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Banquo
climber
Amerricka
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Topic Author's Original Post - May 7, 2015 - 02:44pm PT
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WARNING: Long boring post probably not of interest to most.
I have been sharpening carbide tipped drills for a while and have had mixed success. Sometimes they drill very fast but sometimes the tips shatter after a few blows or they continuously bind. So I decided to do a little research and practice sharpening to see if I could get more consistent results.
The material we call carbide is a more correctly called tungsten carbide and is also referred to as cemented carbide. It is a composite material manufactured by a process called powder metallurgy. Although it looks like a metal, it is a composite of Tungsten carbide powder, generally ranging in proportion between 70%-97% of the total weight, mixed with a binder metal, usually cobalt or nickel, compacted in a die and then sintered in a furnace. The resulting material is more like a ceramic than a metal. It is a very hard material being a 9 on the Mohs scale where diamond is a 10, quartz a 7 and hard steel perhaps 4.5. It is a very stiff material with a modulus of elasticity more than twice that of steel and about half that of diamond. It has a low coefficient of thermal expansion, 1/3 to ½ that of steel, and a low coefficient of friction. It is a very brittle material with virtually no plastic deformation before failure. It is a fairly strong material having a tensile strength similar to high strength steel
Tungsten carbide isn’t heat treatable in the same sense as steel but its durability and wear resistance can be increased 2 to 3 times by cryogenic treatment at about -300 degrees F. I haven’t worked with this property since I don’t have access to cryogenic equipment. Liquid nitrogen is -320 degrees F so if you have access to some you might be able to improve your drill bits. I have no idea if commercial manufacturers take advantage of this. Sharpening carbide on a grinder does create heat and therefore some problems. First, if your bit was cryogenically treated, you will probably eliminate that effect (more research required to know for sure). Second, rapid cooling such as quenching will cause internal cracks due to thermal stresses and the strength of the material is lost. Third, since the thermal expansion rate of steel and carbide are significantly different, changes in temperature can create large stresses.
The shape of the tip is critical to drilling efficiency. There are several factors to consider. The more acute the angle at the cutting edge the faster the bit will drill but the more likely it will shatter. I pretty much stick with the angle the manufacturer uses based on the assumption that they know more about it than I do. This may not be true in the future since my hammer blows are probably much harder and they are perhaps more concerned with a durable tip than a fast tip. Some people like straight cutting edge perpendicular to the shank of the bit but my thought is that a tip with angled edges helps keep things centered.
Symmetry of the tip is critical in keeping the bit from binding and drilling oversize. The point of the tip must be centered along the cutting edge and centered between the facets of the tip. The angle of the two cutting edges must be the same also.
To sharpen bits you will need a bench grinder with a silicon carbide wheel (usually green), an angle gauge, a square with a measuring rule and some sort of coolant (water will do). Wet grinding would be best but I don’t have a wet grinder and assume you don’t either. Make sure to dress the wheel so the grinding face is true.
After dressing the wheel, I set up the grinder by tilting the support so that when I place the shank of the drill on the support, the face of the wheel is flush with one of the large facets of the drill tip. Note that the factory tip may have different angles on different facets. Pick one, the other or something in between. To grind the bit I simply place the shank of the drill on the support, rotate the bit about the two other axes until the edge being ground is square to the wheel (parallel to the grinder axle) and move the tip of the bit into the wheel. Make sure to move the bit across the face of the wheel as you grind so the wheel wears evenly. Carbide wears out the wheel fairly fast and you don’t want a curved grinding surface. Grind each of the four facets in turn just a little bit while keeping the bit cool by dipping in water after every few seconds of grinding. Keep grinding the four facets until you have a sharp edge. If the bit gets hot, do not quench it but let it air cool as slowly as possible.
Once you have a sharp edge, use the angle gauge to check that both cutting edges are at the same angle. If they are not, grind some more and adjust how much you rotate the bit to compensate.
Once you have the edges sharp and the angles equal check that the tip is centered in both planes. If it is not centered, grind the two facets that will bring it back to center.
With patience you will end up with two sharp edges and a centered tip. My result is somewhat different than the factory one because they used two slightly different angles for the large and small facets. I suppose I could do this but it makes things more complex so I have skipped it.
If your bits shatter when you drill, you have probably got them too hot and then cooled them too fast. If your bits bind while drilling, the tip is probably not symmetric and centered.
