The worker had not received any training on what to do if the chain became dislodged.
He decided to fix the problem by climbing over the outfeed's barrier to reach the second chain drive and by placing the chain back on the chain drive using his right, gloved hand while the machine was running.
The worker had fixed the machinery by doing this previously, the summary said.
When he went to put the chain back on to the drive, the glove got caught in the chain and the tops of his middle, index and little fingers were amputated.
His glove containing the amputated parts of his fingers were found wrapped around the chain drive by WorkSafe inspectors the day after the incident.
At the time of the accident, Claymark employed 600 workers at six sites in the North Island and was New Zealand's largest manufacturer and export of premium pine timber products.
The summary said an emergency stop button that was closest to the worker had been damaged and the red "activating mushroom" was missing.
While the button was not prominent as required, it was able to stop the Optimiser when triggered.
The summary said an ambulance transported the worker to Rotorua Hospital where he underwent surgery on his hand.
He had further surgery on his index and little fingers on July 27.
The man started work at Claymark in 2015 and had been operating the Optimiser for a week before the accident happened.
A WorkSafe investigation found guarding around nip points of chains on the Optimiser was inadequate, it had no documented safety procedures for workers to follow when a chain needed fixing and had not trained workers on this.
WorkSafe also found an emergency stop, the closest one to the injured worker, was not clearly marked with a red mushroom.
Despite Claymark recording it knew where all the emergency stops were, it had not trained the injured worker where they were on the Optimiser.
It also found despite having a system for monitoring maintenance, including emergency stop buttons, repairs had not been made.
The summary said WorkSafe issued a prohibition notice on the Optimiser which required Claymark to install guarding, assess and install correct emergency stops and conduct a risk assessment of the Optimiser.
Claymark hired an engineering firm to assess the Optimiser and as a result, abandoned the use of chains and instead adopted a fixed steel table which was fully enclosed.
Improvements were also made to other areas of the machine including fixing guards along the front of the machinery and Claymark updating safe operating procedures.
Claymark will be sentenced in the Rotorua District Court on November 5.
Lawyer Fraser Wood represented Claymark and Anna Longdill appeared for WorkSafe.