The tool must have a specific geometry (known as tool geometry) for effective cutting and smooth surface finish. According to the tool geometry, the cutting tools can be classified into solid cutting tools and carbide tipped tools.
Machining is one type of manufacturing operation where excess material is gradually removed by shearing in the form of chip from a preformed blank. A rigid, hard, wedge shaped device, called cutting tool, is employed for compressing the work material and thereby shearing the excess layer of material.
So the purpose of cutting tool (also called cutter) is to compress a particular layer of work material in order to shear it off. Therefore, cutter must have wedge shape with sharp edge for smoothly and efficiently removing material requiring minimum power. At the same time cutter material should be sufficiently hard so as to withstand intense rubbing occurred during machining. Along with the definition and example, purposes, various features, designations, materials and classification of cutting tools are discussed the following sections.
- In machine tool: Cutting tools. Metal-cutting tools are classified as single point or multiple point. A single-point cutting tool can be used for increasing the size of holes, or boring. Turning and boring are performed on lathes and boring mills. Multiple-point cutting tools have two or more cutting edges Read More; application of abrasives.
- These tools are dual-purpose tools with two different types of bits on the head: One side is a cutting edge similar to an axe blade for chopping stumps and logs. The other edge, called the grubbing edge, is used to dig out roots and cut trenches.
Cutting tool is a wedge shaped and sharp edged device that is used to remove excess layer of material from the workpiece by shearing during machining in order to obtain desired shape, size and accuracy. It is rigidly mounted on the machine tool. A relative velocity between workpiece and cutting tool is also provided by various mechanical and other arrangements for cutting action.
Cutting tool is basically the cutter used in machining operation. Various machining operations utilize different cutters and thus various names are available for these cutters based on the application. A list of commonly used cutting tools is provided below.
- Single point turning tool—cutter for turning operation performed in lathe
- Drill—cutter for drilling operation performed on drilling machine or lathe or milling machine
- Milling cutter (or mill)—cutter for milling operations performed on milling machine
- Fly cutter—cutter for fly milling operation performed in milling machine
- Shaper—cutter for shaping operation performed in shaping machine
- Planer—cutter for planing operation performed in planing machine
- Boring bar—cutter for boring operation performed in drilling or boring machine
- Reamer—cutter for reaming operation performed in drilling machine
- Broach—cutter for broaching operation performed in broaching machine
- Hob—cutter for hobbing operation performed in hobbing machine
- Grinding wheel—abrasive cutter for grinding operation performed in grinding machine.
Geometry of a cutting tool encompasses inclination and orientation of various planes and cutting edges of the tool as well as nose radius. Tool designation basically refers displaying various features of the cutting tool in a symbolic but standardized manner. There are various tool designation systems and each has specific style of representing such features. Among various systems, the commonly used systems for turning tool designation are enlisted below.
- Tool In Hand system
- American Standards Association (ASA) system
- Orthogonal Rake System (ORS) or ISO Old System
- Normal Rake System (NRS) or ISO New System
- Maximum Rake System (MRS)
During machining, part of the cutting tool remains in physical contact with the workpiece and thus experiences severe cutting temperature and insistent rubbing. The material of the cutting tool must have the capability to sustain such high cutting temperature as well as cutting force. Every tool material must possesses certain properties such as high hardness, high hot hardness, high strength, higher melting point and chemically inert even at high cutting temperature. As a thumb rule, the hardness of the tool material should be at least 1.5 times of the hardness of the workpiece for smooth cutting action.
Suitable coating can also be applied on the tool to improve various desired properties. However, a coated tool does not allow easy re-sharpening by grinding when the edges are worn out after prolonged use. Now-a-days, insert based tools are available where small interchangeable inserts can be attached or clamped on large shank. These inserts perform cutting action and thus worn out gradually. When wear exceeds the tolerable limit, the inserts can be replaced by a new one, while the shank can be used repeatedly. Some of the tool materials commonly available in todays’ market are enlisted below.
- High Speed Steel (HSS)
- Tungsten carbide
- Ceramics
- Cubic Boron Nitride (cBN)
- Diamond
Cutting tools can be classified in various ways; however the most common way is based on the number of main cutting edges that participates in cutting action at a time. On this basis, cutting tools can be classified into three groups as given below.
- Single point cutting tool—Such cutters have only one main cutting edge that participate in cutting action at a time. Examples include turning tool, boring tool, fly cutter, slotting tool, etc.
- Double point cutting tool—As the name implies, these tools contain two cutting edges that simultaneously participate in cutting action at a pass. Example includes drill (common metal cutting drill that has only two flutes).
- Multi-point cutting tool—These tools contain more than two main cutting edges that can simultaneously remove material in a single pass. Examples include milling cutter, broach, gear hobbing cutter, grinding wheel, etc.
- Book: Machining and Machine Tools by A. B. Chattopadhyay.
- Book: Metal Cutting: Theory And Practice by A. Bhattacharya.
- Book: Manufacturing Process for Engineering Materials by S. Kalpakjain and S. Schmid.
- Book: Geometry of Single-point Turning Tools and Drills – Fundamentals and Practical Applications by V. P. Astakhov.
