'Drilling' holes in closed cell foam - how?

Something hot?

A router will give you a flat bottomed hole, and accurate control of the depth. You could probably stick a router bit in the pillar drill if you wanted...

"A router bit in the pillar drill", that's an idea which will be easy to try, thank you. I may actually get round to trying it later today. I will report on results.

red-hot poker?

It worked pretty well in that it cut away the foam neatly but it didn't produce a flat bottomed hole, it cut a neat circular channel. I think maybe some Forstner bits may get closer to the flat bottom I'm after.

A Forstner bit would certainly be my first choice. Even they have a small locating spike in the middle, so the bottom won't be totally flat unless you grind off the spike.

Aldi and Lidl often have cheap sets which would probably be sufficient for your purposes.

If a heated rod can't be used then:

- make a D bit. Take a rod (drill, silver steel, bolt) of correct diameter, grind off a section so that the profile is a D shape (flat section just less than the diameter), file a bit of relief on the leading edge (might not need that for foam). or

- buy a slot drill of the right size (google)

The hole in the middle won't matter. I've ordered a cheap set from Amazon for the princely sum of £14.99. it has the 10mm, 12mm and 15mm diameters I need (plus 16 others!).

They should arrive today so we'll see! :-)

Although nothing in the size you want, the hole cutters made of plastic used for cutting shallow holes in (exterior) foam insulation are somewhat open probably to allow balled up foam to easily escape during drilling.

That is simply a hole saw. and if the pilot hole it leaves is not an issue why not get a proper set..?

But I see you already spent more at amazon :-)

It is not a normal hole saw since it has radial teeth as well - it is designed to mill out a cylindrical pocket, not just cut the annulus.

A hole saw doesn't work for blind holes. It would simply cut a slot round the piece of foam I want to extract but it wouldn't remove the middle.

I have lots of hole saws, very useful for cutting holes right through things but not for blind holes.

Usage

What kind of router cutter was it (straight flute, or up cut / down cut spiral)?

Also small router cutters normally expect fairly high RPMs - 20K ish - so the cut may not be as clean at drill speeds.

It was a 'straight' cutter for making parallel sided round holes. It's from a set I bought many years ago (20+) from Screwfix I think. It cut quite well (i.e. cleanly) the issue is that the cutters on the sides project a couple of mm beyond the cutter on the end that removes material from the hole. Thus for my 2mm or 3mm deep holes it didn't work too well.

Grind the protruding bits off the bottom of the cutters ...

No, it does have "end" cutters on the cruciform section ...

... and the forstner bits work well in my little pillar drill making a neat, flat bottomed hole with a little depression in the middle that doesn't matter at all as it's going to have a thin screw through there anyway.