When tools cut a workpiece and generate chips, the workpiece is subjected to large plastic deformation. At this time, some of the force necessary for this plastic deformation is directed through the tool. This force is known as cutting resistance. Cutting resistance not only causes deformation of the workpiece and tool holders, but is also important in deciding such factors as the cutting power required, the workpiece holding method and the machining procedure.
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Depth of cut (h)
Cross section of cut
Chip thickness (hc)
The figure shows the relationship between the cutting edge and the workpiece. Plastic deformation of the chip is induced mainly from the cutting edge to the machined surface in the direction of an angle φ. This is called the shear angle.
Here, the shear angleφ, will change even though the depth of cut, h, is fixed. As the shear angle increases, the chip thickness, hc, decreases. On the contrary, as the shear angle decreases, the chip thickness increases.
When Ch is larger, the workpiece is subjected to larger deformation.
Cutting resistance is the force needed to create plastic deformation of a workpiece and change it into chips, and with a larger shear angle the cutting resistance is lower, while with a smaller shear angle the cutting resistance is higher.
The ratio of the depth of cut to chip thickness is the cutting ratio, Ch, and is found with this formula.
Decreasing cutting resistance
To decrease the cutting resistance the shear angle has to be large. The shear angle is mainly decided by the ductility of the workpiece material, the rake angle and rake face friction. Rake face friction changes according to temperature as well as the combination of workpiece material and tool material. In other words, tool material, rake face roughness, cutting speed and coolant affect the cutting resistance. As a result, the following is an effective way to decrease cutting resistance.
- Enlarge the rake angle.
- Choose a tool material with a low affinity (less prone to welding) to the workpiece material.
- Increase the cutting speed.
- Use coolant.
Principal force, feed force, back force
The direction in which cutting resistance is generated depends on a number of factors such as the workpiece material, tool geometry, depth of cut, feed and cutting speed etc. Measuring the actual cutting resistance exactly is very difficult.
A simplified explanation of the three directions of cutting force are:
(1) Principal force, a force tangential to the direction of rotation.
(2) Feed force, a force opposite to that of the direction of the feed.
(3) Back force a force acting in the opposite direction from the depth of cut.
Effect of backforce
When turning with a true rake angle that is too large, the feed force becomes minus, and with a cutting edge inclination angle that’s too large, the back force becomes minus. During the machining of small or thin parts when the back force becomes a minus value, deflection is generated by a force with which the workpiece is drawn into the tool and this causes the diameter in the middle of the component to become smaller.
The illustration shows the effect of back force (minus) on the workpiece when working towards the workpiece.