Steve Kidd

Acoustic emission monitoring of tool wear during the face milling of steels and aluminium alloys using a fibre optic sensor. Part 2: Frequency analysis

Abstract An investigation of the relationship between tool wear and the energy of acoustic emission (AE) produced during various face milling finishing operations is presented. A model detailing how the AE energy, quantified by the r.m.s. value, varies depending on the material and the detailed tool geometry formed by flank and crater wear is described. Validation of the model was achieved in a series of practical machining tests covering a range of materials and tool types which resulted in various different wear forms. In all these wear tests a non-contact fibre optic interferometer was employed for AE detection directly from the workpiece. This sensor makes absolute, calibrated measurements of AE, unlike conventional contacting piezoelectric AE transducers, which may suffer uncertainties due to their frequency response and variations in transmission path. The fibre optic instrument is thus advantageous for studying variations of AE energy with tool wear.

Fabrication of single-mode fibre optic Fabry-Perot interferometers using fusion spliced titanium dioxide optical coatings

Techniques are described for the deposition of titanium dioxide coatings onto the end face of single-mode optical fibres by thermal and electron beam evaporation. The absorption and reflectivities of the coatings were measured, and maximum reflectivities of 32% were achieved. Coated fibre ends were fusion spliced to similar, but uncoated, fibres using an electric arc. Optimum parameters for the arc were determined, thus yielding reflective splices having reflectivities of up to 25% and tensile strengths of 9 N. These reflective splices were used to form fibre optic Fabry-Perot interferometers yielding visibilities of up to 92%. Application of these reflective splices and interferometers as physical sensors is discussed.

Surface finish parameters as diagnostics of tool wear in face milling

Abstract We describe a systematic series of wear tests in which surface profiles were measured at intervals during face milling of En24 steel, an aluminium alloy and a stainless steel. Profiles were measured by a stylus instrument and, in the case of the aluminium alloy, by a novel optical fibre instrument mounted on the machine headstock. Spatial frequency analyses are presented that show the long wavelength amplitude increases with insert wear, with a similar tendency in the high spatial frequencies (above the feed per tooth). Variation in tool set affected insert wear rate and the evolution of surface profile. Insert chipping damage was seen to increase the low spatial frequency content. The Ra parameter correlated with insert wear only for single point cutting.

Scheme for Extending the Bandwidth of Injection-current-induced Laser Diode Optical Frequency Modulation

Abstract It is shown that optical frequency modulation of laser diodes via control of their injection current is dominated by thermal effects for modulation frequencies in the range up to about 1 MHz, and that the modulation bandwidth is determined by the complex thermal structure of the laser. A theoretical model is developed, based on the thermal properties and the heat transfer between the laser chip, its submount and its heat sink. The theory is validated by experiments which indicate that the 1 dB modulation bandwidth of the typical laser diode used was about 10 kHz. The design of a simple equalization circuit for the laser diode is described, and its application was shown to increase the modulation bandwidth to about 300 kHz. It is shown that the equalization technique is valuable in extending the bandwidth of phase demodulation techniques commonly employed in interferometric fibre optic sensors.

Interferometric fibre sensors for measurement of surface heat transfer rates on turbine blades

Abstract We describe a prototype sensor for surface heat transfer measurements based on a miniature fibre Fabry-Perot (FFP) interferometer. These sensors are being developed for a particular application where heat transfer data are currently obtained using conventional platinum thin film resistance thermometers. The specification that the sensors must exceed is: (i) temperature resolution of 25 mK over a 50 K range; (ii) temporal response of 10 μs; (iii) an ability to operate as a calorimetric heat transfer gauge. The sensor consists of a short length of single mode optical fibre (∼ 3 mm) to which low reflectivity coatings have been applied at each end. It is illuminated and interrogated by an arbitrary length of addressing fibre. A laser diode is used as the source and we have exploited the facility to frequency modulate the diode in a novel signal processing scheme. To determine the performance of the sensor, short duration heat pulses derived from a pulsed Nd: YAG laser were applied to one end of the FFP. The response time was found to be 8 μs and the sensor operation as a calorimeter was verified.

A Fiber Optic Probe for Gas Total Temperature Measurement in Turbomachinery

This paper describes the design, operation, construction, and demonstration ofa new type of high-bandwidth unsteady temperature sensor based on fiber optics, and capable of operating in a high-speed multistage research compressor with flow representative of jet engine conditions. The sensing element is an optical coating of zinc selenide deposited on the end of an optical fiber. During evaluation in aerodynamic testing, a 1 K gas temperature resolution was demonstrated at 9.6 kHz and an upper bandwidth limit of 36 kHz achieved.

Measurement of aspects of surface form using an optical differential height measurement technique

We describe a fibre interferometer for making non-contact differential height measurements of machined metal surfaces without removal from the machine tool. The system comprises a probe which produces two small laser spots ( diameter, 3 mm apart) on the surface, an optical fibre addressing lead and signal processing unit incorporating an optical source and detector. The probe is an extrinsic fibre optic Michelson interferometer whose arms terminate in the laser spots on the test surface, and the signal processing unit calculates the difference in local surface height between the two spots. Differential height profiles of machined aluminium surfaces and optical mirror surfaces are presented.

Surface features as indicators of tool chipping in single point face milling of aluminium

Abstract We describe a set of wear tests in which machined surfaces were produced by single point face milling of an aluminium alloy. The objectives were firstly to identify surface characteristics associated with tool chipping, and secondly to demonstrate that a fibre optic surface profiling probe capable of in-situ operation on the machine tool had the ability to provide the same wear information as a metrological interference microscope. Spectral energy of the surface profiles decreased significantly with insert damage in a spatial frequency band around the tooth passing frequency of the cutter and also at higher spatial frequencies.

Fibre Optic Interferometric Heat Transfer Sensors for Transient Flow Wind Tunnels

A new type of heat transfer sensor has been developed in which the sensing element is a short length (∼3mm) of single mode optical fibre acting as a Fabry-Perot interferometer. The reflected light intensity follows a periodic transfer function, and the phase change is proportional to the sensor’s spatially averaged temperature. We present results from three optical fibre sensors embedded as calorimeter gauges in a ceramic nozzle guide vane end wall model exposed to a transient heat flux of ∼100 kWm−2 in the Isentropic Light Piston Facility at DRA Pyestock Famborough, and validated by comparison with previous data from platinum thin film resistance gauges. The optical sensors exhibit high spatial resolution (∼5μm), high heat transfer resolution (∼1kWm−2), and wide temperature measurement bandwidth (100kHz) with intrinsic calibration. No electrical connections to the measurement volume are required and multiplexing is possible.Copyright

A Fibre Optic Probe for Gas Total Temperature Measurement in Turbomachinery

This paper describes the design, operation, construction and demonstration of a new type of high bandwidth unsteady temperature sensor based on fibre optics, and capable of operating in a high speed multi-stage research compressor with flow representative of jet engine conditions. The sensing element is an optical coating of zinc selenide deposited on the end of an optical fibre. During evaluation in aerodynamic testing, a 1 K gas temperature resolution was demonstrated at 9.6 kHz and an upper bandwidth limit of 36 kHz achieved.Copyright