BROACHES AND BROACHING MACHINES
Process:-
Broaching (Fig 1) is the process in which a cutting tool called broach is pushed or pulled through a hole or a surface to remove metal by axial force. Both roughing and finishing can be done in a single pass.
A broach is a cutting tool with a series of cutting edges A broach has three cutting zones such as roughing teeth, semi-finishing teeth and finishing teeth. (Fig 2)
The broach tapers from the roughing teeth to the finishing teeth. Normally all the finishing teeth are of the same size.
Individual teeth have a land, cutting edge, hook or face angle, gullet and pitch.
The pitch of a broach is determined by:
q The length of the cut
q The material being cut
q The size of the gullet
The length of the tool is determined by
o The amount of metal that must be removed
o The length of the machine stroke
o The required accuracy and the degree of finish
The size of the tooth gullet is related to the material to be cut and the type of chips produced.
The radius of the tooth root is designed so that the chips can be carried away from the cutting edges
Chip breakers are provided to control the chips
Broaches are mostly made of high speed steel. Some of the broaches have inserted carbide cutting edges brazed on to the body of the broaching tool.
The beginning hole can be drilled or cored in a casting. The finished hole can be round, square, rectangular or of any required shape. (Fig 4)
Broaches can be used to cut single or multiple keyways in pulleys, gears, splines in hubs and teeth in internal gears, ratchets, etc.All machinable metals and non-metals can be broached.External broaching can also be done. (Fig 5)
Broaching machines:-
Three types of broaching machines are normally used.
► Vertical broaching machine
► Horizontal broaching machine
► Continuous surface broaching machine
Vertical broaching machine (Fig 6):-
The majority of the broaching machines used in industry are of the vertical type, their main advantage being that minimum floor space is required.
This machine can be of pull-down, pull-up or push-down version
Pull-up machines are available with capacities of 6 to 50 tonnes, strokes of up to 2m while pull-down machines come in the range of 2 to 50 tonnes with strokes of up to 2.5m.
Pull-down broaching is more common than pull-up broaching
Modern broaching machines generally have hydraulic drives
Horizontal broaching machine (Fig 7):-
This is invariably a pull-type. It is available with capacities ranging from 2 to 60 tonnes and strokes up to 3m.
When the stroke required is large and the ceiling of the building is limited, the horizontal version is preferred.
Heavy duty horizontal surface broaching machine available with capacities of up to 100 tonnes and s1 of up to 9m.
Surface broaching can also be performed on horizontal broaching machines using special fixtures.
Rotary continuous surface broaching machine:-
The work pieces are loaded on a series of work holding fixtures which are mounted on a moving endless chain. Thus the workpiece is fed to the machine, guiding it accurately through a rigid section of the stationary tool where broaching is done.
For very high production rate, this type of machine is used.
In this type of broaching, the workpiece must have one open side. Half rounds such as connecting rod caps are best for chain-type continuous broaching.
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