Single point cutting tool
A single point culling 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 culling edge a-b, is formed by the intersection of the face and side flank. The end cutting edge a-c is formed by the intersection of the face and the end flank. The chips are cut from the work the piece by the side-cutting edge, the point ‘a’ where the end and side-cutting edges meet is called the nose of the tool. Figure 1 is for a right hand tool.
Figure-1 : Nomenclature of Single Point Cutting 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 flank of the tool
Face : The surface on which the chip slides is called the face 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 toot which removes the material from the work piece. The total cutting edge consists of side cutting edge (major cutting edge), end cutting edge (minor culling edge and the nose)
A single point culling tool may be either right-or left hand cut tool depending on the direction of feed.
In a right cut tool, the side culling 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 tail stock 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 fed from left to right.
Figure-2 : Principle Surfaces and Planes in Metal Cutting
The various types of surfaces and planes in metal cutting are explained below with the help of Fig2. 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 work piece. In a lathe tool, the basic plane coincides with the base of the tool.
Designation of Cutting tools
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:
- American Standards Association System (ASA) or American National Standards Institute (ANSI).
- Orthogonal 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 operation, this is illustrated in Fig.
The typical right hand single point cutting tool terminology is given in Fig. gives the three views of the single point cutting tool, with all the details marked on it.
Figure-3 : Tool Terminology
The various tool angles are defined and explained below:
Side cutting edge angle (SCEA) Cs: Side cutting edge angle, Cs, also known as lead angle, is the angle between the side cutting edge and the side of the tool shank.
End cutting-edge angle (ECEA) Ce: This is the angle between the end cutting Edge and a line normal to the tool shank.
Side relief angle (SRA) θs: 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) θe. 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) αb: 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 lace of the tool from the nose towards the shank.
Side-rake angle (SR) αs: 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, Cs. It is the angle which prevents interference as the tool enters the work materials. The tip of the tool is protected at the start of the cut. 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 0º to 90º. 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 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 15º to 30º, for general machining. shape of the work piece will also determine the SCEA. To produce a 90″ 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, Ce. 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 Too large an ECEA takes away material that supports the point and conducts away the heat. An angle of 8º to 15º 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 tut off and necking tools often have no end cutting angle.
Side-relief angle, (SRA) and End-relief angle (ERA). These angles (denoted as θs and θe in the figure) are provided so that the flank of the tool clears the work piece surface and there is no rubbing action between the two. Relief angles range from 5º to 15º 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 work piece 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
Back and Side rake angle (αb, αs): 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, αb” 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, αs”. The rake angles may be positive, zero, or Cutting angle and the angle of shear are affected by the values for rake angles. Larger the rake angle, smaller the cutting angle (and larger the shear angle) and the lower the cutting force and power. However, since increasing the rake angle decreases the cutting angle, this 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 form digging into the work. cutting speeds are, therefore, always used with negative rakes, which require 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 culling edges as an equivalent positive rake angle insert (as will be discussed ahead). So, to machine a given number of components, smaller number of negative rake inserts is needed as compared to positive rake inserts.
The use of positive rake angles is recommended under the following conditions:
- When machining low strength ferrous and non-ferrous materials and work hardening
- When using low power machines.
- When machining long shafts of small diameters.
- When the set up lacks strength and rigidity.
- When cutting at low cutting
The use of negative rake angles is recommended under the following conditions:
- When machining high strength alloys.
- When there are heavy impact loads such as in interrupted machining.
- For rigid set ups and when cutting at high speed.
Nose Radius, Nose radius is favourable in 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 feeds.
= 0.4 mm to 1.2 mm far disposable carbide inserts for common use.
= 1.2 to 1.6 mm for heavy duty inserts.
Tool Designation: The tool designation or tool signature, under ASA system is given in the order given next: Back rake, Side rake, End relief, Side relief, End cutting edge angle, Side cutting edge angle and nose radius that is.
αb ~ αs ~ θe ~ θs ~ Ce ~ Cs ~ R