What are the methods for precision CNC machining of holes?

Compared with the machining of external cylindrical surfaces, precision CNC hole machining is carried out under far less favorable conditions and is generally more difficult. The reasons are as follows:

1.The size of the cutting tool used for precision CNC hole machining is limited by the hole diameter. Therefore, its rigidity is poor, making it prone to bending deformation and vibration.

2.When using a fixed-size cutting tool, the machined hole diameter directly depends on the corresponding tool size. As a result, manufacturing errors and wear of the tool directly affect the dimensional accuracy of the hole.

3.During CNC hole machining, the cutting zone is located inside the workpiece, where chip removal and heat dissipation are difficult. Consequently, both dimensional accuracy and surface finish are hard to control.

1. Drilling and Reaming

(1) Drilling
Drilling is the first operation in CNC machining a hole from solid material. The diameter of drilled holes is generally less than 80 mm. There are two basic drilling methods: one where the drill rotates and another where the workpiece rotates.

(2) Reaming (Boring-out)
Reaming is performed with a reamer to enlarge an existing, cast, or forged hole and improve its dimensional accuracy and surface finish. Reaming can serve as a semi-finishing process prior to fine hole machining, or as the final machining step for holes with lower precision requirements.

2. Reaming (Finishing Reaming)

Reaming is one of the finishing methods for holes and is widely used in production. For smaller holes, compared with internal grinding or fine boring, reaming is a more economical and practical precision machining method.

3. Boring

Boring is a precision CNC machining process that enlarges a pre-formed hole using a single-point cutting tool. Boring operations can be performed on a boring machine or a lathe.

4. Honing

Honing is a superfinishing process that uses a honing head fitted with abrasive sticks (oilstones) to refine the surface of a hole. During honing, the workpiece remains stationary while the honing head, driven by the machine spindle, rotates and reciprocates axially. Under a certain pressure, the abrasive sticks remove a very thin layer of material from the surface, producing a crosshatched pattern of fine lines. To ensure the abrasive grains’ paths do not repeat, the rotational speed of the honing head and its reciprocating stroke per minute must be relatively prime numbers.

5. Broaching

Broaching is a high-efficiency precision machining method performed on a broaching machine using a specially designed broach. Broaching machines are classified as horizontal or vertical, with horizontal broaching machines being the most common type.