After reading this article you will learn about:- 1. Meaning of Cutting Tool 2. Types of Cutting Tools 3. Angles 4. Signature.
Meaning of Cutting Tool:
A cutting tool in metal working can be defined as “any tool that is used to remove metal from the work piece by means of shear deformation”. Frequently, it also refers as a tool bit. In order to perform effective cutting operation, the cutting tool must be made of a material harder than the work material to be cut. Also, the tool must be able to withstand the heat generated during machining process.
The tool must have a specific geometry (known as tool geometry) for effective cutting and smooth surface finish. According to the tool geometry, the cutting tools can be classified into solid cutting tools and carbide tipped tools.
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There are two surfaces adjacent to the cutting edge of the tool:
(a) Rake surface.
(b) Flank surface.
(a) Rake Surface:
Rake surface directs the flow of newly formed chip. It is oriented at a certain angle is called the rake angel ‘a’. It is measured relative to the plane perpendicular to the work surface. The rake angle can be positive or negative.
(b) Flank Surface:
The flank surface of the tool provides a clearance between the tool and the newly formed work surface, thus protecting the surface from abrasion which would degrade the finish. This angle between work surface and the flank surface is called the relief or clearance angle.
Types of Cutting Tools:
Various cutting operations require various types of cutting tools. To achieve good surface quality, proper cutting tool selection is very important.
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Following are some important parameters to be considered while selecting a cutting tool for particular machining operation:
i. Geometry.
ii. Material to be machined.
iii. Shape and Size of part.
iv. Type of operation required.
v. Machine tool quality.
vi. Surface finish required.
vii. Holding facility.
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viii. Machining parameters such as feed speed and depth of cut selected.
The various types of cutting tools are shown in Fig. 9.11.
The major classifications of cutting tools are following:
(i) According to Construction:
(a) Solid tool.
(b) Carbide tipped tool.
(ii) According to Number of Cutting Edges:
(a) Single point tool.
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(b) Multipoint tool.
(c) Formed (Tailor designed) tool.
(iii) According to Shape:
(a) Square.
(b) Circular.
(c) Left hand.
(d) Right hand.
(e) Round nose.
(f) Straight nose.
(iv) According to Operations:
(a) Turning.
(b) Drilling.
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(c) Threading.
(d) Knurling.
(e) Boring.
(f) Forming.
(g) Parting-off.
(h) Reaming.
(v) According to Type of Cutting Tool Material:
(a) H.S.S.
(b) Carbide.
(c) Ceramics.
(d) Diamond.
Cutting Tool Angles:
The face and the flank are pain surfaces, the cutting edge can be assumed to be a line. These surfaces and the edges are inclined with respect to some reference plan or line. The inclinations are called tool angles.
These angles are defined by various names. They are provided for various purposes. Consider the case of the face abgf, as shown in Fig. 9.12. It is a plane surface no doubt, but can have some inclinations. This surface may be parallel to the base or say to horizontal surface, or it can be inclined upward or downward with respect to the horizontal plane. Again it may have inclination sideward also. So in general the face can have two inclinations simultaneously, backward and sideward. Similarly the flank (Principal flank abed or auxiliary flank adef) can have two inclinations.
For efficient machining operation, the cutting tool must be provided with necessary tool angles. A tool with proper geometry (cutting edge and tool angles) cuts the metal effectively. Therefore reducing the chattering, breaking of the tool with less heat generation. Fig 9. 14. (a) and (b) shows a single point cutting tool with various cutting edges and tool angles.
From the geometry of cutting tool the various cutting tool angles are:
Rake Angle (α):
(a) Black rake angle.
(b) Side rake angle.
Clearance or Relief Angle (γ):
(a) End clearance relief angle.
(b) Side clearance relief angle.
Cutting Edge Angle:
Cutting Tools (types)the Mechanic Parts
(a) End cutting edge angle.
(b) Side cutting edge angle.
(i) Back Rake Angle:
It is the angle between the face of the tool and plane parallel to its base. It is also known as front rake angle or top rake angle.
(ii) Side Rake Angle:
It is the angle between the face of the tool and the shank of the tool.
(iii) End Clearance (Relief) Angle:
It is the angle between the front surface of the tool and a line normal to the base of the tool. It is also known as front clearance angle.
(iv) Side Clearance (Relief) Angle:
It is the angle between the side surface of the tool and a line normal to the base of the tool.
(v) End Cutting Edge Angle:
It is the angle between the end cutting edge of the tool and a line perpendicular to its shank.
(vi) Side Cutting Edge Angle:
It is the angle between the side cutting edge of the tool and shank of the tool.
(vii) Nose Radius:
Nose radius is one which connects the side and end cutting edge. Now, we will discuss the functions and effects of cutting tool angles on cutting process.
Functions of Back Rake Angle:
(a) It helps to control the chip flow in a convenient direction.
(b) It reduces the cutting force required to shear the metal and consequently helps to reduces power requirements and increase tool life.
(c) It also helps counteract the pressure against the cutting tool from the work by pulling the tool into the work.
