The nature of a carbide edge and what care it likes the best?

[rons]

So I mostly use brazed carbide bits for roughing out SS or anything else. Some of them are marked C5.
And I have Iscar brand inserts for the lathe and try to save those for the finish.

Somebody said to me once that a carbide edge can start out real sharp and then quickly becomes an intermediate sharpness and stays that way for a long time.
And that HSS will stay real sharp longer in the beginning phase of use but then dulls out and becomes unusable.
This was concerning wood planer or jointer blades but it should apply to metal.
True or False?

I'm grinding the carbides and they can withstand red hot. As long as they don't get dunked in water.
So my work never gets them red hot or chips that are blue.
Will a intermediate sharp carbide edge eventually chip at some lifespan hours? Would intermittent use prolong the hours? Pecking?
True or False?

I use a diamond stone to true up these brazed carbide bits and what I notice is that if I dress them more often the chipping goes down.
When a break occurs I have to grind out more of the edge than if I periodically sharpened the edge.
Is there a formula for this?

Another issue is hand cranking. Starting on a uneven surface allows me to feel the vibrations on the crank handle.
Does a smooth CNC motion save cutting bits versus a human?

 

[johansen]

One of the gear snobs here (no offence) has the data to prove that hobs last longer if sharpened more frequently.

And this makes sense if you think about it in terms of the surface area of the cutting edge rubbing on the chip. Dull = more heat because of both more friction and, more surface area of poor geometry. Dull= more cutting forces needed which destabilizes the cut.

The silicon in the wood dulls carbide.

The metallic binders between the carbides get abrasively and chemically removed, exposing the carbide grains which then break out.
Hence the "micrograin" carbide sold to help that problem.

So yes you can sharpen hhs to a finer point because the grains are smaller.. does it really stay sharper longer than carbide would, if used in the exact same conditions? Good question.

 

[rons]

It's misleading to the operator standing outside of the cutting zone. A not so sharp carbide still looks like it's doing good.
When the insert chips the operator just loosens the set-screw and rotates the tri-tip insert.

I'm looking at my edge with a 10x magnifier while turning. The accumulation of the almost fused metal on top of the carbide
tends to force the carbide down and up as the new metal coming off the cut is hitting the cold stuff on the carbide.
This is my opinion as to how a break might occur.

 

[CarbideBob]

The accumulation of the almost fused metal on top of the carbide
tends to force the carbide down and up as the new metal coming off the cut is hitting the cold stuff on the carbide.
This is my opinion as to how a break might occur.

Called chip weld. Here the material "binds" to the top of the carbide and its binder.
Then the load increases and it rips out a chunk of the tool. Rinse and repeat.
Do not ever grind carbide "red-hot". It is now junk. It may look okay but you have put microcracks in it.
These are only .005 to .020 in but will kill tool performance.

So edge and hand finishing.
Way back before coatings every carbide tool salesman gave out "hone stones".
If your tool chipped too early or chattered you would hit it with this Sic stone, effectively dulling it.
This is know as honing a insert. Now we do it with brushes or a vibe. A small radius on the edge of .0005 to the big of .004.
And then there are T or K-lands. These plus a hone in cases.
This dulls the tool but strengthens the edge. (what the puck, dull tool going in lives longer? How can that be?)
When you hand finish you get some of this. Now way around it.

You are asking questions with long answers. I will try to help if I can.

 

[DanASM]

I like the questions as I use brazed carbide a lot, and have been sharpening and experimenting with it myself.

I do find that stoning the edge of my carbide form tools after each 12' bar gets run greatly helps the finish and tool life. My form tools are not coated and AL can stick to them pretty early on.

When grinding brazed carbide, I do it with a diamond wheel on a surface grinder by hand. The carbide gets hot and I let it air dry slowly before picking it up again to grind some more material away. Rapid temp change will cause it to crack.

I like my HSS tools sharp, but they cant run the same speeds as carbide. Carbide likes to chip a lot in unstable conditions, getting the tool within .002 under center will greatly increase tool life. Chatter and vibrations will kill carbide, where the same tool in HSS will just keep on trucking along (at a lower speed of course).

Pecking is generally a no no, as it can cause vibrations and chipping. Temp changes from intermittent cuts can hurt as well.

 

[john.k]

I used to cut up hardened gears ,and I found that once the carbide insert breaks down into a rounded point ,then it lasts for a long time with the interrupted cut......as long as the chip comes off bright red hot ,or even orange ,the carbide will last indefinitely.

 

[Conrad Hoffman]

I thought I knew what the C-numbers meant, but it seems not. Some good info in this thread- https://www.practicalmachinist.com/forum/threads/c2-c3-or-c5-carbide-question.79944/
Carbide has good compression strength, but is lousy in tension. Don't thread with carbide, stop the machine and then withdraw the tool. It's apt to chip the edge, especially if you're doing very fine threads. I like my carbide tools (non-CNC manual stuff) as sharp as possible, just as sharp as my HSS. An Accu-Finish will do it. IMO, a green wheel is near worthless for getting carbide truly sharp. You need diamond.

 

[eKretz]

Carbide can have an edge put on to just as fine an apex as HSS, but it's way more brittle. There are tiny spaces in between the grains filled with binder that are the weak spots, but they're usually very small indeed. The grain size of the carbide is only important in that it affects the intergranular spacing that is filled with binder, as I understand it. T-land and K-land are extremely useful in heavy cut, high hardness and interrupted cut scenarios. A hone does the same thing but helps more for lighter cuts and high speed finishing. The edge needs to be tailored to the individual application to optimize tool life.

 

[EmGo]

No offense but for doing three parts per week on the south bend, this is all just pulling the weewee ...

Which is fine but then other beginners take it seriously and get all wrapped up in silly shit instead of learning to machine. And then they come here and spout off all this nonsense while their 1/8" cnmg insert is hanging out a foot trying to turn a 1/2" 316 bar 14" long between the chuck and a live center with 6' hanging out the other end of the headstock, to .0001" (or 3 microns, whichever is more ridiculous) ... sigh.

 

[rons]

Called chip weld. Here the material "binds" to the top of the carbide and its binder.
Then the load increases and it rips out a chunk of the tool. Rinse and repeat.
Do not ever grind carbide "red-hot". It is now junk. It may look okay but you have put microcracks in it.
These are only .005 to .020 in but will kill tool performance.

Well, I expected you to come into this one and give me a spanking.
Is 1800 rpm too fast?
A brazed carbide's support is just some steel shank. In grinding the carbide and steel are getting hit with the abrasive
at the same time. So the wheel is loading up with carbide and steel particles. Comments if any?

So edge and hand finishing.

I'm using a diamond stone I bought for my woodworking.
I do the wheel if I have to shape a bit back to original profile.
Even if there microcracks I probably hand stone those away.
In front of the TV watching the news or a doc takes a long time.
Definitely not worth paying somebody regular wages to do this.

Any info gathered here will reduce cracks and failures. Then that means less time re-sharpening.

 

[dian]

there are so many grades of carbides. and so many materials to cut. what was actually the question?