Lathe machines are probably the most common and important machine in a mechanical tool room. In this post, we will understand a lathe machine’s working principle, its types, and features, and the operations we can perform on it.
What is the working principle of a Lathe machine?
- The article that we are machining using the lathe machine is known as the job.
- The tool that we attach to the lathe machine to perform a particular operation on the job is known as a tool/cutting tool.
- In a lathe machine, the job is held between two centers. These centers support the job and hold it firmly in place. The job needs to be held down securely.
- The job rotates about its own axis.
- The cutting tool is placed on the tool post. (Check out the diagram for a four-way tool post below.)
- Due to the relative motion of the job and the cutting tool, the material gets removed from the job in the form of chips and shavings.
What are the different types of lathe machines? How are lathe machines classified?
There are seven main types of lathe machines that we will be looking at in this course:
- Speed lathe
- Engine/Center lathe
- Bench lathe
- Tool room lathe
- Capstan and Turret lathe
- Automatic lathe
- Special purpose lathe.
How does an engine/center lathe machine work?
We are going to completely understand every working principle and every function of the engine/center lathe machine. Let’s kick things off with a gander at the block diagram of the lathe machine.
Block diagram of a lathe machine (Engine/Center lathe)
The figure highlights the main parts of a lathe machine. We will dive into each of them and understand their purpose and functions. The main parts of a lathe machine are:
- Feed mechanism
Bed of a lathe
- The bed is the foundation of the lathe machine on which all the other major parts lie.
- The top of the bed has two longitudinal guideways.
- These guideways are made with precision to ensure the accurate alignment of other parts.
- The tailstock can also slide on these guideways.
- The bed of a lathe machine is strong enough to resist vibrations due to cutting forces.
- The bed of the lathe machine is made using cast iron.
The headstock of a lathe
- It is permanently fastened at the left end of the lathe on the bed.
- The headstock serves the following purposes:
- It supports the spindle and provides provisions for a rotating live center.
- The headstock provides the power required for rotating the work at various speeds. It also provides power for tool movement. The headstock receives this power from an electric motor, and it uses pullets and gears for obtaining different speeds on the spindle.
- The external surface of the front end of the headstock is threaded to provide attachment facilities for job holding devices such as chucks and drives etc.
- In modern lathes, pulleys are replaced by gears.
The tailstock of a lathe
- The tailstock of a lathe is mounted at the right end of the bed. It is opposite in position to the headstock.
- The tailstock can move along the bed of the lathe to accommodate different lengths of the work.
- Here are some of the uses of the tailstock of the lathe:
- The tailstock is used as a support when the job is turning between the centers.
- It is used to support the job when a chuck is used at the headstock too.
- It is also used for attachments of tools like drills, tap, etc. These tools or the dead center can be attached at the front of the spindle, which has a tapered hole.
- The tailstock can be moved forward or backward by using a screw.
- The spindle is also movable and can be moved in and out of the barrel.
- A handwheel is used for lateral movement.
The carriage of a lathe
The carriage of a lathe is a part that provides attachments whose function is to support, guide, and control the cutting tool. These are the main parts of the carriage of the lathe:
- Cross slide
- Compound rest
- Tool post
- The saddle is an H shaped casting. It fits on the bed.
- The saddle slides between the headstock and tailstock.
- It carries the cross slide and the toolpost.
- The cross slide is mounted on the saddle and supports the compound rest.
- The cross slide is used to move the cutting tool in a perpendicular direction to the axis of the lathe.
- It moves using a feed screw. A handwheel controls the feed screw.
- The compound rest is mounted on the cross slide. It supports the tool post.
- The compound rest can swivel in any desired angle in the horizontal plane.
- The base of the compound rest is graduated in degrees to indicate its current angular settings.
- It is movable.
- The compound rest of a lathe is used for obtaining angular cuts, short taperings as well as to position the tool.
The tool post
- It is mounted on the compound rest and supports the tool.
- The tool post’s job is to hold the tool in place and to enable us to adjust it to a convenient position.
- Four-way tool posts can hold four tools at once.
- An apron is attached to the saddle where it hangs.
- The apron contains gears, clutches, and levers for operating the carriage.
- There is a hand wheel for manual movement of the carriage.
Legs of the lathe
- The legs are the support of the lathe machine. They carry the entire load of the machine.
- The legs are made up of cast iron. They can be secured to the ground using bolts for extra stability.
- The left-hand leg houses the electric motor.
Feed mechanism of the lathe
- The relative motion between the tool and the work is known as ‘feed.’
- So, the feed mechanism consists of all the parts that cause or support this relative motion. The carriage is a part of the feed mechanism. Similarly, the gears, the levers, and the pulleys which transmit motion from the headstock spindle to the carriage are all parts of the feed mechanism of the lathe.
