Skiving instead of broaching2017/02/20 LIEBHERR VERZAHNTECHNIK GMBH
Gear-skiving is a challenging process. That applies both to the geometry of the tools used as well as to their durability, as they are exposed to enormous wear and tear. Liebherr-Verzahntechnik GmbH has perfected the design of skiving wheels and produces tools of supreme quality and extended durability. Broaching is usually the process of choice for producing internal gear teeth – for instance ring gears for planetary transmissions. The broaching process however has several disadvantages. Tool costs are the highest compared with all other gearing tools. Ordering lead times of between six and twelve months are the rule. Broaches often feature poor concentricity as well because of unavoidable warpage that occurs during cutting. Investment is therefore very high and only makes commercial sense if quantities are high, as in the vehicle manufacturing industry. The process of gear-skiving offers a new and very interesting alternative. The principle on which gear-skiving is based is more than 100 years old. However the process was never successfully applied to serial manufacturing in the past, because the service lives of the tools were very limited. Liebherr-Verzahntechnik GmbH has now developed special tools, which can withstand the very high stresses that occur during the skiving process. Process makes more stringent demands on the tools The tool’s clearance and true-rake angles are constantly changing during contact. “Enormous forces act on the gear-skiving tool during the continuous cutting and pressing process”, is how Ottmar Kern describes the problem. He is Head of Gear-Cutting Tool Sales at Liebherr’s Ettlingen plant and helped develop the cutters. Simultaneous cutting and reshaping make higher demands of the materials used to make the tools. Liebherr uses two different materials as well as a combination of the two: 1. Tools made of PM steels have a cost advantage, but are subject to greater wear and tear. 2. Carbide tools are more expensive both in terms of material and manufacturing costs and on account of longer grinding times and greater consumption of abrasives. Since they cost roughly three times as much as PM-steel tools, use of carbide tools only makes commercial sense if production quantities are high. 3. PM-steel tools with carbide inserts are a compromise that is not suitable for every application, given the required assembly clearance between indexable insert and fixture. These combination tools can however lengthen rough-machining lifetimes and deliver premium quality post-finishing using conventional PM / HSS skiving cutters. Ottmar Kern firmly believes in the success of gear-skiving and of his tools: “We anticipate that this process will gain acceptance, given the higher productivity it delivers. Ring gears can be skived in higher quantities for all industries using planetary drives.” Here he has vehicle manufacturing (incl. e-drive trains), trucks, construction machinery, aerospace engineering and many other planetary gearbox applications in mind.