rotor hammer bit life, only 2 holes in granite?


Original Post
Jason Minahan · · Unknown Hometown · Joined Sep 2013 · Points: 0

Howdy all,

New 3/8" SDS Plus bit doesn't last 2 holes. What am I doing wrong?

I believe I'm drilling into quartz granite.

I've got a 20V cordless dewalt dch273b SDS rotor hammer (don't judge, it's what I've got).

3/8" dia, going 4" deep with this Bit

Also used friend's other bit, not sure brand, but it was new.

Couldn't finish 4th hole with 2 bits.

Drills quickly first couple inches, then eventually just stops and won't drill anymore. Tip is mushed.

After first hole, the steel under the carbide tip is starting to turn blue. After second hole, it's very blue. I can lick my fingers and sizzle the tip even after short bursts.

Definitely set to turning and hammering mode (same as in pic in link)

Definitely set to correct direction. (I triple checked all this after smoking first bit)

I'm pulling tip out to clean chips regularly ~ every inch or so.

About 10lbs of force? My friend thinks I'm leaning on it too hard, I didn't think so initially, now, not sure. I feel like it's the amount of force I'd use to drill a hole in say, aluminum w/ a regular sharp bit. Would it make such a huge difference that a bit which (according to one site) should last 350 holes in concrete won't even last 2?

Obviously doing something wrong. I don't want to order replacements until better idea what I'm doing wrong.

Does anyone pull the bit and quench their tip with water every inch? Try and flood the hole with water?

I'm having a tough time believing that a Hilti or Bosch would make that much difference, especially if they're hitting even harder and about same RPM.

Jason Todd · · Cody, WY · Joined Apr 2012 · Points: 958

I'd guess you are pushing too hard. I use very little pressure when drilling holes, let the bit do the work. 

I get 20-30 holes/bit (3/8 x 4) in granite with 30-40% quartz. Each hole takes <60 sec. The bit loses guage, or proper diameter before it actually wears out. Hilti TE6A drill.

Lurker · · Verdi, NV · Joined Nov 2007 · Points: 2,835

I've used these Bosch bits (https://www.fastenal.com/products/details/0258510) in extremely hard quartzite that would mushroom other bits within a few holes.  Best bits I've ever found, last forever, and I think I was getting them for about $6-7 on eBay.

LL2 · · Santa Fe, NM · Joined Sep 2016 · Points: 0

As Jason said, just let the bit do the work, don't push. Let bits cool a bit between holes. I have only drilled limestone myself.

rocknice2 · · Montreal, Quebec · Joined Nov 2006 · Points: 3,018

Get a better drill bit. I use the Hilti one. Haven't worn one out yet. I drill mostly in granite type rock.

https://www.mec.ca/en/product/5014-665/TE-CX-3-8"-x-6"-SDS-Drill-Bit.

Even those Bosch bits with a flat carbide at the tip are crap. Haven't tried the new Bosch bits with the full carbide head like the Hilti. 

Billcoe · · Unknown Hometown · Joined Mar 2006 · Points: 655



Sounds like you either don't have it in the correct mode (I know you say you do), or the drill itself is broken. It's not hammering right. Here's how you'd know, take it out to your driveway and drill a hole in an inconspicuous spot. If it goes in easy, it's you or the rock you are drilling (probably the rock). It's suspect that it's not the bit. Double check the bit is spinning the right direction...sometimes it's the obvious things.


But you should have a better idea once you put a hole or 2 into concrete. 

ryan laird · · Denver, CO · Joined Sep 2008 · Points: 1,880

20V rotary hammer drill should have plenty of power. You don't need to lean on the drill like a metal cutting bit. Let the hammer do the work. My guess would be that you are drilling the hole in reverse or the hammering/rotating mechanism on the drill is faulty. I've seen new route developers drill holes in reverse. Maybe the direction is reversed on the drill. If you are looking down at the hole, the bit should be turning clockwise. You should not have to stop to clear the hole of chips/drill dust when drilling on a vertical wall. The flutes move the cuttings. Also the cuttings should be dust, not chips.

I doubt it was the bits, but the bits you posted from McMaster are unbranded which usually means they are crap. When you do buy bits, go with Bosch, Hilti (expensive) or Powers. Here is a link for a 4-pack of full-head carbide bits. 

http://www.ebay.com/itm/4pk-3-8-x-6-1-2-SDS-Bulldog-Xtreme-Rotary-Hammer-Bit-Bosch-Tools-HCFC2061-New-/222008061921?hash=item33b0b62fe1:g:kyIAAOSwPh5ZJKW7

Try drilling some holes in concrete like recommended above, to confirm your tool is working correctly. If it is not, contact DeWalt who is now owned by Black 'n Decker/Stanley. They also own Powers, so you can get some replacement bits too if the tool was faulty.

