Taking a bite: dental bur manufacturing
You’re sitting nervously in the dentist’s chair. Dr. Hurt is preparing your abscessed tooth for a root canal. Your mouth is pried open wide enough to swallow a baseball, and all you can hear is the high-pitched whine of the drill while a fine mist of bone particles, water and blood swirls through the harsh glare of the dental light.
Is this really a good time to ask your dentist where he buys his cutting tools?
If you want to avoid the gut-wrenching pain that comes from a dull or cheaply made dental bur, you should give that some thought before you sit down. The quality of your visit to the dentist is largely determined by the tools he or she buys.
But how are those incredibly tiny and sharp cutting tools manufactured in the first place, and what makes one better than the next?
One size will not do
There’s no tooth fairy when it comes to dental bur selection. Michael Quigley, senior engineer at SS White Burs Inc., Lakewood, N.J., said there are hundreds of shapes and sizes available, ranging from 0.5mm-dia. tools to 3mm- to 4mm-dia. “piranhas.”
Grinding processes for dental burs are similar to those used for medical and industrial tools. (Left to right): dental implant drill, industrial bur, orthopedic bur (shoulder and knee surgeries), orthopedic bur with guide pin, and two nickel-titanium root canal reamers. Image courtesy Rollomatic.
“We offer burs for use by dentists performing oral surgery, general and cosmetic dentistry, as well as tools used by dental labs for grinding and smoothing dentures, crowns and bridge work,” he said.
Burs are grouped into two families: those that cut, meaning carbide tools similar to endmills, and those that grind, basically miniature versions of diamond-plated grinding wheels (see sidebar below). “With carbide, the tooth material is shredded and moved away from the cut site, whereas grinding burs remove material by abrasion,” said Quigley. “There are pros and cons to each, and it’s up to the individual dentist to decide which works best for him and his patients.”
But aren’t teeth simple to cut? Not really, according to Quigley. Teeth have multiple layers, including a hard enamel outer shell. Under that is the dentin, which—when healthy—is also hard, but more flexible than the brittle outer surface. But when the tooth decays, dentin softens and becomes sticky.
There may be restorative materials to deal with as well—old amalgam fillings, porcelain crowns and composite resins—all of which can be difficult to cut. One must efficiently cut them all.
Back to the grind
The manufacturing process for dental burs is similar to those used for other industrial tools. A typical carbide dental bur begins as a carbide blank that is then welded to a stainless-steel shank. The flutes are then ground in a single operation.
Michael Shaluly, president of Mastercut Tool Corp., Safety Harbor, Fla., noted, “Burs are obviously much smaller than many industrial tools, so you need to carefully support them when you’re grinding the flutes. Sometimes you need to slow down your feeds so you don’t get deflection. Other than that, dental burs are made using the same grinding process [as industrial tools].”
Mastercut specializes in laboratory burs. It makes the two-piece burs just described, as well as solid-carbide tools. “Laboratory burs are larger, as a rule, than those used by dentists, and have shank sizes up to ¼" in diameter (dental burs are typically half that). We call them milling burs, but they’re no different than an endmill.”
What about grinding machines? “Some toolmakers design their own machines,” said Shaluly, “but we use standard CNCs from Rollomatic.”
Eric Schwarzenbach, president of Rollomatic Inc., Mundelein, Ill., agreed that tool support is vital when grinding. “We have a system where we clamp the bur right behind the head,” he said. “A dental bur is very weak on the shank, and if you don’t have a good support system, you can’t grind an accurate geometry. It will deflect all the time.”
Schwarzenbach said that dental tool grinding machines are much smaller than standard units. “We offer a compact, 5-axis machine for dental tools. The bur business is very competitive, with volumes of 60 million to 70 million burs per year worldwide. Prices for these tools are very low, so manufacturers have to optimize their processes.” (The bur the dentist sticks in your mouth might cost less than $2.)
