar to a lathe but has a vertical axis of work-piece rotation.

The cutting tool (usually a single point made of M-2 and M-3 high-speed steel and C-7 and C-8 carbide) is mounted on the tool head, which is capable of vertical movement (for boring and turning) and radial movement (for facing), guided by the cross-rail. The head can be swiveled to produce conical (tapered) surfaces.

In horizontal boring machine, the work-piece is mounted on a table that can move horizontally in both the axial and radial directions. The cutting tool is mounted on a spindle that rotates in the headstock, which is capable of both vertical and longitudinal movements. Drills, reamer, taps, and milling cutters can also be mounted on the machine spindle.

Boring machine are available with a variety of features. Although work-piece diameters are generally 1 m-4 m(3ft-12ft),work-piece as large as 20 m(60ft) can be machined in some vertical boring machines. Machine capacities range up to 150 kw (200hp).these machines are also available with computer numerical controls, which allow all movements to be programmed. With such controls, little operaror involvement is required and consistency and productivity are improved. Cutting speeds and feeds for boring are similar to those for turning.(For capabilities of boring operations)

Jig borers are vertical boring machines with high –precision bearings. Although they are available in various sizes and used in tool rooms for making jigs and fixtures, they are now being replaced by more versatile numerical control machines.

Design considerations for boring. Guidelines for efficient and economical boring operations are similar to those for turning. Additionally, the following factors should be considered:

a. Whenever possible, through holes rather than blind holes should be specified.(The term blind hole refers to a hole that does not go though the thickness of the work-piece )

b. The greater the length –to –bore-diameter ratio, the more difficult it is to hold dimensions because of the deflections of the boring bar due to cutting forces.

c. Interrupted internal surfaces should be avoided.

(2)Fundamentals of Machine Tools

In many cases products form the primary forming processes must undergo further refinements in size and surface finish to meet their design specifications. To meet such precise tolerances the removal of small amounts of material is needed. Usually machine tools are used for such operation.

In the United States material removal is a big business-in excess of $ per year, including material, labor, overhead, and machine-tool shipments, is spent. Since 60 percent of the mechanical and industrial engineering and technology graduates have something connection with the machining industry either through sale, design, or operation of machine shops, or working in related industry, it is wise for an engineering student to devote some time in his curriculum to studying material removal and machine tools.

A machine tool provides the means for cutting tools to shape a workpiece to required dimensions; the machine supports the tool and the workpiece in a controlled relationship through the functioning of its basic members, which are as follow:

(a) Bed, Structure or Frame. This is the main member which provides a basis for, and a connection between, the spindles and slides; the distortion and vibration under load must be kept to a minimum.

(b) Slides and Sideways. The translation of a machine element (e.g. the slide) is normally achieved by straight-line motion under the constraint of accurate guiding surfaces (the slideway).

(c) Spindles and Bearings. Angular displacements take place about an axis of rotation; the position of this axis must be constant within extremely fine limits in machine tools, and is ensured by the provision of precision spindles and bearings.

(d) Power Unit. The electric motor is the universally adopted power unit for machine tools. By suitably positioning individual motors, belt and gear transmissions are reduced to a minimum.

(e) Transmission Linkage. Linkage is the general term used to denote the mechanical, hydraulic, pneumatic or electric mechanisms which connect angular and linear displacements in defined relationship.

There are two broad divisions of machining operations:

(a) Roughing, for which the metal removal rate, and consequently the cutting force, is high ,but the required  dimensional accuracy relatively low .

(b) Finishing, for which the metal removal rate, and consequently the cutting force, is low, but the required dimensional accuracy and surface finish relatively high .

It follows that static loads and dynamic loads, such as result form an unbalanced grindingwheel, are more significant in finishing operations than in roughing operations, The degree of precision achieved in any machining process will usually be  influenced by the magnitude of the deflections, which occur as a result of the force acting.

Machine tool frames are generally made in cast iron, although some may be steel casting or mild-steel fabrications. Cast iron is chosen because of its cheapness, rigidity, compressive strength and capacity for damping the vibrations set-up in machine operations, To avoid massive sections in castings, carefully designed systems of  ribbing are used to offer the maximum resistance to bending and torsional stresses. Two basic types of ribbing are box and diagonal. The box formation is convenient to produce, apertures in walls permitting the positioning and extraction of cores. Diagonal ribbing provides greater torsional stiffness and yet permits swarf to fall between the sections; it is frequently used for lathe beds.

The slides and slideways of a machine tool locate and guide members which move relative to each other, usually changing the position of the tool relative to workpiece .The movement generally takes the form of translation in a straight line, but is sometimes angular rotation, e.g. tilting the wheel-head of a universal thread-grinding machine to an angle corresponding which the helix angle of the workpiece thread. The basic geometric elements of slides are flat, vee, dovetail and cylinder. These elements may be used separately or combined in various ways according to the applications . Features of slideways are as follows :

(a) Accuracy of Movement. Where a slide is to be displaced in a straight line, this line must lie in two mutually perpendicular planes and there must be no slide rotation. The general tolerance for straightness of machine tool slideways is 0~0.02mm per 1000mm; on horizontal surfaces this tolerance may be disposed so that a convex surface results, thus countering the effect of  "sag" of the slideway.

(b) Means of Adjustment. To facilitate assembly, maintain accuracy and eliminate "play" between sliding members after wear has taken place, a strip is sometimes inserted in slides. This is called a gibstrip. Usually, the gib is retained by socket-head screws passing through elongated slots;and is adjusted by grub-screws secured by lock nuts.

(c) Lubrication. Slideways may be lubricated by either of the following systems:1)Intermittently through grease or oil nipples, a method suitable where movements are infrequent and speed low.

2) Continuously e.g. by pumping through a metering valve and pipe-work to the  point of application; the film of oil introduced between surfaces by these means must be extremely thin to avoid the slide “floating”.If sliding surfaces were optically flat oil would be squeezed out,resulting in the surfaces sticking. Hence in practice slide Sill"faces are either grourld using the edge of a cup wheel,or scraped. Both processes produee minulte surface depressions,which retain‘‘pocket” of oil, and complete separation of the parts may not occur at all points.

(d) Protection.To maintain slideways in good order, the following conditions must be met:

1) Ingress of foreign matter,e.g.swarf,must be prevented. Where this is no possible,it is desirable to have a form of slideway,which does not retain swarf,e.g. the inverted vee.

2) Lubricating oil must be retained.The adhesive property of oil for use on vertical or inclined slide surface is important; oils are available which have been specially developed for this purpose. The adhesiveness of oil also preverts it being washed away by cutting fluids.

3) Accidental damage must be prevented by protective guards.

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