Lathes are versatile machines that can be used to perform various machining operations.
Performing operations on a lathe require specific cutting tools, specially designed for each operation.
No matter how powerful your machine, the end results of your machining operations are dependent on the selection of the right tool for the operation.
But what sets these tools apart and how can you identify the right tool for your application?
Each tool has a specific geometry that makes them ideal for a particular machining operation.
In this article, I've discussed lathe cutting tools in detail and classified them on the basis of operation, structure, material, and feed direction.
What is a Lathe Cutting Tool?
Generally, lathe cutting tools consist of a sharp edge that is pressed against the surface of the rotating workpiece to remove the material at the desired depth. The cutting tool is either mounted on a tool post (metal lathes) or positioned on a tool rest (wood lathes).
The path traced by the cutting tool on the surface of the workpiece determines its final shape.
Unlike other machining operations, lathe machines consist of a rotating workpiece, mounted on the chuck, generally a 3 jaw or 4 jaw chuck, and a stationary cutting tool, known as a lathe cutting tool.
As a result, the cutting tools used in lathes are significantly different from the tools used in other machines like routers, mills, drills, etc.
Moreover, depending upon the control of the cutting tool movement, lathe machines can be either manual machines or automatic CNC machines.
However, the tools used in all these lathes have a similar nomenclature and geometry.
Geometry of a Single Point Cutting Tool
The shank of the cutting tool is the part that is clamped in the tool holder/tool post.
It is the thickest part of the cutting tool and generally has a rectangular cross-section.
Flank is the side of the cutting tool, which along with the face, forms the cutting edge.
A single-point cutting tool generally consists of two flanks: major flank and minor flank.
Face is the surface over which the chip slides when performing the cutting operation.
Cutting edge lies on the face of the cutting tool. It is the edge that performs the material removal action as the tool glides over the workpiece.
Generally, a single-point cutting tool has two cutting edges: side cutting edge and end cutting edge.
As a result, the cutting tool performs the cutting action on two surfaces at right angle to each other.
Nose is the corner of the cutting tool where the two cutting edges (side cutting edge and end cutting edge) meet.
It is slightly curved to provide greater strength, increase tool life, and produce a smoother cut.
Side rake angle
It is the angle between the tool face and the line perpendicular to the tool body.
The side rake angle determines the chip flow direction, and increasing the side rake angle of a turning tool reduces the thickness of the chip.
Side relief angle
When viewing from the front, the side relief angle is the angle made by the major flank with the shank surface perpendicular to the base of the cutting tool.
It provides clearance to prevent rubbing of the major flank against the workpiece during a longitudinal feed direction.
End relief angle
When viewing the tool from a side, the end relief angle is the angle made by the leading edge of the flank with a line normal to the base of the tool.
This angle provides clearance to prevent abrasion of the minor flank against the workpiece.
Back rake angle
Back rake angle determines the angle of inclination of the face of a cutting tool.
It is the angle made by the face with a plane parallel to the base, passing through the tip of the face.
Generally, a positive rake angle increases the sharpness of the tool but reduces its strength, and is therefore used for cutting soft materials.
Whereas negative rake angle increases the strength and facilitates easy chip flow, thereby making it ideal for machining hard materials.
However, it increases the cutting force, making the process prone to vibrations and high heat generation due to friction.
On the other hand, cutting tools with zero rake offer neutral cutting power, have a simple design, and are easier to manufacture.
End cutting edge angle
It is the angel made by the end cutting edge with a line running perpendicular to the body and tangential to the nose.
Similar to relief angels, the end cutting edge prevents interference of the tool and the machined surface of the workpiece.
Side cutting edge angle
It is the angel made by the side cutting edge with a line running parallel to the body of the cutting tool.
This angle affects the cutting forces and chip thickness while improving the performance of the tool by reducing the impact load.
Nose radius is the radius of curvature of the tip of the tool.
It improves the surface finish and increases the tool life by preventing sudden breakage from impact loading.
Different Types of Lathe Cutting Tools
Apart from being hand-held or mounted on a tool post, lathe cutting tools can be classified on the basis of their operation, structure, material, and feed direction.
On the Basis of Operation
A turning tool is used for material removal along the length of the workpiece.
