Swiss-style turning machines are gaining broader acceptance as more industries find new applications for tiny, precision parts. While Swiss-machined parts were once used mainly by watchmakers, today’s medical, electronic, and automotive engineers routinely design components too small and precise for conventional lathes. One Swiss machine with modern carbide cutting tools turns a titanium hip joint implant just 0.2 inch diameter within ±0.0004 inch, tolerances five times tighter than possible with turret lathes. Bar- or wire-fed Swiss machines can also make such parts economically and with far more quality than casting or extrusion processes. Advanced cutting tools and precision toolholders can maximize Swiss machining productivity. However, to exploit the latest tooling advances, shop managers should draw on the expertise of knowledgeable tool suppliers.
The right tools for Swiss machines can boost throughput and improve the quality of finished parts. For example, switching from brazed carbide parting tools to more wear resistant coated carbide inserts with sharper edges reduced cycle time on an automotive fluid connector from 11 seconds to just 5 seconds. The 0.4 inch long steel part measures less than 0.2 inch diameter and requires a specific geometry and surface texture to grip a hose. The more consistent performance of the modern carbide inserts also produces a higher-quality finish that is more uniform from batch to batch.
Slow Speeds, Shallow Feeds
Swiss-machined parts are generally 0.02 to 2 inch diameter. They can range from short screws with a 1:1 length-to-diameter ratio all the way up to 12 inch long surgical pins less than 0.25 inch diameter. For long, slender parts, Swiss machining is sometimes the only practical production method. The variety of parts is growing. Automotive engineers are incorporating Swiss-machined valves and connectors into air bags, anti-lock braking systems and other new features. Miniaturized electronics require extra-small connectors made to extremely tight tolerances.
Modern tooling can make a dramatic difference in Swiss machining throughput, accuracy and finish. Throughput on the previously mentioned hip joint doubled when the shop replaced brazed tools with uncoated carbide inserts. The fine-grained and wear-resistant carbide made it possible to Swiss-machine the titanium parts twice as fast. Throughput increased from 100 parts per day with the brazed tools to 200 parts per day with the modern indexable inserts. Equally important, the extra-sharp, positive-rake cutting edge of the carbide inserts held consistently tight tolerances and improved surface finish from 32 Ra to 16 Ra.
The Sharper Edge
Compared with regrinding brazed carbide and solid high speed steel cutters, replacing indexable carbide inserts improves the cost effectiveness and quality of Swiss machining. Small valves, camera parts and other components are widely Swiss-machined with modern carbides and cermets. Uncoated carbide resists wear when turning titanium and other difficult aerospace materials. Fine-grain carbide compositions provide extra-sharp cutting edges for turning stainless steels. Nickel-free cermet inserts improve the finish of alloyed steels.
Stainless steel bone reamers used to implant artificial hips, for example, are Swiss-machined with precision geometry inserts of different carbide grades. The reamers range from 8 to 12 inches long, and some taper from 0.75 inch diameter down to 0.375 inch diameter. Even at relatively low speeds and feeds, the “gummy” stainless steel poses machining challenges. One carbide grade protected by a three-layer coating resists thermal and mechanical shock and prevents a built-up edge when roughing stainless steel. An alternative grade with the same coating on a cobalt-enriched substrate withstands the sustained heat of finishing cuts.
Productive Swiss machining nevertheless requires the right combination of carbide grades and insert geometries. Cutting inserts need edges ground extra-sharp to reduce cutting pressure and potential part distortion. They also require high positive rake angles for lower cutting forces that enhance surface finish and accuracy. The shape of the part dictates how low cutting forces must be. Long, slender components, those with a lone overhang, and parts that require a very good surface finish and exceptionally tight tolerances demand high positive rake inserts. Tough sticky materials such as stainless steels and titanium also require positive rake to prevent built-up wear.
To make the highest precision cuts with near-absolute repeatability, Sandvik Coromant designed a new generation of turning inserts, specifically for Swiss machining. The new inserts combine high positive rake with sharp edges for cleaner, smoother cuts. Both overall size and thickness of the new inserts are held to the tightest tolerances—±0.001 inch. A polished top surface enhances chip flow when machining difficult materials. To machine a range of workpiece alloys, the new inserts are available in uncoated carbide, coated carbide and uncoated cermet.
Accurate, repeatable toolholding is also essential to productive Swiss machining. To fit automatic sliding-head machines, a line of specially qualified Swiss machining holders is available with no tool offset between the insert tip and shank. Keeping the tool setup close to the collet minimizes vibration and protects the accuracy of machined parts.
New screw-clamp holders for standard inserts are designed to fit the gang-style tool setup common on Swiss machines. The optimized holders expand the range of carbide inserts available for Swiss machining. They have ground 3/8 and 1/2 inch shanks to hold tighter tolerances. The stronger, more rigid design and secure clamping make it possible to do multiple turning and grooving operations with a single tool.