12 June 2017

Broaching process

Introduction :

Broaching is one of the most precise and productive processes in metalworking areas. In this process removal of a layer of material of desired width and depth usually in one stroke by a slender rod or bar type cutter having a series of cutting edge. It can be performed on a broaching machine.

The broaching term may have derived from ancient Roman word braces, which meant an object having projecting teeth. The operation itself dates only to the 1850s when broaching tools, then called "drifts" were hammered in blacksmith shops through the work or pushed through with an arbour process.

Basic principles of broaching :

Broaching is a process of machining in which removal of a layer of material of desired width and depth usually in one stroke by using a slender rod or bar type cutter. This rod or cutter having a series of cutting edges with gradually increased protrusion. Broaching and shaping are similar processes while in shaping, attaining full depth requires a number of strokes to remove the material in thin layers step by step by gradually in feeding the single point tool. Whereas, the broaching process can remove the whole material in one stroke by using a bar type cutter is called broach. 

By using the broaching process various forms of holes and various size of the section, internal and external through straight or helical slots or grooves or external surfaces of different shapes can be made. Teeth of external and internal splines and small spur gears are also been made by broaching.

Broaching process

Construction And Operation of Broaching :

Construction of any cutting tool is characterized mainly by :
  • Configuration
  • Material 
  • Cutting edge geometry
  • Configuration of the broaching tool

Pull and push-type broaches both are made in the form of slender rods or bars of the varying section having along with one or more rows of cutting teeth with increasing height. But push-type broaches are subjected to compressive load and therefore they are made shorter in length to avoid buckling.

Broaching tool configuration

The essential elements of the broach are following below :

  • Pull end for engaging the broach in the machine.
  • The neck of shorter diameter and length.
  • The front pilot for initial locating the broach in the hole.
  • Roughing and finishing teeth for metal removal.
  • Finishing and burnishing teeth.
  • Rear pilot and follower rest or retriever.

Broaches are designed mostly pull-type to facilitate alignment and avoid buckling. 

The length of the broach is mainly depended on :
  • Type of the broach ( Pull or Push type ).
  • Number of cutting edges.  
  • Pitch depending upon the work material. 
  • The maximum thickness of the material layer to be removed.
  • Nature and extent of finish required.
Broaches are generally made from solid rod or bar. The broaches of large section and complex shape are made by separate sections or inserting separate teeth.

Cutting motion of broaching

The material of broach :

In broaching process for cutting broaches are used. That is made of materials having the usual cutting tool material with high hardness, strength, toughness and good heat and wear resistance properties.

For ease of manufacture and resharpening the complex shape and cutting edges, broaches are mostly made of HSS (high-speed steel). Nowadays cemented carbide segments or replaceable inserts are used especially for stronger and harder work materials like steels and cast iron. TiN-coated carbides provide much longer tool life in broaching.

Broaching operation :

Broaching operation can done by a series of following sequential steps :
  • Selection of broach and broaching machine
  • Mounting and clamping the broach in the broaching machine
  • Fixing workpiece in the machine
  • Planning tool - work motions
  • Selection of the levels of the process parameters and their setting
  • Conducting machining by the broach.
Methods of broaching process :

  • Pull broaching :
In this type of broaching the work is held stationary and the brooch is pulled through the work. broaches are usually long and are held in a special head. Pull broaching is used mostly for internal broaching but it can do some surface broaching.
  • Push broaching :
In this type of broaching the work is held stationary and the broach is pushed through the work hand and hydraulic arbor press are popular for push broaching, this method is used mostly for sizing holes and cutting key ways. 
  • Surface broaching :
In this type of broaching either the work or the broaching tool moves across the other. This method has rapidly become an important for surface finishing. Many irregular or intricate shapes can be broached by surface broaching, but the tools must be specially designed for each job.
  • Continuous broaching : 
In this type of broaching the work is moved continuously and the broach is hell stationery. The path of moment maybe the straight horizontal or circular. This method is very suitable for broaching number of similar works at a time.

Different Types of broaches and their applications :

Broaching is widely used machining process, wherever feasible, for high productivity as well as product quality. There are different types of broaches have been developed and are used for wide range of applications.
Broaches can be broadly classified in several aspects such as 

Type of operation :
  • Internal broaching 
  • External broaching 
Method of operation :
  • Push type 
  • Pull type
Type of construction :
  • Solid
  • Built-up
  • Inserted tooth 
  • Progressive cut 
  • Rotor cut 
  • Double jump or overlapping tooth
Function :
  • Surface
  • Keyways
  • Round hole 
  • Splint
  • Spiral 
  • Burnishing 
Broaching of inside surfaces is called internal broaching or hole broaching and of outside surfaces, external or surface broaching. Internal broaching tools are designed to enlarge and cut various contours in holes already made by casting, forging, rolling, drilling, punching etc. It offers a combination of a high degree of accuracy and excellent surface finish, combined with high output rate and low downtime.

A push Broach is one that is designed to be pushed through the workpiece by special press or a push broaching matching. Because of tendency to band under compressive loads the push broach must be short and stocky, which means fever teeth are in broach and less material can be removed for each pass of the tools.

In a pull broach the tool is entirely in tension and long slender broaches are possible having a large number of teeth, consequently more stock can be removed for each pass.

When a brooch is made is one piece, it is called solid broach. Internal broaches are usually of the solid type. Broaches are sometimes build up of several sections and sometime made up of a series of teeth inserted in a block of steel. 

Surface broaches are usually of the build up or inserted tooth type. 

Progressive cut broaches have teeth a part of which are of the same height and have different width. In progressive cut broaching, metal is removed in thick layers by each tooth from only part of work-surface. The last teeth of a progressive cut broach remove a thin layer over the entire profile of work surfaces in ordinary cut broaching. 

Rotor cut broaches are used for removing large amount of materials in holes where forging or castings where a primary cutting operation is not desired. Teeth are staggered around the periphery at different sections so as to shear the work and allow cheap clearance. This would be an ideal tool to use for making a square hole from around cast one. 

A burnishing process makes a glazed or finished surface in steel, cast iron or nonferrous hole. Burnishing teeth are round and do not cut but compress and rub the surface metal. The amount of stock left for burnishing should not exceed 0.025 mm.

Advantages of broaching :
  • Very high production rate. Mainly higher than milling, planing, boring etc.
  • High dimensional, high accuracy and high class of surface finish of the product is possible.
  • Roughing and finishing in single stroke of the same cutter.
  • Needs only one motion of cutting, so design, construction, operation and control are simpler.
  • Extremely suitable and economic for mass production.
  • Expertise not needed.
  • Remarkable finished face.
  • Short cycle time with high accuracy.
  • Little skill is required to perform a broaching operation. In most cases the operator merely loads and unloads the work piece.
Limitations of broaching : 
  • Only through holes and surfaces can be machined.
  • Usable only for light cuts.
  • Cutting speed cannot be high.
  • Defects or damages in the broach severely affect product quality.
  • Design, manufacture and restoration of the broaches are difficult and expensive.
  • Economic only when the production volume is large.
  • Very large work piece can't be broached.