This tool reduces the diameter of the workpiece to produce the desired shape and size. The process is known as turning.
These tools can be classified into two types: rough turning and finish turning.
Rough turning tools have a geometry ideal for removing maximum volume of material with minimum cycle time.
Whereas finish turning tools have a comparatively smaller cutting edge and are used to produce a smooth finish with accurate measurements.
These tools can be used for performing various different operations, like step turning, taper turning, forming, etc.
Chamfering is the operation of producing a slanting edge, inclined at an angle with the vertical axis.
Generally, a turning tool, with its cutting edge set at an angle to the face of the workpiece, is used for chamfering.
However, if the angle of inclination is high or the volume of chamfering requirement is high, a specially designed chamfering tool with a chamfered cutting edge is used.
Thread cutting tool
A thread cutting tool is used for cutting a spiral thread pattern on a cylindrical workpiece.
These tools have a specific cutting edge sharpened to match the width and shape of the desired threads.
Generally, the nose angle of a thread cutting tool is dependent upon the angle of thread to be cut. It is around 60° for metric threads and 55° for B.S.W threads.
Similarly, the cross-section of the cutting edge of the tool affects the pitch of threads.
Generally, when designing a threading tool, it is recommended to shape it in such a way that its width is around half the value of the thread's pitch.
The width of the cutting edge should be equal to half the pitch of the thread.
Apart from the pitch and size, the selection of the threading tool also depends upon the shape of the threads.
Square threads can be cut by using a specially customized tool suitable for your application.
When customizing a square thread cutting tool, it must be remembered that the forward side clearance angle of the tool should be 5° greater than the helix angle of the required square thread.
Whereas the clearance angle at the back end of the flank should be 5° less than the helix angle of the square thread.
Therefore, it is important to select a thread-cutting tool based on the shape, size, and pitch required for your threading operation.
Internal thread cutting tool
An internal thread-cutting tool is similar to a boring rod tool with a thread-cutting edge at its end.
This tool is used for cutting internal threads on parts such as nuts, caps, etc.
Unlike other processes that utilize both cutting edges of the tool, facing operation uses only the side cutting edge of the tool to remove the material.
Facing is generally performed to remove a thin layer of material and produce a smooth surface finish.
Therefore, a facing tool is similar to a finish turning tool used at the face of the workpiece.
Grooving tool, as the name suggests, is used for producing grooves on a cylindrical surface of the workpiece.
The shape of the groove is determined by the shape of the cutting tool. These tools can be V-shaped, square, or any special shape as required for the application.
A square grooving tool can also be used as a parting tool, where the tool is gradually pushed into the thickness of the workpiece until the part cuts off and falls on the bed of the lathe.
A forming tool is a combination of turning tool and grooving tool. These tools are used to produce relatively complex shapes in a single go.
A turning tool can also be used for performing forming operations, but a precisely designed forming tool reduces the cycle time and increases accuracy.
Boring tool is used to machine the internal surface of a hole and increase its diameter.
Generally, a boring tool consists of a boring bar with a cutting tool mounted on its end at right angle to the length of the bar.
The bar consists of slots to clamp the cutting tool and tighten it with an Allen key.
Final diameter of the hole is determined by the length of the cutting tool from the center of the boring bar.
A boring bar can hold multiple tools of different lengths to produce a hole with varying diameters.
Depending upon the type of boring tool used, it can be mounted on the tailstock (for long workpieces) or on the tool post (for very small boring depth).
A parting tool is similar to a grooving tool but has a narrow width (3mm - 12mm) to minimize material removal during cutting the workpiece.
These tools are generally forged and have a length greater than the radius of the workpiece.
Parting tool is an end-cutting tool with only a single edge performing the material removal action.
These tools have no side rake angle but a slight back rake facilitates easy chip clearance.
Furthermore, parting tools should be provided with clearance on all sides to prevent easy material removal from the sides of the cutting edge.
Knurling is the process of producing patterns of indents on the surface of the workpiece to enhance its grip.
It is generally performed on screwing objects that need a firm grip to apply force during screwing and unscrewing them.
A knurling tool consists of two or more metal rolling wheels with the desired pattern embossed on them.
This tool is clamped in the tool post and pressed against the rotating workpiece to remove the material and produce the desired pattern on the circumference of the workpiece.