I also sharpen twist drills ever since I learned to over 35 years ago. Once you get a knack for sharpening drills, it really isn’t too hard to do. Here’s a good video about sharpening twist drills which is in some ways similar to what I have described.
https://www.youtube.com/watch?v=y0SQkzScQk0
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jonnyrig
climber
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Thanks. Been wondering what exactly the tips were made of; but too lazy to look it up. I wonder what the effect of changing the tip angles would be, since hand drilling would be more akin to chiseling, as opposed to any sort of genuine rotary motion that a drill would impart. I bet you could glean some data from the guys who do competitive hand drilling at the mining expo's.
Anyhow, informative post.
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Reilly
Mountain climber
The Other Monrovia- CA
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Banqi has done his homework. The bible for all kinds of sharpening and
the metallurgy of tooling is The Complete Guide To Sharpening by
Leonard Lee, the founder of Lee Valley Tools.
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giegs
climber
Flagstaff, Arizona
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What speed are you running your green wheel at?
Usually when I have to sharpen carbide I have a ton to do so I'll just work one for a couple seconds, put it to the side letting it cool, and work my way down the line. I don't think I've ever damaged a chisel or bit by overheating but I've always wondered just how hot is too hot. I'm probably far too cautious.
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tolman_paul
Trad climber
Anchorage, AK
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The reason the factory bits are sharpened to such blunt angles is that while carbide is very hard it is also very brittle and without the cutting edge being nearly 90 degrees the edge would chip before being worn away by the rock.
While a sharper angle will cut faster, there is a balance between getting an edge that is sharp enough to cut but not so sharp that the edge chips.
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Banquo
climber
Amerricka
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Topic Author's Reply - May 7, 2015 - 04:44pm PT
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Reilly- I'll check that out, does he cover tungsten carbide?
What speed are you running your green wheel at?
I have no idea but I wish it was slower. It's a cheap hardware store grinder that vibrates so bad the dishes rattle in the kitchen at the far side of the house.
tolman_paul - I'm not actually changing the angle of the cutting edge, not much anyway. I am mostly getting rid of the rounded cutting edge. So, sharper to me isn't changing the angle but refining the edge.
Grippa - I haven't tried the 4 edged tips yet but want too. I doubt you could sharpen them but it seems like they would last longer than two and perhaps they do drill faster. Figuring out what drills faster can be hard to do since the rock varies, the arm varies, etc. What I've been doing id drill in a block of granite I have in the garage using the same hammer, same body position, same rock, etc. My elbow is in bad shape these days so I'm not testing anything. The Bosch bulldog tips look interesting too.
Locker - bits don't last long for me. I usually blow out the tips before the sides wear down.
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tolman_paul
Trad climber
Anchorage, AK
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I know Hilti bits of old were the smallest dia when new and while they seemed to hold an edge better than most, they'd be useless for drilling bolt holes long before you ran out of carbide to sharpen.
I also think rotohammers do more to abrade the sides of the bit and hence reduce the dia of the bit than hand drilling, but I haven't hand drilled enough bits to make an accurate judgement on that.
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tolman_paul
Trad climber
Anchorage, AK
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I agree SDS bits don't come sharp from the store, but wanted to caution others that would think a more accute angle would cut faster should be cautioned that thinking a "sharper" angle should cut faster doesn't hold with carbide due to it's brittleness. From the picture your clearance angle looks sharper than I'm used to seeing on a carbide but, but it could just be the perspective of the camera.
Typical bench grinders run ~3600 rpm assuming a 6" grinder. Diamond or cubic boron nitride (CBN) wheels are also an option for sharpeneing carbide and the run smoother and cooler than silicon carbide.
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Reilly
Mountain climber
The Other Monrovia- CA
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He does cover carbide but, as he notes, seemingly monthly advances in tungsten
carbide technology make any news old news. Since his company mainly makes
woodworking tools the book is geared to that. There is a large chapter on
bits of all sorts although not 'concrete' bits. There is a fair bit (sorry)
on sharpening twist bits which would apply in terms of techniques.
