## 1 Introduction  
During a pre-clinical prosthetics course, dental students learn the principles of performing dental preparations for fixed partial dentures (FPD). One of the major problems students face in the course is spatial perception and orientation, which can be manifested in poor identification of the tooth axis and difficulty in identifying undercuts in the preparation, leading to inadequate preparation. Furthermore, the difficulty arises when assignments require indirect vision using a dental mirror, which requires manual skills for working with both hands. [1, 2]. Learning this skill is considered difficult due to the natural difficulty of the brain to control two new complex motor actions that require coordination of both hands. Tooth preparation for FPD is a clinical challenge for students because the designs must meet important biomechanical principles in order to meet accepted standards to receive a permanent crown [3]. Two of the basic principles in preparing for FPD are the principle of resistance and retention. One of the goals of retention is to have a single insertion path that is parallel to the tooth's longitudinal axis. The insertion path is determined by tooth preparation, and tilting of the opposite walls properly is critical to the retention of the preparation. Optimally, we would like the insertion path to be parallel to the tooth's longitudinal axis, so that it should always be 90\({}^{\circ}\) to the occlusal surface and inclined at a convergence angle of 6-12\({}^{\circ}\)[4; 5; 6]. Researchers found that dental students performed on averaged 12.7 degrees of wall convergence, while the clinical preparations average was 22.8 degrees [7; 8]. Another study that modeled 909 dental preparations for FPD found that the average convergence of preparations was 19.2 degrees [9]. In order to increase the accuracy of dental students' preparations, researchers have developed innovative digital design software for helping students understand the tooth's axis, marginal design, and undercuts, and objectively self-evaluate their preparation [10; 11; 12]. For example, PrepCheck (Dentsply Sirona, Wien, Austria) makes use of CAD/CAM (computer-assisted design and computer-assisted manufacturing) technology by comparing scans of tooth preparations with a standardized \"master\" preparation [13]. Although accumulating evidence has confirmed the effectiveness of such new assessment methods, primarily in relation to self-assessment and self-directed learning of manual skills, most studies have claimed that further investigation of CAD/CAM technology for the purpose of assessment and education of dental students is recommended [14; 15]. Other researchers believe that manual performance can be facilitated by means of a positive transfer of learning. Accordingly, repeated training of one motoric task leads to significant improvement in the performance of another motoric task that has similar components [16]. They developed training tools to enhance performance in preclinical courses. For example, [PERSON] et al. developed an indirect training device called Mirroprep, which consists of a high-profile quality steel sheet with a mirror mounted to its rearmost wall and a replica of a dental drill holding a short pencil. The test sheet consists of a curved track course in different outline forms, in which the student must track the course on the pad bounded by two lines with the pencil. They showed that mirror vision can be learned and improved while practicing on the Mirroprep; the result was a high degree of transfer in manual performance [17; 18]. The disadvantage of training tools that have been developed is that the actual conditions of the training were not similar enough to the phantom laboratory environment. In this study, a simple portable manual training tool was developed for training at home that entails the general principles and identical elements as in the pre-clinical prosthetics course and can be used as a supplement to conventional training as needed.