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What is Bore turning?

Bore turning, also known as boring, is the use of turning methods to expand the inner hole of a workpiece or process the inner surface of a hollow workpiece. It can be processed using most cylindrical turning techniques. During cylindrical turning, the length of the workpiece and the selected tool size do not affect the tool overhang, thus being able to withstand the cutting forces generated during the machining process. When boring and turning internal holes, the depth of the hole determines the overhang. Therefore, the aperture and length of the part have great limitations on the selection of cutting tools, so it is necessary to optimize the machining plan by integrating various influencing factors.

The general rule for inner hole machining is to minimize tool overhang and select the largest possible tool size to achieve the highest machining accuracy and stability. Due to the spatial limitations of the machining part aperture, the selection of tool size will also be limited, and chip removal and radial movement must also be considered during machining. To ensure the stability of inner hole machining, it is necessary to select the correct inner hole turning tool during machining and apply and clamp it correctly to reduce tool deformation, minimize vibration, and ensure the quality of inner hole machining. The cutting force in internal hole turning is also an important factor that cannot be ignored. For a given internal hole turning condition (workpiece shape, size, clamping method, etc.), the magnitude and direction of the cutting force are important factors in suppressing internal hole turning vibration and improving machining quality. When the tool is cutting, the tangential and radial cutting forces cause the tool to deviate, gradually moving the tool away from the workpiece, resulting in cutting force deviation; the tangential force will attempt to forcefully press down the tool and move it away from the centerline, reducing the back angle of the tool. When the diameter of the turning hole is small, it is necessary to maintain a sufficiently large back angle to avoid interference between the tool and the hole wall.

During machining, radial and tangential cutting forces cause the inner hole turning tool to deviate, usually requiring forced cutting-edge compensation and tool vibration prevention. When a radial deviation occurs, the cutting depth should be reduced to reduce the chip thickness.
From the perspective of tool application, several factors can improve the quality of inner hole machining:
(1) Selection of blade groove type:
The groove type of the blade has a decisive impact on the cutting process. Generally, for internal hole machining, a sharp cutting edge with high edge strength is selected as the front corner groove type blade.

(2) Selection of tool main deviation angle:
The main deviation angle of the inner hole turning tool affects the direction and magnitude of radial force, axial force, and composite force. A larger principal deviation angle produces a larger axial cutting force, while a smaller principal deviation angle results in a larger radial cutting force. In general, the axial cutting force in the direction of the toolbar usually does not have a significant impact on the machining; therefore, choosing a larger main deviation angle is advantageous. When selecting the main deviation angle, it is recommended to choose one as close as possible to 90 ° and not less than 75 °. Otherwise, it may cause a sharp increase in radial cutting force.
(3) Selection of tool tip radius:
In the inner hole turning process, the small tip radius should be the preferred choice. Increasing the tip radius will increase radial and tangential cutting forces and the risk of vibration trends. On the other hand, the deviation of the tool in the radial direction is affected by the relative relationship between the cutting depth and the tool tip radius. When the cutting depth is less than the radius of the tooltip, the radial cutting force continuously increases as the cutting depth deepens. The cutting depth is equal to or greater than the tool tip radius, and the radial deviation will be determined by the main deviation angle. The Rule of thumb for selecting the tool tip radius is that the tool tip radius should be slightly less than the cutting depth. In this way, the radial cutting force can be minimized. At the same time, under the condition of ensuring the minimum radial cutting tool, using the maximum tip radius can obtain a stronger cutting edge, a better Surface finish, and a more uniform pressure distribution on the cutting edge.

(4) Selection of blade treatment:
The rounding of the blade’s cutting edge (ER) also affects the cutting force. Generally speaking, the rounding of the cutting edge of non-coated blades is smaller than that of coated blades (GC), which should be considered, especially when overhanging long tools and machining small holes. The blade’s back face wear (VB) will change the tool’s back angle relative to the hole wall, which may also become the root cause of affecting the cutting effect of the machining process.

(5) Effective discharge of chips
In the internal hole-turning process, chip removal is also very important for the processing effect and safety performance, especially when processing deep and blind holes. Short spiral chips are ideal for internal hole turning, as they are easier to discharge and do not cause significant pressure on the cutting edge when the chips break. When machining, if the chip is too short and the chip-breaking effect is too strong, it will consume higher machine power and tend to increase vibration. Long chips can make it more difficult to remove them. Centrifugal force presses the chips towards the hole wall, causing residual chips to be squeezed onto the surface of the processed workpiece, posing a risk of chip blockage and damage to the tool. Therefore, it is recommended to use tools with internal cooling when turning internal holes. In this way, the Cutting fluid will effectively discharge the chips out of the hole. When machining through holes, compressed air can also be used instead of Cutting fluid to blow out chips through the spindle. In addition, selecting appropriate blade grooves and cutting parameters can also help control and discharge chips.

(6) Selection of Tool Clamping Methods
The clamping stability of the cutting tool and the stability of the workpiece are also very important in inner hole machining, as they determine the magnitude of vibration during machining and whether this vibration will increase. It is very important for the clamping unit of the cutter bar to meet the recommended length, surface roughness, and hardness.

The clamping of the tool holder is a key stability factor. In actual machining, the tool holder may deviate, and the deviation of the tool holder depends on the tool holder material, diameter, overhang, radial, tangential cutting force, and the clamping of the tool holder in the machine tool. The slightest movement at the clamping end of the toolbar will cause the tool to deviate. Modern high-performance tool holders should have high stability during clamping to ensure no weak links during processing. To achieve this, the inner surface of the tool clamping must have a high surface finish and sufficient hardness. For ordinary tool holders, the clamping system can achieve the highest stability by fully clamping the tool holder on the circumference. The overall support is better than the tool holder directly clamped by screws. It is more suitable to clamp the tool holder onto a V-shaped block with screws, but it is not recommended to directly clamp the cylindrical handle tool holder with screws, as a direct action of screws on the tool holder can damage the tool holder.