Trepan

A parachute mine after being defused and partially dismantled by the Royal Navy Bomb Disposal Team

Posthip Scott sends us an entertaining story about a WW2 bomb squad in southern England. One paragraph talks about how they got inside a bomb:

A hole was then cut in the mine’s casing using a trepanner. This cutting tool was made of non-magnetic materials so that it could be used on magnetic mines and was driven by compressed air. After some time a four-inch circular hole had nearly been cut through the casing.

Okay, that sounds like how they cut the holes in the mine casing in the picture up top, but a treppaner? The only time I have heard that technique being used is when Stephen Maturin (the surgeon in the Patrick O'Brian sea stories) opened up the top of a guy's head to replace the damaged portion of his skull with a silver plate.

Brace and bit cranial trephine, Germany, 1701-1800

Stephen had a new "Lavoisier's trephine" and Google turned up the above image.

Trepaner
The bit at the center serves as a pivot to keep the cutting bit at the end of the arm on track.

This is probably more like the tool that the bomb squad guys used. A tool like this would normally be used in something like a milling machine, that is, a machine that could hold the tool and the target rigidly in place so that the depth of the cut made by the bit at the end of the rotating arm could be accurately controlled. It seems unlikely that they would have used an actual milling machine for this project, what with milling machines being in high demand for war production and the high probability of it being destroyed should the mine explode. I am sure the bomb squad guys cobbled up something.

The part that bugs me is the claim that is "was made of non-magnetic materials". I don't know that there are any non-magentic materials that could be used to cut through the mine's thick steel shell. I supposed you could use a grindstone of some sort, but that doesn't produce a sharp, well defined cut. And would it really need to be made of non-magnetic material? After all, the mine's casing is made of steel, and a magnetic fuse is going to be designed to react to the presence of the massive amount of steel that is a ship. So maybe the small amount of steel found in this tool wouldn't be enough to trigger a magnetic fuse, but any support and / or driving mechanism would, so those parts would need to be made of brass or wood or something similar.

WW2 UK Mine Disposal Locations

WW2Talk has some more pictures of bomb disposal activity at the stone frigate HMS Mirtle.