Walter Schumacher and his son Stefan run the German company Walter Schumacher Impuls Technik GmbH (SIT), which specialises in the development and production of special valves and stand-alone valve solutions. The product range includes valves for all types of media ranging from high-vacuum valves to high-pressure valves rated up to 800 bar.

One special solution made from aluminium consists of several helium valves, various sensors, pressure relief valves and throttles. To achieve the required functionality for the application, the quality of the mating surfaces has to be between Ra 0.012 and 0.014µm (12 to 14 nanometres) to ensure an effective seal. It is being achieved without the need for polishing using a Mazak turn-mill centre with a bespoke workholding attachment and employing a diamond tool from Horn and available in the UK from Ringwood-based subsidiary Horn Cutting Tools.

Walter Schumacher said: “Sealing hydraulic valves that are subjected to oil pressures of around 450 bar is relatively simple. However, valves for controlling the flow of helium or other gases have to tolerate pressures up to 800 bar and manufacturing them is technically a very difficult challenge that not many people want to take on.”

Before transitioning to the new production process, SIT relied on a specially developed polishing procedure to finish the valve blocks after machining. However, the time and labour entailed encouraged the manufacturer to optimise machining of the valve surfaces. After a move to new company headquarters in Bretzfeld, investment in a new turn-mill centre had reached the planning stage but the machine builder was yet to be chosen.

Stefan Schumacher added: “We approached a few potential suppliers with our requirements. Mazak immediately expressed a high level of interest and proposed a complete valve block machining solution based on an Integrex i-200ST.”

He said that the machine is rigid and stable and the spindle runs smoothly, with low vibration. In addition, Mazak and another partner designed a workholding attachment for the 300mm long x 110mm wide x 30mm high valve block. The attachment is mounted directly on the spindle flange in place of the chuck, reducing the number of clampings from nine to two.

Once the machine concept had been decided, the next step was planning the tooling. Horn product manager Jürgen Schmid said: “After initial contact had been made and the machining task outlined, it quickly became clear to me that the finishing process should be performed using our DTM milling system.”

An alternative tool manufacturer was also considered. However, Horn was successful in achieving the required result on the first attempt, while the other manufacturer needed three tries. Walter Schumacher said: “This made our choice clear, not only because we were immediately satisfied with the result, but also because of how impressed we were by the tooling company’s expertise. We use other tool systems as well from Horn's Tübingen factory and are very happy with them.”

The tool in question is a 125mm diameter face mill with six insert pockets but only two cutting edges. A polycrystalline diamond-tipped pre-cutting insert achieves a defined allowance of 0.02mm, while a monocrystalline diamond-tipped insert mounted opposite it in the cutter body finishes the surface. Carbide balancing inserts occupy the four other seats but are not involved in the machining.

The tool is finely balanced at Horn to ensure it runs without vibration. The inserts in the DTM body can be finely positioned in the axial direction via an adjusting screw. Every 10deg rotation moves the insert by 0.01mm. It means that the axial run-out of the individual cutting edges can be adjusted with high precision. An internal coolant supply ensures targeted cooling of the contact zone and enables efficient chip removal. The low mass of the aluminium tool body protects the milling spindle and reduces energy consumption compared with using a steel body.

Mr Schmid continued: “One difficulty was the large number of holes pre-drilled into the surface to be milled. Necessarily, high-polish milling must be the final machining operation, as otherwise very fine chips from drilling, reaming and thread milling would damage the reflective mirror surface. Even fingerprints can render the sealing surface unusable.”

The interrupted cut across the drilled holes posed no problem for the Horn tool. During processing, the milling cutter moves once longitudinally over the workpiece at a rotational speed (N) of 5,000rev/min and with a feed rate (Vf) of 500mm/min. Cutting speed (Vc) is 1,960m/min. To reduce re-cutting of chips, the tool is set at a minimum lead angle of 0.008deg. A commercially available emulsion is used for the coolant.

Stefan Schumacher concluded: “We are very satisfied with the performance of the Horn tool. In series production, we achieve surface qualities between Ra 0.012 and 0.014µm. Craftsmanship is required for producing our monocrystalline diamond cutting edges, which are ground by hand to achieve a high-polish machined surface.

“An air-bearing grinding table with a top made of granite provides optimal conditions. The cutting edge must be absolutely free from imperfections when optically inspected under a microscope at 200x magnification.”