- There are three types of feed of the tool:
- Longitudinal feed: When the tool moves in parallel to the lathe axis, via a carriage.
- Cross-slide: When the tool moves in a perpendicular direction to the lathe axis.
- Angular feed: When the tool moves at an angle to the lathe axis with the help of compound rest.
What are the three cutting parameters of lathe machines?
- Cutting speed: This is the speed at which material is removed by the tool from the workpiece. It’s generally expressed in meters/minute. In lathes, the cutting speed is obtained from the rotary motion given to the work.
- Feed: It is the distance which the tool moves for each revolution of the work. We express it in mm/revolution.
- Depth of cut: It is the depth of penetration of the tool into the workpiece during machining. It is usually expressed in millimeters. In a lathe, depth of cut is the perpendicular distance between the machined and unmachined parts of the work.
What are the different operations that can be done on a lathe machine? What are the different applications of a lathe machine?
Here are the different operations that can be done on a lathe:
- Parting off
- Drilling and Reaming
- Thread Cutting
Let’s understand each of these operations one by one.
- In Turning, two centers support the job.
- The job rotates on its own axis.
- The tool is fixed on the toolpost.
- In this operation, the diameter of the work is reduced due to the relative motion between the tool and the job to a desirable dimension. The tool moves in parallel to the axis.
- If the diameter reduces uniformly throughout the length of the work, it’s known as straight turning.
- When different diameters are obtained over the length, it’s called step turning.
- If the diameter is reduced in an angle, it’s called taper turning.
- Shoulder turning, eccentric turning, and rough turning are the other three types of turning operations.
- In Facing, two centers support the job. The job rotates on its own axis.
- The tool is fixed on the toolpost. And it is held slightly inclined to the job though its motion is perpendicular to the axis of the job due to the cross-slide.
- In facing operation, the ends of the work are made flat, and the length of the job is reduced.
- The cutting tool moves perpendicular to the job. Due to the relative motion between the job and the tool, part of the job gets removed in the form of chips.
- Job supported by two centers and rotates on its own axis.
- The tool is fixed on the toolpost. It’s held at a small angle to the job and moves perpendicular to the job.
- Due to the relative motion, the edges of the job undergoes tapering.
- Chamfering is a safety feature. It is done to make sure that the sharp edges of something don’t harm someone who is handling the job.
- The job is attached to a chuck.
- It rotates about its own axis.
- The parting tool is mounted on the tool post, and the feed is perpendicular to the axis of the job.
- In parting off, we remove a part of a particular length from the remaining job.
- The job is supported by two centers. It rotates about its own axis.
- Knurling operation is done to provide some grip on the job. E.g.: watch turners.
- The knurling tool is kept on the toolpost and pressed perpendicularly to the job until the patterns on the rollers are formed on the job.
- These patterns are usually square or diamond in shape.
- Note that material is not removed, knurling only creates an impression.
Drilling and Reaming
What is the difference between drilling using a lathe and drilling using a drilling machine?
In the drilling operation that we perform on a lathe machine, the job rotates while the tool stays still. Whereas in a drilling machine, the job stays still, and the tool rotates.
- The job rotates about its own axis, but the tool (drill) is fit into the tailstock instead of the toolpost.
- In a drilling operation, a hole is made into the job.
- The drill is fed along the axis of the rotating job, and due to relative motion, we get a hole of a desirable diameter.
- After drilling, reaming is the process where the hole undergoes correction to a desirable size.
- A small amount of material is removed in reaming.
- The job is attached to a chuck. The job rotates on its own axis.
- The boring tool is fitted to the tool post.
- In the boring operation, an already drilled hole is enlarged through relative motion between job and tool.
- However, boring cannot be done prior to drilling.
- If an operation is carried out up to a certain length of the job, it’s called counter-boring.
- Work is held between two centers and rotates about its own axis.
- Thread cutting is an important operation where helical grooves are produced on the job by feeding the tool longitudinally on the rotating job.
- The longitudinal feed should be equal to the pitch of the grooves. Automatic movement of the carriage is required for constant pitch.
Longitudinal feed = pitch/revolution
- The ratio of longitudinal feed to the speed of headstock spindle should be such that the relative speeds of work and lead screw cause the grooves.
- Change gears are used to engage relative motion between the lead screw and half nut, achieving automatic motion of the carriage.
- The speed of the lead screws controls the pitch.
This is an excellent two video series that makes a machinist hammer from scratch. A lot of the operations that we have seen in this post are shown. Check it out. Next up, we have a similar guide for drilling machines for you.