Analysis of Boring Tool Holders: The Key to Enhancing Internal Hole Machining Accuracy

Indexable insert toolholders have been used in cutting tools for many years. In modern manufacturing environments, they are widely employed in boring, turning, milling, and drilling operations. There are numerous reasons why toolholders have become so popular in the shop floor:

• When more precise insertion positioning is required, the small-tool holder enables adjustable tool positioning. As a standalone component, the small-tool holder can be precisely and independently moved relative to other cutting elements of the tool, thereby compensating for manufacturing imperfections.
• The tool holder protects the expensive tool body. Collisions are a common failure mode in machining operations. Even with the enhanced verification capabilities of modern CAD/CAM systems and the use of tool-monitoring systems, the likelihood of collision between the cutting edge of the tool and the workpiece remains high. Proper positioning of the tool holder can prevent all forms of misuse and safeguard the tool body from severe damage.
• The small-knife clamp utilizes standard components manufactured by multiple suppliers in the open market, thereby reducing tooling costs.
• The knife holder is made from high-quality alloy steel that has undergone appropriate heat treatment, resulting in a softer and lighter blade material and thereby enhancing the overall quality of the tool.
• The multi-tool holder enables users to configure multiple stations on a single tool, allowing numerous different operations to be integrated into a single machining sequence. This capability plays a crucial role in increasing material removal rates, particularly in high-volume production.

Figure 1: Typical small-knife holder design as specified in ISO 5611

The typical design of an ISO 5611 cartridge (see Figure 1) includes:

1. A solid body made of heat-treated alloy steel; 2. A cutting insert made of cemented carbide, ceramic, or ultra-hard cutting materials such as PCD or CBN; 3. Insert-retaining screw; 4. Capless screw; 5. Axial adjustment screw; 6. Cartridge clamping screw; 7. Washer; 8. Optional thread-retaining element or helical coil.

The standard itself does not specify magazine design; it only defines the principal dimensions. Over time, however, various manufacturers collaborated to establish a common basic design, which is why all magazines today look essentially the same.

Over the past three decades, manufacturers have made significant improvements in machine tool design and performance. Spindles have become faster and more rigid, accuracy has increased, and advanced communication and data acquisition systems have enhanced machine efficiency and output. Similar breakthroughs have also been achieved in the development of new cemented carbide and ultra-hard cutting materials. However, cutting tool design—particularly tools employing ISO small toolholders—has lagged behind these advances in other areas of the industry.

The principal standard for indexable insert holders and their dimensions, ISO 5611 (Indexable Insert Holders, Type A—Dimensions), underwent a major revision in 2015 and now comprises 12 substandards, ranging from ISO 5611-1:2015 to ISO 5611-12:2015. It is derived from the German standard DIN 4985.

Figure 2: A small tool holder typically installed in accordance with ISO 5611

Several factors are particularly outdated (see Figure 2) and undermine the machine tool’s achievements:

• Using standard socket set screws with a coarse thread pitch results in an adjustment resolution that falls short of modern requirements. For example, for cartridge 10CA (where “10” denotes the distance from the cutting edge in millimeters, “C” stands for cartridge, and “A” indicates Type A), the screw has an M4 diameter and a thread pitch of 0.7 mm. This means that one complete revolution of the screw will cause the cartridge to move radially by 0.7 mm.
• The ratio L/l >> 1 further reduces the solution rate of the adjustment.
• Due to the rigidity of the cylinder body, adjustment can only be achieved by tightening the tensioning clamp screw 6 (see Figure 1). In practical operation, the smaller screw 4 and the larger screw 6 work in opposition to each other (see Figure 2).
• Due to the clearance between the cartridge case body and the case base, the cartridge case body lacks robust support under cutting forces.

These shortcomings result in a cumbersome adjustment process that must be repeated multiple times. To make the adjustment, the operator typically reduces the torque applied to clamping screw 6 (see Figure 1). The operator then engages adjusting screw 4 and secures clamping screw 6 to the required specification. During this process, the shot-blasting body undergoes slight deformation, causing the target position of the insert to shift. From this point onward, the outcome depends on the operator’s experience: some operators start over by loosening the clamping screw, while others use the radial screw to fine-tune the cutting-edge position. It is impossible to predict how tightly the radial adjustment screw may become jammed by the clamping threads. Moreover, the hexagonal or torque tool used to drive the screw is often of a small size, which may not be sufficient to overcome the friction.

Figure 3: The new concept of fine adjustment for the small-tool holder is detailed. (Fr represents the radial cutting force.) The figure shows an arrow piercing a stone.

To address the shortcomings of conventional designs, Arrow Through Stone Company has developed a cartridge concept (see Figure 3), which recombines the new cartridge with cutting-edge technologies in the machine tool industry.

