End mills
An end mill is a milling tool with a slender shank that has a cutting edge on the periphery and on the end face, and each cutting edge can be involved in cutting at the same time or can be cut separately. End mills are used in a wide range of machining fields such as side machining, grooving, etc. The mainstream of end mills used to be high-speed steel solid end mills, and now with the progress of coating technology and tool material technology, coated carbide solid end mills and indexable end mills are gradually popularized, and are widely used in molds, high-hardness materials, difficult-to-machine materials and other processing fields.
(1) The influence of down milling and conventional milling on machining When the integral end mill is machining, the axial cutting depth is generally large and the radial cutting depth is small, which is not achieved by the face milling cutter. Therefore, end mill machining is unstable and prone to high-frequency vibration. The end mill can be imagined as a face mill with an extended axial cutting edge, so it generally adopts the cutting method of climb milling, which has many similar characteristics with the face mill when choosing forward milling: when the feed mechanism of the machine tool has a gap, conventional milling should also be used, and conventional milling can also prevent the chipping of the cutter teeth when the hardness of the workpiece skin is large. But it is precisely because of the longer cutting edge of the end mill that it has some disadvantageous tendencies in reverse milling compared to face mills. Skews occur when the end mill mills are milled on the side. In the downmilling state, the end mill will deflect in the opposite direction of the workpiece due to the cutting force, and the deflection of the end mill will cause the machining surface of the workpiece to be skewed, as shown in Figure 2-6-29.
In conventional milling, the end mill is also affected by the cutting force and deflects, and its deflection direction is the direction of the bite of the artificial part, and as a result, the machining surface will produce undulating valleys. The amount of deflection is maximum at the moment before the bottom edge leaves the workpiece, so the A part of the cutting edge is machined in the valley, as shown in Figure 2-6-30. As a result, the grooving surface is inclined to the conventional milling side, as shown in Figure 2-6-31.
(2) The influence of various structural parameters of the end mill on its function: 1) Outer diameter. The larger the outer diameter of the end mill, the smaller the deflection deformation under the same cutting conditions, and the general diameter is doubled, and the deflection of the end mill becomes 1/16 of the original tool. When the depth of cut increases, the cutting force will increase, and the end mill is prone to deflection deformation, so it is necessary to use large diameter tools as much as possible without affecting the cutting conditions. 2) Blade length. Generally, when selecting an end mill, the blade length should be greater than the length of the machined part, but the longer the length, the less favorable the rigidity of the tool. Because the longer the cutting edge means the longer the cutting groove, and the cross-sectional area of the cutting groove is smaller than the cross-sectional area of the tool holder, which is less rigid than the shank part.
3) Helix angle. The helix angle is the angle between the axis of the end mill and the spiral cutting edge, and in the outer periphery is the axial inclination angle of the peripheral edge. A larger helix angle means a larger rake angle around the outer circumference of the tool, the sharper the tool and the lighter it is to cut.
However, a larger helix angle will produce a larger feed force, and when processing a thin plate workpiece or a workpiece with insufficient rigidity in the vertical direction, it is easy to cause workpiece deflection deformation or high-frequency vibration, which will affect the machining quality. The large helix angle leads to a decrease in the cutting force, and the surface roughness value of the machined surface is significantly reduced, so the helix angle of the end mill used for finishing is relatively large. The helix angle also affects tool life. Increasing the helix angle increases the contact length of the cutting edge and reduces tool wear, but an excessively large helix angle will reduce the strength of the cutting edge, which will adversely affect the tool. 4) Number of blades. The higher the number of blades, the higher the feed per revolution, and the higher the machining efficiency. If the cutting length of the tool reaches the service life increases, it also extends the tool life. However, as the number of cutting edges increases, the gap between the cutting edges decreases, and chip evacuation performance deteriorates. In addition, the increase in the number of cutting edges involved in cutting also increases the cutting force. The chip removal is not smooth, and it is easy to make the cutting edge of the end mill bite together with the chip, which affects the machining accuracy and may also cause the damage of the cutting edge. Therefore, if you plan to use a large depth of cut, it is best to use an end mill with a small number of blades.
