DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Bricks, Masonry and Concrete > weak hammer drill, bad bits, or ?




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Old 06-25-2010, 08:27 PM  
Dave928
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Default weak hammer drill, bad bits, or ?

i'm trying to drill 2 2" deep 5/16" hole into my concrete slab. i've already gone thru 4 masonry bits and the hole is only about 1/4" deep. i get a little bit of concrete dust and then nothing. it just won't go any deeper. i've tried light pressure and heavy pressure. low speed and high speed. there's nothing in the hole, i didn't hit a piece of rebar.

it's a brand new 18v DeWalt DCD950KX hammer drill, $300.

what am i doing wrong? would a roto-hammer be better? or do i just have extra-strong concrete? crappy drill bits?

thanks,



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Old 06-25-2010, 09:37 PM  
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Well, despite what you may have been told, the truth is that cordless drills don't have as much power as corded drills, and percussion type hammer drills just don't deliver the powerful hammering action that the piston in cylinder arrangement in rotary hammers do. So, in a nutshell, your cordless hammer drill is just not hammering hard enough on the concrete with each stroke to do any damage to it.

Understanding the answer lies in the difference in the way twist drill bits and masonary drill bits work:

Twist drill use a sharp cutting edge at the front of the bit to CUT a hole in wood or metal. A masonary bit works by BATTERING a hole in the masonary. To do that, it has to hit the concrete hard enough to bash the concrete into pieces. Percussion type hammer drills have a very high number of blows per minute, but each blow is only effective if it bashes the masonary in front of the bit into dust. So, percussion type hammer drills are all you need for relatively soft masonary like bricks, blocks or mortar because those materials are soft and batter into dust easily. Once you get into concrete or hard rocks, then you need a much more powerful hammering action to make dust out of that stuff. 50,000 blows per minute will get you nowhere if none of those blows are powerful enough to do any damage to the concrete. Rotary hammers have far fewer blows per minute, but each blow is powerful enough to bash the concrete into pieces, and that's why rotary hammers work so much better in these harder masonary materials.

I have a Makita corded percussion type hammer drill that delivers 49,000 blows per minute and it sounds like a screaming Ninja when I'm using it. It's OK for small holes in relatively soft masonary, but a 5/16 inch hole in concrete would be about the most I would expect of it. It drills into concrete, but quite slowly. That's cuz each blow doesn't do much damage.

I also have a Hilti TE-10 rotary hammer that uses SDS drill bits and delivers 400 blows per minute, and that sucker will drill a 5/16 inch hole in concrete about as fast as a cordless drill can drill a 5/16 inch hole in wood. And, it's all because each blow is powerful enough to bash some concrete into dust.

Rotary Hammer Corner

PS: Before they had electric tools, the way you would drill a hole in hard rock (to set dynamite charges, say) would be with two men. One man would hold a tool with a bit on the front of it that looked like any one of those below:

and the second man would hammer on the top of the tool with a sledge hammer. After each blow, the first man would turn the tool so the teeth on the front of the tool would always hit in a different spot. The tool bit would essentially batter a hole through the rock one blow at a time. That's still the way modern rotary hammers work, but just with two employees less.

(All of the above are bits for "rock hammers", which are the pneumatic drills used in mines. Note that each bit has at least one hole in it. That's because these bits use compressed air inside the hollow drill shafts to blow the dust created out of the hole. The compressed air does the same job as the spiral flutes on a twist drill bit or a masonary drill bit.)



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Old 06-25-2010, 10:00 PM  
Dave928
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cool, thanks. a friend has a rotary hammer, i'll see if he can drop by.

it's even worse than that. it's a 5/32 hole, not 5/16. i typo'd.

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Old 06-26-2010, 12:18 AM  
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I'd call that false advertising.

If a company can call a tool a "hammer drill" and charge $300 for it, and it won't even drill a 5/32 inch hole 2 inches deep in concrete, then it shouldn't be called a hammer drill.

I expect you hit a particularily hard stone, like maybe a bit of granite or something. I'd switch to a cheap Fuller or Stanley 5/32 inch HSS bit and see if you bring up metal cuttings. If not, that would confirm it's not rebar. You'll wreck the bit, but it would be worth knowing what's causing the problem.

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Old 06-26-2010, 05:46 AM  
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I suspect that you hit something other than concrete.

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Old 06-26-2010, 11:43 AM  
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If I were Dave, I'd try drilling the other hole. If I couldn't do better on the second hole than the first, then I'd return that toy for a refund.

Either he's hitting steel, or that thing he bought for $300 shouldn't be marketed as a "hammer drill"

I don't think you can buy masonary bits in 5/32. I think that must be a 5/32 inch Tapcon bit to predrill holes for Tapcon screws.

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Old 06-26-2010, 11:57 AM  
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I have an 18V DeWalt hammer drill and it works fine. I wouldn't call it the super duty model but for the occasional hole it does the job. The bit has to be sitting on rebar or a hard stone not to work. I agree with trying another hole in another place.

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Old 06-26-2010, 12:00 PM  
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Dave, I know this sounds really stupid... but do you have the drill in forward and not reverse? I've done this before without out looking, heck we all have done it once, or maybe more.

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Old 06-26-2010, 12:00 PM  
Dave928
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Quote:
Originally Posted by Nestor_Kelebay View Post
I don't think you can buy masonary bits in 5/32. I think that must be a 5/32 inch Tapcon bit to predrill holes for Tapcon screws.
the first one was. then 2 other versions from different stores. they are for the 3/16 tapcon screws.
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Old 06-26-2010, 12:02 PM  
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Quote:
Originally Posted by oldog/newtrick View Post
Dave, I know this sounds really stupid... but do you have the drill in forward and not reverse? I've done this before without out looking, heck we all have done it once, or maybe more.
yes, correct direction.


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