New thread grinding process developed for ball screws

A new thread grinding process for ball screws has improved the surface topography of the screw threads, resulting in improved running properties and having a positive effect on service life and quality.

Since the 1960s, August Steinmeyer has specialized in the production of ball screws. As part of the further development and optimization of its products, the company has recently established a new thread grinding process for the spindles of its products. This makes it possible to achieve smoother running of the nut. Extensive tests have proven that the improved running properties have a positive effect on the service life and quality of the ball screws.

Whether machine tools or special machines, medical or measuring technology, robotics or handling, aviation or aerospace - ball screws are used in a wide variety of designs in different industries. The economic efficiency, reliability and precision of the installed components are always important. Important assessment criteria for the quality of ball screws are smooth running, noise development, vibrations, running-in behavior and energy efficiency. "In order to meet our customers' increasing demands on the quality of our ball screws, we have recently optimized our grinding processes," reports graduate engineer Nima Jandaghi, Head of Technology at August Steinmeyer. The aim is to maintain the company's current market share and, if necessary, even attract new customers. "We have not only improved cooling, flushing and cleaning, but also expanded tool selection and process parameters that play a role in grinding the spindle thread."

Spindles with ground ball threads currently account for about 80 percent of the company's sales. They can be manufactured with very high accuracies and are used in applications that require a high degree of precision. "Spindles with rolled or whirled ball threads, on the other hand, are highly dependent on material behavior and therefore cannot always be manufactured to such a high level of accuracy as the ground variants," explains Jandaghi. Tests carried out at August Steinmeyer show that even with the ground variant, microscopic irregularities can occur on the raceway surface of the screw thread. These lead to vibrations, uneven running characteristics and noise, and are associated with increased wear and thus a shorter service life of the ball screws.

The specialists from the research and development department of the ball screw manufacturer worked intensively on finding a remedy by optimizing the grinding process. Grinding is often the last sub-process in the manufacture of high-quality and precise components for mechanical and plant engineering and is therefore at the end of the value chain. In this process, functional surfaces such as running, visible and sealing surfaces are produced with high surface quality and very tight geometric tolerances. An efficient and at the same time safe manufacturing process is therefore a prerequisite for economical production. High-precision thread grinding is one of the manufacturing processes that require an extraordinary amount of experience. The experts at August Steinmeyer know what needs to be taken into account when manufacturing these complex components. In many cases, therefore, the capabilities of the machines and tools are not fully utilized during grinding so as not to jeopardize process reliability. "It is important to optimally adapt the process parameters to the production times and the required quality of the spindles," says Jandaghi. "Simply utilizing the full capacity of the machine will not necessarily improve quality." In addition to the three cardinal variables of cutting speed, feed rate and infeed, the selection of suitable grinding tools and their conditioning, i.e. profiling, dressing, structuring, sharpening and cleaning, have a significant influence on the process result.

With increasing wear of the grinding tool, important process parameters such as normal and tangential forces as well as the maximum temperature in the contact zone between tool and workpiece change. So-called clogging is regarded as a parameter worthy of attention in the wear mechanism: It describes the adhesion of parts of the workpiece material to the abrasive grains, or the clogging of chips in the pores of the grinding wheel. Clogging causes strong friction and high temperatures in the contact zone. They can weld with the workpiece material and tear out small parts of it, damaging the surface. As the clogging progresses, this can cause a change in the macrostructure of the grinding wheel. Sharp edges in particular are subjected to severe stress as a result, for example during flute grinding. As a result, the wheel can lose its profile, causing dimensional and shape errors on the workpiece.

The grinding process is also a machining process with geometrically indeterminate cutting edges. A large part of the mechanical energy supplied is converted into heat: Firstly, through friction and plastic deformation of the workpiece material during grain engagement, and secondly, during shearing of the chip during chip formation. A large part of the heat generated in the grinding contact zone flows into the workpiece and can thus cause thermal damage and edge zone effects.

Cooling lubricants have important tasks in the grinding process - namely to reduce the occurring friction processes through a good lubricating effect. Thus, heat generation can be effectively reduced and dissipated from the contact zone. In addition, the flushing effect of the cooling lubricant can prevent clogging of the grinding wheel.

To improve the quality of ground workpieces, August Steinmeyer has invested in a new oil system that allows higher pressures and better filtration. On the other hand, the company has also developed and is now using new nozzles for cooling the grinding contact zone and additional coolant nozzles for in-process cleaning of the grinding wheel surface. "Thus, a better supply of the cooling medium into the grinding contact zone and a more effective cleaning of the grinding wheel is possible," Jandaghi explains. As a result, welds on the abrasive grain are reduced and no stress pockets are formed in the grinding wheel pore, and the heat supply into the workpiece is significantly reduced. The company also successfully uses the innovative cooling and cleaning technology developed in-house in the grinding processes for the production of spindle threads. Thus, any comma formation and irregularities on the surface of the spindle thread can be avoided.

"One of the most important results of a grinding process is the refinement of the surface of the workpiece," Jandaghi emphasizes. "The surface topography of our spindle threads could be improved by about 50 percent with our new grinding process." Surface roughness, along with several other parameters - such as geometric accuracy or texture, describes the quality of the ground workpiece. Roughness is influenced by many factors, including wheel topography, clogging, wear, cutting conditions, workpiece material and grinding direction. "Thanks to the smooth interaction of the selected tools, i.e. grinding wheel and dressing tool, as well as cutting conditions with optimized flushing of the grinding wheel and cooling of the grinding contact zone, we can produce the end product - in our case the spindle - with an even higher manufacturing quality than before," concludes Jandaghi. Thanks to the improved grinding process, August Steinmeyer can thus offer ball screws that meet the increasing requirements of high-precision applications and are particularly durable, economical and efficient thanks to optimized running properties. (OM-3/23)