Nibbling is a progressive-punching process to shear sheet metal to create blanks of complex shapes, which are later bent into 3-dimensional products. It is particularly suitable for low- to mid-volume production and complex-shaped workpieces because it is fairly fast and does not require the construction of mating tool- and die-sets. However, the amount of time and effort required before the actual machining commences for the selection of suitable standardised punches, for the computation of the feed to achieve the required surface roughness, for the definition of the tool paths and for the preparation of NC program paper tapes, may make this material-processing method relatively unattractive, if the volume of production is low. To overcome the obstacles mentioned, PC-based computer software has been developed. The use is required only to define the workpiece profile by draughting the drawing using AutoCAD, a popular and easily available CAD software. The software implementation is achieved by interfacing AutoCAD to run an external program coded in a combination of C and AutoLisp. The software processes the geometry entity file created by AutoCAD to determine the definition of the external and internal profile of the workpiece. It then: uses a number of heuristic rules to select suitable punches; computes the feed for each line segment to achieve the required surface-roughness; and generates and presents graphically the tool paths with respect to the workpiece for the user to verify before the generation of the paper tape. It will also allow the user to select punches manually for any portion of the profile and where the automatically- or manually-selected punches are not capable of punching out any particular portion of the workpiece, the software will highlight the portion for the user to modify. This paper discusses: the method used to extract information about the workpiece profile from the AutoCAD DXF file; the organization and processing of this information to allow the automatic punch selection; the method used to generate the tool paths; and the interface of AutoCAD and the external program. © 1990.
