ENERGY AND POWER IN MACHININGThe cutting force system in a conventional, oblique-chip formation process is shown schematically in Figure below,

1) Fc: Primary cutting force acting in the direction of the cutting velocity vector. This force is generally the largest force and accounts for 99% of the power required by the process.

2. Ff : Feed force acting in the direction of the tool feed. This force is usually about 50% of Fc but accounts for only a small percentage of the power required because feed rates are usually small compared to cutting speeds.

3. Fr : radial or thrust force acting perpendicular to the machined surface. This force is typically about 50% of Fr and contributes very little to power requirements because velocity in the radial direction is negligible.

TOOL GEOMETRY:

The side rake angle and the back rake angle combine to form the effective rake angle. This is also called the true rake angle or resultant rake angle of the tool.

Increased rake angle reduces the compression, the forces, and the friction, yielding a thinner, less deformed, and cooler chip. Unfortunately, it is difficult to take much advantage of the desirable effects of larger positive rake angles, since they are offset by the reduced strength of the cutting tool, due to the reduced tool section, and by its greatly reduced capacity to conduct heat away from the cutting edge. To provide greater strength at the cutting edge and better heat conductivity, zero or negative rake angles are commonly employed.

TOOL FAILURE AND TOOL LIFE:

Wear zones: Gradual wear occurs at three principal locations on a cutting tool. Accordingly, three main types of tool wear can be distinguished, 1) crater wear 2) flank wear 3) corner wear

1) Crater wear: Consists of a concave section on the tool face formed by the action of the chip sliding on the surface. Crater wear affects the mechanics of the process increasing the actual rake angle (thus reducing cutting forces) of the cutting tool and consequently, making cutting easier. At the same time, the crater wear weakens the tool wedge and increases the possibility for tool breakage. In general, crater wear is of a relatively small concern.2) Flank wear: Occurs on the tool flank as a result of friction between the machined surface of the work piece and the tool flank. Flank wear appears in the form of so-called wear land and is measured by the width of this wear land, VB, Flank wear affects to the great extend the mechanics of cutting. Cutting forces increase significantly with flank wear. If the amount of flank wear exceeds some critical value (VB > 0.5~0.6 mm), the excessive cutting force may cause tool failure.3) Corner wear: Occurs on the tool corner. Can be considered as a part of the wear land and respectively flank wear since there is no distinguished boundary between the corner wear and flank wear land. We consider corner wear as a separate wear type because of its importance for the precision of machining. Corner wear actually shortens the cutting tool thus increasing gradually the dimension of machined surface and introducing a significant dimensional error in machining, which can reach values of about 0.03~0.05 mm.

Tool life:As cutting proceeds, the amount of tool wear increases gradually. But tool wear must not be allowed to go beyond a certain limit in order to avoid tool failure. The most important wear type from the process point of view is the flank wear; therefore the parameter which has to be controlled is the width of flank wear land, VB.This parameter must not exceed an initially set safe limit, which is about 0.4 mm for carbide cutting tools. The safe limit is referred to as allowable wear land (wear criterion), VBk . The cutting time required for the cutting tool to develop a flank wear land of width VBk is called tool life, T, a fundamental parameter in machining. The general relationship of VB versus cutting time is shown in the figure (so-called wear curve). Although the wear curve shown is for flank wear, a similar relationship occur for other wear types. The figure shows also how to define the tool life T for a given wear criterion VBk .

The slope of the wear curve (that is the intensity of tool wear) depends on the same parameters, which affect the cutting temperature as the wear of cutting tool materials is a process extremely temperature dependent. Parameters, which affect the rate of tool wear are 1) cutting conditions (cutting speed V, feed f, depth of cut d) 2) cutting tool geometry (tool orthogonal rake angle) 3) properties of work material

From these parameters, cutting speed is the most important one. As cutting speed is increased, wear rate increases, so the same wear criterion is reached in less time, i.e., tool life decreases with cutting speed.

Rake angle has two major effects during the metal cutting process. One major effect of rake angle is its influence on tool strength. An insert with negative rake will withstand far more loading than an insert with positive rake. The cutting force and heat are absorbed by a greater mass of tool material, and the compressive strength of carbide is about two and one half times greater than its transverse rupture strength.

The other major effect of rake angle is its influence on cutting pressure. An insert with a positive rake angle reduces cutting forces by allowing the chips to flow more freely across the rake surface.

Negative Rake: Negative rake tools should be selected whenever work piece and machine tool stiffness and rigidity allow. Negative rake, because of its strength, offers greater advantage during roughing, interrupted, scaly and hard-- spot cuts. Negative rake also offers more cutting edges for economy and often eliminates the need for a chip breaker. Negative rakes are recommended on insert grades which do not possess good toughness (low transverse rupture strength).

The disadvantages of negative rake are, Negative rake requires more horsepower and maximum machine rigidity. It is more difficult to achieve good surface finishes with negative rake. Negative rake forces the chip into the work piece, generates more heat into the tool and work piece, and is generally limited to boring on larger diameters because of chip jamming.

