CNC lathes are high-precision, high-efficiency automated machine tools. Using CNC lathes can improve processing efficiency and create more value. The emergence of CNC lathes has freed enterprises from outdated processing technologies. The processing technology of CNC lathes is similar to that of ordinary lathes. However, since CNC lathes complete all turning processes continuously and automatically in a single setup, the following aspects should be noted.
Reasonable selection of cutting parameters
For high-efficiency metal cutting, the workpiece material, cutting tool, and cutting conditions are the three key factors. These determine machining time, tool life, and machining quality. An economical and efficient machining method necessarily involves the rational selection of cutting conditions.
The three key elements of cutting conditions—cutting speed, feed rate, and depth of cut—directly cause tool damage. As cutting speed increases, the tool tip temperature rises, leading to mechanical, chemical, and thermal wear. A 20% increase in cutting speed reduces tool life by half.
The relationship between feed conditions and tool flank wear occurs within a very small range. However, a large feed rate leads to increased cutting temperature and greater flank wear. Its impact on the tool is smaller than that of cutting speed. While the depth of cut does not have as significant an impact as cutting speed and feed rate, the formation of a hardened layer in the workpiece during small depths of cut can still affect tool life.
Users should select the cutting speed based on the material being processed, its hardness, cutting conditions, material type, feed rate, depth of cut, etc.
The optimal processing conditions are selected based on these factors. Regular, stable wear to the end of the service life is the ideal condition.
However, in actual operation, the selection of tool life is related to tool wear, changes in the machined dimensions, surface quality, cutting noise, and machining heat. When determining machining conditions, it is necessary to conduct research based on the actual situation. For difficult-to-machine materials such as stainless steel and heat-resistant alloys, coolants can be used or rigid cutting edges can be selected.
How to determine the three elements of cutting process?
How to correctly select these three elements is a major topic in the course "Principles of Metal Cutting". This article on metal processing extracts some key points, outlining the basic principles for selecting these three elements:
(a) Cutting speed (linear speed, circumferential speed) V (m/min)
To select the spindle speed per minute, you must first know the appropriate cutting speed V. The selection of V depends on the tool material, workpiece material, machining conditions, etc.
Tool material:
For cemented carbide cutting tools, V can be achieved at high speeds, typically exceeding 100 m/min. Technical parameters are usually provided when purchasing cutting tools.
What linear speed can be selected when processing what materials? For high-speed steel: V can only be relatively low, generally not exceeding 70 m/min, and in most cases below 20~30 m/min.
Workpiece material:
For high hardness, use a low V<sub>m</sub>. For cast iron, use a low V<sub>m</sub>; when the tool material is cemented carbide, a V<sub>m</sub> of 70-80 m/min is acceptable. For low-carbon steel, V<sub>m</sub> can be above 100 m/min. For non-ferrous metals, V<sub>m</sub> can be even higher (100-200 m/min). For hardened steel and stainless steel, V<sub>m</sub> should be lower.
Processing conditions:
For roughing, use a lower V; for finishing, use a higher V. If the rigidity of the machine tool, workpiece, and cutting tool system is poor, use a lower V. If the CNC program uses S in spindle speed per minute, then S should be calculated based on the workpiece diameter and the cutting speed V: S (spindle speed per minute) = V (cutting speed) * 1000 / (3.1416 * workpiece diameter). If the CNC program uses a constant cutting speed, then S can directly use the cutting speed V (meters per minute).
(ii) Feed rate
F primarily depends on the surface roughness requirements of the workpiece. For finishing, where high surface finish is required, a smaller feed rate (0.06~0.12 mm/spindle revolution) is used. For roughing, a larger feed rate can be used. It mainly depends on tool strength; generally, a value above 0.3 is acceptable. A larger tool clearance angle results in lower tool strength, limiting the feed rate. Additionally, the machine tool's power and the rigidity of the workpiece and tool should be considered. CNC programs use two units for feed rate: mm/min and mm/spindle revolution. The units used above are mm/spindle revolution. To use mm/min, the formula can be used: Feed rate per minute = Feed rate per revolution * Spindle revolutions per minute.
(iii) Depth of cut
For finishing, a depth of cut (radius value) of 0.5 or less is generally acceptable. For roughing, the depth of cut depends on the workpiece, cutting tool, and machine tool. Generally, on a small lathe (maximum machining diameter less than 400mm), when turning normalized 45# steel, the radial depth of cut should not exceed 5mm. It's also important to note that if the lathe spindle uses a standard frequency converter, when the spindle speed is very low (below 100-200 rpm), the motor output power will significantly decrease, resulting in a very small depth of cut and feed rate.
Choose the right cutting tools
1. When roughing, select high-strength and durable cutting tools to meet the requirements of large depth of cut and large feed rate.
2. When precision machining, select high-precision and durable cutting tools to ensure the required machining accuracy.
3. To reduce tool change time and facilitate tool setting, mechanically clamped tools and inserts should be used as much as possible.
Choose the right fixture
1. Use general-purpose fixtures to clamp workpieces whenever possible, and avoid using special-purpose fixtures;
2. The positioning references of the parts coincide to reduce positioning errors.
Determine the processing route
The machining path refers to the trajectory and direction of the tool relative to the workpiece during the machining process of a CNC machine tool.
1. It should be able to guarantee machining accuracy and surface roughness requirements;
2. The machining path should be shortened as much as possible to reduce the tool idle travel time.
Relationship between machining route and machining allowance
Currently, given that CNC lathes are not yet widely used, excess material on blanks, especially material containing forged or cast hard skin layers, should generally be machined on conventional lathes. If CNC lathe machining is necessary, then the program should be arranged flexibly.
Fixture installation points
Currently, the connection between the hydraulic chuck and the hydraulic clamping cylinder is achieved by a pull rod. The key points for hydraulic chuck clamping are as follows: First, use a wrench to remove the nut on the hydraulic cylinder, remove the pull tube, and pull it out from the rear end of the spindle. Then, use a wrench to remove the chuck fixing screws to remove the chuck.
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