How to Determine the Cutting Quantity and Feed Rate in CNC Lathe Programming

When programming, the programmer must determine the amount of cutting for each process. When selecting the cutting amount, it is necessary to fully consider various factors that affect the cutting, correctly select the cutting conditions, and reasonably determine the cutting amount, which can effectively improve the machining quality and output. Factors affecting cutting conditions include: rigidity of machine tools, tools, tools and workpieces; cutting speed, cutting depth, cutting feed rate; workpiece accuracy and surface roughness; tool life expectancy and maximum productivity; type of cutting fluid, cooling method; Workpiece material hardness and heat treatment conditions; number of workpieces; machine tool life.

Among the above factors, cutting speed, cutting depth, and cutting feed rate are the main factors.

The speed of cutting directly affects the cutting efficiency. If the cutting speed is too small, the cutting time will be longer and the tool will not be able to perform its function. If the cutting speed is too fast, although the cutting time can be shortened, the tool will easily generate high heat and the tool life will be affected. There are many factors that determine cutting speed. To sum up, there are:

(1) Tool material. Different tool materials, the maximum allowable cutting speed is also different. High-speed steel cutting tool cutting speed is less than 50m/min, carbide cutting tool high-temperature cutting speed of up to 100m/min, ceramic cutting tool high-temperature cutting speed up to 1000m/min.

(2) Workpiece material. The hardness of the workpiece material will affect the cutting speed of the tool. When the same tool is used to process hard materials, the cutting speed should be reduced. When machining softer materials, the cutting speed can be increased.

(3) Tool life. If the tool usage time (life) is long, a lower cutting speed should be used. Conversely, higher cutting speeds can be used.

(4) Depth of cut and feed. Depth of cutting and the amount of feed, cutting resistance is also large, cutting heat will increase, so the cutting speed should be reduced.

(5) The shape of the tool. The shape of the cutter, the size of the angle, and the sharpness of the cutting edge all affect the selection of the cutting speed.

(6) Coolant use. A machine with good rigidity and high precision can increase the cutting speed; otherwise, it needs to reduce the cutting speed.

Of the above factors that affect cutting speed, the influence of the tool material is the most important.

The cutting depth is mainly conditioned by the rigidity of the machine tool. If the rigidity of the machine tool allows, the cutting depth should be as large as possible. If it is not limited by the machining accuracy, the cutting depth can be equal to the machining allowance of the part. This will reduce the number of passes.

The spindle speed is determined based on the allowable cutting speed of the machine and the tool. It can be selected by calculation method or look-up table method.

Feed rate f (mm/r) or feed speed F (mm/min) is selected according to the part's machining accuracy, surface roughness, tool and workpiece material. The maximum feed rate is limited by the stiffness of the machine and the feed drive and numerical control system.

When selecting the cutting amount, the programmer must select the cutting amount suitable for the characteristics of the machine tool and the best durability of the tool according to the requirements of the machine tool specification and the tool durability. Of course, analogy can also be used to determine the amount of cutting. Regardless of the method used to select the cutting amount, the durability of the tool must be ensured to complete the machining of one part, or the durability of the tool is not less than one work shift, and the minimum can not be less than half the shift time.