In addition to overcoming the resistance of the workpiece material, the tool geometry will also affect the actual cutting effect and even the result. Choosing proper tool geometry can increase tool life, maintain machining accuracy, reduce cutting power, etc. Common tool related geometry is as follows: 1. Tool edge angle; 2. Chip discharge groove; 3. Over center and over center tools; 4. Number of blades
01
Tool edge angle
1.1 Tool edge angle --- rake angle
The bevel can be changed from a positive value to a negative value, as shown in the following figure. In terms of cutting force and required power, the tool tip angle formed by positive and oblique angles is small, the tool can easily cut into the workpiece, and the chip flows out smoothly, which can reduce the cutting pressure, so the cutting efficiency is high. However, too large positive bevel angle forms a sharp blade, so the blade is fragile and easy to wear or crack. On the contrary, the negative bevel angle has a strong cutting edge, which is suitable for cutting high-strength materials.
1.2 Tool edge angle clearance angle
It is also called clearance angle, which is positive. Its function is to avoid the interference of single friction or physical phenomena between the cutter belly and the workpiece surface when the cutter cuts into the workpiece, as shown in the following figure. The small clearance angle gives the cutting edge greater support, which is generally used for work-piece materials with high strength mechanical properties. Large clearance angle can make the blade sharp, but the strength of the blade is reduced, which is easy to wear or crack. It is suitable for soft or low strength workpiece materials.
1.3 Tool edge angle Helix Angle
The groove of milling cutter is spiral, which can be divided into left spiral and right spiral, as shown below. When the cutting edge enters the workpiece during cutting, as shown in the figure below on the right, the cutting force F will instantly increase to the maximum. When the cutting edge leaves the workpiece, the cutting force will rapidly decrease, which is the reason for the vibration during cutting. The effect of helix angle at this time can prevent the cutting force from concentrating too much in one direction and scattering it in the other two directions - the horizontal component FH and the vertical component FV. When helix angle γ The larger the value is, the larger the horizontal component FH will become, causing the tool to swing during cutting; Helix angle γ The smaller the value is, the greater the vertical component FV will become. When the force holding the tool is insufficient during cutting, the tool will disengage from the handle, which is very dangerous when rotating at high speed. The common helix angle is 30 ˚、 thirty-eight ˚、 forty-five ˚、 sixty ˚。
02
Chip discharge chute
The ideal chip processing condition is that the chip will not interfere with or scratch the workpiece surface or impact the tool and hurt the worker when it flows out, so the chip should be able to naturally break into small pieces and be discharged to other places. Therefore, the chip control should not only consider the chip flow direction, but also make the chip break automatically. In order to meet this requirement, a design is generally made on the top surface of the tool. The mechanism that can automatically limit the chip length is called chip chute or chip breaker. The purpose is to enable the chip to curl rapidly and force the chip to break by the curling stress. The general chip removal groove design is shown in the lower right:
Groove width W: curl is formed when chips are generated. If the groove width is too large, the curl radius is large, and the curl stress generated is not enough to break the chips; If it is too small, on the contrary, when the generated stress is too large, the cutting edge is easy to crack.
Groove depth H: it affects the stability of chip flow. If it is too deep, the force required for the chip to curl when flowing to the groove shoulder is large, which is easy to cause the blade to break; If it is too shallow, the chip may leave automatically when it does not flow to the slot shoulder, making the chip flow difficult to control.
Groove shoulder R: refers to the part where the chip rolls up from the chip breaking groove, which directly affects the size of the curling force. If the radius is too large, the chip is easy to slide up, and the curling stress may not be enough to break the chip; If the radius is too small, the chips are easy to be blocked and slide on, which will produce great extrusion stress.
03
Tools passing the center and not passing the center
When making a discarding round nose knife, the diameter D of the knife is usually much larger than the R angle of the blade, so the blade will not cross the center at the middle of the bottom and there will be a region without a blade, that is, there is no cutting capacity in this region, as shown in the left figure. When the workpiece in the shape of machining hole or groove is encountered, the processing problem in the lower right figure will occur.
Although the size of the tool can enter these areas, because the blade does not cross the center, the blade will not cut the material in the middle and leave the yellow columnar residual material in the figure. With the deeper processing, the height of the residual material will increase, and finally it will hit the bottom of the tool, causing damage to the tool. The tool that passes through the center means that its blade passes through the center, so there is no such problem, so it is also called drilling tool.
04
Number of blades
The relationship between the number of cutting edges of a milling cutter and the cutting effect will vary depending on the workpiece material, the shape of the milling cutter, the brightness of the machining surface, and so on. A milling cutter with more cutting edges can obtain a more smooth and smooth machining surface because it has more cutting edges. However, because there is not enough chip space to accommodate the chips, it is vulnerable to chip interference, and the strength of the blade will be weak.
Therefore, for general rough cutting, high feed, especially for soft materials, large chip space is required, and the best way to provide chip space is to reduce the number of edges and increase the blade, which can not only increase the chip space, but also increase the strength of the blade, and the number of regrinding times and life of the milling cutter can also be increased. Therefore, when considering the processing method, heavy and rough cutting should choose the milling cutter with fewer blades and coarse teeth; For fine and finish machining, the milling cutter with more blades and finer teeth should be selected.
