Model-based tool design for the manufacturing of hypocycloidal internal profiles by polygon turning

Polygon turning is a machining process for manufacturing internal and external hypocycloidal profiles, which are becoming increasingly important for shaft-hub-connections. The highly productive process can be integrated on lathe machines. Existing analytical models are not sufficient to address the main challenge when designing tools for manufacturing internal profiles. In this work a dexel-based numerical approach is presented to calculate the limits of tool design. The findings are made usable for tool design with an analogy model and are finally validated experimentally using an alternative drive system for bone screws as an example

Gear skiving with minimum twist errors – Modeling and optimization of flank twist in gear skiving [Verschränkungsarmes Wälzschälen – Modellierung und Optimierung der Verschränkung beim Wälzschälen]

Over the last 15 years, gear skiving has established itself as a highly productive gear cutting process for the production of internal gears and gears with near interference contours. As with all processes with crossed axes, gear skiving generally results in a pronounced natural twist when gears with lead crowning or other flank modifications are produced. In practical applications, the unintended profile angle changes over the tooth width resulting from the twist leading to unwanted contact patterns and unfavorable NVH behavior. In this work, a contact line-based method for tool profile calculation for gear skiving is developed based on conical-screw gear theory. The relationship between contact line and natural twist errors is worked out. The process and tool design strategies for minimizing the twist are elaborated and finally, an adaptive process kinematics for low-twist error gear skiving is presented

Implications of carbon, nitrogen and porosity on the γ→α′ martensite phase transformation and resulting hardness in PM-steel Astaloy 85Mo

This study aims at a thorough characterization of the relationship of interstitially solved carbon and nitrogen on the γ → α′ transformation in PM steels, the accompanied volume change and the resulting hardness. Furthermore, the investigations include multiple porosity levels of 6.9, 7.2 and 7.35 g/cm$^{3}$ to characterize porosity effects. Dilatometric samples were carburized and carbonitrided to seven distinct compositions to account for common compositions in the process of thermochemical case hardening heat treatment. The dilatometric samples were rapidly austenitized and quenched and the dilatometric response was evaluated. To fully characterize the martensitic transformation of PM steels, X-ray diffractometry evaluated the amount of retained austenite after quenching. Conclusive results of iterative quenching procedures along with elemental analysis after heat treatment give distinct evidence that PM steels underlie rapid decarburization. This effect ultimately leads to an erroneous evaluation of the martensite transformation kinetics, especially the often proposed effect of porosity on M$_{S}$. However, a distinct effect on the accompanied volume change from austenite to martensite is proposed. To account for an interplay of solved carbon and nitrogen, an effective nitrogen contribution of 25% based on carbon equivalent is proposed. Utilizing the effective content, the impact of nitrogen can be projected on carbon within the range of common carbon and nitrogen contents, and a good predictability of the martensite transformation can be achieved. Regarding the resulting hardness, a dependency solely on carbon is suggested. The overall hardness shows a typical maximum at approximately 0.6–0.7 wt%, irrespective of the solved amount of nitrogen

Concentric Scanning Strategies for Laser Powder Bed Fusion: Porosity Distribution in Practical Geometries

Besides the optimisation of process parameters such as laser power or scan speed, the choice of the scan path represents a possibility to optimise the laser powder bed fusion process even further. The usual hatching strategy creates a homogeneous microstructure but makes it necessary to switch the laser off and on after each scan vector, which can slow down the fabrication. Moreover, the end of each scan vector is a location susceptible to the creation of keyhole pores. In this work, these disadvantages were meant to be avoided by using scan strategies that consist of longer paths and thus less end of track points. To this end, an open-source tool to tailor the LPBF G-code to geometric part features and advanced path configurations was developed and embedded into a co-visualization platform. With this tool, specimens built with four different types of paths were fabricated and the effect of these alternative scan strategies on pore distributions and path neighbourhood was investigated using micro-computed tomography. In the examined example geometry, a spiral scan pattern reduced the distance the laser had to jump between scanning by 78%. However, with the alternative path patterns, the defect architecture was strongly dependant on the part geometry and increased the overall porosity to 0.42%. Respective alleviation approaches are therefore necessary and are discussed in the remainder of this work