Drill bit seems to have suffered heat damage and 'blued', holes taking much longer than usual.

Hey guys, I'm normally drilling in softer rock and can put in a hole in one straight shot with no problem. However, this weekend I was drilling in some very hard coastal granite. I've heard people let their bits cool off for X seconds after a certain period of time or swap out bits. I noticed my drill was losing significant performance, with my M12 and a 4Ah battery I can normally get 8+ holes on the softer granite of the interior, but I was able to get a grand total of 7 holes with 2 batteries. It seems like I've caused some heat damage to my bit with some blueing and gouging on the shaft near the tip. Is this the likely cause of the diminished performance? Is the bit fucked now? The tip is carbide and looks fine. What are you suggestions for drilling in harder rock in terms of keeping the bit cool? Spraying water, swapping bits out?

Hmm.. could be the the harder rock but I usually get 40-50 holes per bit before I need to swap out due to the shoulders shrinking.  I have never swapped out bits while drilling unless as I mentioned the shoulders shrink and it becomes difficult to hammer in the bolt.  My guess is that its the 12v drill with a low amp hour battery that is causing things to slow down and take longer.  I would try a higher amp hour battery.   The tip on your drill looks fine and bluing below the tip shouldn't effect drilling.   Also, how old is the drill you are using?  Drills don't last for ever and over time they start losing hammer efficiency.   I just retired a drill of mine because it was taking much longer to to drill a hole.  Since it was an old drill I couldn't find a shop to refurbish it so I ended up buying a new one.  

John Serjeantson wrote:

Hey guys, I'm normally drilling in softer rock and can put in a hole in one straight shot with no problem. However, this weekend I was drilling in some very hard coastal granite. I've heard people let their bits cool off for X seconds after a certain period of time or swap out bits. I noticed my drill was losing significant performance, with my M12 and a 4Ah battery I can normally get 8+ holes on the softer granite of the interior, but I was able to get a grand total of 7 holes with 2 batteries. It seems like I've caused some heat damage to my bit with some blueing and gouging on the shaft near the tip. Is this the likely cause of the diminished performance? Is the bit fucked now? The tip is carbide and looks fine. What are you suggestions for drilling in harder rock in terms of keeping the bit cool? Spraying water, swapping bits out?

Can be common with that style of bit in hard rock. I drill granite and volcanics more so than limestone and have had plenty of blued bits. Hilti said it was from heat damage and due to the material (rock) hardness. I'm using a 21v Hilti drill with 8 Amp batteries so plenty of power.

You may well still be able to drill holes ok with the bit. It's not necessarily worn out completely. Certainly be listening for a change in the pitch of noise from the drilling because that indicates when the head is about to snap off! I carry a magnet pen in my rigging bag for retrieving snapped heads.

I have tried using a smaller diameter bit to drill a pilot hole for the final diameter afterwards but it's a real hassle so only do that if the final diameter is 16mm or 17mm and a deep hole.

Alternatively try a different brand and see if there's any difference?

Kevin Mokracek wrote:

I usually get 40-50 holes per bit 

Jesus, I need to do some development on whatever rock you're getting that on. I get 2-6 on my quartzite.

John Serjeantson wrote:

Hey guys, I'm normally drilling in softer rock and can put in a hole in one straight shot with no problem. However, this weekend I was drilling in some very hard coastal granite. I've heard people let their bits cool off for X seconds after a certain period of time or swap out bits. I noticed my drill was losing significant performance, with my M12 and a 4Ah battery I can normally get 8+ holes on the softer granite of the interior, but I was able to get a grand total of 7 holes with 2 batteries. It seems like I've caused some heat damage to my bit with some blueing and gouging on the shaft near the tip. Is this the likely cause of the diminished performance? Is the bit fucked now? The tip is carbide and looks fine. What are you suggestions for drilling in harder rock in terms of keeping the bit cool? Spraying water, swapping bits out

Why are you using a 4 cutter bit? Anecdotally I have found the sharper (not rebar cutting) 2 cutter bits to drill faster and give more hole per battery. Less time drilling should lead to a cooler bit

Ricky Harline wrote:

Jesus, I need to do some development on whatever rock you're getting that on. I get 2-6 on my quartzite.

On Quartz monzonite I can get 40-50 easy.  On Rhyolite I get a little less and on marble and limestone probably 20 3/8 by 2.75".  

Blake M wrote:

Why are you using a 4 cutter bit? Anecdotally I have found the sharper (not rebar cutting) 2 cutter bits to drill faster and give more hole per battery. Less time drilling should lead to a cooler bit

The 4 bit are all that's available now near my home.  I have found they do drill a little slower but they tend to drill cleaner and a more true hole.   If I could still get the two bit Bosch locally I would probably keep using them but the 4 bit are fine.

Ricky Harline wrote:

Jesus, I need to do some development on whatever rock you're getting that on. I get 2-6 on my quartzite.

Let the drill do the drilling, not you.

I have the m12 and m18 drills. Same thing happens to me with my m12 in hard granite after about 4 holes, I get probably 15-20 with the m18. It has to do with the power of the drill and how slowly it drills the hole causing the bit to heat up. I haven't found a solution with the m12 other than buying cheap bits in bulk. 