(d) It provides keenness to the cutting edge and improves the surface finish.
Functions of Side Rake angle:
(a) It performs similar functions as performed by back rake angle.
(b) Side rake angle along with back rake angle controls the chip flow direction.
(c) It partly counteracts the resistance of the work to the movement of the cutter.
(d) For example, brass requires a back and side rake angle of almost 0°, while aluminum uses a back rake of 35° and a side rake of 15°.
Functions of End Clearance (relief) Angle:
(a) It allows the tool to cut freely without rubbing against the work surface.
(b) This angle varies from 0° to 15°, and usually 8°.
(c) Excessive relief angle reduces strength of the tool.
Functions of Side Clearance (relief) Angle:
i. It avoids the rubbing of flank against the work piece when the tool is fed longitudinally.
ii. This angle is 6° to 10° for steel, 8° for aluminum.
iii. It maintains that no part of the tool besides the actual cutting edge can touch the work.
Functions of End Cutting Edge Angle:
i. It avoids rubbing between the edge of the tool and workspace.
ii. It influences the direction of chip flow.
Functions of Side Cutting Edge Angle:
i. Increase in side cutting edge angle tends to widen and thin the chip.
ii. An excessive side cutting edge angle redirects feed forces in radial direction which may cause chatter.
Open The Cutting Tool
Functions of Nose Radius:
i. A sharp point at the end of tool is undesirable, because it is highly stressed, short lived and leaves groove in the path of cut.
ii. Therefore Nose Radius is favourable for long tool life and good surface quality.
iii. It affects the tool life, radial force, and surface quality of work piece.
iv. If nose radius is too large chatter will occur.
v. There is an optimum value of the nose radius at which the tool life is maximum.
vi. If the nose radius exceeds optimum value, the tool life decreases.
vii. Larger nose radius means larger area of contact between tool and work piece. Resulting more frictional heat is generated. Also, cutting force increases due to which the work part may start vibrating and chattering, if work part holding is not very tight.
viii. The recommendations for use of more nose radius are.
R= 0.4 mm for delicate components.
R = 0.4 mm to 1.2 mm for disposable carbide inserts for common use.
R = 1.2 mm to 1.5 mm for heavy duty inserts.
R ≥ 1.5 mm for heavy depth of cut, interrupted cuts and heavy feeds.
Significance of Rake Angle:
1. The rake angles may be positive, zero or negative.
2. An increased rake angle will reduce the strength of the cutting edge.
3. Rake angle affects the values of cutting angle and the shear angle.
4. Larger the rake angle, smaller the cutting angle (and larger the shear angle).
5. In general, the small rake angle is used for cutting hard metals and a larger rake angle is used for cutting soft and ductile metals.
6. The use of negative rake angle started with the employment of carbide cutting tools. When positive rake angle is used, the force on the tool is directed towards the cutting edge, tending to chip or break it, as shown in Fig. 9.15(a).
7. Since the carbide material is brittle and lacks shock resistance, it 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 protects to the cutting edge, as shown in Fig. 9.15 (b).
8. The use of negative rake angle increases the cutting force. This can compensate by higher cutting speeds. Therefore, high cutting speeds are always used with negative rake angles. High cutting speeds require high power of the machine tool.
9. The use of index able inserts also need the use of negative rake angles.
10. A negative rake angle insert has twice life than an equivalent positive rake angle insert.
11. Negative rake angle increases cutting edge strength, because the cutting force acts on the middle of cutting edge.
12. Positive rake angle decreases cutting edge strength, because the cutting force acts on the end or corner of the cutting edge.
13. Positive rake angle recommendations are:
(a) When machining low strength metals and alloys, such as aluminum and copper alloys, mild steel, etc.
(b) Where cutting at low speeds.
(c) When set up has low strength and rigidity.
(d) When low power machines used.
(e) When tool materials are H.S.S. and cast alloys.
14. Negative rake angle recommendations are:
(a) When machining high strength metal and alloys, such as stainless steel, alloy tool steel, titanium alloys, etc.
Table 9.4. Gives the recommended rake angles for various combinations of work and tool materials:
Tool Signature:
Tool signature is the specification or nomenclature of the tool which provides information regarding various tool angles and nose radius.
It includes seven parameters in specified order as given below:
(i) Back rake angle.
(ii) Side rake angle.
(iii) End relief (clearance) angle.
(iv) Side relief angle,
(v) End cutting edge angle.
(vi) Side cutting edge angle.
(vii) Nose radius.
For example:
(a) If the tool signature is 12, 15, 7, 6, 10, 15, 0.8
Means,
Back rake angle (degree): 12
Side rake angle: 15
End relief angle: 07
Side relief angle: 06
End cutting edge angle: 10
Side cutting edge angle: 15
Nose radius (mm): 0.8
(b) If the tool signature is -10, 15, 8, 6, 8, 5, 0.5
Here, also the meaning is back rake angle is negative 10 degree, side rake angle is 15 degree, End relief angle is 08 degree, side relief angle is 06 degree, End cutting edge angle is 08 degree, side cutting edge angle is 05 degree and nose radius is 0.5 mm.
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