Lyndal, I tuned in late to this thread but I would like to offer some comments from an engineer’s point of view in the design of a boring bar for very deep hollowing. Setting aside cost, the main issues are the stiffness of the bar, the weight of the bar and the minimum size opening that the bar must penetrate. Ultimately, the last factor will determine whether any design is feasible. For any given opening size, there are some interesting trade-offs to be made regarding the choice of a cross-section of bar. There are three obvious candidates: square bar, round bar and round pipe.
Consider first the relative stiffness or bending strength of the three types of bars. The stiffness is proportional to a quantity called the “second moment of areaâ€of the cross-section. Without dragging equations into the discussion I will summarize the comparative strengths of bars that will fit through a given sized opening. Taking the stiffness of the round bar as 1.00, the relative stiffness of the square bar is 0.424, that is, it will deflect 1/0.424 = 2.36 times as much under a given load as a round bar that fits through the same sized hole. The stiffness of pipe depends on both the inside and outside diameters, but suffers surprisingly little loss of stiffness unless the walls are rather thin. For example, if the wall thickness is 10% of the outside diameter, the stiffness drops only to 0.590 of that of a solid rod. It is immediately clear that a pipe with a 10% wall thickness will be stiffer than a square bar for the same opening size constraint.
Now consider the weights of these respective alternatives. Again, taking the weight of the round bar as 1.00, we find that the weight of the rectangular bar is 0.637 and the pipe with 10% wall thickness is 0.36. Thus a square bar fitting a given opening would not only be far weaker than a pipe of 10% wall thickness but would weigh almost twice as much.
I would summarize these results as follows. If absolute maximum stiffness is required for a given access opening size, then a round bar is the answer, but the weight of the bar also will be maximum. If the opening size requirement can be relaxed just a little, then a pipe can provide the same rigidity as a slightly smaller round bar and weigh a small fraction as much as the round bar. The square bar seems to have no significant advantages in any scenario.
I believe that excess weight would be a problem for two reasons: First, it would increase the static friction force that the turner must overcome to begin to move the tool. Second, it would increase the force necessary to overcome momentum when stopping or redirecting the tool.
For anyone interested in the mathematical details, a Google search for “beam deflection†and for “second moment of area†will provide what you want.
