CUTTING TOOLS & NOMENCLATURE

CUTTING TOOLS

INTRODUCTION :- 

Metal cutting or "Machining" is the process of producing a workpiece by removing unwanted material from a block of metal, in the form of chips. This process is most important since almost all the products get their final shape and size by metal removal, either directly or indirectly in the form of chips.

The Mechanics Of Chip Formation

A typical metal cutting process can be schematically represented as in Figure below. A wedge-shaped tool is made to move relative to the workpiece. As the tool makes contact with the metal, it exerts a pressure on it resulting in the compression of the metal near the tool tip. This induces shear-type deformation within the metal and it starts moving upward along the top face of the tool. As the tool advances, the material ahead of it is sheared continuously along a plane called "Shear plane". This shear plane is actually a narrow zone (of the order of about 0.025 mm) and extends from the cutting edge of the tool to the surface of the workpiece. The cutting edge of the tool is formed by two intersecting surfaces. The surface, along which the chip moves, upwards is called "Rake surface" and the other surface which is relieved to avoid rubbing with the machined surface, is called "Flank". The angle between the rake surface and the normal is known as "Rake angle" (which may be positive or negative), and the angle between the flank and the horizontal machined surface is known as the "relief or clearance angle". Most cutting processes have the same basic features as in Figure, where a single point cutting tool is used (a milling cutter, a drill, and a broach can be regarded as several single-point tools joined together and are known as multi-point tools).

Single Point Cutting Tool

A single point cutting tool consists of a sharpened cutting part called its point and the shank. The point of the tool is bounded by the face (along which the chips slide as they are cut by the tool), the side flank or major flank the end flank, or minor flank and the base. The side cutting edge, is formed by the intersection of the face and the end flank. The chips are cut from the workpiece by the side-cutting edge. The point "a" where the end and side-cutting edges meet is called the nose of the tool. Figure is for a right hand tool. Below, we give the definitions of the various tool elements and tool angles:-

Shank: It is the main body of the tool.

Flank. The surface or surfaces below and adjacent to the cutting edge is called the flank of the tool.

Heel: it is the intersection of the flank and the base of the tool. Nose. It is the point where the side cutting edge and end cutting edge intersect.

Cutting Edge:It is the edge on the face of the tool which removes the material from the workpiece. The total cutting edge consists of side cutting edge (major cutting edge), end cutting edge (minor cutting edge and the nose).

A single point cutting tool may be either right or left hand cut tool depending on the direction of feed. In a right cut tool, the side cutting edge is on the side of the thumb when the right hand is placed on the tool with the palm downward and the fingers pointed towards the tool nose. Such a tool will cut when fed from right to left as in a lathe in which the tool moves from tailstock to headstock. A left-cut tool is one in which the side cutting edge is on the thumb side when the left hand is applied. Such a tool will cut when cut from left to right 

Primary Cutting edge

The various types of surfaces and planes in metal cutting are explained below with the help of Figure, in which the basic turning process is shown. The three types of surfaces are :-

1. The work surface, from which the material is cut.

2. The machined surface which is formed or generated after removing the chip.

3. The cutting surface which is formed by the side cutting edge of the tool.

The references from which the tool angles are specified are the ‘cutting plane' and the 'basic plane' or the 'principal plane'. The cutting plane is the plane tangent to the cutting surface and passing through and containing the side cutting edge. The basic plane is the plane parallel to the longitudinal and cross feeds, that is, this plane lies along and normal to the longitudinal axis of the workpiece. In a lathe toot, the basic plane coincides with the base of the tool.

By designation or nomenclature of a cutting tool is meant the designation of the shape of the cutting part of the tool. The two systems to designate the tool shape, which are widely used are: -

1. American Standard Association System (ASA) or American National Standard institute. (ANSI).
2. Orthoganal Rake System. (ORS).

ASA System, in the ASA system, the angles of tool face, that is its slope, are defined in two orthogonal planes, one parallel to and the other perpendicular to the axis of the cutting tool, both planes being perpendicular to the base of the tool For simple turning operations, this system is illustrated in Figure.

The typical right hand single point cutting tool terminology is given in Figure below. It gives the three views of the single point cutting tool, with all the details marked on it.