Positive Rake: When cutting tough, alloyed materials those tend to "work-harden," such as certain stainless steels, when cutting soft or gummy metals, or when low rigidity of work piece, tooling, machine tool, or fixture allows chatter to occur. The shearing action and free cutting of positive rake tools will often eliminate problems in these areas.

The back rake angle affects the ability of the tool to shear the work material and form the chip. Positive rake angles reduce the cutting forces, resulting in smaller deflections of the work piece, tool holder, and machine. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. Generally speaking, the higher the hardness of the work piece, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range, depending on the type of tool (turning, planing, end milling, face milling, drilling, etc.) and the work material.

Externally and Internally Reversible Process:

The most common source of external irreversibility in the system is the heat transfer through a finite temperature difference.

Examples of reversible processes:1) Boiling of water from a heat source from a same boiling temperature.2) Theoretical Isothermal Compression of a gas.3) Frictionless movement4) Restrained compression or expansion5) Electric current flow through a zero resistance6) Restrained chemical reaction7) Mixing of two samples of the same substance at the same state.

Examples of Irreversible processes:1) Theoretical Polytrophic compression with heat rejection to atmosphere.2) Movement with friction3) Unrestrained expansion4) Energy transfer as heat due to large temperature non uniformities5) Electric current flow through a non zero resistance6) Spontaneous chemical reaction7) Mixing of matter of different composition or state.

THE ROLE OF THE GATING SYSTEMThe gating system conveys the material when molten metal is poured in to the mould and delivers it to all actions of the mould cavity. The speed or rate of metal movement is important as well as the amount of cooling that occurs while it is flowing. Slow filling and high loss of heat can result in misruns and cold shuts. Rapid rates of filling, on the other hand, can produce erosion of the gating system and mold cavity, and might result in the entrapment of mold material in the final casting.

The shape and length of the channels affect the amount of temperature loss. When heat loss is to be minimized, short channels with round or square cross sections (minimum surface area) are the most desirable. The gates are usually attached to the thickest or heaviest sections of a casting to control shrinkage and to the bottom of the casting to minimize turbulence and splashing. For large castings, multiple gates and runners may be used to introduce metal to more than one point of the mold cavity.

Gating systems should be designed to minimize turbulent flow, which tends to promote absorption of gases, oxidation of the metal, and erosion of the mold. Short sprues are desirable, since they minimize the distance that the metal must fall when entering the mold and the kinetic energy that the metal acquires during that fall. Rectangular pouring cups prevent the formation of a vortex or spiralling funnel, which tends to suck gas and oxides into the sprue. Tapered sprues also prevent vortex formation. A large sprue well can be used to dissipate the kinetic energy of the falling stream and prevent splashing and turbulence as the metal makes the turn into the runner.

Pressurized Gating System The total cross sectional area decreases towards the mold cavity Back pressure is maintained by the restrictions in the metal flow Flow of liquid (volume) is almost equal from all gates Back pressure helps in reducing the aspiration as the sprue always runs full Because of the restrictions the metal flows at high velocity leading to more turbulence and chances of mold erosionUn-Pressurized Gating System The total cross sectional area increases towards the mold cavity Restriction only at the bottom of sprue Flow of liquid (volume) is different from all gates aspiration in the gating system as the system never runs full Less turbulence

Goals of Gating System To minimize turbulence to avoid trapping gasses into the mold To get enough metal into the mold cavity before the metal starts to solidify To avoid shrinkage Establish the best possible temperature gradient in the solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part. Incorporates a system for trapping the non-metallic inclusionsRiserRiser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage which takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally.Risers are known by different names as metal reservoir, feeders, or headers.Shrinkage in a mold, from the time of pouring to final casting, occurs in three stages.1. during the liquid state2. during the transformation from liquid to solid3. during the solid stateFirst type of shrinkage is being compensated by the feeders or the gating system. For the second type of shrinkage risers are required. Risers are normally placed at that portion of the casting which is last to freeze. A riser must stay in liquid state at least as long as the casting and must be able to feed the casting during this time.Functions of Risers Provide extra metal to compensate for the volumetric shrinkage Allow mold gases to escape Provide extra metal pressure on the solidifying mold to reproduce mold details more exactDesign Requirements of Risers1. Riser size:For a sound casting riser must be last to freeze. The ratio of (volume / surface area)2of the riser must be greater than that of the casting. However, when this condition does not meet the metal in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers.2. Riser placement: the spacing of risers in the casting must be considered by effectively calculating the feeding distance of the risers.3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers, although considers as best, are difficult to cast. To increasevolume/surface area ratio the bottom of the riser can be shaped as hemisphere.

