Maximization of the effective impulse delivered by a high-frequency/low-frequency planetary drill tool

Abstract

Ultrasonic tools are used for a variety of cutting applications in surgery and the food industry, but when they are applied to harder materials, such as rock, their cutting performance declines because of the low effective impulse delivered by each vibration cycle. To overcome this problem, a technique known as high-frequency/low-frequency (or alternatively, ultrasonic/sonic) drilling is employed. In this approach, an ultrasonic step-horn is used to deliver an impulse to a free mass which subsequently moves toward a drilling bit, delivering the impulse on contact. The free mass then rebounds to complete the cycle. The horn has time between impacts to build significant vibration amplitude and thus delivers a much larger impulse to the free mass than could be delivered if it were applied directly to the target. To maximize the impulse delivered to the target by the cutting bit, both the momentum transfer from the ultrasonic horn to the free mass and the dynamics of the horn/free mass/cutting bit stack must be optimized. This paper uses finite element techniques to optimize the ultrasonic horns and numerical propagation of the stack dynamics to maximize the delivered effective impulse, validated in both cases by extensive experimental analysis.

A Parametric Study for the Design of an Optimized Ultrasonic Percussive Planetary Drill Tool

“…Compared to previous models of USDC/UPCD style devices (including the work in [4], [6] and [10]) which compute the effective horn mass and then attribute the rest of the material to the transducer mass, this current work proposes an integrated modelling technique to represent the transducer and horn itself. This is important because the input signal is an AC voltage to the piezoceramic rings (which generates the modelled mechanical vibrations) and the output signals are then the piezoceramic ring vibrations, transducer radiating face vibrations and horn tip vibrations.…”

“…The identified parameters of the dynamic stack are shown in TABLE III for a 5g free-mass, an 80g drill-bit and a 1.925kg transducer. The contact time of collision between the horn tip and free-mass has been estimated in [4] using finite element analysis, estimating a contact time of 1.2 × 10 −5 seconds. As the free-mass and drill-bit have the same material properties as the horn, the contact time for the free-mass and drill-bit is also assumed to be 1.2 × 10 −5 seconds.…”

“…The USDC is driven by a piezoelectric Langevin-style transducer which operates at around 20kHz. This vibration is amplified by a resonant step horn [4] and then used to excite a percussive stack which consists of a free-mass and a drill-bit.…”

“…The energy of multiple ultrasonic vibration cycles is thus accumulated and transferred to the free-mass and hence to the drill-bit/rock interface when an impact occurs [6]. The rate of progress is broadly proportional to the rate at which effective impulse is delivered, where this rate is equal to the growth of the bit/rock contact-pressure time integral exceeding the compressive strength of the rock [4].…”

A Motion Control System Design for an Ultrasonic Percussive Coring/Drilling Unit

“…This mechanism uses a piezoelectric transducer to convert electrical pulses into mechanical vibrations with an amplitude of a few micrometers and an ultrasonic frequency of around 20kHz. The vibration is further amplified by an ultrasonic metal horn which is coupled acoustically to resonate at the same frequency as the piezoelectric transducer [4] , before driving a free-mass oscillating between the horn tip and the drill bit. The free mass oscillation is chaotic and varies from tens of Hz to about 1kHz [5] , so the acoustic energy of several ultrasonic vibration cycles is accumulated and transferred to the bit/rock interface [6] , causing fracture when the local stress level exceeds the strength of the rock.…”