The phenomenon of spiralling or helical multi-lobe formation in holes, produced by the BTA (Boring and Trepanning Association) machining, is experimentally investigated for the solid boring tool. The causes leading to spiralling are deduced from this investigation. The experimental approach pursued in exploring the problem involved the running of the machine, at analytically predicted critical speeds and observing the reoccurrence of the phenomenon. It has been established that sprialling is caused by defectiveness of the tool (radial oversize of the circle-land with respect to the leading pad around the circumference) and the coincidence of the lateral natural frequency of the boring bar-tool asssembly, with five cycles per revoution of the tool, relative to the workpiece. It has also been established that spiralling occurs only in five lobes for the commercially available BTA-solid tool and is a consequence of the standard position of the circle-land, relative to the leading pad. The trials are repeated a number of times with various workpiece materials, to assert validity of the observations.

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The design concept and the analysis of multi-edge BTA deep-hole machining tools are presented. The main feature of the design is that the tool cutting edges are unsymmetrically located on the boring head. This provides a stabilizing cutting force resultant necessary for self-guidance of the boring tool. The role of this stabilizing force for machining accurate holes of high length-to-diameter ratio is explained and demonstrated by examples. The principles of tool self-guidance and the range of application for various drilling methods are discussed and examples are given. The results show that, besides achieving higher material removal rate, the use of unsymmetrical multi-edge cutting tools provides similar performance to that of single cutting-edge boring tools in terms of tool stability and bore accuracy. Results also show that by controlling the cutting force resultant at a predetermined value based on feed rate, cutting conditions and tool geometry, a full-film lubrication at wear-pads of the boring head can be insured during the boring operation.

The design procedure of optimal multi-edge BTA deep-hole machining tools with unsymmetrically located cutters and preliminary test evidence are presented. Based on a mathematical model of cutting forces in terms of fundamental cutting parameters of the tool, a multivariable, nonlinear objective function was derived and modified to an unconstrained type with bounded decision variables. A numerical, direct search method, accelerated in distance, was selected to minimize the objective function. This procedure insures, on one hand, a predetermined cutting force resultant necessary for tool guidance; on the other hand, it minimizes the variation of cutting edge pressure. A relatively fast computer routine was adapted to provide the optimal tool parameters, which then were used to design cutting head prototypes. Two trepanning heads of three and two cutters were manufactured and tested at production facilities. The test results showed that the cutting force resultant was well predicted in both heads and that they were well guided. Much higher feed rates were possible compared to those achieved with single-edge tools without any loss of hole accuracy straightness or surface finish.

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