The selection of lathe tool holders for external turning requires consideration of factors such as machining form, tool strength, and economic efficiency.
1. Tool holder selection is primarily based on the machining form. The type of tool holder that can be used varies with the turning part (external round, end surface, copying, etc.) and the direction of the tool's movement (forward or backward feed).
(1) The machining form that each tool holder can correspond to is determined by the main cutting edge angle when the tool bit is installed. Generally, when 90° vertical cutting (right-angle machining) is not required, if a tool holder with a main cutting edge angle of less than 90° is chosen, a square tool bit holder can be selected, which is more economical. When end surfaces are machined using a backward feed method, due to the requirements of chip handling, a tool holder with a main cutting edge angle of more than 105° and the corresponding tool bit must be selected. When the main cutting edge angle is below 95°, chip handling is very difficult and not recommended. When the main cutting edge angle is below 90°, backward machining cannot be performed. For chamfer machining, a tool holder with a main cutting edge angle of 45° to 60° should be selected. A negative side cutting edge angle is used exclusively for end surface cutting.
2. The selection of external round and end surface turning tool bits is as important as the selection of tool materials, and it is necessary to consider the machining process, workpiece material, cutting conditions, etc. Choosing the best tool bit can improve machining efficiency and reduce machining costs.
(1) The selection of tool bit shape requires a comprehensive consideration of machining form, cutting edge strength, clamping strength, and economic efficiency. With the increasing popularity of CNC lathes, tool bits that can machine both external rounds and end surfaces are most widely used. The 80° rhombic tool bit is suitable for a wide range of rough to finish machining. For copying machining, 55° rhombic or 35° rhombic tool bits are used. Although the cutting edge strength is not as good as that of the rhombic shape, it can correspond to the most extensive range of machining forms. The choice between 55° and 35° should be based on the shape of the workpiece. There are also tool bit shapes suitable for thread machining, slot machining, and cutting off, etc.
(2) The larger the tool bit tip angle, the higher the cutting edge strength, which is beneficial for interrupted cutting, but is subject to the constraints of the machining form. In stable cutting processes such as continuous cutting, using a regular triangular tool bit with slightly lower cutting edge strength but more edges is more effective. Selecting an unequal-sided and unequal-angle hexagonal tool bit with a tip angle of 82° can make up for this deficiency. When the feed depth is small, using an equilateral unequal-angle hexagonal (80°) tool bit that ensures cutting edge strength is also effective. Circular tool bits have the best strength and are most suitable for use when a good machining surface is required. Due to the large back force, vibrations are likely to occur when machining long and thin-walled workpieces, and the management of changing the cutting edge angle is also more difficult. Tool bits with larger size and thickness, or vertically mounted tool bits, have greater clamping strength and are suitable for heavy cutting. The cutting edge length of the 80° rhombic tool bit can be positioned on both sides, so the clamping strength is large, which is conducive to interrupted cutting and heavy cutting.
(3) When using negative angle tool bits, square tool bits are the most economical because a single side of a square tool bit has 4 edges, and both sides have 8 edges that can be used, and the tool tip angle is 90°, which is high in strength. Next is the regular triangular tool bit with 3 edges on one side and 6 edges on both sides.
(4) The tool tip radius refers to the size of the arc at the tip of the tool bit. The larger the tool tip radius, the higher the machining surface accuracy and the greater the tool tip strength, but it will cause an increase in radial force, which is prone to causing vibrations and making chip handling more difficult. In addition, the cutting edge position recedes, and the machining diameter increases. Conversely, as the tool tip radius decreases, the machining diameter decreases accordingly. The general range of tool tip radii used is 0.4 to 1.2 mm, but from the perspective of tool tip strength, a larger tool tip radius should be chosen for heavy cutting, and a smaller tool tip radius should be chosen for precision machining.
The selection of tool bit form for internal hole machining is basically the same as for external machining. However, during internal hole machining, the overhang of the tool is larger, and heavy cutting cannot be performed, so there is almost no need to consider the clamping strength of different shapes.
(1) As the tool tip radius increases, the back force increases. The back force causes the tool holder to bend and deform, and it is necessary to prevent vibrations. When the feed depth is small, attention should be paid to the deterioration of chip handling and the change in chip discharge direction caused by the increase in tool tip radius. During internal hole machining, chips must be discharged from the inside of the workpiece, and a slight change in the discharge direction can cause difficulties in chip discharge.
(2) When performing internal hole machining with small diameters, to prevent interference between the tool bit and the inner wall surface, if a negative angle tool bit is used, a large negative rake angle should be selected. This will increase the cutting force and is prone to cause high-frequency vibrations, so a positive angle tool bit with a back angle is usually used for internal hole machining. If the processing diameter is larger, from an economic point of view, it is more appropriate to use a negative angle tool bit.
