Metal cutting
Cutting tools are classified into two major groups:-
1) Single point cutting tools
2) Multipoint cutting tools
Single point cutting tool:-
The tool terminating in a single point has been termed a single-point cutting tool. The tool is made of either carbon steel, high-speed steel, or carbide bar.
The cutting edge is prepared by grinding. It has one cutting edge which is partially formed by the end cutting edge and largely by the side cutting edge.
Its nose is given a small radius and it is never a sharp point.
Multipoint cutting tool:-
They have more than one cutting edge to remove excess material from the workpiece. For example, milling cutters, drills, reamers, broaches, and grinding wheels are multipoint cutting tools.
Terminology of single point cutting tool (as per ASA)
Back rake angle:-
It is the angle between the line parallel to the tool axis passing through the tip and the rake face and angles are measured in a plane perpendicular to the base.
While machining stronger or brittle materials, a smaller rake angle is used. For example- machining brass, and zero degrees rake angle are chosen.
If tool material is brittle like ceramic and carbides, a negative back rake angle has to be provided because we want to make the tool stronger. By providing the negative back rake angle, the energy required to overcome the friction increases which increases the overall power requirement in machining but a negative rake angle is required for tool stability because due to impact created by fayalite tool may break.
When machining slots or keys the cutting tool may be given by a negative back rake and a positive side rake.
For machining ductile material tool life increases by increasing the back rake angle because of decreases in contact length between chip and tool over the rake angle.
The decreases strength of the tool when tool life decreases. The optimum back rake angle of ductile material is around 10-15 degrees.
Side cutting edge angle:-
It is the angle between the side cutting edge and the line extending the shank. The angle is measured in a plane parallel to the base. Feed represents uncut chip thickness and depth of cut represents the width of the chip.
Side cutting edge angles increase the possibility of chatter vibrations. For machining shoulder, a small negative side cutting edge angle of 2-5 degrees is given on tools. Normally the value of side cutting edges angle varies between 15-30 degrees.
Side rake angle:-
It is the angle between the rake face and the line passing through the tip perpendicular to the tool axis and the angle is measured in a plane perpendicular to the base. Normally this angle varies between 5-15 degrees.
Side relief angle:-
It is the angle between the side flank and the line passing through the tip perpendicular to the base and the angle is measured in a plane perpendicular to the tool axis. This angle varied in the range of 5-15 degrees. To avoid this rubbing side relief angle is provided.
End cutting edge angle:-
It is the angle between the end cutting edge and the line passing through the tip perpendicular to the tool axis and the angle is measured in a plane parallel to the base. The normal value of this angle is in the range of 8-15 degrees.
End relief angle (clearance angle):-
It is the angle between the end flank and the line passing through the tip perpendicular to the base and the angle is measured in-plane parallel to the tool axis. Machining of ductile materials elastic recovery will be more so large clearance angle has to be provided. Relief is provided to the side and the end flanks in order to minimize physical interference or rubbing contact with the machine surface of the workpiece.
Nose radius:-
A larger nose radius gives a better surface finish. Increasing the nose radius also improves the tool life, so that a higher cutting speed can be employed. However, the cutting force and possibility of chatter increase when the nose radius is increased excessively.
ASA TOOL SIGNATURE:-
Back rake angle – side rake angle – end relief angle – side relief angle – end cutting edge angle – side cutting edge angle – nose radius (in inches)
Normal or orthogonal rake system(ORS):-
A number of lines are drawn perpendicular to the side cutting edge in the horizontal plane and the line given the maximum slope is called the normal rake angle. If the side cutting edge angle is zero, the normal rake angle is equal to the side rake angle.
Tool signature in the system is given as:-
I – N – side relief angle – end relief angle – end cutting angle – approach angle – R(mm)
Where:-
I – the angle of inclination
N – Normal rake angle
The angle of inclination is the angle of the tool axis with cross-side motion.
Type of metal cutting process:-
The metal cutting processes are of two types.
1) Orthogonal cutting process:- it is also known as the two-dimensional cutting process. It occurs when the major cutting edge of the tool is presented to the workpieces perpendicular to the direction of feed motion.
2) Oblique cutting process:- it is also known as the three-dimensional cutting process. The form of cutting occurs when the major cutting edge of the tool is presented to the workpieces at an angle perpendicular to the direction of feed motion.
DISTRIBUTION OF HEAT IN METAL CUTTING:-
In the metal cutting, there are three places for the distribution of heat.
1) Primary deformation zone
2) Secondary deformation zone
3) Work tool contact zone
Primary deformation zone:-
There will be a region in the material in which crystal structure break. This region is called the primary deformation zone and around (70-80)% of the total energy supplied will be consumed in this region. This is because of the plastics deformation of the metal. a portion of this energy will also come out in the form of heat.
Secondary deformation zone:-
Around (20-25)% of energy is consumed to overcome the drag between the chip and rake face. This region is called the secondary deformation zone. 100% of energy comes outs in the form of heat.
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