Carbide Cutters.

[WilDun]

After having had a discussion in another thread on the merits (and failings) of coated carbide cutters versus HSS, I really can't understand why the metal used for the cutter underneath the coating is still tungsten carbide, (or is it?).
It appears to me that if the coating gets chipped, the metal underneath plays a very insignificant part in the cutting process thereafter, so why isn't it just steel? - how is tungsten carbide relevant here?

[DareBee]

Carbide is harder. So it can be ground to a sharper cutting edge. It also is way more ridgid (no cutter flex). The guys doing hard-milling even use carbide bodied insert cutters for their PVD inserts.

[ger21]

Originally Posted by DareBee Carbide is harder. So it can be ground to a sharper cutting edge.

Coming from woodworking, I've always read that HSS can take a sharper edge than carbide, but carbide will stay sharp longer.

[HuFlungDung]

From a metal working viewpoint, carbide is desirable because it has higher strength at red heat. You might seldom see this when milling, but when roughing on lathe jobs, the engaged portion of the tip of the carbide will glow a steady, bright red. It nonetheless, keeps right on cutting.

The coating on top of the carbide is there to improve the lubricity and hardness of the tool. A hard coating needs to be backed up with something that is also very hard. Carbide makes a good base material meeting this requirement.

[WilDun]

Originally Posted by DareBee Carbide is harder. So it can be ground to a sharper cutting edge. It also is way more ridgid (no cutter flex). The guys doing hard-milling even use carbide bodied insert cutters for their PVD inserts.

The point I was making was that the carbide itself does not actually seem to be capable of doing any cutting, and I have noticed that when the Titanium Nitride (or whatever) coating wears off, it's cutting ability is almost gone. This leads me to believe that it must be an inferior grade of carbide (hardness wise) underneath.
I agree on rigidity, perhaps because carbide doesn't have the "ring" you get from steel - it seems more "dead" - this would be desirable.
I've heard that carbide tooling is made by the sintering process, and I would have thought that this would have made them much easier to break than steel.

Going way back, working on a centre lathe and manual milling machine, I remember using grey tungsten carbide tips ( insert type as opposed to cemented or brazed on) on both the lathe and mill, they appeared to me to be uncoated and I was able to sharpen them (very successfully) on a soft green stone, or on a diamond impregnated stone.
Nowadays this would be impossible on the coated modern inserts, and I realize that it would be highly impractical in a high volume CNC shop.

I guess what I'm really asking is, where to get those high grade uncoated carbide inserts we used to have? - (for my home workshop when I retire soon). I don't want to have to be paying out continually on replacements for chipped coated lathe insert/milling cutters when doing one off experiments.
These (in my opionion) are useless in this type of situation.

Any young salesmen that I ask, just stare blankly at me - "uncoated???
whaddyamean uncoated??"

[HuFlungDung]

By the time the coating wears off, the tool is worn out, and the cutting edge has lost its keeness. The coatings are extremely durable, and do not "wear off" unless they are an unsuitable coating for the material being cut. In such cases, the workpiece material welds itself continually to the tool and then breaks away. This causes erosion and cratering which will break down the coating.

I don't think there is anything better about the grade of plain uncoated inserts. There are however, many different grades of carbide, so they will have differing properties. The coating is only .0001" thick, so the carbide most definitely has the actual shape of the cutting edge ground into it.

I use regular old inserts on a regular basis for making finishing tools for manual lathe work. The carbide will cut if you give it an edge.

[Konrad]

I still buy uncoated [brazed] bits in different sizes from Kennametal....but you can also
use the worn out ones, coated, [throe away inserts] and grind a new edge to it.
Konrad

[WilDun]

Thanks for all the answers,
I think maybe I'll review my money saving options and try that sharpening thing again! I was sure that the coatings were a bit thicker than .0001", but that doesn't really matter, what matters to me is that I can still sharpen them!
BTW, I have just found a stash of the old tips (all sorts) which I used about 25 years ago, at the back of my workshop.

[DareBee]

As far as buying uncoated goes, I can get them from any of my dozen (or so) tooling suppliers, even the old "brazed on a chunk steel" ones.
I also often get my coated C/bides re-sharpened, If it is just the tips I have them sharpened from the end only (ie shortened); and if they are 1/2" dia or bigger I will resharpen for the guys to use in the manual machines.
They still cut better than HSS.

[WilDun]

It's a pity that I live in the South Pacific, otherwise I could have taken advantage of your sharpening service, I think the postage costs would be horrendous!
No doubt there will be a host of people down here who will do it for me, but I'll have to try and locate them and I'll probably end up doing it myself eventually!

Regarding HSS lathe tools, there is one area where they will beat carbide hands down, that is when you are trying to turn down a piece of crappy old mild steel to a smooth finish and your final roughing cut is a mess, and effectively you've only got a couple of thou to go. A good sharp piece of tool steel (with a slight nose radius) used at low feed rate and low revs, and plenty of cutting oil will save the day every time!

Thanks again for all the help.

[carbidecraters]

Carbide is harder. So it can be ground to a sharper cutting edge

I was going ot give you a little hell on this but you kinda explained it. Basically the carbide has a denser structure but cant take the impact load a high speed steel insert can or M8 cobalt high speed mix. Micrograin carbides are great because they take more shock load without chipping and have a slightly more uniform composition (like cryogenics.......well kind of). Some materials dont like carbide at all...Like titanium. When you turn or mill a piece of titanium you need a cutter that will actually act as a heat sink if it breaks down and the chip isnt taking all the heat. Work hardening occurs and in the aerospace industry this used to be a big problem. Parts made out of a huge chunk of titanium when a endmill smoked it would take 3-4 new end mills to get the tool path back. 304,316 and other stainless steels are similar but some are more forgiving that titanium. Sharper edge carbides are NOT ideal due to chipping and snapping of the tip. If you look closely as say a ingersoll(iscar) or kennametal or carboloy insert it isnt a knife edge if its carbide. Carbide requires a shoulder relief to add structural strength to the insert. so that statement isnt exactly true...not that I am a know all

[masterblaster]

I see the biggest and best advantage of coated carbides is SPEED. Carbide is the hardest, most durable material for cutting. If your tool breaks down it wasn't the coating that failed it was your application. I.E. chip load ,speeds,feed,deep of cut. Carbide will break down faster if used in a convential cutting appliacation. Carbide is brittle if used in a interrupted cut manner. Carbide cuts best under full load.Also it is the best for climb cutting, which is the fastest cut because it is a compressive force cutter. That's why lathe inserts are all ways carbide. The coatings are just for lubricity and to keep the material from welding to the tool. If you are looking to work in your small shop and have low volume work, then HSS will be less expensive and readily resharpened for you. Coated carbides are for high volume, high speed,high feed applications. If you were to use a coated insert on that lathe finish you were looking for, you could get a glass finish by really cranking up the speed and boosting up the feed. In a nutshell, carbide for really high speeds and feeds. Way out lasts and out performs any HSS