Basic Mechanisms Of Tool Wear

The various mechanisms that contribute to wear process are as below .

• Mechanical overload causing micro breakages (attrition). 

• Abrasion
• Adhesion
• Diffusion

Attrition :
The grains of the various components of the tool material hold together at grain boundaries. Those on the rake face and on the flank are supported on at least half of their surfaces and can therefore be rather easily broken out, embedded in the machined surface and in the underside of the chip, and dragged over the tool surface. Some of them may then break out other grains and produce a kind of chain effect.
 

Abrasion :
Abrasion is the commonly known wear process in which a harder material scratches a softer material over which it is sliding under normal pressure. This mechanism is significant for tool wear only in those instances where the workpiece material is very hard or contains hard particles: cast iron with grains of cementite, various metal containing hard inclusions like hypereutectic aluminum with SiC grains, steel killed with aluminum and containing Al2O3 , and so on. The machined surface is cooler than the tool flank, and it may happen that tool material is softened more than some of the constituents of the workpiece materials, which creates the conditions for abrasion.
 

Adhesion :
In the conditions of the intimate contact between the tool and the freshly created surfaces on the workpiece and on the underside of the chip, welding of the workpiece surface and of the chip to the tool can often be observed. The extreme case is the built-up edge, which is formed in the low and middle speed range. Layers of workpiece material welded to the tool are found in ductile materials like in ferritic and Tehran International Congress on Manufacturing Engineering  Tehran, Iran austenitic steels, titanium alloy, and nickel-based alloys. The welded layers and points are periodically sheared away. This mechanism contributes to flank wear as well as to the formation of the crater.
 

Diffusion:
Diffusion is an important mechanism and plays a significant role at higher cutting speeds in some workpiece/tool material combinations. The diffusion rate, that is, the amount of atoms of material penetrating into another material, depends on the affinity of the two, very strongly on temperature, and on the gradient of concentration of the penetrating atoms in the solvent material. The latter aspect is very special in cutting, because the chip materials that absorbs the atoms of the tool material is continuously being carried away, and all the time new, virgin, unsaturated material is always arriving.

Orthogonal and Oblique Cutting

Orthogonal Cutting: 

The cutting condition when chip is expected to flow along the orthogonal plane is known as orthogonal cutting.pure orthogonal cutting is orthogonal cutting when principle cutting angle is 90 degree.

Oblique Cutting:

 When the chip does not flow on orthogonal plane i.e chip deviates from orthogonal plane then it is called oblique cutting.

Basic Difference Between Orthogonal and Oblique Cutting: 

 In oblique cutting inclination angle(lamda) have some value but in orthogonal cutting there is no lamda i.e lamda=zero.

Causes of orthogonal cutting:

• Restricted cutting effect.
• tool nose radius-ing.
• inclination angle (Lamda not equal to zero).

Cutting Processes

Cutting processes work by causing fracture of the material that is processed. Usually, the portion that is fractured away is in small sized pieces, called chips. Common cutting processes include sawing, shaping (or planing), broaching, drilling, grinding, turning and milling. 

Although the actual machines, tools and processes for cutting look very different from each other, the basic mechanism for causing the fracture can be understood by just a simple model called for orthogonal cutting. In all machining processes, the workpiece is a shape that can entirely cover the final part shape. The objective is to cut away the excess material and obtain the final part. This cutting usually requires to be completed in several steps – in each step, the part is held in a fixture, and the exposed portion can be accessed by the tool to machine in that portion. 

Common fixtures include vise, clamps, 3-jaw or 4-jaw chucks, etc. Each position of holding the part is called a setup. One or more cutting operations may be performed, using one or more cutting tools, in each setup. To switch from one setup to the next, we must release the part from the previous fixture, change the fixture on the machine, clamp the part in the new position on the new fixture, set the coordinates of the machine tool with respect to the new location of the part, and finally start the machining operations for this setup. Therefore, setup changes are time-consuming and expensive, and so we should try to do the entire cutting process in a minimum number of setups; the task of determining the sequence of the individual operations, grouping them into (a minimum number of) setups, and determination of the fixture used for each setup, is called process planning

TOOLCHIP INTERFACE TEMPERATURE IN TURNING PROCESS

The cutting temperature is a key factor which directly affects cutting tool wear, workpiece surface integrity and machining precision according to the relative motion between the tool and workpiece. The amount of heat generated varies with the type of material being machined, cutting parameters, contact length between tool and chip, cutting forces and friction between tool and workpiece material. 

The temperatures which are of major interests are: average shear zone temperature, average (and maximum) temperature at the chip-tool interface, temperature at the work-tool interface (tool flanks), average cutting temperature. Temperature on the chip-tool interface is important parameters in the analysis and control of machining process. Total tool wear rate and crater wear on the rake face are strongly influenced by the temperature at chip-tool interface. Much research has been undertaken into measuring the temperatures generated during cutting operations.

To measure the tool temperature at the tool chip interface many experimental methods have been developed. The main techniques used to evaluate the cutting temperature during machining are tool-work thermocouple, embedded thermocouple and thermal radiation method. Design and develop control system to control the temperature lead to decrease tool wear and better surface finish. Production research activities in a real production environment supported by statistical experimental procedures enable continuous improvement of control processes and further cost .

Milling Cutter Nomenclature

As far as metal cutting action is concerned, the pertinent angles on the tooth are those that define the configuration of the cutting edge, the orientation of the tooth face, and the relief to prevent rubbing on the land.

