Cobalt Drill Bits For Hardened Steel

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SharperThanSteel

Joined
Mar 18, 2014
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23
Hi guys,
I'm making my first knife. Its been extremely fun and very rewarding. About 3 weeks ago I saw that I could make a knife out of an old lawn mower blade. I know that its not the best for a knife because of its mystery steel composition but it was free and I wanted to practice all the techniques on something that didn't matter if I wrecked. I have worked on the knife now for about 28 hours and it has come out far better than I could have ever imagined. When I went to go drill the holes in the tang for the handle nothing really happened. Sadly since I only had the cheap standard drill bits I didn't get anywhere. :grumpy: After some research I have come to the conclusion that I either need a cobalt drill bit or a carbide tipped bit. I am just getting into the hobby of knife making so i'm not crazy about buying a $20 carbide tipped bit. I am wondering if I can use a cobalt bit since they are a lot cheaper? I only need to drill two 1/4" holes in the tang. What are your opinions about the cobalt bits? If I keep an interest in knife making, I plan on getting a carbide tipped bit down the road anyway. But if I can avoid the extra expense at least for this or even my next knife it would be great.
Thanks for the help. From what I have seen so far this is a great forum with lots of knowledge and respect for other members. :thumbup:
 
Sometimes it's helpful to fill out your location. If you're close to me you could come by and I'd pop some holes in there for you.

A split point cobalt drill will cut up into the 40's, so it depends on the lawnmower blade, but it takes quite a bit of feed pressure to cut hard steel regardless of the cutter material. This creates a tendency to over feed once the web of the drill has exited the back side of the workpiece. So if you use cobalt, feed it hard enough to keep it cutting and back your work with something to restrain your feed as the point exits the back or you'll have problems.

Clamp your work or otherwise restrain it, because when drilling difficulties arise you'll sometimes get a helicopter that eats up your hands in a blink.
 
Ok thanks. I guess i'll pick up a cobalt bit and give it a try. It's only $5 so if it doesn't work I can always use it for something else and just buy a carbide tipped bit. Also does anyone know if I can use a masonry bit instead? I assume not but if anyone has gave it a try I would like to know the results. Thanks!
 
Lawn mower blades are usually pretty soft because they have to deal with hitting rocks. My neighbor put his blade in backwards and was cutting grass with the blunt back edge. Most L/M blades can be readily sharpened with a file. If a file will easily sharpen your chunk of steel, then you should have little difficulty drilling it with an ordinary high speed steel drill bit.
 
This doesn't appear to be very soft at all. I tried cutting it with a hacksaw and it took 30-45 minutes just to cut 2 inches. I switched to my angle grinder and it was still slow but it beat using the hacksaw. Anyway the ordinary bit had no luck it would cut about an eighth of an inch and just stop.
 
You can save money by buying a carbide tipped masonry bit. They are super cheap and work well. Just remember to go very slow and push hard.
 
SharperThanSteel said:
Ok, so what cuts better. A carbide tipped masonry bit or a cobalt bit? Any suggestions? Thanks.
Click to expand...

I guess the answer would depend on the material and hardness. Since you only know that it is kind of hard steel, get both and try them and get back to us.
 
I'd just pick up a cobalt bit locally and try.

You could buy a carbide bit but it would cost more than a bar of nice steel. I have great luck with Niagara end mills on hardened steel but they are in the $20 range.

28 hours... sounds like you are working hard on it, keep it up! :D My first took over thirty.
 
Today I bought a cobalt bit at the hardware store. I went home and excitedly tested it out. It cut very fast with little pressure until I made it about 1/2 to 3/4 the way through. Then it pretty much stopped cutting. I thought maybe I just dulled the bit so I tried to cut another hole in my test piece. Once again it sailed through until it made it 1/2 to 3/4 the way through. I tried using the bit with and without oil and there was not really any change. My guess is that the mower blade is softer on the outside and harder on the inside considering it stops at about the same depth every time. Should I get a carbide tipped masonry bit or would I have the same result? Any other suggestions or ideas to try? Thanks again!
 
Sounds like you may be work hardening the steel. Did you notice any discoloration of the steel or the chips being produced?
One technique that can be used is to feed hard but back off quickly as soon as things get "grabby" then repeat as needed to complete the hole. Slow speeds (rpm) are also key.
 
