What is 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.
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.
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.
Depth of cut (h)
Chip thickness (hc)
Cross section of cut
Decreasing cutting resistance
As previously stated, 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:
(πDm) of the formula below is the periphery of the work. The periphery x n(RPM) is the amount of movement of the tooth per minute (mm).
Usually expressed in m/min. Divide by 1,000 to change from mm to m.
- 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.
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
The ratio of the depth of cut to chip thickness is the cutting ratio, Ch, and is found with this formula.
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.