Firestone · · California · Joined Nov 2015 · Points: 509

Does anyone use lubrication for drilling? It sounds like hassle to bring liquids along with drilling supplies. If the cooling makes drill bits last twice as long I might consider it reasonable.

John Barritt · · OKC · Joined Oct 2016 · Points: 1,053

Cheap bit...........

tenesmus · · Unknown Hometown · Joined Jan 2004 · Points: 2,190

Granite is usually kind of soft and easy to drill. Like the other people said: You're probably leaning on the drill.

An intimate friend tried to get me to make myself be more patient when I'm working with really hard rock:

Start the hole. Go nice and easy for about 30 seconds.

Stop. Pull out. Carefully brush.

Drill another 30 seconds or so.

Stop. Pull out. Carefully brush. Maybe blow on it gently, if you're into that or if you think the hole really needs it.

Drill another 30 seconds, trying hard not to get overzealous or push too hard. 

Brush till its clean. Blow on it a little. 

You might be surprised by the way your drill bit and the hole react. 

Also, I've never needed any drilling fluid as this process seems to self lubricate. Not sure what that's about.

Ney Grant · · Pollock Pines, CA · Joined Feb 2014 · Points: 1,005

Definitely hammer drill dependent.  Could be your 20V dewalt is like my 12V M12 Milwaukee.  

We recently put up a route ground-up, so we used the lightweight M12.  It sucked and took 3-4 minutes per hole (long time while crimping holds) and it blued the end of the drill.  This is 10mm (basically 3/8) Hilti (Petzl) bit.

Later I put some bolts in on a sport climb on rap, so I used a big heavy Makita.  Same bit, which we thought we may have ruined and burned up.  I did 15 holes with the same bit, each hole took about 20 seconds!  A world of difference.  This is all in hard granite.

Basically, the little lightweight drills spin more than hammer.  I think the key is for that SDS bit to really slam back and forth to create the hammering motion.  A larger more powerful drill with more mass seems to help greatly.   

I've found steep rock self cleans with the flutes of the bit, but shallow rock requires cleaning while drilling (I just keep pulling the drill out).  Otherwise the dust sits down in the hole and interferes, even leading to an oversized hole and a spinning bolt.

Trevor. · · Boise, ID · Joined Apr 2012 · Points: 834

I'll echo everyone else, in that you're probably pushing on the drill too hard. When drilling, my goal is to put the minimum amount of pressure on the drill to activate the hammering action. I've never felt the need to clean the hole mid-drill or use any lubrication, but if the going is really slow, sometimes I'll pull out halfway and let the bit cool if it seems like it's getting really hot. 

Jon Nelson · · Bellingham, WA · Joined Sep 2011 · Points: 4,695
rocknice2 wrote:

Get a better drill bit. I use the Hilti one. Haven't worn one out yet. I drill mostly in granite type rock.

https://www.mec.ca/en/product/5014-665/TE-CX-3-8"-x-6"-SDS-Drill-Bit.

Even those Bosch bits with a flat carbide at the tip are crap. Haven't tried the new Bosch bits with the full carbide head like the Hilti. 

The linked page says "15-20 holes in granite". Does anyone know what exactly that means? For example, do they measure a standard reduction in speed, or a standard reduction in hole-cutting diameter?

I also wonder if anyone has results on side-by-side drillings with different bits to directly compare the speed it takes to drill to a given hole depth?

Jon Nelson · · Bellingham, WA · Joined Sep 2011 · Points: 4,695
Firestone wrote:

Does anyone use lubrication for drilling? It sounds like hassle to bring liquids along with drilling supplies. If the cooling makes drill bits last twice as long I might consider it reasonable.

I've heard of someone who has put water into the drilled hole, but not to aid the drilling part, rather to help lubricate the bolt insertion. 

One might dip the bit in a cup of water to cool it quickly and then let it dry. 

But it might be counter-productive to put water in the hole during the drilling. I've never tried it, but I imagine that the increase in conduction rate is not that great (thermal conductivity of granite is about 1/100th that of some typical metals), yet the ground rock dust could coagulate and impede the drilling action. 

Once, I had a bit tip that appeared to melt and instantly become useless. I might have been making the same mistake as the OP. 

caesar.salad · · earth · Joined Dec 2012 · Points: 85
tenesmus wrote:

Granite is usually kind of soft and easy to drill. Like the other people said: You're probably leaning on the drill.

An intimate friend tried to get me to make myself be more patient when I'm working with really hard rock:

Start the hole. Go nice and easy for about 30 seconds.

Stop. Pull out. Carefully brush.

