Printing machine for thermal transfer printing of images on articles, comprising a frame structure, stationary in operation, support means fixed to the frame structure and arranged to support the article on which at least one image is to be printed, a thermal transfer assembly with a transfer pad having an active surface which is heated in operation, the thermal transfer assembly being placed above said support means and being movable vertically relative to the frame structure between a raised rest position and a lowered working position, motor means arranged to cause a vertical translational movement of the thermal transfer assembly relative to the frame structure between said raised rest position and lowered working position, supply and guide means arranged to advance, along a predetermined path extending in part between the active surface of the transfer pad and said support means, a flexible printing ribbon which on a side thereof facing said support means carries, at predetermined intervals, images formed of a thermally transferable ink, and control means, characterized in that it further comprises position sensor means arranged to provide electric signals indicative of the vertical position of the transfer pad, and in that said control means are arranged to carry out a learning phase wherein said control means cause a descent run of the thermal transfer assembly until the transfer pad is brought into engagement with an article of a predetermined type positioned on said support means, and through said position sensor means detect and store reference data characterizing said descent run, and a subsequent printing phase, wherein said control means cause a subsequent descent run of the thermal transfer assembly, towards and up to an article of the same type positioned on the support means, detecting and comparing with said reference data corresponding data which characterize the current descent run, and stopping said printing phase when the comparison of said data is indicative of an anomaly in the current descent run. a frame structure, stationary in operation, support means fixed to the frame structure and arranged to support the article on which at least one image is to be printed, a thermal transfer assembly with a transfer pad having an active surface which is heated in operation, the thermal transfer assembly being placed above said support means and being movable vertically relative to the frame structure between a raised rest position and a lowered working position, motor means arranged to cause a vertical translational movement of the thermal transfer assembly relative to the frame structure between said raised rest position and lowered working position, supply and guide means arranged to advance, along a predetermined path extending in part between the active surface of the transfer pad and said support means, a flexible printing ribbon which on a side thereof facing said support means carries, at predetermined intervals, images formed of a thermally transferable ink, and control means, characterized in that it further comprises position sensor means arranged to provide electric signals indicative of the vertical position of the transfer pad, and in that said control means are arranged to carry out a learning phase wherein said control means cause a descent run of the thermal transfer assembly until the transfer pad is brought into engagement with an article of a predetermined type positioned on said support means, and through said position sensor means detect and store reference data characterizing said descent run, and a subsequent printing phase, wherein said control means cause a subsequent descent run of the thermal transfer assembly, towards and up to an article of the same type positioned on the support means, detecting and comparing with said reference data corresponding data which characterize the current descent run, and stopping said printing phase when the comparison of said data is indicative of an anomaly in the current descent run.