Sevenoaks magistrates heard from the Health and Safety Executive's Katherine Buss, who said that Joyce had become trapped after he accidentally hit some controls.
Buss added that although Joyce had worked on similar machines in the past, there had been a technical breach in this instance as there was no manual available on site.
Durtnell admitted falling short of safety standards in this respect and also of failing to provide sufficient supervision and equipment.
In its defence the firm argued that the relative simplicity of the job and Joyce's experience and training as a fitter, meant there had been no need to supply a manual.
Joyce joined the £30m turnover company, which employs about 100 full-time staff, from school, and excelled in his NVQ course.
At the hearing, defence barrister Christopher Russell described Joyce as "the paradigm of a safety-conscious employee", but pointed to crucial procedures not carried out.