A Broaching Operation is a machining process that is used to create a well-finished hole or accurately remove material from the surface of the workpiece. A special type of tool is used for broaching Operations, known as a broach. A broach is a tapered tool with multiple progressive cutting teeth. Multiple cutting edges in broaching Operations create a finished surface hence reducing the requirement of surface finishing processes. Broaching is usually done for completion shapes that include non-circular holes, keyways, etc. High pressures are required for a broaching process, unlike other machining processes.
Principle of Broaching Process
The principle of cutting involves a relation between force, pressure, and area. When the area is minimal for a given amount of force the pressure exerted will be more. This is the reason why a sharp cutting tool can remove material from the workpiece. In case of broaching the tool with multiple progressive edges is pushed or pulled over the surface of the workpiece. The material removal starts from the narrow end of the taper to the broad end; this results in a finished product and the loads on each tooth are reduced to some extent.
History of Broaching
The broaching Operation has its existence back in the 1850s. It was initially used to cut keyways in pulleys and gears. After the first World War was over, the broaching Operation was used for machining rifles and guns. In the 1930s broaching machines with high tolerance and faster operations were introduced. As a result of which broaching became a key machining process for several industries.
Characteristics of Broaching
- The exact size of the hole is confirmed by the last set of teeth.
- Both roughing and finishing operations are completed by a single pass of the broach.
- High tolerance is provided using a broach.
- Broaching is cheaper than milling and boring.
- Both internal and external surfaces can be broached.
- Broaching was originally developed for keyways but now the principle is used for various surface operations as well.
- What makes broaching different from other machining processes is that the feed is built into the tooth.
- A variety of materials can be broached.
Types of Broaching
There are various types of broaching used for different purposes. Broadly broaching Operation is classified into two types
- Internal broaching
- External broaching
Internal broaching refers to the broaching operation in which the whole broach enters into the surface of the workpiece and then comes out from the opposite direction creating a finished cut. Internal broaching is used to produce holes, keyways, gear teeth, etc. Internal broaching is also used for enhancing the dimensions of a previously drilled hole due to its high accuracy. The previously drilled hole must be larger than the smallest diameter of the broach. At first, the broach is put into the drilled hole and then it is pushed or pulled through the hole. Push broaching can be used in the case of a hydraulic machine while a pull one is used for a higher production rate. Different types of holes can be produced using internal broaching methods like circular holes, keyways, square holes, triangular holes, etc.
External broaching refers to the broaching operation where the material removal takes place from the surface of the workpiece with the help of a broach. External broaching is also known as surface broaching. External broaching can be done on surface broaching machines. Simple surface broaching can also be done on an internal broaching machine. The external broach is made to slide over the workpiece which is resisted using a fixture. This sliding of the teeth on the workpiece results in the removal of material from the surface of the workpiece. During the return stroke, a clearance is created between the workpieces and the broach to prevent scratches due to the blunt edges.
Types of Broaches
There are two basic types of broaches used according to the type of broaching. A broach tool is generally made of high-speed steel (HSS) or cast iron with cutting edges of a harder material.
The two basic broach types are
- Internal broaches
- External broaches
The basic diagram of an internal broaching tool or an internal broach is given below.
- Total broach length is the total length of the broach from start to end.
- Stroke length is the length that should at least come out from the other side of the previously drilled hole.
- Stroke length consists of a pull end from where the broach is pulled. It also consists of a neck and front pilot.
- The rare end of the broach consists of a rare pilot and follower end.
- There are a series of teeth with sharp edges to perform the cutting action.
- The cutting length consists of roughing teeth and semi-finishing teeth.
- The first roughing tooth has the smallest area of cross-section and is increased gradually. Roughing teeth are responsible for maximum material removal and are more in number when compared to other teeth.
- After the series of roughing teeth comes semi-finishing teeth.
- The area of a cross-section of the first semi-finishing teeth is greater than that of the last roughing teeth.
- These are used to provide an armature surface finish on the workpiece.
- Then comes finishing teeth with the largest diameter and lowest numbers. These are used to give a fine surface finish to the workpiece.
The basic diagram of an external broach is given below.
- An external broach works on the surface of the workpiece.
- Length is the length of the teeth that are gradually increased in case of an external broach.
- Rake angle is the inclination of a tooth w.r.t the horizontal surface of the tool.
- Pitch is the distance from the beginning of one tooth to the other tooth.
- The horizontal length of the tooth from its cutting point to the end of the cutting edge is called land.
- Gullet radius is the curve provided between two consecutive teeth. It is useful for the removal of chips.
- The clearance angle is the angle made by the land and the surface of the workpiece with a cutting point in between.
Modes of Broaching
There are two modes or two ways in which broaching is done. The first one is pull-type broaching and the second one is push-type broaching.
Pull type broaching:-
In a pull-type broaching Operation the broach is pulled from one end of the broach.
- The force Acting on a pull-type broach is tensile.
- The number of teeth is more.
- The broach is pulled until the finish is not obtained.
