# weak hammer drill, bad bits, or ?



## Dave928 (Jun 25, 2010)

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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## Nestor_Kelebay (Jun 25, 2010)

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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## Dave928 (Jun 26, 2010)

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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## Nestor_Kelebay (Jun 26, 2010)

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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## kok328 (Jun 26, 2010)

I suspect that you hit something other than concrete.


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## Nestor_Kelebay (Jun 26, 2010)

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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## CraigFL (Jun 26, 2010)

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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## oldognewtrick (Jun 26, 2010)

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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## Dave928 (Jun 26, 2010)

Nestor_Kelebay said:


> 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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## Dave928 (Jun 26, 2010)

oldog/newtrick said:


> 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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## Nestor_Kelebay (Jun 26, 2010)

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



If it's brand new, and putting a normal twist drill into it shows that you're not hitting metal, maybe you can take it back to the store for a refund.  That's what I'd do.

Calling a tool that can't drill a 5/32 inch hole more than 1/4 inch deep into concrete a "hammer drill" isn't just misleading advertising, it's fraud.

You might just have a defective product, too.  Any hammer drill, even a cordless one, should be able to do what you're wanting to do.


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## Dave928 (Jun 26, 2010)

it did it.  5 tries later.  it wasn't the drill or the bits.  must be a lot of rock in the slab.  in clean concrete it did fine, even with the bits i thought were shot.

thanks for the input.  now on to the drywall...


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## kok328 (Jun 26, 2010)

Having drilled my fair share of holes into concrete, it had to be something other than concrete that you were hitting.
I've even had success drilling into concrete with a masonry bit and a plain cordless drill.
If you hit metal, the tip of a masonry bit will turn into a blob of metal.  It will look as if it melted (technically it did).
Glad you finally got through.


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## Dave928 (Jun 27, 2010)

that's the funny thing, the bits are fine, they just stopped.


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