A TOOL TECHNOLOGY

The complimentary angle of SCEA is called the "Approach angle".

End cutting-edge angle (ECEA): This is the angle between the end cutting edge and a line normal to the tool shank.

Side relief angle (SRA) : It Is the angle between the portion of the side flank immediately below the side cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the side flank.

End relief angle (ERA) : It is the angle between the portion of the end flank immediately below the end cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the end flank.

Back-rake angle (BR) : It is the angle between the face of the tool and a line parallel to the base of the tool and measured in a plane (perpendicular) through the side cutting edge. This angle is positive, if the side cutting edge slopes downwards from the point towards the shank and is negative if the slope of the side cutting edge is reverse. So this angle gives the slope of the face of the tool from the nose towards the shank.

Side-rake angle (SR): It is the angle between the tool face and a line parallel to the base of the tool and measured in a plane perpendicular to the base and the side cutting edge. This angle gives the slope of the face of the tool from the cutting edge. The side rake is negative if the slope is towards the cutting edge and is positive if the slope is away from the cutting edge.

Importance of Tool Angles

Side cutting-edge angle it is the angle which prevents interference as the tool enters the work material. The tip of the tool is protected at the start of the cut, Figure, as it enables the tool to contact the work first behind the tip. This angle affects tool life and surface finish. This angle can vary from 0 degree to 90 degrees. The side cutting edge at increased value of SCEA will have more of its length in action for a given depth of cut and the edge lasts longer. Also, the chip produced will be thinner and wider which will distribute the cutting and heat produced over more of the cutting edge.

On the other hand the larger this angle, the greater the component of force tending to separate the work and the tool. This promotes chatter. Satisfactory values of SCEA vary from 15 to 30 degrees, for general machining. The shape of the workpiece will also determine the SCEA. To produce a 90 degree shoulder, zero degree SCEA is needed. NO SCEA is desirable when machining, castings and forgings with hard and scaly skins, because the least amount of tool edge should be exposed to the destructive action of the skin.

End cutting-edge angle. The ECEA provides a clearance or relief to the trailing end of the cutting edge to prevent rubbing or drag between the machined surface and the trailing (non-cutting) part of the cutting edge. Only a small angle is sufficient for this purpose. Too large an ECEA takes away material that supports the point and conducts away the heat. An angle of 8 10 15 degrees has been found satisfactory in most cases on side cutting tools, like boring and turning tools. Sometimes, on finishing tools, a small flat (1.6 to 8 mm long) is ground on the front portion of the edge next to the nose radius, to level the irregular surface produced by a roughing tool. End cutting tools, like cutoff  and necking tool, tools often have no end cutting edge,

Side-relief angle, (SRA) and End-relief angle  (ERA) :  These angles are provided so that the flank of the tool clears the workpiece surface and there is no rubbing action between the two. Relief angles range from 5 deg. to 15 deg. for general turning. Small relief angles are necessary to give strength to the cutting edge when machining hard and strong materials. Tools with increased values of relief angles penetrate and cut the workpiece material more efficiently and this reduces the cutting forces. Too large relief angles weaken the cutting edge and there is less mass to absorb and conduct the heat away from the cutting edge.

Back and Side rake angle: The top face of the tool over which the chip flows is known as the rake face. The angle which this face makes with the normal to the machined surface at the cutting edge is known as "Back-rake angle", and the angle between the face and a plane parallel to the tool base and measured in a plane perpendicular to both the base of the tool holder and the side cutting edge, is known as "Side-rake angle". The rake angles may be positive, zero, or negative. Cutting angle and the angle of shear are affected by the values for rake angles. Larger the rake angle, smaller the cutting angle and lower the cutting force and power. However, since, increasing the rake angle decreases the cutting angle, these leaves less metal at the point of the tool to support the cutting edge and conduct away the heat. A practical rake angle represents a compromise between a large angle for easier cutting and a small angle for tool strength. In general, the rake angle is small for cutting hard materials and large for cutting soft ductile materials. An exception is brass which is Machined with a small or negative rake angle to prevent the tool from digging into the work.

