On-line drill condition monitoring by measurement of drill spindle vibration and dynamics

On-line drill wear monitoring in the manufacturing of electronic circuit boards from epoxy-glass composite was investigated. An indirect monitoring approach using spindle speed and vibration was found to be appropriate for production use, and was evaluated analytically and experimentally.; A simple analytical model was developed to describe the dynamic characteristics of cutting forces on the drill. Numerical results showed that cutting forces with harmonic spectra are generated, and that thrust axis vibrations are likely to be most useful for monitoring wear. Measured acceleration spectra from a card drilling machine showed the predicted spectral structure.; A technique was developed and used for calculating mean torque from estimates of spindle speed derived from an encoder signal. The properties of this method were examined analytically and numerically, and the resulting torque estimates were found to be insensitive to torsional and lateral vibrations at integer multiples of the encoder frequency.; A factorial experiment was conducted to examine the usefulness of various spectral and non-spectral vibration responses and torque in detecting known differences in drill wear. The results were evaluated by analysis of variance and discriminant analysis, and significant response differences were found between new and worn drills.; A progressive wear test was conducted to study the relationship between the dynamic responses and advancing drill wear. Although many of the differences observed earlier were found to be due to wear-in effects, several harmonics of thrust axis acceleration were found to be sensitive to advanced drill wear. Using linear discriminant functions, a drill wear monitoring scheme which is capable of reliable drill wear classification was developed.