- Has a wide range of applications.
- Production rate increases.
- Less risk of tool failure.
Push type broaching:-
In the push-type broaching Operation the broach is pushed through the surface of the workpiece.
- The force acting in case of push broaching is compressive.
- The number of teeth is less.
- The broach is pushed until the finish is obtained.
- Has limited applications.
- The production rate is lessened.
- There are high risks of tool failure.
Types of Broaching Machines
There are three basic types of broaching machines depending on their orientation and functions.
- Vertical broaching machine.
- Horizontal broaching machine.
- Continuous broaching machine.
Vertical Broaching Machine:-
A vertical broaching machine shown in the diagram below is the most widely used broaching machine.
- A vertical broaching machine consists of a cast iron base to support the whole setup. It is also used as a damper to absorb vibrations.
- It consists of a column that acts as a support element for the slide and the broach.
- There exists a worktable with attached fixtures where the workpiece can be clamped.
- The worktable is used to correctly position the workpiece to be broached.
- A vertical broaching machine can be automated or can be used manually.
- It promotes both internal and external broaching Operations.
- It also promotes push as well as pull-type of broaching Operations.
- The workpiece to be machined is placed on the worktable and is fixed firmly with the help of fixtures.
- The pulling mechanism is located at the base of the machine.
- As soon as the workpiece is clamped rigidly the broach is pushed or pulled towards the workpiece to be machined.
- Material removal takes place due to the action of progressive cutting edges.
- When the operation is finished the broach is moved up and removed from the workpiece and the finished product is removed by loosening the fixtures.
- The metal chips are carried down with the broach and are removed from the workpiece.
- A vertical broaching machine is suitable for heavy industrial purposes.
- Pull-up and push-down machines are available with the capacities of producing forces up to 500kN and a stroke length of 2 to 2.5m. (for internal broaching)
- Surface broaching machines are available with the capacities of producing forces up to 300kN and a stroke length of 2.5m.
- Down broaching is more suitable than up broaching because up broaching produces a problem of chip disposal.
Horizontal Broaching Machine:-
The diagram of a horizontal broaching machine is shown above.
- A horizontal broaching machine consists of a cast iron base to support the whole setup.
- It is also provided with a column to support the slide and different mechanical parts.
- It consists of a horizontal sliding arrangement that holds the broach.
- It is also provided with a worktable and fixtures for situating and clamping the workpiece respectively.
- Horizontal broaching machines are suitable for operations with long stroke lengths.
- They generally pull types due to the long stroke length involved.
- Material removal again takes place due to the action of progressive cutting edges on the workpiece.
- After the operation is done the broach is taken out and the workpiece is removed by loosening the fixtures.
- They occupy more space when compared to vertical broaching machines.
- Horizontal broaching machines for internal broaching have capacities up to 600kN and a stroke length of 3m.
- While horizontal broaching machines for surface broaching have capacities up to 1000kN and stroke length of 9m.
Continuous Broaching Machines:-
Continuous broaching machines are used when a large number of small parts are needed to be broached simultaneously.
There are two basic types of Continuous broaching machines.
- Rotatory continuous broaching machine
- Chain type continuous broaching machine.
Rotary Continuous Broaching Machine:-
In this type of Continuous broaching machine, the workpieces are clamped on a circular rotating disk and the broach tool is held stationary. As the disk rotates the workpiece comes in contact with the broach and material removal takes place due to the Action of rotational forces. The workpieces are accurately guided to the broach to remove material.
Chain Type Continuous Broaching Machine:-
In a chain-type continuous broaching machine the workpieces are held on the chain or a belt with the help of fixtures. The broach is held stationary at a single position. As the chain rotates the workpieces are moved horizontally and are passed through the broach which removes the material from the surface of the workpiece.
NOTE:- both the continuous type broaching machines are suitable for external broaching Operations only.
Advantages of Broaching Operations
- A broaching Operation provides high accuracy in a single pass.
- Good surface quality is obtained with the help of a broaching process. It minimizes the requirement of surface finishing processes.
- It can be used to make complex holes inside the workpiece.
- A broach is highly durable and does not get damaged.
- There are no requirements for a highly skilled operator. A semi-skilled operator can also operate a broaching machine.
- Applications of lubricants are easy and quick.
- It is an economical process for the mass production of machined parts.
Disadvantages of Broaching Operations
- The initial cost of a broach is high.
- It can be costly for small-scale production.
- The sharpening or grinding of a broach tool is time-consuming and difficult.
- The fixtures and the workpiece must be strong enough to withstand the cutting forces.
- It is only suitable for removing small amounts of metals.
Applications of Broaching Operations
Broaching is a very important production process that is capable of producing parts with good surface finish and dimensional accuracy. Some of the applications of broaching include
- Broaching is used in automotive industries for machining gear teeth.
- It is also used in producing keyways for pulleys.
- It is used in the casting and forging process for cutting grooves.
- Broaching is also used in agricultural-based industries for machining different parts.
- It is used for punching complex shapes in the workpiece.