A worker's hand was dragged towards machinery after his glove became snagged at a factory in South Yorkshire.
The man, who was working at the site of Hallamshire Engineering Service, then based in Dinnington, Rotherham, required stitches and dislocated a bone on his wrist.
The firm, of Belle Vue, in Wakefield, admitted breaching health and safety regulations and, at Sheffield Magistrates' Court on July 13, was fined £5,000 and ordered to pay nearly £2,763 in costs.
The man, employed by The Works agency, was polishing a metal shaft on a lathe using emery cloth when the accident happened in September 2014. He had only been working at the site for four days.
A Health and Safety Executive (HSE) investigation found insufficient safety assessments had been carried out for the process and there were no clear guidelines for workers on how to polish safely.
HSE inspector Laura Hunter said: "Those in control of work have a responsibility to devise safe methods of working and to provide the necessary information, instruction and training to their workers in the safe system of working.
"This incident could so easily have been avoided by simply carrying out correct control measures and safe working practices."