02-07-2014, 10:15 AM
BROACHING
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INTRODUCTION
Before understanding the process of broaching and the related machining tools it is required to know the clear difference between single point and multipoint cutting tool.
A single point cutting tool normally consists of a single cutting point or an edge used for cutting. The example of single point cutting tool is turning tool used for turning tool is generally has more than one cutting points or cutting edges in action at a time. Example of multipoint cutting tool are milling cutter or a broaching tool.
Broaching is one of the metal machining operations done by a multipoint cutting tool called broaching tool or broach. The tool is made reciprocating linearly relative to the workpiece in the direction of tool axis. The relative movement, necessary fixtures for workpiece and the broach are provided by a machine tool called broaching machine. The broaching operation is depicted in Figure 2.1. The broaching is a high productivity method as so many cutting edges work to machine the workpiece at a time. The tool may be pulled or pushed through the surfaces to be finished. Surfaces finished by broaching either internal or external. External broaching is performed on the outside surface of the workpiece to create a pre-decided shape with dimensional accuracy and high degree of surface finish. Internal broaching is done on the internal surfaces of the workpieces. This way internal surfaces are brought to exact size with the required surface finish. Examples of the different shapes produced by internal and external broaching are shown in Figure 2.2.
BROACH
As we know that the broach is a broaching tool, it consists of a series of distinct cutting edges called cutting teeth along its length. Feed is accomplished by the increased step between any two successive teeth on the broach. The total material removed in a single pairs of the broach is the cumulative result of all the teeth in the tool in action (it is not necessary that all the teeth available in the broach in action at a time). The cutting speed of the broach is decided by the linear travel of the tool with respect to the workpiece. The shape of the cut surface (machined surface) is determined by the contour of the cutting edges on the broach. Generally broaches are made of high speed steel (HSS). In some cases the broaches are made of cast iron and their cutting edges are made of cemented carbide inserts. These inserts are fastened to the right place by mechanical means or brazed. A typical broach is shown in Figure 2.3 along with its nomenclature.
DIFFERENT TYPES OF BROACHES
A broach is normally made in one piece and it is called a solid broach. Different types of the broaches are described below.
Progressive Cut Broaches
These broaches have cutting edges, a part of which are of the same height along the broach but have different widths. In progressive cut broaching, metal is removed in thick layers by each tooth from only part of the work surface. The last tooth of a progressive cut broach remove a very thin layer over the entire profile of the work surface as in ordinary cut broaching.
Rotor Cut Broaching
Rotor cut broaches are used for removing large amount of material in holes in forgings or castings where a primary cutting operation is not desired. Their teeth are staggered around the periphery at different sections so as to shear the work and allow chip clearance. This is recommended for making square holes from a round cost one.
Burnishing Broaches
Burnishing broaches are used for making glazed and finished surface on a steel workpiece. These are used for finish the hole too. Burnishing teeth are rounded and not cut but compress and rub the surface of metal. The amount of stock is intentionally left for burnishing. Its thickness should not be more than 0.025 mm.
BROACHING METHODS
On the basis of method of broaching the operation of broaching is classified as discussed below.
Pull Broaching
Workpiece is clamped to the broaching machine in stationary position and the broach is pulled through the work. Broaches are usually long and are held in a special head. Pull broaching is mostly used for internal broaching.
Push Broaching
Workpiece is held in the broaching machine in stationary position and broach is pushed through the portion of workpiece to the machined. Normally push broaching is done by hand and arbor presses (hydraulic press). This method is also recommended for internal broaching like for sizing and finishing the holes, cavities, and key ways.
Surface Broaching
Any one of two, either workpiece or the broach (tool) is kept moving and other is kept stationary. The method is widely used as surface finishing operation. In case of surface broaching, the broaching tool is specifically designed for the shape to be finished.
Continuous Broaching
In continuous broaching the broach is held stationary in the broaching machine and workpiece is moved continuously. The teeth of movement of the workpiece may be either straight, horizontal or circular. This is generally used for broaching a large number of similar workpieces at a time.
BROACHING OPERATION
Broaching operations are used for processing of various internal and external surfaces. This is also recommended for processing of round and irregular shaped holes ranging from 6 to 100 mm size which may have flat or contoured surfaces. Some of the cavities and holes are directly machined by broaching operations, the example is spline hole. Sometimes execution of broaching operation requires more than one pairs. The teeth of a gear or spline may be broached altogether or one or a few at a time. Normal indexing operation (see milling machine operation for details) is done for cutting teeth by broaching. After completion of broaching of one tooth indexing action is performed and another tooth is broached and this continues till all the teeth are broached.
FIXTURES USED IN BROACHING
Fixtures plays important role in broaching operation. Fixtures are used for providing an accurate pre-decided movement to broach with proper clamping action. Accuracy and rigidity of fixture make the operation of broaching repeatable and fit for mass production. Functions of broaching fixture are summarized as given below :
(a) Fixture holds the workpiece accurately and rigidity and moves it to the exact cutting position.
(b) Guide the movement of the broach in pre-decided manner relative to the workpiece.
© Fixture is also responsible to give feed motion to workpiece after completion of the cut.
[b]MERITS AND DEMERITS OF BROACHING OPERATION[/b]
Broaching is widely used for faster finishing operations in metal working so it is a fit case for mass production. Merits and demerits of broaching operations are described below.
(a) It gives high rate of production so recommended for mass production.
(b) Production run time in case of broaching is very large as a broach has very long life. The whole processing load is shared by so many teeth.
© Due to faster operation and longer tool life, it is relatively cheaper.
(d) Both rough cutting as well as finished cutting can be completed in a single pairs of tool.
(e) Little skill or employment of semi-skilled manpower is sufficient to perform broaching operation.
(f) Broaching provides accurate and excellent quality of surface finish. It is capable to maintain tolerance of the order of 0.007 mm and surface finish CLA value upto 0.8 .
(g) It is also capable to process internal and external surface including intricate shaped cavities.
(h) Broaching makes the effective use of cutting fluids as it facilitates the flow of cutting fluid into the cuts.
MATERIAL OF BROACH
Common broach material is 18-4-1 stainless steel. As its name indicates, it has 4% chromium, 1% vanadium and 18% tungsten. This is corrosion and wear resistant steel. Carbide is also recommended for broach making, these broaches are used for broaching brittle material like cast iron in automobile industry. Inserted bit type and cemented carbide type broaches are also preferred to reduce the cost of broaches
SUMMARY
Broaching is one of the machining operations which uses broach as tool. Broach is a multipoint cutting tool and works on the same principle as that of a single point cutting tool in lathe machine. It is a high productivity method as so many cutting edges work together to machine a workpiece. In broaching operation any one of two either broach or workpiece reciprocates and other remains stationary. Broaching can be done on internal surfaces as well as external surfaces of workpiece so accordingly the broaching operation is named as internal broaching and external broaching respectively. Generally three parameters are controlled in a broaching operation which are cutting speed, feed rate and depth of cut. Broach material can be any from low hardness material cast iron to very high hardness material cemented carbide depending upon what type of workpiece is to be processed by it. Nomenclature of a broach is explained in the unit. Depending upon the working broaches are classified as progressive cut, rotor cut, and burnishing broaches. Different type of broaching methods are described in the unit depending upon the type of work to be done one a workpiece. The broaching operations have their major advantages as longer tool life, faster machining work and limitation is special fixture and machine are required to carry out this operation. Depending upon the working principle, construction and applications, broaching machines are categorized into different categories. The detailed categorization of the broaching machines is described in the unit.