Flat Tool Cross-Wedge Rolling
Cross-wedge rolling (CWR) is a forging technique in which a heated billet is formed by rolling it between two flat (or cylindrical parallel-axis) die plates. This article describes the CWR process and equipment as applied to the production of railroad screws and other similar products.
The cross-wedge rolling (CWR) technique was conceived at the end of the 19th century and was believed to be ahead of its time. CWR is considered to be the most cost-effective technology for large-scale, high-volume commercial production. The essence of the process is that the billet experiences elastoplastic spinning between two wedge tools that are mounted on plates and move in parallel toward each other or are mounted on parallel-axis rolls and rotate in the same direction.
The CWR Sequence
The CWR process begins as an initial billet is fed to the “rolling axis” of the mill work zone and positioned across the leading parts of the tools, which can move in a linear (shuttle) or rotational mode (Fig. 1). The leading parts of both tools cut into the billet from diametrically opposite sides, causing it to rotate and form a circular groove. As the heated billet is rolled across the tooling, the groove becomes larger by traversing the facets of the tooling. As it does so, the excessive metal mass is displaced in the axial direction, and the billet is shaped and elongated. As a result of rolling, the billet acquires the negative shape of the tool. At the final stage of rolling, the shape is calibrated and the excessive metal is trimmed from the rolled part with the help of shears mounted on each side of the tools.
CWR is efficient in the production of multi-diameter bodies of revolution with various section profiles and diameter differentials of four times or more. The process is well suited to the manufacture of transmission shafts, hydraulic equipment valves, axles, ball joints, spindles, augers, pump and electric motor shafts, mining machinery cutter housings, helical expansion bolt shields, and shaped intermediate billets of high accuracy for further no-flash or low-waste plastic deformation.
Hands-on experience in the operation of roller and flat tool mills has defined the most expedient areas in which each design may be used. Flat tool mills are most efficient in the production of complexly shaped high-accuracy parts when changeover is often required. Roller mills are most suitable for large-scale production of one or two parts, especially short ones, with relatively low accuracy requirements (Fig. 2).
The primary advantages of the CWR method of metal forming include:
- Use of hot-rolled steel as the initial material (no preliminary pre-rolling preparation is required)
- High equipment productivity
- Favorable material grain structure
- High simplicity of maintenance
- High accuracy and maximum proximity of CWR-formed billets produced to finished-part dimensions
- Minimum material waste
- Use of the same equipment to produce a wide range of parts
- Short (5 minutes) tool changeover time
- High tool strength (up to 1,000,000 parts with re-sinking)
CWR Case Study: Railroad Screws
The current technology used to produce railroad screws features rolling with three forming rollers. The railroad screw’s helical surface consists of a 100-mm-long single-thread section and a 30-mm-long smooth transitional section. The initial cylindrically shaped billet is subjected to heading on a press, followed by the removal of flash and the formation of a tapered section to ensure engagement of the billet with forming rollers and rolling itself.
The thread parameter accuracy is within the ±0.5-mm range and maximum rod bending is 1.5 mm. The main shortcomings of this process are the short service life of forming rolls (50,000 parts), the necessity of having an additional tapering operation and up to 10% part defects due to excessive rod bending.
The Engineering Center of AMT Engineering, together with ERS Engineering Corp., have conducted research and industrial implementation of the CWR process to produce helical surface parts (railroad screws). CWR makes it possible to expand the technological capabilities of the plastic deformation of railroad screws regarding part parameter accuracy and the succession of forming operations (Fig. 4).
By using CWR, better deformation quality and accuracy of threaded sections is achieved, as is the avoidance of rod section distortion (Fig. 3).
Additionally, CWR offers a forming speed equal to or greater than conventional techniques by making the tool-rolling facet taper angles larger than or equal to the helical-surface inclination angle. The tool design helps remove superfluous metal from the forming die and contributes to additional straightening of the part by stretching, which also prevents distortion. A higher production rate is achieved through reduction of the time spent on forming the part’s helical surface by combining rod-rolling and thread-cutting operations.
The cost of parts produced using this process is 10-20% less than that of parts made conventionally and, most importantly, more-accurate parts are produced. Specifically, diameter accuracy is within ±0.2 mm and rod bending does not exceed 0.5 mm. Rod distortion-related rejection is completely avoided. According to a Russian manufacturer’s data, the tool-kit life is as high as 2,000,000 parts with re-sinks. Since 2005, this company has installed five of AMT Engineering’s WRL60 lines.
Tooling for the CWR process is made from high-quality steel (M2 or equivalent), which provides for its high strength. The tool consists of separate segments. Thus, it can be produced and restored using versatile metalworking equipment. Only the shape-forming segments of the tool in direct contact with the heated billet are subject to restoration. Therefore, only distinct tool segments need to be fabricated to replace the worn ones in the course of part production, which reduces the cost of the tool and the products made on it.
Further improvement of railroad-screw production technology - aimed at increasing the productivity and ensuring cost-effective use of resources – is possible with the screw production line developed by the Engineering Center of AMT Engineering of Belarus (Fig. 5).
The production process includes the following automatic operations:
- Cutting of two-screw billets from a rod whose D1 is equal to D3 of the maximum circle inscribed in the screw head
- Heating to 1000-1100°C
- Forming of the initial billet by CWR with the purpose of amassing optimum metal volumes in the heads and rods for subsequent screw shaping
- CWR of the shaped billet rod section, with the formation of helical surfaces and separation into two semi-finished railroad screws at the forming temperature of 950-1000°C
- Closed (no-flash) forging of the screw head at the semi-hot deformation temperature of 800-850°C
The railroad-screw production rate is 900-1,200 parts per hour. The line’s operation is controlled by a PLC-based, state-of-the-art control system that optimizes the process throughout the entire sequence of operations. The line requires only one operator.
Another basic control element is the intelligent data-analysis system, which can perform interactive analysis and generate a detailed web report of the manufacturing process and the use of production means and materials. All the information important for analysis and networking is shown on the control-panel monitor. This improves production efficiency, reduces raw-material and power costs, and contributes to the achievement of better and more-stable production results.
Our experience in the CWR process and its automated implementation leads us to expect further improvement of CWR technology in the development of energy-saving and low-waste rolling methods. Shaping conditions are also optimized in order to produce high-accuracy shaped preforms for subsequent no-flash forging or finished parts that do not require machining.
Co-author A. Roudovich is General Manager of the Engineering Center of AMTengineering, Belarus; co-author S. Brayman is President of ERS Engineering Corp., West Bloomfield, Mich. They can be reached at 248-538-9082, firstname.lastname@example.org or email@example.com. For additional information visit www.ersengine.com.