The actual time needed to grind a dental bur is short. “Typical cycle time for a dental bur is less than 1½ minutes, while a large lab bur could take up to 4 minutes,” Schwarzenbach said.
Tri Hawk Inc., Morrisburg, Ontario, prides itself on making dental tools for the bur connoisseur. “There’s a huge difference in the manufacturing process for high-quality burs,” said Gustel Fischer, president of Tri Hawk. “Our tools go into an air-driven handpiece [used by the dentist] that may run up to 500,000 rpm. If you have the slightest vibration or whipping—say a defective or worn bearing—you’ll find out how well the bur is welded. Also, the tool shank must be totally round. If not, the bur will slip in the chuck.”
Fischer said there’s more to good dentistry than quality tools. “A good dentist has the chucks and bearings in his hand tools changed every month. This prevents vibration and assures the correct speed.” µ
Diamond shines in dental work
Carbide’s not the only kid on the dental bur block. “The dental industry is moving away from fluted carbide tools in favor of diamond-plated, high-speed-steel burs,” said Daniel Lenk, director of sales for grinding machine builder Unison Corp., Ferndale, Mich.
Stainless steel surgical burs typically have fewer teeth than industrial tools and a negative rake angle to make them cut less aggressively. Image courtesy Unison.
What’s driving this? Lenk explained that some of it is simple preference, since manufacturers and dentists alike go with what they’re used to. But diamond tools are also simple to make—just grind or turn the form on the blank and send it out for electroplating. As a rule, diamond-plated steel tools outlast carbide and—depending on the manufacturer—cost about the same, making them an economical choice in many applications.
Steel is also a frequent choice for surgical tools. “Surgical burs are just larger versions of dental burs,” said Lenk. “While dental burs go down to 0.5mm in diameter, most surgical burs are in the 6mm-dia. range, and are used to cut bone and cartilage.”
Another difference with surgical burs is the inclusion of a tube that allows irrigating water to flow through the center of the tool into the surgical site, while debris—bits of bone, tissue and other yucky stuff—is sucked back up through an outer tube (see photo above, top two tools).
Burrs created during manufacturing represent one of the biggest challenges to using steel for surgical tools, according to Lenk. “You have to remove those after grinding. If you don’t, those bits of metal can break off inside the patient during surgery.”
To avoid this, manufacturers use various deburring methods, including chemical etching and polishing, Lenk explained. “Some companies hit the edges with a nylon wheel to knock the burrs off. The problem is that everything you do after the grinding operation dulls the tool. It’s a fine balance between removing the burr and keeping a sharp edge.”
The hooks and barbs of root canal
Ever had a root canal? They can hurt—a lot. But they’re also kind of interesting. If you’re like me, you sat there in an anesthetic-induced haze and wondered, “What the heck does he keep sticking into that truck-sized hole he just drilled in my mouth?”
Turns out the hole isn’t really that big—only a few hair-widths in diameter—and that tool he’s using? It’s called an endodontic file.
Daniel Lenk, director of sales for grinding machine builder Unison Corp., Ferndale, Mich., explained that these little nightmares are made from Nitinol, the same stuff they use for heart stents and coronary angioplasty guidewires.
“The smallest ones are 0.0015" in diameter at the tip, with a small hook on the end,” said Lenk. “That’s what the dentist uses to rip the nerve out of the hole.” Other models resemble long, elegantly tapered torture devices. “We make a single-flute, positive rake file that measures 0.003" in diameter at the tip, and tapers out to 0.015" about an inch back.”
Why Nitinol, and how do you cut something this small?
“It gets pretty hairy,” he said. “Nitinol is just some gummy, terrible stuff. But you can’t use high-speed steel for something this small—when you get below 0.01" in diameter or so, you can’t keep it straight. It just goes wherever it wants, whereas the Nitinol wire holds its shape.”
You’ll be glad it does the next time you’re strapped into the dental chair for a root canal.