How Does CNC Boring Work, and Which Boring Tool Should You Choose?

CNC Boring Overview

Boring Accuracy Requirements

Boring is one of the main machining operations performed on a machining center. It can accurately control the dimensional and geometric tolerances of a hole system and can correct errors left by the preceding operation.

Most cylindrical holes produced by boring are critical mating or supporting holes in machine components, so they require relatively high dimensional accuracy. General mating holes are commonly controlled to IT7 or IT8, spindle-housing bores may require IT6, and less demanding holes are often controlled to IT11.

For high-accuracy holes in brackets and sleeves, and for critical holes in housings, geometric accuracy should generally be controlled within one-third to one-half of the hole-diameter tolerance. Center-distance error between bored holes is commonly held to ±0.025 to 0.06 mm, while the parallelism error between two hole axes is controlled to 0.03 to 0.10 mm. Typical bored surface roughness is Ra 1.6 to 0.4 µm.

Boring Process Stages

Hole boring often proceeds through rough boring, semi-finish boring, and finish boring. The required stages depend on the hole tolerance, workpiece material, and component structure.

Rough Boring

Rough boring is an important preparatory operation for cylindrical holes. It machines cast or forged blank holes, or holes that have already been drilled or counterbored, to establish suitable conditions for semi-finishing and finishing. It also exposes blank defects such as cracks, sand inclusions, and blowholes at an early stage.

Rough boring normally leaves 2 to 3 mm of stock per side for semi-finish and finish boring. Precision housing components may also require tempering or aging after rough boring to relieve the internal stress generated during machining before finish boring is performed.

Large cutting parameters create high cutting forces and temperatures and severe tool wear during rough boring. A rough-boring tool must therefore have sufficient strength and impact resistance. Its geometry should reduce cutting force and the load imposed on the machining system while allowing effective heat dissipation.

Semi-Finish Boring

Semi-finish boring prepares the hole for finish boring by correcting the uneven allowance left after roughing. A high-accuracy hole may require two semi-finishing passes. The first removes the most uneven stock, and the second removes the remaining allowance to improve dimensional and geometric accuracy and reduce surface roughness.

Semi-finish boring generally leaves 0.3 to 0.4 mm of stock per side for finish boring. A hole with less demanding accuracy can sometimes proceed directly from rough boring to finish boring.

Finish Boring

Finish boring uses a higher cutting speed and a lower feed rate to remove the small allowance left by rough or semi-finish boring and produce the specified internal surface accurately.

After rough boring, the clamping plates should be loosened and the workpiece reclamped to reduce the influence of clamping deformation on final accuracy. A typical finish-boring radial depth of cut is at least 0.01 mm, and the feed is at least 0.05 mm/rev.

Types of Boring Tools and How to Select Them

The cutting section of a machining-center boring tool is fundamentally similar to an external turning tool. Boring on a machining center normally uses a cantilevered tool, however, so the boring bar must provide adequate rigidity and accuracy. Tools are available for different cutting conditions and can be classified by the number of cutting edges as single-edge or double-edge boring tools.

Single-Edge Boring Tools

Most single-edge boring tools have an adjustable structure. The illustrated forms (a), (b), and (c) are used for through holes, stepped holes, and blind holes. Screw 1 adjusts the cutting diameter, while screw 2 locks the setting.

A single-edge tool has relatively low rigidity and can vibrate during cutting, so a larger approach angle is selected to reduce radial force. The machining diameter is set by moving the boring tool, which makes adjustment inconvenient and reduces efficiency. This design is mainly used for one-off and small-batch production. Its simple construction and broad adaptability nevertheless make it widely used.

Double-Edge Boring Tools

A simple double-edge boring tool has two symmetrical cutting edges at opposite ends, and both edges cut simultaneously. The balanced cutting action cancels much of the radial force acting on the boring bar. This permits larger cutting parameters, reduces the required bar rigidity, limits vibration, and improves cutting efficiency.

Indexable double-edge boring tools are widely used because their inserts are easy to replace, require no regrinding, and are easy to adjust. Symmetrical cutting can produce high boring accuracy. Compared with a single-edge tool, feed per revolution can be approximately doubled, increasing productivity.

Large-diameter holes can be machined with an adjustable double-edge boring tool whose head permits a wide adjustment range. The maximum boring diameter can reach 1,000 mm.

Fine-Adjustment Boring Tools

Fine-adjustment boring tools are commonly used for finish boring on machining centers. Their radial dimension can be adjusted within a defined range, with a scale resolution of 0.01 mm.

To adjust the tool, first loosen the clamping screw, then rotate the graduated adjusting nut until the cutting head reaches the required diameter, and finally retighten the screw. This type of boring tool has a relatively simple structure and provides high accuracy, good versatility, and strong rigidity.