When high speed milling is used to finish hardened die steel, one of the main factors to be observed is to use shallow cutting. The cutting depth shall not exceed 0.2/0.2 mm (AP / AE: axial cutting depth / radial cutting depth). This is to avoid excessive bending of the toolholder / cutting tool and to keep the machined die with small tolerance and high accuracy.
It is also important to choose a clamping system and tool with good rigidity. When using monolithic cemented carbide tools, it is very important to use tools with the largest core diameter (maximum bending rigidity). A rule of thumb is that if you increase the diameter of the tool by 20%, for example from 10 mm to 12 mm, the bending of the tool will be reduced by 50%. It can also be said that if the tool overhang / extension is shortened by 20%, the tool bending will be reduced by 50%. The tool holder with large diameter and taper further improves the stiffness. When ball end mills with indexable inserts are used (see mold manufacturing sample c-1102:1), if the handle is made of integral cemented carbide, the bending rigidity can be increased by 3-4 times.
When high speed milling is used to finish the hardened die steel, it is also very important to select the special groove and grade. It is also important to select coatings with high thermal hardness such as TiAlN.
11) When should forward milling be used and when should reverse milling be used?
The main suggestion is: use down milling as much as possible.
When the cutting edge is just cutting, the chip thickness can reach its maximum value in down milling. In the reverse milling, it is the minimum value. Generally speaking, the tool life in up milling is shorter than that in down milling because the heat generated in up milling is significantly higher than that in down milling. When chip thickness increases from zero to maximum in up milling, more heat will be generated because the friction of cutting edge is stronger than that in down milling. The radial force is also obviously high in up milling, which has a negative effect on the spindle bearing.
In down milling, the cutting edge is mainly subjected to compressive stress, which is much better than the pulling force produced in up milling on cemented carbide blade or integral carbide tool. There are exceptions, of course. When using solid carbide end mills (see tools in die sample c-1102:1) for side milling (finish machining), especially in hardened materials, the top milling is preferred. It’s easier to get a better straightness and a smaller wall angle of 90. If there is no coincidence between different axial feed, the tool mark is also very small. This is mainly due to the direction of the cutting force. If a very sharp cutting edge is used in cutting, the cutting force tends to “pull” the knife toward the material. Another example that can be used for up milling is the use of an old manual milling machine, which has large lead screw clearance. Up milling produces cutting force to eliminate clearance, which makes milling action more stable.
12) Profile milling or contour cutting?
In cavity milling, the best way to ensure the success of down milling tool path is to use contour milling path. Milling the outer circle of a milling cutter (e.g. ball end mill, see die manufacturing sample c-1102:1) along contour lines often yields high productivity because more teeth are being cut on larger tool diameters. If the speed of the machine spindle is limited, contour milling will help to maintain the cutting speed and feed rate. With this tool path, the change of working load and direction is also small. This is particularly important in high-speed milling applications and hardened material processing. This is because if the cutting speed and feed rate are high, the cutting edge and cutting process will be more vulnerable to the adverse effects of working load and direction changes, which will cause changes in cutting force and tool bending. Profiling milling along the steep wall should be avoided as far as possible. The chip thickness is large at low cutting speed when profile milling is carried out. In the center of the ball knife, there is also the danger of breaking the edge. If the control is poor, or the machine tool has no pre reading function, it can not decelerate fast enough, and it is most likely to cause the edge breakage in the center. The reason is that the chip thickness is the maximum under the favorable chip speed.
In order to obtain the longest tool life, the cutting edge should be kept continuous cutting as long as possible in the milling process. If the tool enters and exits too frequently, the tool life will be significantly shortened. This will aggravate the thermal stress and thermal fatigue on the cutting edge. It is more advantageous for modern cemented carbide tools to have uniform and high temperature in the cutting area than large fluctuation. Profiling milling path is often a mixture of up milling and down milling (zigzag), which means that the cutter will be frequently fed and withdrawn during cutting. This kind of tool path also has bad effect on the quality of die. Every time you eat a knife, it means that the tool is bent, and there is a mark of lifting on the surface. When the tool exits, the cutting force and tool bending decrease, and there will be a slight “over cutting” of the material in the exit part.
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