Inner hole turning, also called boring, is the method of turning to expand the inner hole of the workpiece or processing the inner surface of the hollow workpiece, can be used to process most of the outer circle turning process. In cylindrical turning, the length of the workpiece and the size of the selected tool bar do not affect the tool overhang, so it can withstand the cutting force generated during the processing. When boring and turning the inner hole, the hole depth determines the overhang, therefore, the aperture and length of the parts have a great limitation on the choice of the cutter, so we must optimize the processing scheme by integrating the influencing factors.
The general rule of inner hole processing is to minimize the tool overhang and choose the largest tool size as possible, so as to obtain the highest machining accuracy and stability. Due to the spatial limitations of the aperture of the processed parts, the choice of tool size will be limited, and chip removal and radial movement must be taken into account during processing. In order to ensure the stability of the inner hole machining, it is necessary to select the right inner hole turning tool and apply and clamp it correctly to reduce the tool deformation and minimize the vibration to ensure the machining quality of the inner hole. In the inner hole cutting cutting force is also an important factor to be reckoned with, and for a given inner hole cutting conditions (the shape, size, clamping way, etc.), the size and direction of the cutting force is inhibition of inner hole cutting vibration, improve the quality of machining, the important factors when cutting tool in cutting, tangential and radial cutting force cutting force make the tool produces deflection phenomenon, Slowly move the tool away from the workpiece, causing the cutting force to skew, and the tangential force will attempt to force down the tool and move the tool away from the centerline, reducing the back Angle of the tool. During machining, radial and tangential cutting forces cause the deflection of the inner hole lathe tool, which usually requires forced cutting edge compensation and tool vibration prevention. When radial deviation occurs, cutting depth should be reduced and chip thickness should be reduced.
From the perspective of tool application, the factors to improve the machining quality of inner hole are as follows:
(1)Selection of blade groove type:
The groove type of the blade has a decisive influence on the cutting process. The groove type of the front corner blade with sharp cutting and high edge strength is generally used in the inner hole processing.
(2)the selection of the main deflection Angle of the tool:
The main deflection Angle of inner hole turning tool affects the direction and magnitude of radial force, axial force and synthetic force. A larger principal deviation Angle will result in a larger axial cutting force, while a smaller principal deviation Angle will result in a larger radial cutting force. In general, the axial cutting force in the direction of the tool bar will not have a great impact on the machining, so it is advantageous to choose a larger main Angle of deviation. When selecting the main deviation Angle, it is recommended to choose the main deviation Angle as close as possible to 90°, and not less than 75°, otherwise, it will lead to a sharp increase in radial cutting force.
(3) Selection of tool tip radius:
Small tool tip radius should be the first choice in inner hole turning process. Increasing the tip radius will increase the radial and tangential cutting forces and also increase the risk of vibration trends. On the other hand, the radial deflection of the tool is affected by the relative relationship between the depth of cutting and the radius of the tool tip. When the cutting depth is less than the tip radius, the radial cutting force increases with the deepening of cutting depth. The cutting depth is equal to or greater than the tip radius, and the radial deviation will be determined by the main deviation Angle. A good rule of thumb for choosing a tip radius is that it should be slightly less than the depth of cut. In this way, the radial cutting force can be minimized. At the same time, the use of a maximum tip radius ensures a stronger cutting edge, better surface texture, and a more uniform pressure distribution on the cutting edge while ensuring a minimum number of radial cutting tools.
(4)Selection of cutting edge treatment:
The cutting edge rounding (ER) of the blade also affects the cutting force. In general, the cutting edge round of an uncoated blade is smaller than that of a coated blade (GC), which should be taken into account, especially when long tool overhang and machining holes. Backface wear of the blade (VB) will change the back Angle of the tool relative to the hole wall, and this may be the root of the influence of cutting action in the machining process.
(5)effective removal of chips
In inner hole turning, chip removal is also very important for processing effect and safety performance, especially in deep hole and blind hole processing. Shorter spiral chips are ideal for inner hole turning because they are easier to be discharged and do not cause great pressure to the cutting edge when the chip is broken. When the chip is too short, the chip breaking effect is too strong, will consume higher machine power, and there will be a tendency to increase vibration. The chip is too long will make the chip more difficult, centrifugal force will chip pressure to the hole wall, the residual chip is extruded to the surface of the workpiece has been processed, there will be the risk of chip blockage and damage the tool. Therefore, when turning the inner hole, it is recommended to use the tool with internal cooling. In this way, the cutting fluid will effectively drain the chip out of the hole. When processing through hole, compressed air can also be used instead of cutting fluid to blow out chips through the spindle. In addition, the selection of appropriate blade grooves and cutting parameters is also helpful for chip control and discharge.
(6)Selection of tool clamping mode
The clamping stability of the tool and the stability of the workpiece are also very important in the inner hole processing. It determines the magnitude of the vibration during processing, and determines whether the vibration will increase. It is important that the clamping element of the tool bar meet the recommended length, surface roughness, and hardness.
The clamping of the tool bar is the key stability factor, in the actual machining, the tool bar will appear deflection, the deflection of the tool bar depends on the material of the tool bar, diameter, overhanging, radial, tangential cutting force and the clamping of the tool bar in the machine. The slightest movement at the clamping end of the tool bar will cause the tool to deflect. The modern high performance tool bar should have high stability when clamping to ensure that there will not be any weak link in the processing. To achieve this, the inner surface of the tool clamp must have a high surface finish and sufficient hardness. For common tool barbs, the clamping system provides maximum stability by clamping the tool barbs completely around the circumference. The overall support is better than the screw clamping tool bar directly. It is more suitable to clamp the tool bar with the screw on the V-shaped block, but it is not recommended to clamp the cylindrical handle tool bar directly with the screw, because the screw will damage the tool bar directly on the tool bar.