How Boring Milling Machines have evolved
Present day Boring Milling Machines look almost the same as they did years ago but they have undergone many improvements in terms of capability to cut through super alloys, titanium and high tension steels. The machines are required to cut faster with closer tolerances than they did before. In order to address the requirements, modern milling machines have higher horsepower, greater stiffness, wider speed and feed ranges. The integration of computer numerical control (CNC) technology has resulted into faster work with better finishes and accuracy that were not previously attained with the smaller boring mills. Boring Milling Machines are now available in larger sizes and styles but with requirements that often reaches 200 horsepower. Boring mills that were first developed for gun drilling and cannon boring are now being used in many industries.
Importance of boring rigidity for increased productivity
The single point boring tool is round shaft with one insert pocket that is designed to have a far reach to remove internal stock in parts. Sometimes the rigidity of the machining operation is compromised when the diameter of the boring tool is restricted by the size of the hole and the need for additional clearance to evacuate chips. The limits of practical overhang for steel boring bars is four times the shank diameter and when the boring tool exceeds these limits, the rate of metal removal in the boring operation is compromised because of the lack of rigidity and the increased possibility of vibrations.
Vibrations tend to occur while boring even with the use of the best tool for clamping. When there is radial deflection it can affect the machined diameter. The size of deflection of the boring bar depends on its material, diameter, overhang and the size of the radial and tangential cutting forces. In most cases, when the diameter of the boring tool is increased it creates an increased moment of inertia that can counteract deflection. Another way to counteract deflection is to choose a boring bar material that has a higher coefficient of elasticity like cemented carbide.
Sometimes boring deflection can be calculated and if its exact size is known in advance the problem can be avoided. However, even if the approximate deflection is calculated in advance, sometimes the practical outcome is different because clamping is not absolutely rigid and cutting forces cannot be exactly calculated. The best stability is sometimes obtained by using a holder that completely encompasses the boring bar.