Surface-mount box with 20 mm conduit knock-outs on the sides?

In article , Adam Funk writes

The trade deal with this by having a 20mm holesaw (with centre drill) in their toolkit :-)

Kind of obvious when you think of it but it was like an epiphany when I first saw it (2nd year apprentices are generally less impressed, or so I hear).

I imagine they have a 25mm one too for the reverse situation.

On 12 Nov 2014, Adam Funk grunted:

I dunno; but on a related note... I'm currently wiring up some sound-surround speakers with flush-fitted cables, and am using these light-switch boxes:

These are the shallow ones that have a single pre-formed oval hole, which has rounded-off edges to avoid chafing cables and not using a grommet. However, in the other sides they also have knock-outs for two side-by-side holes; these by definition won't have smoothed edges and won't fit a grommet.

I'm curious to know how these are supposed to be used? Just 'carefully'?

Not that I found. I built a jig as I had 20 or so to do and used a 4mm pilot with a stepped hole cutter in the bench drill. This gave a very good finish and the holes ended up in the right place!

No - not "carefully".

IIRC a 20mm round grommet will fit the oval holes. If not, take a 25mm round grommet, cut it and use it to line the edges, trimming to length.

OK, I could probably do with a holesaw set anyway, although I'll have to shop carefully --- the last time I bought one, I had to take it back as I found that the core was too big for my drill chuck.

In article , Adam Funk writes

I've never found the sets to have exactly what I wanted so have bought individuals when I need them.

As an aside, have you noticed on Toolstation (& prob others) that you can find what you need near the top of a page at a higher price than apparently equivalent others further down.

Very noticeable in your case:

Expensive £14.13:

Cheap £6.31:

Cheap still claims to be "Heavy duty, for industrial use". Ok 'punk' brand claims higher a specified cobalt content but for a few boxes . . .

You'll probably find they drill oversize and leave the hole a bit rough but the grommets (or conduit fittings) should cover any sins. Plus go easy on the pressure to avoid the knockouts being disturbed.

In message , fred writes

Except the Bosch ones further down the page are more expensive than the PUNK ones, so don't really support your theory

In article , Chris French writes

The fact that a cheaper alternative is available further down the page was the point and is accurate.