WELDING:

Neutral Welding Flame:

Theneutral flamehas a one-to-one ratio of acetylene and oxygen. It obtains additional oxygen from the air and provides complete combustion. It is generally preferred for welding. The neutral flame has a clear, well-defined, or luminous cone indicating that combustion is complete.Neutral welding flames are commonly used to weld: Mild steel Stainless steel Cast Iron Copper Aluminium There are two clearly defined zones in the neutral flame. The inner zone consists of a luminous cone that is bluish-white. Surrounding this is a light blue flame envelope or sheath. The neutral or balanced flame is obtained when the mixed torch gas consists of approximately one volume of oxygen and one volume of acetylene. In the neutral flame, the temperature at the inner cone tip is approximately 5850F (3232C), while at the end of the outer sheath or envelope the temperature drops to approximately 2300F (1260C). This variation within the flame permits some temperature control when making a weld. The position of the flame to the molten puddle can be changed, and the heat controlled in this manner.Carburizing Flame

Components of a Carburizing Welding FlameThe carburizing flame has excess acetylene, the inner cone has a feathery edge extending beyond it. This white feather is called the acetylene feather. If the acetylene feather is twice as long as the inner cone it is known as a 2X flame, which is a way of expressing the amount of excess acetylene. The carburizing flame may add carbon to the weld metal.

Reducing or carburizing welding flames are obtained when slightly less than one volume of oxygen is mixed with one volume of acetylene. This flame is obtained by first adjusting to neutral and then slowly opening the acetylene valve until an acetylene streamer or "feather" is at the end of the inner cone. The length of this excess streamer indicates the degree of flame carburization. For most welding operations, this streamer should be no more than half the length of the inner cone.

The reducing or carburizing flame can always be recognized by the presence of three distinct flame zones. There is a clearly defined bluish-white inner cone, white intermediate cone indicating the amount of excess acetylene, and a light blue outer flare envelope. This type of flare burns with a coarse rushing sound. It has a temperature of approximately 5700F (3149C) at the inner cone tips.

When a strongly carburizing flame is used for welding, the metal boils and is not clear. The steel, which is absorbing carbon from the flame, gives off heat. This causes the metal to boil. When cold, the weld has the properties of high carbon steel, being brittle and subject to cracking.

A slight feather flame of acetylene is sometimes used for back-hand welding. A carburizing flame is advantageous for welding high carbon steel and hard facing such nonferrous alloys as nickel and Monel. When used in silver solder and soft solder operations, only the intermediate and outer flame cones are used. They impart a low temperature soaking heat to the parts being soldered.Oxidizing Flame

Oxidizing welding flames are produced when slightly more than one volume of oxygen is mixed with one volume of acetylene. When the flame is properly adjusted, the inner cone is pointed and slightly purple. The temperature of this flame is approximately 6300F (3482C) at the inner cone tip.Oxidizing welding flames are commonly used to weld thesemetals: zinc copper maganese steel cast iron

Whenapplied to steel an oxidizing flamecausesthe molten metal to foamandgive off sparks. This indicates that the excessoxygenis combining with the steelandburning it. An oxidizing flame should not beusedfor welding steel becausethe deposited metal will be porous, oxidized,andbrittle. This flame will ruinmostmetalsandshould be avoided, except as noted in below.

A slightly oxidizing flame isusedin torch brazing of steeland cast iron. A stronger oxidizing flame is-used in the welding of brass or bronze. Mapp Gas Welding FlamesThe heat transfer properties of primary and secondary flames differ for different fuel gases. MAPP gas has a high heat release in the primary flame, and a high heat release in the secondary. Propylene is intermediate between propane and MAPP gas. Heating values of fuel gases are shown in table 11-3.

The coupling distance between the work and the flame is not nearly as critical with MAPP gas as it is with other fuels.

Adjusting a MAPP gas flame. Flame adjustment is the most important factor for successful welding or brazing with MAPP gas. As with any other fuel gas, there are three basic MAPP gas flames: carburizing, neutral, and oxidizing (fig. 11-3).

A carburizing flame looks much the same with MAPP gas or acetylene. It has a yellow feather on the end of the primary cone. Carburizing flames are obtained with MAPP gas when oxyfuel ratios are around 2.2:1 or lower. Slightly carburizing or "reducing" flames are used to weld or braze easily oxidized alloys such as Aluminum.

As oxygen is increased, or the fuel is turned down, the carburizing feather pulls off and disappears. When the feather disappears, the oxyfuel ratio is about 2.3:1. The inner flame is a very deep blue. This is the neutral MAPP gas flame for welding, shown in figure 11-3. The flame remains neutral up to about 2.5:1 oxygen-to-fuel ratio.

Increasing the oxygen flame produces a lighter blue flame, a longer inner cone, and a louder burning sound. This is an oxidizing MAPP gas flare. An operator experience with acetylene will immediately adjust the MAPP gas flame to look like the short, intense blue flame typical of the neutral acetylene flame setting. What will be produced, however, is a typical oxidizing MAPP gas flame. With certain exceptions such as welding or brazing copper and copper alloys, an oxidizing flame is the worst possible flame setting, whatever the fuel gas used. The neutral flame is the principle setting for welding or brazing steel. A neutral MAPP gas flame has a primary flame cone abut 1-1/2 to 2 times as long as the primary acetylene flame cone.