Mechanical joining of sheets to tubes by a sheet bending-unbending cycle

Abstract

The utilization of an axial sheet bending-unbending cycle was previously utilized to produce mechanical joints of sheets to rods at ambient temperature by two subsequent operations. In this research, that joining process is now extended to produce a hidden mechanical joint between a sheet and tube that may not have protrusions both at the sheet surface and at the inner tube wall (if a mandrel is inserted there during the joining operation). The process is performed at ambient temperature and starts with the axial bending of a sheet with a pre-drilled hole of smaller diameter than that of the external diameter of the tube. That hole will be enlarged until a point where its diameter matches the external diameter of the tube to allow for its insertion. In a following operation, the sheet is unbent back to its original form, which will force the sheet material near the adjacent region of the hole to flow into a circular slot previously machined in the tube.Although the joining process is successful, the feasibility of the first axial sheet bending operation can be compromised, since it relies on a predetermined vertical displacement to ensure that its pre-drilled centre hole has the same diameter as the external diameter of the tube or rod that is to be inserted inside that hole, so that the mechanical joint is produced after the unbending of the sheet. To address those limitations, a modification is also proposed to the joining process where a redesigned punch tool allows to calibrate the hole diameter and guarantee that the tube or rod can be inserted correctly inside that hole for every joint to be produced while keeping tight tolerances.The combination of numerical modelling and experimentation allowed to determine the working parameters and their respective influence in the overall mechanics of the process. Unit test cells were utilized to prove the feasibility of the new concept and evaluate its pull-out destructive performance, although the joining process can easily be extended to larger tube and sheet dimensions since the plastic deformation is localized.