The use of negative rake angles started with the employment of carbide cutting tools. When we use positive rake angle, the force on the tool is directed towards the cutting edge, tending to chip or break it. Carbide being brittle lacks shock resistance and will fail if positive rake angles are used with it. Using negative rake angles, directs the force back into the body of the tool away from the cutting edge, which gives protection to the cutting edge. The use of negative rake angle, increases the cutting force. But at higher cutting speeds, at which carbide cutting tools can be used, this increase in force is less than at normal cutting speeds. High cutting speeds are, therefore, always used with negative rakes, which requires ample power of the machine tool.

The use of indexable inserts has also promoted the use of negative rake angles. An insert with a negative rake angle has twice as many cutting edges as anequivalent positive rake angle insert.   So, to machine a given number of components, smaller number of negative rake inserts are needed as compared to positive rake inserts. 

 Cutting With Positive And Negative Rake Tools :-

The use of positive rake angles is recommended under the following conditions:

1. When machining low strength ferrous and non-ferrous materials and work-hardening materials.
2. When using low power machines.
3. When machining long shafts of small diameters.
4. When the set up lacks strength and rigidity.
5. When cutting at low cutting speeds.

The use of negative rake angles is recommended under the followinc conditions:

1. When machining high strength alloys.
2. When there are heavy impact loads such as in interrupted machining.
3. For rigid set ups and when cutting at high speeds. Recommended rake angles are given in Table below.

Nose Radius : Nose radius is favorable to long tool life and good surface finish. A sharp point on the end of a tool is highly stressed, short lived and leaves a groove in the path of cut.   There is an improvement in surface finish and permissible cutting speed as nose radius is increased from zero value. Too large a nose radius will induce chatter. The use of following values for nose radius is recommended:

R = 0.4 mm for delicate components.

> = 1.5 mm for heavy depths of cut, interrupted cuts and heavy loads.

   = 0.4mm to 1.2 mm for disposable carbide inserts for common use.

   = 1.2 to 1.6 mm for heavy duty inserts.

Cutting-Tool Nomenclature:-

Cutting-tool nomenclature means systematic naming of the various parts and angles of a cutting-tool. The surfaces on the point of a tool bear definite relationship to each other that are defined by angles. The principles underlying cutting-tool angles are the same whether the tool is a single-point tool, a multipoint tool, or a grinding wheel. Since a single-point tool is the easiest to understand, it will be discussed in great detail. The basic angles needed on a single-point toe may be best understood by removing the unwanted surface from an oblong toe blank of square section. However, the complete nomenclature of the various part of a single-point tool is shown in Figure above.

The parts are shank, face, flank, heel, nose, base, back rake, side rake side clearance, end clearance, end cutting edge, side cutting edge, and lip angle These elements define the shape of the tool.

The Shank is that portion of the tool bit which is not ground to form cutting edges and is rectangular in cross-section.

The faceof the cutting-tool is that surface which face the workpiece.

The heel of the single point tool is the lowest portion of the side cutting and end-cutting edge.

The nose of the tool is the conjunction of the side an end cutting edges.

The base of the tool is the underside of the shank. 

The rake is the slope of the top away from the cutting edge. Each toot has a side and back rake. Back rake indicates that the plane which forms the face or top cf a too! has been ground back at an angle sloping from the nose. Side rake indicates that the plane that forms the face or top of the tool has been ground back at an angle sloping from the side cutting edge.

The side clearance of side relief indicates that the plane that forms the flank or side of a tool has been ground back at an angle sloping down from the side cutting edge. Likewise, the end clearance or end relief indicates that the nose or end of a too! has been ground back at an angle sloping down from the end cutting edge.

The end cutting edge angle indicates that the plane which forms the end of a tool has been ground back at an angle sloping from the nose to the side of the shank, whereas the side cutting edge angle indicates that the plane which forms the flank or side of a tool has been ground back at an angle to the side of the shank. In the main, chips are removed by this cutting edge.

The lip or cutting angle is the included angle when the tool has been ground wedged-shaped.

Single Point Cutting Tools:- 

Here we discuss about the different tipped single point cutting tools.

Tipped single point cutting tools :As already discussed the carbides, ceramics, cast alloys, diamond, CBN and UCON are used as tips or inserts which are either brazed into a prepared seat machined on a tough steel tool shank or are clamped to the shank. The second type of tips or inserts are known as indexable inserts or throwaway tips.

