by Kip Hanson
The soup to nuts of indexable carbide inserts
Machine shops make tooling every day. Fixtures are built, chuck jaws bored, vise jaws milled. Why not add indexable carbide inserts to that repertoire? Contrary to popular opinion, inserts aren’t made by Santa’s elves in the off-season. If you’re feeling adventurous, just follow these easy step-by-step directions and your shop will soon be shaving big bucks off its cutting tool budget.
First, you’ll need some tungsten. China or Russia’s a good place to shop for it, but Canada’s no slouch either, ranking third in the world’s tungsten production. Bring warm clothes, though: most Canadian tungsten is found in the Yukon. Once there, you might be lucky enough to mine some tungsten in its pure metallic form, but most is extracted from tungsten oxide, and requires additional processing.
Once you have a big pile of tungsten ore, you’ll need a ball mill to process it, a heavy-duty machine capable of grinding those rocks into dust finer than flour. That’s an important point, because the tungsten powder used in carbide inserts is made of particles just a few microns across.
Now that you have a suitable quantity of ground tungsten, it’s time to carburize it. Tungsten carbide is nearly as hard as diamond, but pure tungsten—that whitish rock you just dug out of the ground—is malleable enough to be drawn into filaments for incandescent light bulbs. It’s only through carburization that tungsten “mans up” into tungsten carbide.
The carburizing process begins by mixing tungsten powder with graphite. If you have a few thousand old pencils lying about, you might sacrifice them for their leads, otherwise you can buy graphite powder online for several hundred dollars a kilo. Don’t get any on your hands though, and be careful not to breathe it in. It’s nasty stuff.
You’ll also need a furnace, one capable of 1,600° C. Place the tungsten/carbon mixture on a ceramic pizza stone, stick it in the furnace, pump in some hydrogen and take a long lunch break. By the time you return, the graphite—which is really just a form of carbon—will have been “taken up” by the tungsten, atomically binding it to the tungsten particles. Tungsten carbide is born.
As any machinist knows, tungsten carbide is an extremely hard and abrasion resistant material. It’s used in such things as surgical instruments, snowmobile treads, nuclear fuel processing equipment and ballpoint pens. It can also be made into a lathe operator’s best friend, the 80-degree diamond.
Your freshly minted tungsten carbide powder needs a few more ingredients first, however. One of these is cobalt. Since you already blew the company’s holiday party budget on a ball mill, you could always grind your own, but it might be better at this point to order some online. Most of it comes from the African Congo.
However you obtain it, cobalt is critical to carbide insert manufacturing. In fact, if our soon to be born 80-degree diamond were a Rice Krispie treat, cobalt would be the marshmallow that holds the bits of tungsten carbide cereal together. You don’t need much, but without it there won’t be any dessert.
Some Rice Krispie treats need a few cinnamon candies or chocolate bits to make them sweeter. Carbide’s no different. Titanium carbide and tantalum carbide, or TIC and TAC, are common additives used to enhance carbide’s properties, and are especially useful at preventing cratering when machining superalloys.
After you’ve determined the proper recipe, throw everything back into the ball mill together with some alcohol until it’s mixed together. Nitrogen or some other inert gas can be used to dry the mixture when it’s done, prior to baking it in the oven.
A Rice Krispie bar suitable for interrupted cuts or roughing of grey cast iron would be made from grains of tungsten carbide in the 2-3 micron range, together with around 13-percent cobalt to bind them together. That much marshmallow binder makes the insert tough and shock resistant.
Micrograin carbide, on the other hand, is like a very dry Rice Krispie bar. It’s made from fine grain tungsten carbide powder and perhaps 6-percent cobalt, far less than its gooey counterpart. This combination is what makes micrograin ideal for turning a hardened bearing race, or an Inconel rotor shaft.
That said, it takes more than cobalt and tungsten carbide to make a carbide insert. Cobalt might be the ultimate carbide superglue, but only after it’s been heated to its melting point. Until then, forming it into any shape is like trying to make sandcastles without water.
You could mix it with wax. Up until the mid-80s, this was the preferred binder for holding green inserts together prior to sintering. But manufacturers found that wax leaves a sticky residue when it evaporates, messing up the oven. That’s where polyethylene glycol, or PEG, comes in.
An organic compound, PEG is used in everything from laxatives to rocket propellant. In the case of insert moulding, it’s like the missing water in our sandcastle example. Mix of few snowflakes of PEG with the cobalt tungsten carbide mixture before molding the inserts and they’ll hold together just fine until firing.
Speaking of moulds, you’ll need one 17-per cent larger in each direction than the desired insert size. You’ll find out why later. Since this adventure into carbide insert manufacturing is unlikely to become a high-volume affair, a steel mould is adequate, although one made of carbide is fine if the volumes justify it. You’ll also need a matching die, to form the top of the insert and force the tungsten carbide mixture into the mold.
If you think you should be pressing tens or hundreds of these at a time, think again. Even with the big tooling manufacturers, inserts are made like sandwiches, one at a time. Granted, the folks at Seco and Sandvik make those sandwiches very quickly, several of them per second, but the basic process is similar to that used in our homemade insert factory.
Measure out enough tungsten powder to fill the mould, inject it into the cavity, and drive the die home. With an 80-degree diamond, it should only take a few tons of pressure, but it could take five times that for a very large or thick insert.
The actual pressing is quite fast. In production volumes, inserts pop out of the mold like baby aspirin, and several per second is fairly typical. Green inserts have the consistency of blackboard chalk, and are placed on specially made carriers for transport to the furnace. Unless your shop is making enough inserts for the next five years, you can probably get by with something simpler, as long as you’re careful not to handle them before cooking.
Go easy with the furnace, too. Heat an insert too fast and it will crack. In a few hours, however, something magical happens. At 1495°C, the melting point of cobalt, the insert shrinks to 60-percent of its original volume. It happens in an instant, and if you’ve done everything right, it shrinks the exact same amount, every time.
After the inserts have cooled, they’re ready to go to work. If you need a sharp edge, you might choose to grind them on a 5-axis grinder. And depending on your application, you could also send them out for TiN or TiC coating to improve tool life and increase productivity.
Of course, making carbide inserts isn’t nearly this simple. Tooling manufacturers have this down to a complex and high-tech science, and invest millions each year towards improving their wares. So while the steps described here are conceptually accurate, take some advice: spend the $10-$15 and buy your inserts from the experts. SMT
Kip Hanson is a contributing editor. [email protected]
Information based on interviews with Don Graham, manager of education and technical services at Seco Tools Inc., and Scott Lewis, turning product specialist for Sandvik Coromant.