Outside diameter — The diameter of a circle passing through the peripheral cutting edges. It is the dimension used in conjunction with the spindle speed to find the cutting speed (SFPM).

Root diameter — This diameter is measured on a circle passing through the bottom of the fillets of the teeth.

Tooth — The tooth is the part of the cutter starting at the body and ending with the peripheral cutting edge. Replaceable teeth are called inserts.

Tooth face — The tooth face is the surface between the fillet and the cutting edge, where the chip slides during its formation.

Land — The area behind the cutting edge on the tooth that is relieved to avoid interference is called the land.

Flute — The flute is the space provided for chip flow between the teeth.

Gash angle — The gash angle is measured between the tooth face and the back of the tooth immediately ahead.

Fillet — The fillet is the radius at the bottom of the flute, provided to allow chip flow and chip curling.

The terms defined above apply primarily to milling cutters, particularly to plain milling cutters. In defining the configuration of the teeth on the cutter, the following terms are important.

Peripheral cutting edge — The cutting edge aligned principally in the direction of the cutter axis is called the peripheral cutting edge. In peripheral milling, it is this edge that removes the metal.

Face cutting edge — The face cutting edge is the metal removing edge aligned primarily in a radial direction. In side milling and face milling, this edge actually forms the new surface, although the peripheral cutting edge may still be removing most of the metal. It corresponds to the end cutting edge on single point tools.

Relief angle — This angle is measured between the land and a tangent to the cutting edge at the periphery.

Clearance angle — Is provided to make room for chips, thus forming the flute.

Radial rake angle — The angle between the tooth face and a cutter radius, measured in a plane normal to the cutter axis.

Axial rake angle — Measured between the peripheral cutting edge and the axis of the cutter, when looking radially at the point of intersection.

Blade setting angle — When a slot is provided in the cutter body for a blade, the angle between the base of the slot and the cutter axis is called the blade setting angle.

Milling Cutters

Milling is a process of generating machined surfaces by progressively removing a predetermined amount of material from the workpiece, which is advanced at a relatively slow feed rate to a milling cutter rotating at a comparatively high speed. The characteristic feature of the milling process is that each milling cutter tooth removes its share of the stock in the form of small individual chips.

Types of milling cutters
The variety of milling cutters available helps make milling a versatile machining process. Cutters are made in a large range of sizes. Milling cutters are made from High Speed Steel (HSS), others are carbide tipped and many are replaceable or indexable inserts.

Periphery milling cutters — Periphery milling cutters are usually arbor-mounted to perform various operations. Common high-speed steel milling cutters include the staggered tooth cutter, side-milling cutter, plain-milling cutter, single-angle milling cutter, double-angle milling cutter, convex milling cutter, concave milling cutter, and corner-rounded milling cutter.

Light-duty plain mill — A general-purpose cutter for peripheral milling operations. Narrow cutters have straight teeth, while wide ones have helical teeth.

Heavy-duty plain mill — Similar to the light duty mill except that it is used for higher rates of metal removal. To aid it in this function, the teeth are more widely spaced and the helix angle is increased to about 45 degrees.

Side milling cutter — Has a cutting edge on the sides as well as on the periphery. This allows the cutter to mill slots.

Half-side milling cutter — Same as the one previously described except that cutting edges are provided on a single side. It is used for milling shoulders. Two cutters of this type are often mounted on a single arbor for straddle milling.

Stagger-tooth side mill — Same as the side-milling cutter except that the teeth are staggered so that every other tooth cuts on a given side of the slot. This allows deep, heavy-duty cuts to be taken.

Angle cutters — The peripheral cutting edges lie on a cone rather than on a cylinder. A single or double angle may be provided.

Drilling In Tool Engineering

The geometry of the common twist drill tool (called drill bit) is complex; it has straight cutting teeth at the bottom – these teeth do most of the metal cutting, and it has curved cutting teeth along its cylindrical surface .The grooves created by the helical teeth are called flutes, and are useful in pushing the chips out from the hole as it is being machined. Clearly, the velocity of the tip of the drill is zero, and so this region of the tool cannot do much cutting. 

Therefore it is common to machine a small hole in the material, called a center-hole, before utilizing the drill. Center-holes are made by special drills called center-drills; they also provide a good way for the drill bit to get aligned with the location of the center of the hole. There are hundreds of different types of drill shapes and sizes; here, we will only restrict ourselves to some general facts about drills. 

• Common drill bit materials include hardened steel (High Speed Steel, Titanium Nitride coated steel); for cutting harder materials, drills with hard inserts, e.g. carbide or CBN inserts, are used.
• In general, drills for cutting softer materials have smaller point angle, while those for cutting hard and brittle materials have larger point angle.
• If the Length/Diameter ratio of the hole to be machined is large, then we need a special guiding support for the drill, which itself has to be very long; such operations are called gun-drilling. This process is used for holes with diameter of few mm or more, and L/D ratio up to 300. These are used for making barrels of guns.
• Drilling is not useful for very small diameter holes (e.g. < 0.5 mm), since the tool may break and get stuck in the workpiece.
• Usually, the size of the hole made by a drill is slightly larger than the measured diameter of the drill – this is mainly because of vibration of the tool spindle as it rotates, possible misalignment of the drill with the spindle axis, and some other factors.