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I'm in need to buy a rotary hammer to make some holes in a slab about 12 cm thick of armed concrete. The holes will be between 5 and 10 cm in diameter, so I will have to hammer a round opening instead of drilling through (or drill lots of holes in a circle).

The rotary hammers in the market goes from 2,5 to 30J. I have no idea of how much energy I need to this task and I wasn't able to find it anywhere. The price goes up really high (and so is the weight of the tool).

The smaller models (about 2,7J) like Bosch GBH 2-24 costs about 7 days of rental of a bigger model (about 10J), so I'm inclined to buy one if it will work.

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I was surprised how easily a bottom-of-the-line rental rotohammer cut a 3 cm hole through 25+ cm of slab, perhaps 10 seconds. Based on my experience, you could get the bottom of the line unit and actually have time to think while the tool works. –  wallyk Aug 9 at 16:36

2 Answers 2

up vote 5 down vote accepted

It's mostly going to be about speed - if you are patient and like handwork, a star drill and 4 lb / 2 kg hand sledge will drill holes in concrete or granite. If you plan to drill many small holes and then hammer out the plug, the smallest rotary hammer drills will generally do that just fine. The bigger ones may do it much faster, and will do it with bigger bits.

The "nice way" to drill finished holes of the size you want is with a core drill (which just drills around the edge, like a hole saw, leaving a plug or core) but those cost serious money even to rent/hire, usually.

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I'm aware of the core drills. Their cost is the reason I decided to go the rotary hammer way. Just 2 sizes (80 and 110 mm) of those for using with a regular drill costs more than a 2.4 J rotary hammer. –  Luiz Borges Aug 9 at 14:54
    
@LuizBorges - If you go with the rotary hammer drill, you'd want to have a rebar cutter bit on hand if you're going through reinforced concrete. –  Comintern Aug 9 at 16:32

Power drills have a maximum bit size specification shown as: ø. I would take this into account on hammer drills more than it's joules when deciding on a purchase. That's just how well it's going to work and tendency to get bogged-down. When renting, as long as the bit size you need chucks, it's not your concern how bad you dog it out. If it is, don't push so hard and give the bit some time to cool as well as the drill. If you are going to do anything ever again that could remotely be improved by using a hammer drill, buy one.

If this is a one-off, rent the cheapest one they have, butcher both the tool, the job, and be done with it. I used to smash through, too, after drilling a bazillion holes before I had a core bit set. Hilti TE72 is one of my best friends, shes discontinued though. Probably because it still works after 10+ years of being utterly dogged-out and only one 'proprietary' trigger replacement from....Bosch. I see these (so pretty) used for ~$800 and 4" SDS core bits for ~$150.

Friend

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