Joint analysis in rapid fabrication of non-assembly mechanisms Yonghua Chen and Chen Zhezheng Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong Abstract Purpose – The main purpose of this study is to develop a systematic method that can minimize joint clearance for non-assembly mechanism fabrication using a layer-based fabrication technology. Design/methodology/approach – Joint clearance is one of the key factors affecting a mechanism’s performance. Hertz theory is adopted to analyze the joint clearance-penetration displacement relationship and the impact force-displacement relationship. This analysis has indicated the importance of reducing joint clearance. To reduce joint clearance in layer-based fabrication, a drum-shaped roller is proposed for pin joint design in non-assembly mechanism fabrication. Compared to cylindrical pin joint design, a drum-shaped roller joint results in less impact force in mechanism operation. Furthermore, the joint clearance can also be drastically reduced. Findings – Large joint clearance could introduce instability into the dynamic behaviour of a mechanism. By applying a drum-shaped roller, the instability could apparently be alleviated. This has been demonstrated by both simulation and fabrication of a number of mechanisms with and without drum-shaped pin joints. Practical implications – Since the proposed joint design can reduce the joint clearance in rapid fabrication of non-assembly mechanisms, it is possible to expand layer-based rapid fabrication techniques for more mechanism design applications. Originality/value – Layer-based fabrication technologies have two distinct advantages: building parts without geometry restriction; and building sub- systems (static or mobile) without the need for assembly. Only very few previous studies have investigated the applications that can benefit from the second advantage due to the limited accuracy of layer-based technologies in making joints of a mechanism. Through the proposed drum-shaped roller pin joint design together with the proposed joint design guidelines, joint clearance can be reduced significantly. Thus, sub-systems or mechanisms built using layer-based technologies could have accuracy close to the design specification. This will expand the application of layer-based technologies to more mechanism or mobile mechanical system studies. Keywords Rapid fabrication, Rapid prototypes, Joint clearance, Non-assembly mechanisms, Mechanical engineering Paper type Research paper 1. Introduction In the early days, layer-based rapid prototyping (RP) technologies are mainly concerned with producing physical parts as quickly as possible from a design concept, for the purpose of design verification (Jacobs, 1992). Nowadays, with the rapid development of layer-based technologies: more available materials with various mechanical properties to meet a variety of applications, higher accuracy of parts produced, layer-based technologies have been used for rapid fabrication of functional parts and tooling (Khaing et al., 2001; Pessard et al., 2008). However, most reported applications of rapid fabrication are for static parts. There were only limited reports about direct rapid fabrication of mechanisms. In fact, mechanisms are the core sub- systems for many machine designs. With today’s layer-based technologies, it is possible to directly build mechanisms of reasonable accuracy without the need of post-assembly. There were only a few reports about direct fabrication of non-assembly mechanisms using layer-based technologies. Lipson et al. (2005) had built various well-known mechanisms in history using a 3D printer. Their main purpose was to duplicate the mechanisms without further analysis of the mechanism accuracy. Mavroidis et al. (2001) had built major types of mechanical joints such as revolute, spherical, prismatic and universal joints using SLA and SLS machines. For revolute joints, both machines used a joint clearance of 0.5 mm. This clearance was determined based on a series of trial and error test builds. The initial clearance began at 1.0 mm and decreased by 0.1 mm for each successive build until the joint was no longer mobile. The clearances were then increased by 0.05 mm until the joint mobility was clear and freely moving. Rajagopalan and Cutkosky (2001) had formulated a general technique for investigating the kinematic performance on mechanisms fabricated as non- assembly using layer-based machines. The technique only admits deterministic and stochastic error estimation for mechanisms with ideal joints. Errors due to joint clearances, and form errors which are prominent for RP-based mechanism fabrication were not considered. Park et al. (2007) had used RP for the fabrication of a humanoid biped robot which has 27 degrees of freedom, predominantly revolute joints. However, they only used RP to build the The current issue and full text archive of this journal is available at www.emeraldinsight.com/1355-2546.htm Rapid Prototyping Journal 17/6 (2011) 408–417 q Emerald Group Publishing Limited [ISSN 1355-2546] [DOI 10.1108/13552541111184134] This research is supported by a CRCG grant from The University of Hong Kong. The technical support of Frank Tse in making the prototypes is greatly appreciated. 408