. the important of clearance and rake angles in metal cutting
Metal cutting
Knowledge about the principal for metal cutting relating to the various tool angles will assist in carrying out of metal cutting work efficiently.
Workpiece are shaped and brought to the required dimension by the use of cutting tools. While cutting materials the following four important elements interact.
Cutting tool
Tools holding or grinding device
Work holding devices, and work
Although the metal cutting process involves the combination of the above four elements, everything begins with a cutting tool and its impact at the time of contacting the workpiece.
The common metal cutting processes are carried out either by the use of a single point or multi point cutting tools.
Cutting edge are taken as the factors for determining the tool life, surface finish, and the force required for cutting.
The important angles of the cutting edge are
Clearance angle
Rake angle
Wedge angle or tool angle
Clearance angle
This is the clearance or relief provided behind the cutting edge.
If the clearance angle is not provided the tool will rub against the work surface, and will impose a lot of strain before the metal chips are sheared from the work.
The clearance angle improves these conditions and prevents tool rubbing, thus reducing the cutting force.
The clearance angle depends on the of surface being machined.
Normal clearance angles for different surface are
External cylindrical surface -5° to 7°
Flat surface -6° to 8°
Internal cylindrical surface -8° to 10°
Smaller clearance angles tend to rub against the surface while excessive clearance causes chatter, and there is a tendency for the tool to dig in. This will also result in increased tool angle thereby reducing the tool strength.
Rake angle
This is the angle given in the front of the leading edge of the tool.
This reduces the pressure of the chip on the tool face, and thereby reduces the cutting force ;also, it affects the formation of chips and the surface finish.
Positive rake angles produce better finish at low speeds.
Increased rake angle increases the cutting efficiency but decreases the tool wedge angle, and thereby decreases the strength of the tool. Low strength and ductile materials can be cut with increased efficiency with tools having high rake angles.High strength materials are cut with tools having high rake angles. Low strength materials are cut with tools having low positive rake angles.
Zero and negative rake angle tools are stronger and have a long tool life.
Negative rake tools produce poor finish at low cutting speeds but give good finish at higher cutting speeds
Negative rake tools need more power to operate than the positive rake tools.
Rake angle of different metals
Cast iron 0°
Ductile brass 14°
Free cutting brass 0°
Tin bronze 8°
Aluminium alloys 30°
Mild steel 25°
Medium carbon steel 25°
High carbon steel 12°
Teflon 0°
The wedge or tool angle is affected by the amount of clearance and rake angle. More clearance and rake angle will decrease the tool wedge angle thereby reducing the strength of the cutting edge.
The rigidity of tool holders and holding devices contributes to efficient cutting.
The design of the holder depends on the type and shape of the tool and its intended function.
The work holder used depends on the shape of the work and the type of force acting during cutting.
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