This is an incident report of a potentially dangerous shop operation which I experienced. It was not so much a surprise that it happened but the violence caught me off guard.
I was working on a very simple taper candle holder design using a 3” compressed sphere which had been chamfered and textured. I wanted to punch a socket in the center to hold a standard taper candle. This is the link to the source of the compressed sphere stock and images.
The first step was making a series of punches to be dedicated to the project. I forged a 3” round taper in a piece of 7/8” round tool steel, polished it, then cut it into three sections of about 1” length and welded on utility handles. After the faces were dressed by putting a radius on the edges I was ready to test them. I did no heat treatment. See illustration.
With an even coal fire heat I put the compressed sphere in the hydraulic forging press, lubricated the punch with the coal dust grease, which I wrote about on the 25th of October, and punched the smallest starter punch. The press moves slowly but is very powerful and the punch sunk a bit deeper than I wanted so it hung up a couple of seconds before I could knock the punch out. It came out at a dull red heat but fortunately did not upset.
On the second heat I followed with the middle punch with no problem. On the third heat, when I began to reverse the press and come off the punch the grease lubrication exploded and backed the punch out for me with the “muzzle flash” and loud bang. The confined work space didn’t give the punch anywhere to go so it couldn’t become a projectile in this case.
This was exciting enough to stimulate some forensic examination. So what are the factors to consider? Without the lubricant the explosion wouldn’t occur. If I had drilled a small through-and-through pilot hole the hot gas would have escaped at the bottom. Perhaps if the punches were wider relative to their length the escape would be easier. Perhaps of the face had more radius - more ball shaped and less flat - no chamber could form to trap the gas.
When I return to the project the plan will be modified in some way which I haven’t settled on yet. Safety First!