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Cobalt drill bits

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Cordell in AR

01-18-2001 07:00:03




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I have noticed that some drill bits such as DeWalt brand indicates in smaller print on the package as being Cobalt coated bits while in McMaster Carr listing there is no mention of being cobalt coated nor does the Craftsman cobalt bit mention being cobalt coated bits. Is there a difference in the ones saying cobalt treated and the ones that just say cobalt bits?




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Steve U.S. Alloys

01-19-2001 08:15:48




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 Re: Cobalt drill bits in reply to Cordell in AR, 01-18-2001 07:00:03  
I would only add one thing to what Larry and Fred have told you. The correct turning speed of the Co drill is paramount to getting good results. Most people turn them waay too fast.

Also, in drills you are generally going to get what you paid for as with any tool.



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T_Bone

01-19-2001 02:00:22




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 Re: Cobalt drill bits in reply to Cordell in AR, 01-18-2001 07:00:03  
Hi Cordell, I tried some cobalt drill bits about 12yrs ago and threw most of the unused ones in the trash. I don't remember the name as I was buying parts and the guy just gave them to me to try out. They just would not do what a good HSS bit would.

As far as drilling stainless steel (SS), there's a trick I learned 30yrs ago, using HSS bits, vary the bit speed by pulling the trigger switch on and off until the hole is drilled, never letting the drill get to full rpm, about half of drill rated rpm, nor completely stop. One HSS bit can drill many a hole in SS using this method. A 3/8" drill motor is better than a 1/4" for SS because of slower rpm.

The best drill bits we ever found was HSS Dayton brand by WW Grainger. I don't know who made them for WW Grainger but those bits will last.

T_Bone

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Tom

01-19-2001 19:19:13




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 Re: Re: Cobalt drill bits in reply to T_Bone, 01-19-2001 02:00:22  
To drill stainless steel use a sharp drill, a lot of feed pressure, and a slow speed. SS work hardens and if the cutting edge of the drill bit isn't below the workhardened surface of the last cut the edge of the drill will not last long, it will dull and heat which will make it dull and heat more until you have fried the end of the drill and have to start over. But now you have a work hardened surface for your newly sharpened drill point to get through. Better to not get to this point. Also use cutting oil. Learn to shapen drills so you can start with a sharp one.

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T_Bone

01-19-2001 23:51:18




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 Re: Re: Re: Cobalt drill bits in reply to Tom, 01-19-2001 19:19:13  
Hi Tom, Try my method of drilling SS and you'll soon see what I'm talking about. The best cutting fluid for SS is Rapid Tap. The second best is a wax base but can't remember the name but we did a cost study of drilling with and without cutting fluid and different drilling methods other than being on a drill press. The method I described is the most cost effective for drilling SS and without using a cutting fluid.

T_Bone

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Larry Garbarek

01-18-2001 13:45:41




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 Re: Cobalt drill bits in reply to Cordell in AR, 01-18-2001 07:00:03  
Don't know what Dewalt is trying to say.

Cobalt is added to some HSS to give what commonly is called a cobalt drill. It is an alloy, not a coating.

Typically, cobalt drills are a couple of Rc hardness points harder than regular HSS meaning that a cobalt drill can drill stainless steels and other more difficult to cut materials.

They might cost 50% more but they can perform where regular HSS tools fail.

The downside is that cobalt steels are more brittle. Chances are good that you would break a cobalt drill if you tried to use in in a hand held electric drill. You really need a drill press, a good chuck and a good vise to hold work steady when using cobalt drills.

Coatings are a completely different animal and can be applied to any cutting tool whether it is HSS, cobalt, carbide or other material.

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Fred OH

01-19-2001 07:42:25




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 Re: Re: Cobalt drill bits in reply to Larry Garbarek, 01-18-2001 13:45:41  
Larry has it pretty well covered, but I'll add a thing or two. Try sharpening one of those 135 degree split point cobalt drills on your bench grinder. The web is so thick that you have to thin it every time. The chisel point has to push it's way through the metal, no cutting clearance on it! Look at one and rotate it in your hand and think about it. And, like he says, they didn't gain popularity with electric drill motors or people who are removing broken bolts because of their brittleness. They are for production drilling and shouldn't be in the average mechanics toolbox.
For the novices; whenever someone asked me for a bit, I always told them that the bits are in the woodshop. We use twist drills here in the machine shop. L8R----Fred OH

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