As shown in Figure 3, this new type of cassette is constructed on a flexible substrate 1. The insert 2 is secured by a screw 3 at the front of the cassette. The clamping screw 6, together with washer 7 and thread lock 8, is similar to the original design. The key change in the operating principle lies in the integration of a differential screw 4 and a thrust washer 9. The threads on the differential screw have the same lead but different pitches; for example, one thread may be M4 mm while the other is M4.25 mm. This results in a pitch difference of 0.25 mm, which is significantly smaller than the conventional design’s 0.7 mm.

In addition, the radial adjustment screw 4 is positioned at an angle a of 60 degrees, enabling further fine-tuning with enhanced resolution. The actual adjustment resolution of the 10CA magazine is 0.019 millimeters (0.0007 inches) per 90-degree rotation of the adjustment screw. In practical applications, operators can reliably adjust the position of the cutting edge with an accuracy of up to 0.001 millimeter (0.00004 inches).

Another advantage of the thrust pad is that it provides robust support in the region where the radial component of the cutting force (Fr) exerts its greatest influence, while also reducing the L/l ratio (see Figure 4).

Figure 4: Proper installation of the improved knife holder.

A typical feature of this design is the flexibility of the collet body, which is achieved through groove 10 (see Figure 3). This design both requires and enables a constant preloading force between the collet body and the machine tool’s cutting tool. An important benefit of this approach is differential coupling with zero backlash, allowing for smooth adjustment in both the upward and downward directions. Moreover, adjustments can be made without loosening the collet screw, significantly reducing maintenance time and enhancing user-friendliness.

Figure 5: The von Mises stress is shown in the groove region at maximum deformation.

When calculating the nominal preload of the magazine body, Jian Chuan Shi takes the following factors into account:

• An additional maximum deflection of 0.2 mm (0.008 in) is allowed to compensate for cutting-edge wear.
• The impact of centrifugal force on high-speed applications—such as machining aluminum with PCD inserts.
• The maximum stress on the critical cross-section of the body (see Figure 5).

Throughout the machining process, to compensate for wear on the cutting edge, it is necessary to maximize deformation at the front of the insert. The design aims to keep the insert within the linear elastic range of deformation, thereby preventing fatigue failure. After indexing a worn insert and adjusting it to its nominal dimensions, the residual stresses automatically reduce to their nominal levels.

Under high-speed machining conditions, the front portion of the cylinder body is subjected to centrifugal force according to Equation (1):

Fc = 0.01097 m r nrpm2 (1) 2465

Where is it?

m = mass (kilograms)

r = center of mass radius (in meters)

nrpm = revolutions per minute

A typical boring tool for machining multiple surfaces is shown in Figure 6: the front insert is performing roughing, while the other inserts are completing finish machining and chamfering.

Figure 6: One A standard multi-station boring machine featuring a comprehensive internal cooling system with a fine-adjustment box.

The rotational speed, nrpm, is expressed as a power of 2, highlighting the importance of accounting for the effect of centrifugal force on the magazine’s nominal preload. On the one hand, centrifugal force reduces the preload by tending to “lift” the front portion of the magazine off the carrier. On the other hand, increasing centrifugal force elevates the stress in critical sections. For example, if this tool rotates at nrpm = 8,000 rpm and the knife holder at 0 rpm, with a front-end mass of m = 0.0199 kg, the resulting centrifugal force will reach 489 N (110 pounds).

Based on in-depth engineering analysis, Jianchuan Shi has established the geometric constraints for the new small-tool holder while still complying with the requirements of ISO 5611. The new design is fully interchangeable with the original and has been field-proven.

Application analysis by Arrow Through Stone Company indicates that, for the 10 CA-type small tool holder with a length of L = 50 mm, a height of F = 14 mm, and a distance from the cutting edge of H = 10 mm, 75% of precision machining operations can be completed. The company has also identified three primary general-purpose small tool holder types that can meet the needs of most applications. Additional types and sizes of tool holders conforming to ISO 5611 are also available.

Arrow Through Stone recommends using spacers that have been heat-treated to 60 HRC whenever possible. This significantly extends tool life by preventing any plastic deformation at the contact interface between the pit bottom and pusher block 9. (See Figure 3.)

Another advantage of the spacer pad is its ability to be ground down in thickness, allowing the cartridge to accommodate different boring diameters and enabling the use of lighter, softer materials such as aluminum, magnesium, and carbon fiber for the tool body. During machining, the spacer pad absorbs high pressures and provides protection for the cutting insert. To ensure free coolant flow, the spacer pad incorporates a buffer zone.

The main advantages of the newly patented fine-adjustment tool holder design can be summarized as follows: the tool holder complies with the ISO 5611 standard and offers high-resolution adjustment to 0.001 mm; there is no backlash in the adjustment mechanism; the tool holder does not need to be disengaged during adjustment; a sufficient adjustment range of 0.2 mm compensates for wear on the cutting edge; and the support structure beneath the cutting edge provides high stability.

This new design elevates the product to the technological standard of modern machine tools; adopting the new compact tool holder in existing operations can more than double tool life and significantly enhance the end-user experience.