What is Broach tool | types , material , Fixture Used For Broaching
Basic Of Broaching Operation :
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 .
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.
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 along with its nomenclature.
Nomenclature of broach if expressed with its numerical values, called specification of broach. This nomenclature is explained below.
Pull end is made to attach the broach to the broaching machine through the puller head.
This centres the broach in the hole to be finished just before start of processing.
These are the cutting edges which remove larger amount of stocks during cutting. Larger amount removal generates poor quality of surface finish but makes the operation faster.
These are cutting edges removing smaller stocks of material. These are used for final finishing of the surfaces and their accurate sizing.
Rear Pilot and Follower Rest
This is a supporting device to the broach when it is likely to complete its operation of broaching.
It is the width of flank face of the broach normally it is kept slightly inclined to give relief angle to the flank face of broach.
It is the distance between two corresponding points on two successive teeth of a broach. Normally pitch of finishing teeth of a broach is kept comparatively smaller than the rough cutting teeth.
Height of the Teeth
Height of the roughing and finishing teeth gradually increases from the shank to the finishing teeth. This increment is called the cut per tooth, it depends on the material being machined. Normally the cut per tooth is taken from 0.01 or 0.2 mm for the finishing teeth and it may go up 0.2 mm for the cutting teeth.
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 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.
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.
(c) Fixture is also responsible to give feed motion to workpiece after completion of the cut.
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.
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