Russell Houghten wrote:

I have the m12 and m18 drills. Same thing happens to me with my m12 in hard granite after about 4 holes, I get probably 15-20 with the m18. It has to do with the power of the drill and how slowly it drills the hole causing the bit to heat up. I haven't found a solution with the m12 other than buying cheap bits in bulk. 

M12 isn’t sufficient for anything over 1/4. People tend to push too hard on m12 to make it go faster because the drill doesn’t hit as hard as the m18 thus it ends up destroying bits.

Tradiban wrote:

Let the drill do the drilling, not you.

I am. You're welcome to drill some quartzite yourself if you'd like to see. Quartzite is astonishingly more hard than granite. 

Russell Houghten wrote:

I have the m12 and m18 drills. Same thing happens to me with my m12 in hard granite after about 4 holes, I get probably 15-20 with the m18. It has to do with the power of the drill and how slowly it drills the hole causing the bit to heat up. I haven't found a solution with the m12 other than buying cheap bits in bulk. 

So on harder rock you're throwing out bits every 4 holes? That's unfortunate. 

MP makes it hard to reply to multiple comments in one response @Blake M, but essentially, I have a stock of 10mm bolts so my only choice to purchase a not overly priced bit was the 4 cutters. Seems like the move once I've used up those 10mm is to just stock up on 3/8s and purchase a bunch of cheap bits for the areas where the rock is harder. Will use the 4 cutter bit/10mm bolts on the softer rock where it hasn't been an issue.

Tradiban wrote:

M12 isn’t sufficient for anything over 1/4. People tend to push too hard on m12 to make it go faster because the drill doesn’t hit as hard as the m18 thus it ends up destroying bits.

I...disagree with this. I haven't had an issue with my M12 until this last weekend. I've placed maybe 40-50 x 3/8 & 10mm bolts this season, entirely in granite, with no problem. Obviously with harder/more compact granite though I'm going to have to consider this and think of a few ways to mitigate wearing the bits out so fast/buy more cheap bits in bulk.

John Serjeantson wrote:

I...disagree with this. I haven't had an issue with my M12 until this last weekend. I've placed maybe 40-50 x 3/8 & 10mm bolts this season, entirely in granite, with no problem. Obviously with harder/more compact granite though I'm going to have to consider this and think of a few ways to mitigate wearing the bits out so fast/buy more cheap bits in bulk.

Don’t get me wrong, it’ll work, especially in soft rock but it just doesn’t have the power of an M18 and the lag tends to prompt abuse of the bit.

John Serjeantson wrote:

I...disagree with this. I haven't had an issue with my M12 until this last weekend. I've placed maybe 40-50 x 3/8 & 10mm bolts this season, entirely in granite, with no problem. Obviously with harder/more compact granite though I'm going to have to consider this and think of a few ways to mitigate wearing the bits out so fast/buy more cheap bits in bulk.

How many holes are you getting per battery roughly?

Tradiban wrote:

M12 isn’t sufficient for anything over 1/4. People tend to push too hard on m12 to make it go faster because the drill doesn’t hit as hard as the m18 thus it ends up destroying bits.

Thats a bit bold to say. It may not eat like the m18, but it drills granite just fine.
Operator error is not the drills fault.

Mr Rogers wrote:

Thats a bit bold to say. It may not eat like the m18, but it drills granite just fine.
Operator error is not the drills fault.

See my comment to John Serj. The M12 is good enough but not great for 3/8+. It’s size and weight is nice for transport but lacks in performance. 

thank you to all you climbers who bolt and make new route and document them on mp or sell a brochure to the new climbs 

and please consider bolting the 5.5-5.9 moderates in the area when ever possible 

In hard rock i pull the bit out to let it cool a couple of times per hole. Especially when the ambient temp is high. Still only get a few holes in the hardest quartzite here in NC with one bit. I would rather toss the worn bit than have to pound too hard to send the bolt home!

Everyone else has answered your question adequately, but I will add one thing regarding the bluing of the flights.  The only role the flighted section of the bit plays in drilling a hole is to convey the dust away.  It has zero to do with how fast your holes are being drilled. It plays no role in cutting the rock. The only “bit wear factor” that plays a role in the speed of drilling is the condition of the tungsten carbide tip (along with rock hardness, the drill’s power, your technique, and other non-bit things). 

Kent Krauza wrote:

Everyone else has answered your question adequately, but I will add one thing regarding the bluing of the flights.  The only role the flighted section of the bit plays in drilling a hole is to convey the dust away.  It has zero to do with how fast your holes are being drilled. It plays no role in cutting the rock. The only “bit wear factor” that plays a role in the speed of drilling is the condition of the tungsten carbide tip (along with rock hardness, the drill’s power, your technique, and other non-bit things). 

Yea I was aware of the function of the flutes before but figured that blueing of the material may indicate possible, unseen changes to the material elsewhere. I have a mech eng degree but it's been awhile since I was in school and materials was never my strongest subject ;) I thought the same heat that caused the blueing could have created faster wear on the tip but from the sounds of it the likelier explanation is a weaker drill and harder rock causes more wear on bits. The blueing was probably from me running the drill too long and applying too much pressure.