# Structural Optimization Method of a FinRay Finger for the Best Wrapping of Object
## 4 Discussion  
This work comes up with a new and simple method of evaluating the finger wrapping around the object mathematically. Several types of internal structures have been designed, in which their parameters, such as the thickness of the selected walls or the number of internal ribs have changed. To evaluate the wrapping of the finger around the object, a method was determined that measures the ratio between the deflection of the endpoint and the maximum deflection of the finger. Tensile test data was used to set up the material for simulation analysis properly. The tested material is TPU 30D. It is a flexible filament designed for printing on Fused Filament Fabrication printers. The pressed object is a steel roller with a diameter of 20 mm. Three tasks were chosen for the simulation. In the first simulation, the roller is pressed into the finger 8 mm to the depth. Based on the results of this simulation, the deflection coefficients are calculated. In the second simulation, the roller is pressed into the finger with a force of 5 N. The simulation illustrates and points out that from the point of view of wrapping the finger around the object, the magnitude of deformation after loading with a certain force is not very appropriate, as examined by some previously performed works [4; 13]. The third method is done only for few selected types of fingers to compare them with real tests. With this method, the roller load force is gradually increased until the finger bends at the point of contact by 8 mm. The deflection of the point of contact is measured to display a graph of this deflection's dependence on the applied forces. Graphs showing the hysteresis of the printed fingers of structures B and structure A with six ribs were also measured and shown for designers possible needs. Selected finger variations were chosen to demonstrate their ability to grasp various objects. All the selected structures were able to grasp all the selected objects and their contact sides wrapped around their shape appropriately. From the analyses results, it is evident that the greatest wrapping of the fingers around the object occurs in the types of fingers without an internal structure (structure B). For all structures, the dependence between individual parameters (for example, the thickness of selected walls or the number of inner ribs) on the deflection at a constant force was observed. The simulations were verified on selected types of fingers printed on a 3D printer and tested on a real test device. A comparison of the calculated deflection coefficients from the first set of simulations and deformation caused by constant force of 5 N from the second set of tests revealed that the amount of deflection at constant load is not a suitable parameter for judging fingers view of their wrapping around the object. The work aimed not to find the most suitable structure that can exist or include all possible structure variants. The aim was to find a methodology for comparing different structures with each other according to their ability to wrap around a wrapped object. This methodology can be used to test other variants of fingers, changes in their other parameters, sizes, and shapes, etc. This work aims to serve the designers in comparing the FinRay finger structures to find the maximum finger wrap around the gripped objects. For the selected structures in this paper, the work shows graphs of deformation dependence on the gripping force for individual parameters of structures (for example, wall thickness), which could be useful for the designers of the fingers. Conceptualization, J.S., Z.B. and M.V. ; methodology, J.S. ; software, J.M., M.V. and P.O. ; validation, J.S., Z.B. and Z.Z. ; formal analysis, J.S. ; investigation, Z.Z. ; resources, J.S. ; data curation, Z.Z. ; writing--original draft preparation, J.S. ; writing--review and editing, Z.Z. ; visualization, J.S. ; supervision, Z.B. ; project administration, Z.B. ; funding acquisition, Z.B. All authors have read and agreed to the published version of the manuscript.