1/1/2004
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Abrasives For Today’s Machine Shops: Will Superabrasives Rule the World
Will Superabrasives Rule the World?
By Brendan Baker
Twenty-five or 30 years ago engine builders could often get away with little more than an all-purpose abrasive wheel. But today’s engines require more specialized blends of abrasives that can grind a variety of alloys and leave an ultra smooth surface finish.
Engine builders now use a variety of abrasives on a multitude of machines – from valve grinders to crank grinders – to cut aluminum, cast iron and steel. Most abrasives are made up of various mixes of stones and bonding material. The stones are the designated cutters that do the work and the bond keeps the operation cool while exposing new stones to keep cutting.
There are essentially two types of abrasives in the automotive aftermarket: conventional abrasives and superabrasives. Conventional abrasives are used in most shops in some form or another. They’re made up of either silicon carbide or aluminum oxide and come in many different shapes and sizes. These types of abrasives are used in everything from large crankshaft grinding wheels to hand porting tools.
Superabrasives are made of synthetic diamonds, polycrystalline diamonds (PCD), or cubic boron nitride (CBN). They last much longer than conventional abrasives and continue to cut cleanly all throughout their life spans.
In recent years, superabrasives have become popular among production engine remanufacturers (PERs) as well as the OEMs. However, superabrasives are not for every shop because the expense per part to machine them makes them attractive mostly to high volume manufacturers. For the most part, machine shops still use conventional abrasives.
"There’s definitely still a place for conventional abrasives," says Martin Wimberly, Radiac Abrasives, Salem, IL. "If you look at a conventional abrasive under a microscope, it has fine hairline cracks where it fractures and exposes sharp teeth. A superabrasive wheel plows through the material and the crystallite doesn’t breakdown like an aluminum oxide or black silicon carbide."
An aluminum oxide wheel will cut steel, but there are 50 different compounds of aluminum oxide according to Chris Jenson, Goodson Tools and Supplies, Winona, MN: "We added a lot of porosity and made the aluminum oxide wheel soft so it will cut through, break down and keep grinding. What you don’t want is for a wheel to glaze, load up and become inactive and burn."
If a wheel does glaze or burn it’s usually either too hard or too fine of an abrasive wheel, say our experts. The grit is either too fine, there’s improper coolant or improper dress, all of which will make a wheel function poorly.
"If a wheel isn’t dressed properly from doing the previous flywheel it may glaze up on the next one, so I recommend a rough dress prior to grinding any flywheel. It’s just a good habit. It means the abrasive wheel is open and ready to cut," says Jenson.
An aluminum oxide abrasive is like grains of salt. While you’re grinding, the grains will break open. When they break they make a sharp edge. Eventually, the corners become rounded and the wheel loads up and glazes if it is not dressed enough or the material is too hard.
Bond is the glass that holds these wheels together. "Bond material is just like your Mom’s cookies. Every manufacturer has its own blend. It’s very hard to dial in this process," says Jenson. "We try to use the 80/20 rule. Meaning, for 80 percent of the wheel’s grinding normally it will work like it’s supposed to. But there’s always that oddball material out there that you run into occasionally and it will give you a hard time. In those cases you just have to redress several times to get the job done."
Superabrasives
The first diamond superabrasives were manufactured in the mid-’50s but didn’t really catch on until the mid- to late- ’70s. Originally, natural diamonds were used for cutting because of their strength, but they fell into short supply. So manmade (synthetic) diamonds began to replace natural diamonds in the world of industrial cutting tools.
In recent years, the price of synthetic diamond has dropped significantly. Back in 1980 some of the cheapest diamond materials cost $2.20 a carat. Today these diamonds cost about .16 cents a carat. "Back in the ’70s and ’80s a $250 wheel would have been out of reach for most automotive machine shops," says Tom Corcoran, American Superabrasives, Red Bank, NJ. "But today that wheel is only $35. And if that wheel will last for over 50 engines, it costs less than $1 per engine."
There are two substances that are considered superabrasive: the hardest substance is diamonds (PCD) and all its forms. The second hardest substance is CBN. "Superabrasives last longer, do a better job and give a better finish in almost every application compared to conventional abrasives," says Corcoran. "It’s just a matter of whether it’s cost effective for your shop. A low volume shop may not need superabrasives unless it’s looking for speed, because diamonds cut much faster and produce the best quality finish."
According to Radiac’s Wimberly, superabrasive flywheel cutters have been the biggest hit among engine builders in the last few years. "These products have revolutionized flywheel products. It’s so much faster, cleaner and cost efficient than any other abrasive product to date. It’s not that the price has dropped very much but it reduces the cost per part ground, plus less labor and machine runtime."
The biggest part of the automotive aftermarket rebuilding industry uses conventional grinding wheels and there will always be a place for those wheels just because of the nature of the types of products being ground. However, there are some operations that superabrasives can be an advantage.
"There are certain areas where you have opportunities for CBN or PCD," says Wimberly.
"Those areas are the facing type operations, like flywheels, cylinder heads and block surfaces. For cutting cylinder heads there is CBN/PCD pucks or buttons. They’re round radius 3/8˝ and 1/2˝ diameter buttons used on surfacing machines that will surface 35˝ a minute with a six-inch wide head.
