On the prediction of impact noise, Part IX: The noise from punch presses

Abstract

The punch press is a further classical example of impact noise: the force is built up in the punch relatively slowly, and the whole machine is strained until the load on the material being punched reaches a yield and shortly afterwards a fracture level. At this stage the strain energy in the machine and its workpiece must be redistributed and this leads to vibration of the whole machine and to noise radiation. In this paper work which has been carried out at ISVR over a number of years is presented. It describes the way the Energy Accountancy Equation [1] can be modified to relate the noise radiated directly to the sum of the squares of the large rates of change of force against time, and illustrates clearly the way that noise control, with use of passive or active methods in designing the punch tooling, can be related directly to the one parameter 10 log Σ[ f( t)] max 2. It is shown that noise levels can be reduced by up to 30 dB under ideal conditions by fracture pulse tailoring while still cutting metal, but that more realistically, 10 dB reductions may be obtained below those of current practice by practical tool design. An explanation is given of why large open presses are never likely to achieve the proposed factory noise levels and that the noise from double-sided presses can be controlled effectively by tooling modifications and by the addition of damping. The paper describes work carried out on passive and active cancellation systems used to arrest and springback of the press structure following workpiece material fracture and explains the practical limitations of such systems. Active techniques are limited by the difficulties obtained, with brittle hard materials, of finding a trigger and load freezing system which can operate fast enough to reduce adequately the force parameter 10 log Σ ( f max( t)) 2. Where punching finish allows, it has been found that well-designed shear and/or cutting with low percentage clearance is superior to active cancellation, since this provides a surer method of reducing the above force parameter, especially for harder materials. The Energy Accountancy Equation suggests that the use of increased structural damping can be an effective method of noise reduction, particularly if the punch press structure is initially only lightly damped. Experiments on a third-scale model of a 200 tonne double-sided press suggest that this damping needs to be added selectively in areas where the vibrational energy density is high and coincides with areas of high noise radiation. Thus, in the experiments described, 12 dB reduction has been obtained under such conditions, only 1 or 2 dB reduction being obtained when damping of the base plate has been unaltered.