In Fig. 12.73, the profile angle is the angle formed by the sides of the nose of the tool (or by the cutting edge and the following or relief edge); the front rake is the amount of slope from the cutting edge back towards the body or shank of the tool; the side rake is the side slope given to the top of the tool; front clearance is the angle formed by the front of the tool and a line at right angles to the base; side clearance is similar to front clearance, and the cutting angle is formed by the top and front faces of the tool.
The proper selection of these angles in a tool is very important. Comparing the cutting action with a wedge trying to penetrate into a metal, it may be appreciated that for easier penetration the angle of wedge should be acute. If it is too acute, then the front edge will not be sufficiently supported and instead of penetrating will break off.
A cutting tool has not only to penetrate the work but it has to continue its cutting action and offer resistance to bending and abrasion action for some considerable time. It is, therefore necessary to give the cutting edge sufficient support to enable it to withstand the forces acting against it.
Profile Angles:
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The profile angle of a front tool is formed by the cutting edge and the following edge or relief edge. Fig. 12.74 shows various types of profile angles and the applications for which they are used.
For heavy cuts on stiff pieces of work, the cutting edge of the tool is generally rounded to a radius not exceeding about 15 mm. This form of tool is less liable to cause chatter marks on the surface of the work. Its general usefulness in a centre lathe, however, is less than a tool provided with a small nose radius.
Front Rake:
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The front rake of a tool is determined by the cutting angle and front clearance and is dependent upon the hardness and tenacity of the metal being cut. In the case of hard and brittle metals (like cast iron, brass etc.), the turnings break off short and thus considerable support is required to be given to the cutting edge. Therefore, only a small amount of front rake can be allowed in such case.
On softer and more ductile metals like mild steel and wrought iron the front rake can be increased. For very soft metals and alloys like copper and aluminium in which case the penetration is comparatively easy and the cuttings do not readily break up, the front rake can be considerably increased.
Side Rake:
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The side rake or downward slope of the tool from the cutting edge depends on:
(i) Hardness and tenacity of the metal being cut,
(ii) Depth of cut,
(iii) Feed per revolution, and
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(iv) Class of surface finish desired.
The front rake and side rake on the cutting tool help in reducing the power absorbed during cutting operation, penetration becomes easier, and the surface finish is also greatly improved. On the other hand, increasing these rakes too much will make the cutting edge weak enough to stand up the abrasive and bending action of the work for a reasonable length of time.
The most efficient and satisfactory angles can be determined by performing the actual tests under a fixed set of conditions.
Front Clearance:
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It is the angle formed by the front of the tool and a line drawn at a tangent to the work at right angles to the lathe centres.
The diameter of the work has some influence on the amount of front clearance e.g., a tool having small front clearance may work well on a work of small diameter quite satisfactorily, but it will rub without cutting on a job of larger diameter. For hard and tough metals and alloys, the front clearance has to be kept minimum in order to obtain the maximum support to the cutting edge of the tool.
The rate of feed also has a considerable influence on the front clearance and must be taken into account. If excessive clearance than that desired is provided then chatter marks are likely to appear on the finished surface of the work.
Side Clearance:
For coarse feed, the side clearance should be given liberally to allow the necessary tool advance without rubbing below the cutting edge. This aspect is very important when cutting coarse pitched square threads.
Cutting Angle:
Although many fixed and variable factors, in addition to the nature of the metal being cut, contribute towards determining the most efficient cutting angle, but due to the advantages of specialisation in grinding the tool in tool room department, it is usual to standardise shapes and angles of the tools.
