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doug931
08-03-2004, 11:55 PM
In a previous post I mentioned getting a tool chest full of odds and ends. Included were half a dozen Super Cobalt lathe bits. some already ground, but most still untouched. My research shows that these tools are harder than Carbide. But just how hard are they. can they be used to cut a brocken tap. Will they cut Carbide? One tool has a large chunk brocken off the cutting edge, seems quite brittle. Any opinions, advise welcome .Doug

C. Tate
08-04-2004, 07:45 AM
Doug I am afraid you are mistaken. Cobalt is not harder than carbide. It probably will not cut a quality tap and certinly will not cut carbide. It is very tough and has a very high hot hardness meaning you van run it faster than normal HSS.


CT

suprdvn
08-04-2004, 09:45 AM
My guess is that you have mis-identified the metal. Since you said "One tool has a large chunk brocken off the cutting edge, seems quite brittle.". That tool is probably carbide not cobalt.

Carbide is a slightly darker and much heavier than HSS (high speed steel). Pieces of carbide are often brazed to other metals.

Correct me if I'm wrong: Cobalt is an alloy used in HSS. You wouldn't know a tool had Cobalt unless that word was stamped or etched in.

Carbide will cut HSS such as a broken tap. But know that the end mill will likely break in the process.

Evan
08-04-2004, 10:42 AM
Cobalt is used in some grades of HSS but not all. Some cobalt super-alloys contain little to no iron at all. The closest HSS alloy I know of to carbide is Micro-Melt Maxamet alloy by Carpenter. It goes up to HRC70 or more which is just under carbide grades but has much better toughness. It contains 9% cobalt.

See:

http://www.cartech.com/news/wr_news_hardness.html

Also see:

http://www.hitachimetals.com/products/hi_specialty/highspeed_steel.html

[This message has been edited by Evan (edited 08-04-2004).]

WJHartson
08-04-2004, 10:54 AM
Cobalt has a property that will allow it to gain strength up to about 2200 degrees F. As Evan said it is used in some of the super alloys. I use to use some of the Haines alloy 188 in a hot cyclone. It cost about $35 a pound in the late 70's. The control element in the alloy was Fe at 2% max.

Cobalt in cutting tool will allow the tools cutting edge to hold up better under higher temperature during cutting. I don't think you can cut carbide with it.

Joe

brunneng
08-04-2004, 12:54 PM
I do know that the cobalt drill bit will cut through a cold spot in an arbor very sweetly. I converted a boring head arbor to a shell mill holder and while not hardened, it did have plenty of hard spots that would catch on one side of my HSS bits and chew them up.

I would imagine that they would work real nice on cutting materials that you would like to use carbide on but can't get the rpm's high enough for it. I believe I bought some cobalt lathe blanks awhile ago to try on stainless but haven't got to that project yet.

As I under stand it, they take the good points of HSS and just kick it up a notch for taking heavier cuts in harder materials. The cost for a cobalt blank can be anywhere from 2x to 4x the cost of a HSS of the same size depending on cobalt percent.

Evan
08-04-2004, 01:11 PM
Cobalt drills are the prefered type for drilling ferrous metals that work harden easily. Excellent for drilling heat treated 4130 and 4140 steel.

doug931
08-04-2004, 02:38 PM
thanks for the information. The lathe tools are marked with "Super cobalt", qite convencional 1/2 square bits. I suspect they will come in handy as boring bar/head bits.Doug

J Tiers
08-04-2004, 04:54 PM
The cobalt drills do seem to be brittle.

I have had a couple just shear off in situations that HSS would have been OK with. At least I finished the job with HSS.

And, I have never had the cobalt drills succeed where the HSS would not. But then, I don't have a high speed drill press.

Maybe the hot strength and red-hardness would allow drilling hardened material if the material were annealed by the drill friction at high speed? That makes some sense, aside from the problem of starting the drill.

I was advised (here or the "other" board) to use the cobalt drill for a particular job in hardened steel, and all they did was dull into oblivion. Carbide ate it for lunch and spat out the seeds.

Evan
08-04-2004, 05:05 PM
I drilled many a hole in C130 wing spars made of heat treated 4130 using cobalt drills. Carbide was not an option as it would be a major extraction problem if one crumbled in the hole. I never found the cobalt drills to be brittle at all but then there are many different grades and makes. Most important was the same as drilling stainless steel, good feed rate and don't let up.

wierdscience
08-04-2004, 09:00 PM
Yes,there are also surface treated drills which are HSS and coated with cobalt.

I typically don't like cobalt drills,the endmills and toolbits are good,but I have rarely seen the advantage to a cobalt drill.They do work good in tougher steels like Evan mentioned,but that also has to do with the fact that most have a 135 split point as well,the split keeps the drill from wandering and controls the feed rate especially on break through.