What are the commonly used tools for internal boring on CNC lathes?

　　A CNC lathe is a highly precise, high-efficiency automated machine tool. Utilizing a CNC lathe can enhance machining productivity and generate greater value. The advent of CNC lathes has enabled enterprises to move beyond outdated machining technologies. Although the machining processes for CNC lathes are similar to those for conventional lathes, CNC lathes perform all turning operations in a single setup through continuous, automated machining; therefore, the following aspects should be carefully considered. Turning internal bores on a CNC lathe is a common machining method in manufacturing. When performing internal-bore turning, it is essential to understand the cutting tools used, the machining process, the measuring instruments employed, and the quality requirements associated with internal-bore machining.

　　I. Cutting Tools for Internal Hole Machining

　　Depending on the application, internal turning tools can be classified as through-hole tools and blind-hole tools.

　　1. Through-hole turning tool

　　To reduce radial cutting forces and prevent vibration, the principal rake angle of through-hole turning tools is typically set at 60–75°, while the secondary rake angle is 15–30°. To avoid interference between the back face of the internal-bore turning tool and the hole wall without excessively increasing the clearance angle, the back face is usually ground with two distinct clearance angles.

　　2. Blind Hole Turning Tool

　　Blind-hole turning tools are used for machining blind holes or stepped holes, with a principal rake angle of 90–93 degrees. The cutting tip is located at the front end of the tool shank, and the distance between the cutting tip and the outer end of the shank is less than the radius of the hole.

　　II. Internal Hole Machining Process

　　Boring is one of the commonly used hole-machining methods and can be employed for both roughing and finishing operations. To enhance turning rigidity and prevent vibration, select a relatively thick tool shank whenever possible and keep the overhang as short as feasible—just slightly longer than the hole depth. The cutting tip must be aligned with the workpiece center, and the tool shank should be parallel to the spindle axis. For safety, it is advisable to first perform a trial pass inside the hole using an internal-bore cutter before proceeding with boring. When finishing internal bores, ensure that the cutting edge remains sharp; otherwise, chipping or deflection may occur, resulting in a conical bore.

　　III. Measuring Instruments for Internal Bore of Vehicles

　　When the required accuracy for bore diameter is relatively low, a steel ruler, internal calipers, or a vernier caliper may be used for measurement; when higher accuracy is required, an internal micrometer or an internal dial indicator should be employed; for standard bores, plug gauges may also be used.

　　1. Dial caliper:

　　When using a dial caliper to measure hole diameter, ensure that the caliper body is parallel to the end face of the workpiece, and gently rock the moving jaws around the circumference to identify the maximum reading.

　　2. Internal micrometer:

　　The scale lines on this type of micrometer are oriented in the opposite direction to those on an external-diameter micrometer: when the thimble is rotated clockwise, the moving jaw moves to the right, increasing the measured dimension.

　　3. Internal diameter dial indicator:

　　The internal diameter dial indicator is assembled by mounting the dial indicator on a measuring stand. Before measurement, select and install the appropriate fixed measuring head according to the diameter of the workpiece hole, then use a micrometer to set the internal diameter dial indicator to the “zero” position. The minimum reading obtained by oscillating the dial indicator represents the actual standard value of the hole diameter.

　　4. Plug gauge:

　　It consists of a go gauge and a no-go gauge. The go gauge is manufactured to the hole’s minimum limit dimension and must be able to be fully inserted into the hole during measurement, while the no-go gauge is manufactured to the hole’s maximum limit dimension and must not be able to be inserted into the hole. If the go gauge can be fully inserted but the no-go gauge cannot, the hole conforms to the specified dimensional limits.