1.    Brazed Tipped Tools: Here suitable shapes of tool material tips or inserts are brazed to a steel shank. When the tip or insert gets worn out, it is resharpened with the help of special grinding wheels. For resharpening purposes, the tool will have to be removed from the machine involving a resetting operation. The main draw back of a brazed tip is that because of difference in co-efficients of expansion of tip material and tool shank material, the brazing has to be done very carefully.

2.    Mechanically clamped tip tools :In these tools, the tips or inserts are clamped mechanically on to the tool shank. These tips are known as indexable because these have more than one cutting edges which are used one by one by indexing the tip and those are known as throwaway type because once all the edges of the tip have been used, the tip or insert is removed from the tool shank and have been used, the tip or insert is removed from the tool shank and thrown away or is disposed off (disposable tip). 

The most common shapes in which these tips are available are : Square, triangular and diamond shapes. The edges of the inserts may be at 90 degrees to the tip face. The edges may beat a small angle to the face. in the first case, the tips will provide a negative rake angle because these will have to be clamped on to shank with the seating sloping ownwards 10 provide a clearance angle. Here the number of cutting points will be twice the number of edges, because when all the edges on the top face have been used, the insert can be turned over to give an additional equal number of cutting edges.In the second case, positive rake is obtained on the tip. Here, the insert cannot be turned over to use the cutting edges on the bottom face, because the small angle provided on the side of the tip will prevent this.When the cutting edge on the tip gets worn, the clamp is released and the tip is rotated (indexed) to bring a new cutting edge into the cutting position. When all the edges have been used the tip is thrown away and a new tip is substituted.

The various methods for fastening the tip are :

1. Screw fastening
2. Bridge type clamp
3. Pin type clamp

Chip Breaker : During machining ductile materials, continuous type of chips are produced which are difficult to handle and a sharp, hot chip in motion is a hazard. To handle and dispose off the chips conveniently, the continuous chips are broken up into short segments. This is achieved with the help of chip breakers which give an extra bending to the highly work hardened chip to break it into small pieces. The various types of chip breakers are :

1. A groove may be ground into the top face of the tool after leaving a small land from the tip.
2. A step may be ground into the tool.
3. By providing a secondary rake angle and chip breaker projection.
4. In the case of carbide tipped tools, a chip breaker groove is made all around the boundary or a separate plate or step may be clamped on the top of the tool.

Significance of Positive Negative and Zero Rake Angles

Many noteworthy advances in science have emerged from efforts made to overcome deficiencies in an existing set of conditions. Negative-rake cutting is an example, as no doubt it developed from work on the part of the makers of carbide tools to overcome the weakness of this material against the destroying effect of metallic adhesion and the built-up edge when cutting ductile steels. With a positive-rake tool the component of force tending to break off the edge of the tool. When the tendency is aggravated by a built-up edge and the multiplying effect of a worn cavity, the material has not the mechanical strength necessary to withstand the stress imposed upon it and the edge breaks off. If now, the cutting edge is disposed with a negative rake, the force conditions impose thrusts which the tip is well able to withstand; in fact, they are advantageous. No doubt, the conditions of negative rake cutting were so foreign to all previously conceived ideas of cutting mechanism that no one gave serious thought to them until confronted with the situation we have discussed before.

By using the hardest carbides for negative-rake tools, cutting speeds may be increased beyond those possible under positive-rake conditions. The revised arrangement   imposed eliminates the tendency for the built-up edge to form, reduces abrasive wear and, as the speed is increased, reduces the chip force on the tool face. One important characteristic found to exist is that, as the rake is decreased, more and more of the work done is transferred to the chip. This results in cool work and tools, eliminates the necessity of a coolant and, under high-speed conditions, the heat softens the chip and assists in its removal. After experience with positive-rake cutting it is a curious sensation to see the enormous speed and impressive performance of a negative-rake face mill and at the end of the run to find cutter and workpiece relatively cool. The chips, of course, are red hot when they leave the work. Naturally the process is restricted to carbide tools,as under the high speed necessary HSS  would not stand up to the abrasive action of the chips.