Internal turning on lathe is a technical for processing holes on the inner surface of the workpiece, during the process, there may be an improper cutting force and chip removal problems influences the machining quality. According to the influencing factors on internal turning, practical optimization tips and measures can be referenced.

What is Internal Turning?

Internal turning is the process on lathe of enlarging the inner hole of the workpiece or processing the inner surface of hollow parts.

Difference Between Internal Turning and Boring

- The internal turning is also the same as boring, but the internal turning tool is different from the boring tool in size. The internal turned hole is relatively larger and more shallow than the bored hole, the structure and strength are also greater. Generally boring is for producing small and deep holes. 

- In addition, the handle of the internal turning tool is square, using when placed on the tool holder of lathe machine, while the bar of boring tool is round straight handle or taper, often installed on the spindle of the boring machine, they can't be interchanged.

Influencing Factors of Internal Turning Operation

There are many factors affects the quality and results of internal turning operation.

1. In external turning, the length of CNC turning part and the size of the toolbar will not affect the tool overhang, so it can bear the cutting force during the machining process, while in boring and internal turning operation, the hole depth determines the overhang, the hole diameter and length of the workpiece limit the selection of the turning cutting tool. Long overhangs may cause problems with both deflection and vibrations. 

2. The cutting force is also an important factor. For the given internal turning conditions (parts shape, size, clamping type), the direction and magnitude of the cutting force is a factor that minimizes the vibration in the turning and improves machining quality. The tangential cutting force and radial cutting force make the tool deviate, and slowly make the tool away from the workpiece, resulting in the cutting force deviation. The tangential force tries to make the tool down, and the tool away from the centerline, reducing the back angle of the tool. When the diameter of the turning hole is small, the back angle should be large enough to avoid the interference between the tool and the hole wall.

3. Poor chip evacuation, improper clamping, cutting edge angle or tool will all influence internal turning.

Optimization Tips of Internal Turning - How to Improve Internal Turning

From the perspective of cutting tool application, the following aspects can be considered to improve the machining quality of the inner hole of the machined part.

1) Selection of tool nose radius

In the internal turning process, the small nose radius should be the first choice. Raising the nose radius will increase the radial and tangential cutting forces, and also increase the risk of vibration. The tips for selecting is that the nose radius should be slightly less than the cutting depth. In this way, the radial cutting force can be minimized. At the same time, when the radial cutting force is ensured to be the minimum, using the maximum nose radius can obtain a stronger cutting edge, better surface texture and more uniform pressure distribution on the cutting edge.

2) Selection of tool groove

The groove of the cutting tool has a decisive influence on the internal turning process. For the machining of the inner hole, the positive rake angle groove type tool with a sharp cutting edge and high strength are generally selected.

3) Cutting edge angle of the cutter

The cutting edge angle of the internal turning tool affects the direction and magnitude of radial force, axial force and composite force. The larger the cutting edge angle, the larger the axial cutting force, and the smaller the cutting edge angle, the larger the radial cutting force. In general, the axial cutting force towards the toolbar will not have a great influence on the machining, so it is advantageous to choose a larger angle. When selecting the cutting edge angle, it is recommended to select an angle close to 90 ° and not less than 75 °. Otherwise, the radial cutting force will increase sharply.

4) Tool treatment

The rounding of the cutting edge of the tool also affects the cutting force. Generally, the rounding of the cutting edge of the uncoated tool is smaller than that of the coated, especially when the long tool is overhanging and machining the small hole. Flank wear will change the back angle of the tool relative to the hole wall.

5) Clamping method

The clamping stability of cutting tools and the stability of turned components determine the magnitude of vibration and whether the chatter will increase. The clamping unit of the cutter bar should meet requirements in length, surface roughness, and hardness. In order to achieve the stability of tool holder clamping, the inner surface of tool clamping must have high surface finish and enough hardness. For the ordinary toolbar, the clamping system can achieve the highest stability by fully clamping the toolbar. Overall support is better than the cutter bar clamped directly by screws. It is more suitable to clamp the cutter bar on the v-shaped block by screws. It is not recommended to clamp the cylindrical handle directly by screws.

6) Effective chip removal

In the internal turning operation, chip removal also has a great influence on the machining effect and safety performance, especially in the processing of deep holes and blind holes. The short spiral chips are the ideal chips for internal turning, easy to be discharged and will not cause great pressure on the cutting edge when chip breaking. If the chip is too short and the chip breaking effect is too strong, which will consume higher machine power and increase the vibration trend. If the chip is too long, it will make chip removal more difficult. The centrifugal force will press the chip to the hole wall, and the residual chip will be squeezed to the surface of the processed workpiece, which will lead to the risk of chip blockage and then damage the tool. To solve the problems, an internally cooled tool is recommended, or you can use right cutting fluid. When machining through-hole, compressed air can be used instead of cutting fluid. In addition, the cutting parameters will also affect chip control.