Self feeding flat bits

In knocking together a case for something holding a Raspberry Pi, I needed to cut a 24mm hole for some of these:


USB bulkhead

My usual go-to stockists didn’t have a 24mm flat bit (or Forstner), so I went to ebay and had a quick dig around. I found a Bosch “Self Cut” spade bit for cheap (maybe not used, old). It looks like this:


Bosch self-cut bit

Bosch is one of those respectable brands and you won’t go wrong with Bosch tools if you pick the right one for the job.

Speaking of that…

Turns out self feeding bits are wildly unsuitable for the kind of things I do most of the time. They are flat bits but the tip is a screw so it feeds itself into the wood. This should  gives a very consistent depth of cut and chip load. It also means you don’t need to apply any pressure with the drill: it applies an immense amount of drilling pressure using the drill’s torque instead.

They are amazingly, astonishingly aggressive and will happily plough through thick birch ply in seconds (if your drill is up to the task; the level of torque required is vast), and completely split a piece of pine. Note the rather rough cuts with the large amount of tear out:

Holes drilled with self feeding bit

It took much less time to go through the thick birch ply than it took me to drill the holes with a normal bit in a thinner piece of pine.

Great tool, utterly the wrong one for the task.