"When you get into flywheel grinding, you get into a 6˝ over 4-1/2˝ cut flared-style wheel. We also have superabrasives that are used for rod and cap grinding, where you grind the mating surfaces. So there are certain areas where superabrasives have an advantage and have become popular and other areas where a conventional bonded abrasive wheel is the only thing for a particular operation," notes Wimberly.
Our experts agree that superabrasives are faster and produce better results in many cases, but if you abuse one or chunk one it’ll cost much more to replace than a conventional wheel. Some superabrasive manufacturers think that what the OEMs are experimenting with right now will eventually make its way into the aftermarket.
"Vitrified CBN, is what a lot of the OEMs are using. It fractures, where the conventional CBN and Diamond wheels won’t. It lasts longer and stays sharper," Wimberly explains. "But right now, the only fit in the aftermarket for this type of product is a PER because of the cost. A small shop isn’t going to buy an $800 crankshaft wheel, unless that’s all they do." He says it lasts forever with less frequent dressing and maintain a repeatable surface finish.
A general rule of thumb for any type of abrasive, unless you’re looking at it for the cost savings and not necessarily the best result, is you should use a soft abrasive wheel on hard metals and hard wheel on soft metals.
"If you use a hard wheel on hard material it’ll cut but not for long before it loads up the grain structure and quits cutting," says Mart Jeltema, K-Line Industries, Holland, MI. "All the pores in your abrasive will clog up and not cut anymore. The way abrasives keep working is to continually breakdown and expose new grit."
Conventional abrasives are designed to wear out so you have to take into consideration how fast you go through a stone. Using a very soft stone for cutting hard material will wear out much quicker than cutting softer material with a harder stone. If you’re cutting harder material with a soft stone you may have to dress it more often.
"A lot of people like the Ruby wheel; it’s more of an all-purpose wheel," says Jeltema. "But it will load up easily on hard material. It will still cut; you just have to redress it. The thing that engine builders like about the Ruby wheel is that it holds its angle. Once you dress it, it stays at that angle."
It takes longer to grind with conventional abrasives because the bond has to break down in order for the cutting stone to be exposed. "Your bond posts have to break down between crystallites to get into a cutting mode," says Radiac’s Wimberly. "The more bond content you have the longer it will last. It depends on the material you’re cutting and also the amount of coolant you’ve got flowing to the point of contact."
The coolant coming across the abrasive wheel will prohibit breakdown because the heat attacks the bond between the stones. Using less coolant will allow the abrasive to cut more aggressively. With higher bond content, the cutting action will be slower. Subsequently, the more cooling you have the harder the wheel will act. To make the wheel act softer you have to pull coolant away from it or bond away from it. Less bond makes it more aggressive.
"The bond has to be able to release the grain," says Goodson’s David Monyhan. "A grinding wheel has to consume itself to perform properly. It has to wear out or it will not work. If you over-push an abrasive it can glaze or load. When that happens you have to dress the wheel to open the grain up again."
Before helping to select an abrasive for a customer, Monyhan says the first thing he needs to know is what kind of machines are used. He also asks what market they serve (e.g., import, traditional, late model production, etc.). "Once we understand that, we understand dimensionally what we’re dealing with."
The grain structure varies according to the application: "Is it dry? Is it wet? What’s the material that you’re grinding? All that varies depending on what you’re grinding (e.g., valve seats, heads, flywheels, cranks, cams, etc.). That’s why there’s so much variation," says Monyhan.
One of the things that has changed over the last 20 years is the types of alloys that the OEs are using in their engines. "Twenty years ago blocks weighed 60-100 lbs more than they do today," says Goodson’s Chris Jenson, who heads up new product development and is the company’s expert on abrasives. "The manufacturers have added all kinds of composites to strengthen parts and reduce costs, so we had to come up with a stone that would be able to grind this material. We had to come up with a recipe that had the proper hardness, proper abrasive, proper bond and proper porosity for it to cut cool enough and have the proper Ra finish as well."
Jenson says that over the years he’s fielded countless questions about abrasives so he came up with three basic steps for how to choose the correct abrasive. When grinding flywheels, for example, a simple spark test will show what type of material you’re dealing with:
(Jenson reiterates that softer flywheels need to be ground with harder stones and harder flywheels need to be ground with softer stones.)
Cast Iron (soft) flywheels make deep red-orange sparks that travel only a short way before burning up.
Cast Steel (medium) flywheels make light orange to golden sparks that travel farther before burning up.
Billet Steel (very hard) flywheels make bright yellow-white sparks that travel very far.
(source: Goodson 2003 catalog)
One other key to remember about abrasives is that you must dress them properly and frequently. When in doubt, dress it again. There’s no sense in ruining a $4,000 crankshaft because you didn’t dress the wheel. Our experts recommend dressing a conventional abrasive wheel with a high quality diamond. These diamonds are sharper and will last longer than synthetic diamond dressers.
Is there a final answer between choosing a conventional abrasive over a superabrasive? Yes and no. Superabrasives have been perceived to be cleaner and faster and there have been reports of longer tool life. However, grinding with a conventional abrasive is typically more forgiving. Although, you may spend a little more time dressing it and maintaining it. If you’re still undecided about what types of abrasives you need contact your tool distributor. Your tool representative will gladly help you choose the right products for your specific needs.