On the Basis of Structure
Cutting tools can also be categorized on the basis of their structure, which affects their ability to deliver strong cutting force.
Single Body Tools
Single body tools are one of the most commonly used tools in industrial applications.
These tools are forged out of a single piece of metal and ground to have a sharp cutting edge of desired shape, size, and geometry.
Being forged from a single piece of metal gives them strength to deliver high cutting forces without the risk of breaking.
Generally, these tools are made from high-speed tools and are small in length.
Welded tools are manufactured by joining the body/shank and the head/flank of the tool with the help of a welded joint.
Generally, this type of tool consists of a flank made of special metal like carbide and a body made of a comparatively cheaper metal.
Carbide heads are tough and provide long tool life with excellent material removal capability.
However, when compared to a single body carbide tool, a welded carbide tool can deliver lower cutting force and therefore is ideal for machining with shallow depth of cuts.
Clamped tools consist of a long handle or bar that provides slot(s) to clamp the cutting tool on it.
These tools are generally made of two different materials, and used for applications like boring and parting where long tools are required.
Once the cutting tool is rendered blunt, you can replace the cutting head and reuse the handle/bar.
On the Basis of Material
Cutting tools can also be classified on the basis of their material and depending upon the workpiece to be machined, selecting the appropriate tool material can help you achieve optimal results.
High-Speed Steel (HSS) Tools
HSS tools are one of the most commonly used lathe tools that are comparatively cheaper and produce excellent results for roughing operations.
These tools are tough and can be used for turning various metal workpieces.
Furthermore, these cutting tools can be ground to sharpen the cutting edge and reuse the tool for machining.
Carbide is a tough material ideal for machining hard metals like stainless steel, carbon steel, HSS, tool steel, stone, etc.
These tools are known for their ability to deliver strong cutting force and retain their sharpness for long machining hours.
Diamond Tip Tools
Diamond tip tools provide high resistance to wear and heat, making them ideal for machining brittle and tough materials like graphite, aluminum, plastics, and other non-ferrous metals.
Unlike other cutting tools that are prone to work-hardening under high heat conditions, diamond tip tools provide high thermal conductivity and low thermal expansion.
This allows diamond tip tools to be used for long machining hours without affecting the tool sharpness.
Special Coated Tools
Special coated tools, as the name suggests, are generally HSS tools coated with special materials like ceramic, cubic boron nitride (CBN), tungsten carbide, etc.
These materials improve the tool life by enhancing the cutting strength and facilitating easy chip clearance.
Furthermore, these coatings can also improve the thermal coefficient of the tool which is favorable for dry machining.
On the Basis of Feed Direction
The geometry of a tool plays an important role in deciding the optimal feed direction.
Right-hand tools are ideal for applications where the feed direction is set from right to left, i.e, from tailstock to headstock.
When mounted on the tool stock and facing away from the operator, these tools have a side cutting edge angle dipping towards right, which means the tool face has an inclination towards right.
Similarly, left-hand tools are used for machining applications where the feed direction is set from left (headstock) to right (tailstock).
When mounted on the tool stock and facing away from the operator, these tools have a side cutting edge angle dipping towards left, which means the tool face has an inclination towards left.
Round Nose Tools
Round nose tools have a special geometry with no side rake and back rake angles that enables them to be used for both, left to right or right to left machining operations.
These tools are ideal for finishing operations where a smooth surface is required.
Frequently Asked Questions (FAQ)
Can we use a turning tool for facing operations?
Yes, a turning tool can be used for facing operations, provided you incline the tool at an appropriate angle to maintain proper contact between the cutting edge of the tool and face of the workpiece.
Can we use perform drilling operations on a lathe?
Yes, you can perform drilling operations on a lathe by mounting the drill bit on tailstock and feeding it towards the workpiece. However, typical 2-axis lathes can only perform drilling operations along the axis of rotation of the workpiece.
Is it necessary to provide continuous coolant flow during the machining of metals on a lathe?
Yes, it is necessary to provide continuous coolant flow during the machining of metals on a lathe because it prevents overheating which can otherwise damage the workpiece and render the tool blunt. However, you can use special techniques to perform dry machining that requires no or minimal coolant.