High waves, the wind storms against port side and the spray lashes over the deck. The supply ship is only a few meters away from the massive support legs of the offshore platform. However, the ship does not budge an inch from its position. How is this possible? The crew steers it with Voith-Schneider propellers. For about 100 years, Voith has been producing this special drive in Heidenheim, Swabia. The power transmission occurs in the mechanical variant via enormous gearing components. For roughing the gears, the team led by process engineer Dr. Thomas Glaser relies on high-feed milling systems from Paul Horn GmbH and Boehlerit.
Due to its design, a Voith-Schneider propeller (VSP) generates thrust in any direction. It therefore serves as both drive and control. Thanks to short response times to control commands, the VSP also enables fast, safe, and precise maneuvering even under adverse conditions. This system even allows lateral movement (traversing). This drive concept is used on ships that require high maneuverability.
These include, for example, harbor tugs, double-ended ferries, floating cranes, and supply ships for drilling platforms.
Simple principle
In a wheel body, vertically movable and controllable wing blades are mounted. Depending on the type, the number varies between four and eight wings. When the wheel body rotates and the wings simultaneously perform a swinging motion, thrust is generated. The rotational speed of the wheel body and the amplitude of the oscillation determine its power. The phase position of the wings determines the direction. The VSP operates at low speeds during operation.
These are only about 25 percent of conventional ship propellers. The high torques resulting from this require a robust construction of the system. The drive of the VSP is provided by diesel or electric motors.
The power transmission from the motor to the propeller occurs in the mechanical variant via a bevel gear. Depending on the size of the VSP, the dimensions of the gearing components also vary. Due to the sometimes long processing times, Glaser is always looking for ways to optimize the machining processes: 'The production of a bevel pinion with module 21.4 required an adjustment. For the pre-machining of the pinion, we needed a very long processing time on a special hobbing machine.'
The goal was to switch from hobbing to 5-axis simultaneous rough milling to nearly halve the processing time. 'Another important point was the tool life. We stipulated that the tool life must be at least one part per tool,' says Glaser.
High-feed milling cutters from Horn and Boehlerit
Due to the good experiences with Horn solid carbide end mills in hard and finish milling, the Tübingen tool manufacturer was the first point of contact for the milling process of the module 21.4 gearing. Due to the high material removal rate and the required reduction of processing time by 40 percent, high-feed milling systems are used for roughing.
The choice fell, due to the gearing geometry, on three different diameters as well as different cutter types. The first cut is taken over by an indexable insert milling cutter with a diameter of 40 mm and six teeth from Boehlerit. The second feed is taken over by Horn interchangeable head milling cutters of type DGH with four cutting edges and a cutting diameter of 20 mm. For roughing the tooth root of the pinion, a solid carbide end mill of the DSH series with a diameter of 12 mm and also four teeth is used.
High-feed milling cutters cut end-on. Due to the load in the axial direction, the tool spindle is predominantly subjected to compressive stress and the lateral forces are relatively low. Because of the low tendency to vibrate, the tools can safely handle the high loads at the usual tooth feeds of fz = 1 mm at cutting depths ap of up to 1.5 mm. The large radius at the main cutting edge of the indexable inserts creates a smooth cut, ensures an even distribution of cutting forces, and thus long tool lives. On the inside, a small cutting edge radius allows for problem-free and quick immersion. A primary and secondary relief angle leads to a stable wedge angle and good cutting edge stability. Due to significantly higher feeds than with conventional milling, the material removal rate in high-feed milling is much higher, despite the lower feed depths.
Horn and Boehlerit
The sales cooperation between Horn and Boehlerit has existed since September 2016. The aim of this sales cooperation is to bundle both product and sales synergies of two medium-sized companies and to grow together as family-owned businesses in targeted markets. For Horn, the Boehlerit milling program is a suitable product complement to its own milling program. Horn thus expands its leading tool position in technically demanding applications, also to general machining.
Goal achieved
For roughing the gearing, Voith relies on a new fully automated production system from Grob. The system consists of three 5-axis machining centers with automatic linear storage as well as various setup stations. The processing time for pre-gearing could be reduced by 55% through the switch to the new tool concept compared to hobbing. The target of 40% was thus exceeded. The tool life is as required at one part per tool, or one cutting edge per indexable insert milling cutter. 'The performance of the milling cutters has fully convinced us. However, it's not just the tool solutions:
The implementation of such projects also requires competent technical consulting as well as a fast response and delivery time. Horn meets these characteristics for us as a tool partner,' says Glaser.
Company info Voith:
The Voith Group is a globally operating technology company. With its wide portfolio of systems, products, services, and digital applications, Voith sets standards in the markets of energy, paper, raw materials, and transport. Founded in 1867, Voith today has around 22,000 employees, 5.2 billion euros in revenue, and locations in over 60 countries worldwide, making it one of the large family-owned companies in Europe. The Voith Turbo division is part of the Voith Group and a specialist in intelligent drive technology, systems, and tailored services. Through innovative and smart products, Voith offers the highest efficiency and reliability. Customers from numerous industries such as oil and gas, energy, mining and mechanical engineering, shipbuilding, rail and commercial vehicles rely on cutting-edge technologies and digital solutions from Voith.
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