The average tool angles for cutting various materials with high speed steel tool are given below:
Tool Nomenclature as per American Practice: (Refer Fig. 12.75)
Standard commonly followed in India are:
i. Shank:
It is the body of the tool or part on one end of which the cutting edge is formed.
ii. Nose:
It relates to the tip of the cutting end.
iii. Face:
It is the surface against which the chips bear.
iv. Flank:
It is that end surface which is adjacent to the cutting edge and below it when the tool is in a horizontal position as for turning.
v. Base:
It is that surface of the tool shank which bears against the supporting tool-holder or block.
vi. Nose Radius:
For heavy depths of cut and feeds and interrupted cuts, the nose is usually given a radius of 1.5 mm in order to obtain higher tool life and better surface finish. However, too large a nose radius would lead to chatter. The depth of cut should be more than nose radius in order to obtain good surface finish. For fragile work piece which can’t be held securely, nose radius is taken as 0.4 mm.
vii. Rake Angle:
The nominal rake angle is the angle between the tool face and a plane parallel to the base of the tool. Back rake angle is measured in the direction of the tool shank and side rake angle in a direction at right angle to the tool shank. The active or effective rake of a turning tool depends upon its position relative to the axis of the work.
viii. Back Rake Angle:
It is the angle between the face of the tool and the base of the shank, measured in a plane through the side cutting edge and at right angles to the base. If the tool faces downwards from the point towards the shank, (as in Fig. 12.75) then back rake angle is + ve. Positive back rake angle takes the chips away from the machined surface.
If side cutting edge faces upwards too-wards the shank, then this angle is – ve. It would be appreciated that – ve back rake angle strengthens the tool and is used on rigid machines for heavy work.
ix. Side Rake Angle:
It is the angle between the base of the tool shank and the face of the tool measured in a plane perpendicular to the plane through the side cutting edge and at right angles to the base.
If the tool face is sloping upwards towards the side cutting edge, this angle is + ve, and – ve if it is sloping downwards towards the side cutting edge. Positive side rake angle results in lower cutting force and power and thus better cutting action. Negative side rake angle is used for roughing cuts and heavy duty applications.
x. End Cutting Edge angle:
It is the angle between the end or auxiliary cutting edge and the work axis, and it varies between 0 to 30°. It provides the required clearance to the trailing edge and reduces the drag that tends to cause chatter.
xi. Clearance Angle:
It is the angle of the end or side surfaces which are below the cutting edge when the tool is in a horizontal position as for turning. Side clearance angle is the clearance on the side cutting edge, and it is the angle between the portion of the side flank immediately below the cutting edge and a plane drawn from the cutting edge perpendicular to the base. Similar relief is provided on end cutting edge also.
The nominal clearance angle is measured from a plane that is perpendicular to the base of the tool shank. The effective clearance angle may be greater or less than this depending upon the position of the tool relative to the axis of the work. Higher clearance values reduce the wear and result in a clean cut on metals of low strength. Lower values give a better support to the cutting edge and they are suitable for cutting tougher metals.
xii. Relief Angle:
Clearance may consist of two angles (as shown in Fig. 12.75) to reduce the amount of end surface and amount of grinding required in sharpening the tool. In such cases, the angle of that surface which is adjacent to or just below the cutting edge may be called the relief angle end relief or side relief angle, depending upon the location of the surface.
xiii. Cutting Angle:
The true cutting angle is the angle between the face of the tool and a line tangent to the machined surface at the cutting point. The true cutting angle depends upon the position of a turning tool relative to the axis of the work.
xiv. Lip Angle:
It is the angle between the tool face and the ground end surface or flank. If the tool has side and back rake, the lip angle should be measured in whatever plane it is smallest.
xv. Nominal Size:
The size of tool is expressed by giving the width of the shank, the height of the shank and the total length.
xvi. Approach Angle:
It is the angle between the direction of feed and the side cutting edge. It is the complementary angle of the side cutting edge angle. Its value may vary between 0 to 90° or even more. The thickness of chip depends upon the value of approach angle.
For smaller approach angle, the cutting pressure is spread over greater length of the cutting edge and therefore tool life is more. Small approach angle increases radial component of cutting force which has tendency to separate the work from the tool and hence more chatter. For general turning of rigid work, approach angle of 70° is used. For slender shafts, approach angle of 90° is used.