Stuart Stead, 49, was using a hand-held grinder on a high-frequency machine while working on a casting in March 2012.
The disc fitted to the machine suddenly exploded, sending metal fragments across Mr Stead's workbay. A shard of metal went through his visor and struck him in the mouth.
The injuries received by Mr Stead were fatal and his was pronounced dead at the scene.
The grinder had no guard, with one piece of metal found ten metres away after the explosion.
A Health and Safety Executive (HSE) investigation found the disc was nine inches in diameter although the grinder had a maximum permissible tool diameter of only two inches, unless it was guarded.
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The grinder was operating at 12,000 rpm while the disk was only rated for 6650 rpm. The disk was also attached to the grinder using an unregistered tool.
It is understood the investigation also discovered previous incidents where discs had come off of grinders.
The HSE concluded the explosion was caused by the excessive speed of the grinder, coupled with the added load caused by the non-standard attachment, therefore putting stress on the disc beyond its capacity.
The Sheffield firm admitted to breaching section 2(1) of the Health and Safety at Work etc Act 1974.
HSE Inspector Jill Thompson said: "This was a horrific incident that devastated Mr Stead’s wife, his children and wider family. It is also, tragically, one that could have been avoided had the company focused on its responsibility to ensure the safety of their employees.
"The safety of Mr Stead and others was not ensured while using hand-held grinders. If monitoring and supervision had been provided to ensure necessary precautions were followed, this tragic incident would not have happened."
(LM)
