In a single point tool, there are various angles; each of them has a definite purpose. They are listed below in the order of importance: 1. Rake Angle 2. Clearance Angle 3. Cutting Angle 4. Lip Angle.
1. Rake-Angle:
It is the most important angle of the tool. The nominal rake-angle is the angle made by the face of tool and the plane parallel to the base of cutting tool. If the rake angle is measured in the direction of tool shank, it is called back rack-angle and if measured in a direction at right angles to it, then it is called side rake-angle.
The effective rake angle depends upon the position of tool relative to the job axis. The purpose of this angle is to allow the chips to flow plastically over the tool face, so that smoother action can take place.
Rake angle is the only angle on which the strength of the tool depends. Rake angle controls the chip formation, and is in turn governed by the mechanical properties of the material being cut. The force on the tool is reduced by increasing the rake angle but tool is weakened.
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Thus a compromise between the two factors has to be obtained. Here is a table of back and side rake angles for some of commonly used materials in production engineering.
Negative Rake Angle:
In brittle materials like brass, zero rake angle is provided, but in tougher materials like copper, negative rake angles are also used, because of tougher characteristics of the materials. The tougher characteristic has a tendency to cause the cutting edge of the tool to dig into the material and spoil the job surface. Carbide tipped tools usually have negative rake angles.
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The main purpose of using negative rakes is:
(i) To increase the strength of cutting tool point,
(ii) To give better finish,
(iii) To decrease the temperature rise at the tool-tip because more heat flows to chips from tool.
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2. Clearance Angle:
It is the angle of the end of side surfaces which are below the cutting edge. When the tool is in horizontal position, the normal clearance angle is measured from the plane perpendicular to the base of tool shank. The effective clearance depends upon the position of tool relative to the job.
The purpose of this angle is to avoid any frictional drage of the tool on the job and prevent the tool from rubbing on the surface already cut. Its magnitude depends upon the shape of the surface being cut, and is kept as small as possible to avoid weakening of the tool.
3. Cutting Angle:
The true cutting angle is the angle between the face of tool, and the line tangent to the machined surface at the cutting point.
4. Lip Angle:
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It is the angle between the tool face and the ground end surface of flank. It is usually between 60° to 80°.
Nose Radius:
Side and end cutting edges can be joined to form a point but that is not desirable as it leads to high heat concentration at a sharp point. Joining side and end cutting edges by an arc (nose radius) is the common practice.
Provision of nose radius improves tool life, surface finish and reduces cutting force. However, large nose radius results in chatter and that too is not desirable. Therefore, nose radius should be selected properly.
Tool Signature:
It is numerical method of identification of tool standardized by the American Standards Association (ASA) according to which the seven elements comprising signature of a single point tool are always stated in the following order:
(i) Back rake angle,
(ii) Side rake angle,
(iii) End clearance (relief angle),
(iv) Side clearance (relief angle),
(v) End cutting edge angle,
(vii) Side cutting edge angle, and
(viii) Nose radius.
Symbols of degrees for angles and units for nose radius are omitted and only numerical values of those components are indicated.