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How to select boring tools?

Boring tools are the main machining processes of machining centers. It can accurately ensure the dimensional and geometric accuracy of hole systems and correct errors from previous processes. Therefore, selecting the appropriate boring tool is very important.

Boring processing requirements

Boring tools

Most cylindrical holes finished by boring operations are main fitting holes or bearing holes in machine parts, so they have relatively high dimensional accuracy requirements. Dimensional accuracy requirements for general fitting holes are usually controlled within IT7-IT8. Dimensional accuracy for spindle box body holes is IT6. Holes with lower accuracy requirements are generally controlled within IT11. For holes in high-precision supporting parts, brackets and other important box body part holes, their geometric accuracy should be controlled within 1/2-1/3 of the hole tolerance.

Hole distance errors between reamed holes are generally controlled within ±0.025-0.06mm. Parallelism error between two hole axes is usually controlled within 0.03-0.10mm. The surface roughness after boring is usually Ra1.6-0.4μm.

Boring processing method

The boring process of holes often goes through the processes of rough boring, semi-finishing and finish boring. The selection of rough boring, semi-finishing and finish boring processes depends on factors such as the accuracy requirements of the holes, the material of the workpiece and the specific structure of the workpiece.

(1) Single-lip rough boring

Rough boring is an important process in the cylindrical hole boring process. It is mainly used for pre-machining of the cast or forged holes (raw holes) or drilled and expanded holes on the workpiece to lay the foundation for the next semi-finishing and finish boring processes to meet the requirements, and to find defects (cracks, inclusions, blowholes, etc.) in the raw workpiece in time. Generally, rough boring leaves 2-3mm of single side as the allowance for semi-finishing and finish boring. For precision box body workpieces, heat treatment or stress relief treatment is usually required after rough boring to eliminate the internal stress generated during rough boring, and then the final finishing boring is carried out. Since larger cutting parameters are used in rough boring, the cutting forces are larger and cutting temperatures are higher in rough boring, resulting in severe tool wear. Therefore, the rough boring tool is required to have sufficient strength to withstand larger cutting forces and have good impact resistance to ensure productivity and certain boring accuracy of rough boring; the rough boring tool requires suitable geometric angles to reduce cutting forces and facilitate heat dissipation of the boring tool.

(2)Semi-finishing boring

Semi-finishing boring is a preliminary process of finishing boring. Its main purpose is to remove the uneven remaining allowance left during rough boring. For holes with high accuracy requirements, semi-finishing boring is generally done in two steps: The first step mainly removes the uneven remaining allowance left during rough boring. The second step removes the remaining allowance after boring to improve the dimensional accuracy, and geometric accuracy of holes and reduce surface roughness. Semi-finishing boring usually leaves a finish boring allowance of 0.3-0.4mm (single side). For holes with low accuracy requirements, rough boring can be directly followed by finish boring without the need for a semi-finishing process.

(3) Finish boring

Finish boring is based on rough boring and semi-finishing boring. It removes the small remaining allowance left by rough boring or semi-finishing boring using a higher cutting speed and smaller feed rate to accurately achieve the internal hole surface specified in drawings. The clamping plate should be loosened slightly after rough boring, and then tightened again to reduce the influence of clamping deformation on machining accuracy. Usually, the back rake amount of finish boring is ≤0.01mm, and the feed rate ≥0.05mm/r.

Classification of boring tools

Boring tools

In terms of cutting parts, boring tools used on machining centers do not have essential differences from shell end mills, but boring holes on machining centers are usually carried out by overhung processing, so boring tools require sufficient rigidity and better precision. To adapt to different cutting conditions, boring tools have various types:

(1) Single-lip boring tools

The shank and cutting part of this boring tool are integrated, and the cutting part mainly uses cemented carbide. It has a simple structure with one cutting lip. It is compact, small in size, easy to manufacture and widely used. It can ream small holes, blind holes and stepped holes. It can also ream larger holes and end faces when mounted on a universal tool holder or spindle slider. However, its rigidity is poor and prone to vibration during cutting.

(2) Adjustable boring tools

Suitable for adjustable boring tools on transfer lines, coordinate boring machines and CNC machine tools. They have a simple structure, easy manufacturing, convenient adjustment and high precision.

(3) Floating boring tools

Floating boring tools are self-centered during cutting due to balanced forces on both sides. They do not require clamping and can automatically compensate for errors caused by boring tool installation errors, spindle runout or machine spindle deviation. Precision of IT7-IT6 can be achieved.

(4) Machine vise deep hole reamers

This type of reamer has the common characteristics of deep hole tools, with both guide blocks and chip flutes or coolant holes.

Machine vise-deep hole reamers are further divided into rotatable machine vise-deep hole reamers and fixed machine vise-deep hole reamers. Their structures are similar, but the clamping of the inserts and pads is different. The former is more convenient to use, while the latter has better rigidity.

The figure shows a rotatable carbide deep hole reamer held in a machine vise. The guide blocks are made of brazed guide pads and carbide guide blocks, clamped to the reamer body with screws. Screws clamp the insert on the pad to the reamer body. Rotating the adjustment screws drives the tilting axis to move, allowing adjustment of the radial dimension of the reamer.

(5) Modular reamers

To adapt to various hole diameters and depths and reduce reamer varieties, modular reamers are designed. Modular reamers are divided into multiple parts including shank, extender, reducers, reamer shank, head, insert seat and insert, which can be freely assembled based on processing needs.

This greatly reduces the number of shanks and reduces costs. It can also rapidly handle various machining requirements and prolong overall tool life. Modular reamers have obvious advantages over integral reamers. Of course, modular reamers also require high continuous precision, rigidity, repeatability and reliability. In summary, modular reamer systems have great advantages, but modular design alone does not guarantee good performance, which should be evaluated from aspects like connection rigidity, precision, handling and price.

What factors need to be considered when selecting a reamer?

The main factors include workpiece material, machining size and shape requirements, batch size, machine tool type, etc.

What type of reamer should be used for different materials?

Use single-lip reamers for low-carbon steel, floating reamers for alloy steel, replaceable insert reamers for difficult-to-cut materials or where high precision is required.

How does batch size affect selection?

Use general-purpose reamers for small batches and modular reamer systems for long life for large batches.

How to select suitable reamer coefficients?

Select based on material reference tables or determine optimal parameters through trial machining tests.

What model should be selected for high-precision requirements?

Select precision adjustable reamers or floating reamers for high accuracy requirements.

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