Drilling and threading combo tool for hardened steel applications
Tomonori Yoda, OSG Corporation Applications Engineer (Tap Development Division)
Thread milling cutters are considered to be the optimum tooling choice for the threading of difficult-to-machine materials such as hardened steel. With thread mills, the tool load on the cutting edge can be drastically reduced in the processing of hardened steel versus cutting taps. Furthermore, thread mills are able to divide cutting chips into small and manageable pieces even in difficult-to-machine materials with a tendency to form unstable chip shape. Although thread mills are highly reliably due to their low risk of breakage and long tool life characteristics, tooling choice will vary depending on the actual machining environment and specific needs of the manufacturer.
Threading by Cutting Taps
A thread is formed mainly by the chamfer section of a tap. In the machining of difficult-to-machine materials such as hardened steel, heavy tool load is concentrated on the cutting edge, which can lead to tool damage. By increasing the number of chamfers, the tool load on the cutting edge can be dispersed. Specifically, it is possible to disperse the tool load by lengthening the chamfer and increasing the number of flutes in cutting taps. However, this method has limitations. The more flutes, the smaller the individual flute will become. With a limited flute width, chip evacuation, cutting oil lubricity and cooling performance are reduced. Furthermore, the chamfer length is particularly critical for the processing of blind holes, since the length is limited by the pilot hole allowance. These effects and correlations must be taken into consideration in the product development process.
In tapping, a tap can only advance one pitch per revolution. Therefore, the feed per revolution is fixed by the processing pitch. Only the cutting speed can be adjusted, and chip shape varies depending on the difference in cutting speed. Due to these limitations, threading by cutting taps is considered to be a difficult process.
Threading by Thread Mills
Thread mill, in contrast, forms threads by intermittent cutting. Not only can the cutting speed be adjusted, but also the feed amount, making it possible to reduce the tool load on the cutting edge by cutting condition and offer numerous qualities that can resolve common tapping obstacles.
A thread mill’s cutting mechanism is similar to the side milling of an end mill. Even large-diameter threads can be processed in machines with a small horsepower compared to cutting taps. However, up until now, thread mills are not capable of handling work material with a hardness of around 60 HRC. With the release of the AT-2 carbide thread mill – a drilling and threading combo tool, more options are now available for effective thread milling of difficult-to-machine materials such as hardened steel.
OSG’s latest AT-2 carbide thread mill is developed specifically to combine the drilling of a pilot hole and threading into a single procedure even in difficult-to-machine materials such as hardened steel.
Features & Benefits of the AT-2 Carbide Thread Mill with End-Cutting Edge
The greatest feature of the AT-2 carbide thread mill is its ability to process hardened steel materials of approximately 60 HRC without having to prepare a pilot hole.
Figure 1. Left-hand cut configuration: long tool life is achieved by climb milling. Utilization of left-hand cut configuration to enable climb milling of right-hand threads.
Figure 2. Roughing Teeth: distributes tool load.
Figure 3. High-strength cutting edge shape (pat. in Japan): suppresses bending of the tool.
Figure 4. DUROREY coating: OSG original coating specially designed for hardened steel applications.
The tip of the tool incorporates an end mill’s end-cutting edge configuration to allow it to perform helical drilling and threading simultaneously. By eliminating the need for a pilot hole, sudden change in workpiece configuration can be accommodated since it is possible for the AT-2 thread mill to process an internal thread even after the workpiece has been quenched.
Figure 5. Helical drilling and threading simultaneously: no pilot hole is required. The AT-2 thread mill enables stable machining without chip trouble.
With the capability to achieve two processes with a single tool, machining efficiency is enhanced while the initial production cost can also be significantly reduced.
Figure 6. Long and stable tool life in hardened steel application: the AT-2 thread mill achieves higher thread quality compared to cutting taps.
As depicted in figure 6, the AT-2 thread mill exhibits significantly longer tool life in comparison to a carbide hand tap designed for hardened steel, thereby providing reduction in running costs.
The AT-2 thread mill incorporates a number of unique tooling geometries. One of the most prominent features is its left-hand cut configuration as depicted in figure 1. Because the AT-2 thread mill is used without having to create a pilot hole, processing takes place one pitch at a time at hole entry. When processing a right-hand thread by climb milling, a left-hand cut configuration is required. At the time of product development, the right-hand cut configuration is also tested with conventional milling. However, results have shown that climb milling exhibited 20 percent greater durability versus conventional milling. It is important to note that the spindle will rotate counterclockwise during use.
Figure 7. AT-1 one pass thread mill and AT-2 carbide thread mill: suitable work materials and applications.
Compatible with a Wide Range of Materials
The AT-2 has been developed for the purpose of processing hardened steel of 60 HRC, but it is also suitable for other general steels. As illustrated in figure 7, the AT-2 offers a wider processing range than the AT-1 one pass thread mill. However, it is important to note that the AT-2 carbide thread mill is inferior to the AT-1 one pass thread mill and cutting taps in terms of tool life and machining time.
Cutting taps are the fastest as they are able to process one pitch per revolution. The AT-1 is able to achieve 1-pass threading, which the AT-2 is unable to accomplish. With the shorter processing time, it can be assumed that the AT-1 offers longer durability than the AT-2 thread mill.
Although thread milling is a more mature cutting process, it can offer greater reliability, surface finish and precision that outperform conventional tapping. With today’s demand for low quantity and high variation production, the highly versatile AT-2 carbide thread mill provides a new and effective tooling solution that can accommodate a wide range of work materials and applications.
For more information on the AT-2 carbide thread mill