When it comes to machining metal, I think it would be helpful to be able to predict cutting forces mathematically. I realize that the average machinist may not need to do this precisely, but this matters a lot for the design of cutting tools. I dug up the following diagram, showing how a tool cuts into a work piece. It develops a shear angle, which is some function of the tool geometry and the coefficient of friction between the tool and the work piece.
Much can be read about cutting tool theory. However, almost all the research I have done (from sources outside my own experience) presents a precise mathematical theory only for sharp cutting tools. As for dull or rounded cutting tools, mention may be made of a secondary or tertiary shear zone, but there is no method to calculate the size of this zone, or its effect on the overall force applied to the tool.
My background is mechanical engineering, and I am currently designing band saw blades. With band saw blades, unlike milling cutters or turning inserts, the chip load is very small - in some cases, smaller than the microscopic tool edge radius. In that case, the rake angle is almost meaningless, because the tool is pushing down on the material with a blunt edge, rather than digging down under the material. This causes the tool to act as if it had a negative rake angle. Nevertheless, and perhaps paradoxically, a ductile continuous chip is still formed.
One could imagine that a dull tool has a negative rake angle, which is a function of the tip radius and the Undeformed Chip Thickness. You can measure that negative angle with a good microscope. However, this approach does not match observations predictably. If you take a saw tooth and cut a piece of metal bolted to a force gauge, you'll find forces that don't match up to the calculations resulting from the above diagram (I can explain these in more detail if needed).
My point is that, as far as I can tell, machining theory is lacking when it comes to dull cutting tools. Since most tools are at least somewhat dull for most of their working lives, this is a problem.
In this thread, please present and discuss information you may have found or discovered regarding a theory of metal machining for blunt, dull, or honed cutting tools.
Much can be read about cutting tool theory. However, almost all the research I have done (from sources outside my own experience) presents a precise mathematical theory only for sharp cutting tools. As for dull or rounded cutting tools, mention may be made of a secondary or tertiary shear zone, but there is no method to calculate the size of this zone, or its effect on the overall force applied to the tool.
My background is mechanical engineering, and I am currently designing band saw blades. With band saw blades, unlike milling cutters or turning inserts, the chip load is very small - in some cases, smaller than the microscopic tool edge radius. In that case, the rake angle is almost meaningless, because the tool is pushing down on the material with a blunt edge, rather than digging down under the material. This causes the tool to act as if it had a negative rake angle. Nevertheless, and perhaps paradoxically, a ductile continuous chip is still formed.
One could imagine that a dull tool has a negative rake angle, which is a function of the tip radius and the Undeformed Chip Thickness. You can measure that negative angle with a good microscope. However, this approach does not match observations predictably. If you take a saw tooth and cut a piece of metal bolted to a force gauge, you'll find forces that don't match up to the calculations resulting from the above diagram (I can explain these in more detail if needed).
My point is that, as far as I can tell, machining theory is lacking when it comes to dull cutting tools. Since most tools are at least somewhat dull for most of their working lives, this is a problem.
In this thread, please present and discuss information you may have found or discovered regarding a theory of metal machining for blunt, dull, or honed cutting tools.