Drill another 30 seconds or so.

Stop. Pull out. Carefully brush. Maybe blow on it gently, if you're into that or if you think the hole really needs it.

Drill another 30 seconds, trying hard not to get overzealous or push too hard. 

Brush till its clean. Blow on it a little. 

You might be surprised by the way your drill bit and the hole react. 

Also, I've never needed any drilling fluid as this process seems to self lubricate. Not sure what that's about.

uuunnnnnnnnhnhnhhhhhhhhhhhhaahhhhhhhhh....................

Jon Hartmann · · Ojai, CA · Joined Feb 2009 · Points: 1,305

 I'm surprised nobody's talking about the fact that he's using a DeWalt drill.  I've been working in construction for 15 years and I use a DeWalt Hammer drill and a Bosch hammer drill and a Hilty hammer drill and there is a gigantic difference between the three. I only use the DeWalt on stucco on houses and hardly ever on real stone. The Dewalt bits always burn up if you try to use them on real rock. I think the rotations per minute (rpm) are much too low or perhaps the impact that the motor  provides it's not forceful enough. I know the OP only has a 20 V battery Dewalt and I know that he said that that's what he has so don't talk about it, but from my experience in construction that's exactly what the problem is. The number of bits that I've burnt up on that DeWalt drill are enormous. The only reason that I ever use that drill for drilling into rock is because I'm too lazy to pull out the bigger one. My friends were just drilling a bunch of holes in sandstone last month using the 20 V DeWalt drill and decided it was actually easier and better for the bits to hand drill the holes. Granite will obviously be different but I think it's the drill.

Firestone · · California · Joined Nov 2015 · Points: 509
Jon Nelson wrote:

One might dip the bit in a cup of water to cool it quickly and then let it dry. 

But it might be counter-productive to put water in the hole during the drilling. I've never tried it, but I imagine that the increase in conduction rate is not that great (thermal conductivity of granite is about 1/100th that of some typical metals), yet the ground rock dust could coagulate and impede the drilling action. 

Does dipping the bit in water to cool it damage the metal by tempering it in some way?

Awesome info about granite thermal conductivity. And coagulation of rock dust is spot on, "cooling fluid" and rock drilling probably don't mix.

Brian in SLC · · Sandy, Utah · Joined Oct 2003 · Points: 13,772

I think your drill itself should be fine.  You might be pushing too hard.  Let the drill do the work.  Be patient.  Like the 'taco guys mention...let the drill float, hit, and cut.  With your drill, holes might take a lot longer to drill.

Or...the rock you're drilling in is hard.  If its quartzite and not really granite...could be very hard.  That's tough drilling no matter the gear.  What area?

Or...check your bit size.  That drill bit shouldn't be smaller than 0.390".  You wear the sides down and it'll bind and that's also not good for the installation of a bolt either.

The ansi bit size should be:  0.398" - 0.390".  I measure mine all the time.

When the bit wears out, its done.  I save my old bits to drill out placements but not for primary drilling of bolt holes.

And...if the bit heats to the point it turns blue, its done.  Temper is shot.

Jon Nelson · · Bellingham, WA · Joined Sep 2011 · Points: 4,695
Firestone wrote:

Does dipping the bit in water to cool it damage the metal by tempering it in some way?

Awesome info about granite thermal conductivity. And coagulation of rock dust is spot on, "cooling fluid" and rock drilling probably don't mix.

I don't know about the temper -- good question.  I have done that for drilling into wood, and in that case noticed no change in the bit appearance or behavior. A hammer-bit might be different, but given that most bits are not very expensive,  it might be worth trying if heating up is a problem. . 

Mike Slavens · · Houston, TX · Joined Jan 2009 · Points: 35

Do not dip the tip in water to cool it down.  You're not seeing an issue in wood because 1) wood is so soft compared to the bit, 2) you probably can't the bit nearly hot enough in wood to actually change the heat treatment.  However, for a rotary-hammer bit on granite you will be able to get the bit hot enough to potentially change the heat treatment, and it will change for the worse.  

I would guess its a combo of poor quality bit but mostly pushing way too hard.  If the bit is turning colors you are pushing way too hard.  If you push with much force at all you are just going to bind up the bit.  In simple terms like many have said, let the drill do the work and be patient.  Keep just enough pressure on the drill to keep the bit engaged with the rock, no more.  You should easily get multiple batterie charges per bit.

Kevin Mokracek · · Burbank · Joined Apr 2012 · Points: 200

Since we never heard back from the guy I am guessing he had it in the wrong mode.   Probably had it in straight drill mode and not hammer drill mode.   Drill mode in rock will jack up a bit in no time.

Guideline #1: Don't be a jerk.

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