Drilling concrete

"Close-ish" is not good enough. A piece of wood in very firm, uniform contact with the far side might do the trick. Otherwise, is there any way you can accurately measure where your breakthrough point will be, and back drill any oversize hole?

Function of an SDS when it break through if you are pushing the drill, think about how an SDS drill works... Once you get close to full depth stop pushing and take it easy, no gurantee that you won't get some spalling but it shoud be much reduced.

Thanks to everyone for the replies. I think that maybe I didn't make myself clear: the vertical panel is standing on the concrete base, and I'm intending to drill down through the panel and into the base. So in effect I'll have a straight-through hole (in the panel) and a blind hole (in the base). I'm hoping that because the base is up against the underside of the panel, it will prevent any spalling in the latter (and thanks to Paul for the correct term, which had eluded me).

I'm going to try it later this afternoon, so we'll see...

Bert

Avoid going near edges for a start. If drilling through, get someone to firmly hold a large lump of timber over the breakout point, that cuts down the damage.

Try a wooden wedge driven in under the panel.

Thanks Dave and everyone else who responded. I've done this now: the drilling went OK, and if there was any breakout under the panel it must at least have been contained with nowhere to go: there's no sign of any loose chips or other damage either inside or out, and the holes weren't impeded in any way.

The hardest part of the job was driving home the fixing bolts - Multi-Mondi hex heads from Screwfix. I had to use a small socket-and-ratchet tool and the last few turns needed quite an effort. Indeed, the last few turns on the first bolt proved impossible until I realised that I hadn't drilled the hole deep enough...

Bert

next time you could try drilling the smallest size possible then drill from both sides with the bigger drill.