How to improve processing accuracy by using CNC turning parameters

To improve processing accuracy by using CNC turning parameters, comprehensive consideration and precise setting from multiple dimensions such as cutting parameters, tool parameters, machine tool and process system parameters, and programming parameters are required. The following are the specific methods:

Set the cutting parameters reasonably

Cutting speed: The influence of cutting speed on machining accuracy is mainly reflected in cutting heat and tool wear. If the cutting speed is too high, a large amount of cutting heat will be generated, causing thermal deformation of the workpiece. At the same time, the tool wear will be accelerated, affecting dimensional accuracy and surface quality. For instance, when turning steel parts, if the cutting speed exceeds a certain limit, phenomena such as burning and oxidation may occur on the surface of the workpiece, and the dimensional accuracy may also exceed the tolerance due to thermal expansion. Therefore, the appropriate cutting speed should be selected based on the workpiece material, tool material and processing requirements. Generally speaking, when carbide tools are used to turn ordinary carbon steel, the cutting speed can be selected within the range of 80-120m/min. When turning quenched steel with ceramic tools, the cutting speed can be appropriately increased to 150-200m /min.

Feed rate: The magnitude of the feed rate directly affects the roughness and dimensional accuracy of the machined surface. Excessive feed rate will increase the cutting force, causing the workpiece to vibrate and thus affecting the machining accuracy. If the feed rate is too small, although it can improve the surface quality, it will reduce the processing efficiency. For instance, when precisely turning the outer circle, the feed rate is generally set at 0.08-0.15mm/r, which can not only ensure the surface roughness requirements but also guarantee dimensional accuracy. At the same time, the selection of feed rate should also take into account the edge strength of the cutting tool and the accuracy of the machine tool's feed system.

Back depth of cut: Back depth of cut mainly affects cutting force and processing efficiency. Under the premise of ensuring processing accuracy, a larger depth of cut should be selected as much as possible to improve processing efficiency. However, if the depth of cut is too large, the cutting force will increase sharply, causing the workpiece to deform and vibrate, and affecting the machining accuracy. For instance, when performing rough turning, the depth of cut on the back can be 2-5mm, and when performing finish turning, it can be 0.1-0.5mm.

Optimize tool parameters

Tool geometric parameters: Geometric parameters such as the rake Angle, relief Angle, main deflection Angle, secondary deflection Angle and cutting edge inclination Angle of the tool have a significant impact on machining accuracy. When the rake Angle increases, the cutting edge becomes sharp, the cutting force decreases, but the tool strength reduces. An increase in the relief Angle can reduce the friction between the tool and the workpiece, but it will also lower the tool strength. The main deflection Angle and the secondary deflection Angle affect the height of the residual area of the machined surface, thereby influencing the surface roughness. The Angle of the cutting edge affects the flow direction of the chip and the direction of the cutting force. For instance, when turning slender shafts, the main deflection Angle can be appropriately increased to reduce the radial cutting force and minimize the bending deformation of the workpiece.

Tool material: The hardness, wear resistance, toughness and thermal stability of tool materials directly affect the service life and machining accuracy of the tools. The appropriate tool material should be selected based on the workpiece material and processing requirements. For example, when processing common carbon steel, hard alloy cutting tools can be selected; When processing difficult-to-machine materials such as quenched steel and stainless steel, ceramic tools, cubic boron nitride tools or diamond tools can be selected.

Tool wear: Tool wear can lead to an increase in cutting force and a rise in cutting temperature, thereby affecting machining accuracy. Therefore, worn cutting tools should be replaced in time. During the processing, the wear condition of the cutting tool can be judged by observing the shape, color of the chips and the quality of the machined surface, etc. For instance, when the color of the chips darkens and the roughness of the machined surface increases, it indicates that the tool has worn out and needs to be replaced.

Adjust the parameters of the machine tool and the process system

Machine tool stiffness: Insufficient machine tool stiffness can cause elastic deformation of the workpiece under the action of cutting force, thereby affecting the machining accuracy. The rigidity of the machine tool can be enhanced by methods such as increasing the weight of the machine tool bed and optimizing the structure of the machine tool. For instance, adopting an integral cast bed, increasing the width and height of the bed guide rails, etc.

Process system vibration: Process system vibration is one of the important factors affecting processing accuracy. Vibration can be suppressed by methods such as reducing the cutting force, enhancing the damping of the process system, and adopting vibration reduction devices. For example, choosing appropriate cutting parameters, using elastic tool holders, installing shock absorbers, etc.

Machine tool accuracy: The geometric accuracy, transmission accuracy and positioning accuracy of machine tools directly affect the processing accuracy. Regular precision inspection and adjustment of machine tools should be carried out to ensure that they are in good working condition. For instance, regularly inspect the straightness, parallelism and perpendicularity of the machine tool guide rails, and adjust the clearance of the machine tool's transmission components, etc.

Precisely set the programming parameters

Tool compensation: Tool compensation includes tool radius compensation and tool length compensation. Tool radius compensation is used to compensate for the influence of tool radius dimension error on the machined contour, and tool length compensation is used to compensate for the influence of tool length dimension error on the machined depth. When programming, the tool compensation value should be accurately set according to the actual size of the tool. For instance, when the tool wears out, the tool radius will decrease. At this point, the tool radius compensation value should be modified in a timely manner to ensure the dimensional accuracy of the processing.

Machining allowance: The setting of the machining allowance should be reasonable. It should not only ensure that the machining errors and surface defects left by the previous process can be removed, but also avoid the reduction of machining efficiency and machining accuracy caused by excessive allowance. For instance, after rough machining, an appropriate allowance for finish machining should be left, generally ranging from 0.2 to 0.5mm.

Feed path: The selection of the feed path will affect the processing accuracy and processing efficiency. The shortest and most reasonable feed route should be selected as much as possible to reduce the idle travel time of the cutting tool. Meanwhile, during the processing, interference between the cutting tool and the workpiece, fixture, etc. should be avoided. For instance, when turning contours, either climb milling or reverse milling can be adopted, and the appropriate feed path should be selected based on specific circumstances.