I didn't notice any discoloration. Also it never was grabby. The bit would just keep spinning like normal but it wouldn't cut and it would start squeaking. The drill press is also on it's lowest RPM. Today I am going to try again. Maybe I was going too fast or maybe the bit got too hot and it didn't want to cut until it cooled down. I guess i'll find out today.
 
I tested out the cobalt again with more patience and oil. No go. It just goes about halfway through and then makes no progress. I tested it out on a piece of mild steel that was thicker than the mower blade and it cut through that in under 15 seconds. I guess the mower blade is too hard for the cobalt bit. Do any of you think it is worth buying a carbide tip masonry bit? Or is that going to be another waste of money? At this point I was thinking of looking into the carbide bits for metal. Is this something that would work? http://www.knifemaking.com/product-p/td616.htm
Or this? http://www.texasknife.com/vcom/product_info.php?products_id=944
Which is better? A carbide tip or a full carbide drill bit? Also which cutting edge style is better out of the two I listed above?
Thanks!
 
I tested out the cobalt again with more patience and oil. No go. It just goes about halfway through and then makes no progress. I tested it out on a piece of mild steel that was thicker than the mower blade and it cut through that in under 15 seconds. I guess the mower blade is too hard for the cobalt bit. Do any of you think it is worth buying a carbide tip masonry bit? Or is that going to be another waste of money? At this point I was thinking of looking into the carbide bits for metal. Is this something that would work? http://www.knifemaking.com/product-p/td616.htm
Or this? http://www.texasknife.com/vcom/produ...roducts_id=944
Which is better? A carbide tip or a full carbide drill bit? Also which cutting edge style is better out of the two I listed above?
Thanks!
 
SharperThanSteel said:
I didn't notice any discoloration. Also it never was grabby. The bit would just keep spinning like normal but it wouldn't cut and it would start squeaking. The drill press is also on it's lowest RPM. Today I am going to try again. Maybe I was going too fast or maybe the bit got too hot and it didn't want to cut until it cooled down. I guess i'll find out today.
Click to expand...

Are you getting chips or spirals of steel?
 
I tested out the cobalt again with more patience and oil. No go. It just goes about halfway through and then makes no progress. I tested it out on a piece of mild steel that was thicker than the mower blade and it cut through that in under 15 seconds. I guess the mower blade is too hard for the cobalt bit. Do any of you think it is worth buying a carbide tip masonry bit? Or is that going to be another waste of money? At this point I was thinking of looking into the carbide bits for metal. Is this something that would work? http://www.knifemaking.com/product-p/td616.htm
Or this? http://www.texasknife.com/vcom/produ...roducts_id=944
Which is better? A carbide tip or a full carbide drill bit? Also which cutting edge style is better out of the two I listed above?
Thanks!
 
Masonry bits have a geometry to brake rocks not cut steel; if your stubborn enough, could you wear a hole in a piece of metal with one? I guess so. Using a full carbide drill bit generally requires a solid setup.

Are you wasting your time and money? I'd say yes.

And by the way, your mower blade, according to the scrap dealers of america is 1085.

John
 
On second thought. Nathan offered to drill it for you, I'll make the same offer, save the $20.00 on a broken drill bit and spend it on postage and send it to one of us. If you send me a pm I send you my address.

Now I'll up the offer, if you are thinking about making another knife the same way, It's just as easy to drill two as it is to drill one.

John
 
John Moran said:
On second thought. Nathan offered to drill it for you, I'll make the same offer, save the $20.00 on a broken drill bit and spend it on postage and send it to one of us. If you send me a pm I send you my address.

Now I'll up the offer, if you are thinking about making another knife the same way, It's just as easy to drill two as it is to drill one.

John
Click to expand...

Thanks for the offer John. I'll keep it in mind. I think I am going to buy a carbide tipped bit because I plan to make a bunch of knives as a hobby and it would be cheaper if I had my own bit than paying to ship a bunch of knives. I found a place where it looks like I can get a bit for about $10 plus shipping. http://www.biscotoolsupply.com/page_2031_88/carbide-tipped-drill-bit-p2031
Hopefully this will go through it or I don't know what i'll do.
Thanks for all the help guys. Great forum!
 
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