Did you know that tungsten carbide is only 1400-1880 Knoop hardness compared
to 2750 for boron carbide? ;-)
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Banquo
climber
Amerricka
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Topic Author's Reply - May 7, 2015 - 05:28pm PT
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tolman_paul -
Before I got distracted trying to perfect drilling hammers, I spent some time figuring out drill bits. I found some studies looking at carbide tip angle and drill performance. Sharper angle drills faster but is prone to shattering. There must be some optimum angle but would depend on type of rock, how hard the hammer hits, etc.
http://www.supertopo.com/climbing/thread.php?topic_id=2190293&msg=2218575#msg2218575
Locker -
are you power drilling? I've never used a power drill and don't have any interest in them. They probably wear out the edges faster with all the rotation. I'd be embarrassed to carry a power drill on the 5.6 - 5.8 cr*p I put up.
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Banquo
climber
Amerricka
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Topic Author's Reply - May 7, 2015 - 05:29pm PT
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Knoop? Sounds like a Dr. Seuss critter.
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ryankelly
Trad climber
Bhumi
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Thanks for posting, I was curious about why some sharpened bits were binding a while back. I think I got my answer.
Down with Binding Bits
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two-shoes
Trad climber
Auberry, CA
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I also only power drill anymore.
But, what I do is use diamond bits on a large Dremel Moto Tool. I turn the motor at a slow-medium speed, and I use a Optivisor, which are binocular-glasses at about 10 power, I think.
The diamond bits cut pretty fast. I keep the diamond bits moving from carbide bit to carbide bit, not developing anymore heat than when they are drilling in Bosch.
I do all of this freehand. I don't let bits get too dull before I resharpen them.
I don't let the bits get worn too narrow either. Check them with calipers and compare to new bits. Sharpen them about 2-3 times is all.
Bits are pretty cheap if you shop around on line. No use over using them.
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ryankelly
Trad climber
Bhumi
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What is the best non-power drill? (if you don't know why calling it hand drilling is too funny to type you are more mature then me and my friends)
Hurricane?
Petzl?
What else is out there?
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limpingcrab
Trad climber
the middle of CA
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Interesting info. Luckily my wife makes a lot of money so I just buy new bits :)
I appreciate all of the work you put into these topics
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Banquo
climber
Amerricka
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Topic Author's Reply - May 8, 2015 - 06:55am PT
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What is the best non-power drill?
The petzl Rocpec isn't bad. I think the ones I make are better. I make a couple kinds and some people use what I call the bayonet type. I haven't made that type in awhile. I have been making what I call the bomb proof because it is simple and I think it will last practically forever. Nobody has one yet but me. The bomb proof features:
A body made from a high strength 7/8" bolt.
A cast in place urethane rubber grip
A deep socket so less of the usual 6 1/4 x 3/8" drill is exposed
A very short distance between striking surface and bit for efficiency
Two set screws which is better than the one the Pika had but one will work
Set screws with nylon patch that shouldn't work loose like the Pika did
Hard steel dowel pin as a striking surface (don't use a super hard hammer)
A large diameter way for a leash (if you like a leash) so it rotates nicely
http://www.supertopo.com/climbing/thread.php?topic_id=2180408
I know it sounds humble of me but I think the drill holders I make combined with the hammers I make is the best setup in the world - ever.
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ablegabel
Trad climber
Livermore, CA
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Banquo,
That is the same set up that I use, and yes, it is superior to all others in my opinion. I had mine custome made back in the 90's, thanks to a friend of Eddie Joe's. It is still going strong after 20+ years and hundreds of bolts.
As stated above by Two-shoes, sharpening the bits with a diamond grinder is the way to go. Creates far less heat and goes way faster. I start with a 4" wheel on my hand grinder to get the general shape, then move to a smaller wheel on my dremil to finish it off and fine tune. Centering the tip is key. I also use a slightly steeper angle on my bit, but I don't hit as hard with the hammer. I find this saves a lot of energy and cuts very fast , 10 minute bolts in Yosemite granite usually. A little longer on the harder rock like El Cap, high on Reeds, or Supper Nova Wall. I rarely have issues with chipping on the bit. Thanks for the good info.
-Eric Gabel
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tolman_paul
Trad climber
Anchorage, AK
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Bits are pretty cheap if you shop around on line. No use over using them.
True that, I think the last time I bought bits I got a 10 pack of 3/8" bosh for ~$40.
Since most of my bolting is replacing ancient mank in areas open to power, I use a power drill.
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bhilden
Trad climber
Mountain View, CA/Boulder, CO
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Just one word of caution if you are sharpening carbide bits to make them last longer. The problem with carbide SDS bits as they wear is that they also lose some diameter so you need to closely monitor the width of older bits. This is especially a problem when using a power drill as it rotates the bit in the hole much faster than drilling by hand.
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