When cutting metal on a CNC milling machine, the force required for the tool to cut into the workpiece to deform the workpiece material into chips is called the cutting force. Cutting force is an important basis for calculating cutting power, designing tools, machine tools and machine tool fixtures, and formulating cutting parameters. In automated production, the cutting force can also be used to monitor the cutting process and the working status of the tool.
Cutting Force And Cutting Power Of CNC Milling Machine
1. The source of cutting force of CNC milling machine.
The source of cutting force, on the one hand, is the resistance generated by elastic deformation and plastic deformation during the chip formation process; on the other hand, it is the frictional resistance between the chip and the rake face of the tool and the frictional resistance between the workpiece and the flank of the tool .
2. Cutting force and decomposition.
The total cutting force F during cutting is a spatial force. In order to facilitate measurement and calculation to meet the needs of machine tool, fixture, tool design and process analysis, F is often decomposed into three mutually perpendicular cutting component forces Fc, Fp and Ff.
(1) The main cutting force Fc is the projection of the total cutting force F in the main motion direction, and its direction is perpendicular to the base surface. Fc is an important basis for computer bed power, tool strength, fixture design and selection of cutting parameters. Fc can be calculated by empirical formula or unit cutting force kc, (unit is N/mm): Fc=kcAD=kchDbD=kcapf.
(2) The back force Fp is the component force of the total cutting force F perpendicular to the feed direction. It is the main factor that affects the deformation of the workpiece and causes the vibration of the system.
(3) The feed force Ff is the cutting component force of the total cutting force to F in the feed motion direction. It is the main basis for designing and checking machine tool feed mechanism and computer bed feed power.
Cutting force is the total cutting force F decomposed into Fc and FD, FD is decomposed into Fp and Ff, their relationship is Ff=FDsinkr; Fp=FDcoskr.
3. Cutting power of CNC milling machine.
Cutting power refers to the power dissipated by the cutting force during the cutting process, expressed in pm, and the unit is kW. When turning the outer circle, it is the sum of the main cutting force Fc and the power consumption of the feed force Ff. Since the power consumption of the feed force Ff is very small (only 1%5%), the power consumption of the general Ff can be ignored , And Fp does not do work, so we get the formula Pm=Fcυc×10, where Fc is the main cutting force (N); υc is the cutting speed (m/s).
Considering the transmission efficiency of the machine tool, the motor rate PE of the machine tool can be obtained from the cutting power Pm, that is, PE≥Pm/, where is the transmission efficiency of the machine tool, generally 0.75 to 0.85.
The main factors affecting cutting force of CNC milling machine
1. The influence of workpiece material on CNC milling machine.
The higher the strength and hardness of the workpiece material, although the cutting deformation is slightly reduced, the total cutting force is still increased. For materials with similar processing strength and hardness, the plasticity is large, and the friction factor with the tool is also large, so the cutting force is increased; for brittle materials, due to the small plastic deformation, the friction between the chip and the tool rake surface is small, so the cutting force is small
2. The influence of cutting amount of CNC milling machine
(1) The amount ap and feed amount f of the back knife.
When f and ap increase, the cutting area increases and the main cutting force also increases, but the degree of influence of the two is different. In turning, when ap is doubled, the main cutting force is approximately doubled; when f is doubled, the main cutting force is only increased by 68% to 86%. Therefore, in the cutting process, if you consider the main cutting force and cutting power, it is more advantageous to increase the feed rate than to increase the back-cutting amount.
(2) Cutting speed υc.
The influence curve of cutting speed on cutting force when machining 45 steel (ap=4mm, f=0.3mm/r) with YT15 cemented carbide turning tool. When cutting plastic metals, in the built-up edge area, the growth of the built-up edge can increase the actual rake angle of the tool, reduce the chip deformation, and reduce the cutting force; conversely, the reduction of the built-up edge increases the cutting force. When there is no built-up edge, as the cutting speed υc increases, the cutting temperature increases, the friction of the rake face decreases, the deformation decreases, and the cutting force decreases. Therefore, high-speed cutting is often used in production to improve productivity. When cutting brittle metals, υc increases and the cutting force decreases slightly.
3. The influence of geometrical parameters of CNC milling machine tools
(1) Rake angle.
The rake angle has the greatest influence on the cutting force. When cutting plastic metals, the rake angle increases, which can reduce the extrusion deformation and friction of the material to be cut, smooth chip removal, and reduce the total cutting force; when cutting brittle metals, the rake angle has no effect on the cutting force. obvious.
(2) Negative chamfering.
Grinding a negative chamfer on a sharp cutting edge can increase the strength of the edge, thereby increasing the service life of the tool, but at this time, the deformation of the metal being cut increases, which increases the cutting force.
(3) Entering angle.
The influence of the entering angle on the cutting force is mainly through the change of the cutting thickness and the length of the tip arc curve to affect the deformation, thereby affecting the cutting force. The entering angle has a small effect on the main cutting force Fc, but it has a significant effect on the ratio of the back force Fp and the feed force Ff. F’D is the reverse thrust of the workpiece against the tool. Since F’p=F’Dcoskr, F’f=F’dsinkr, increasing the entering angle kr will increase the feed force F’f and the back force F’ p decreases. When turning slender workpieces, a larger entering angle can be selected to reduce or prevent the bending deformation of the workpiece.
4. The influence of other factors.
The friction between the tool and the workpiece material
Under the same cutting conditions, high-speed steel tools have the largest cutting force, followed by carbide tools, and ceramic tools the smallest. Using cutting fluid in the cutting process can reduce the cutting force, and the higher the lubricating performance of the cutting fluid, the more significant the reduction in cutting force. The more severe the tool flank wear, the greater the friction and the greater the cutting force.