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Cutting Edge Tooling - Superabrasives in the Shop
No more than a decade ago, superabrasives such as polycrystalline diamond (PCD) and cubic boron nitride (CBN) were considered too exotic and too expensive for the average engine builder to use. Today, PCD and CBN are mainstream and you can hardly afford not to use them.
By Larry Carley
The need for faster production speeds and higher quality surface finishes has made superabrasives almost mandatory for most resurfacing, honing and grinding operations.
What makes these materials so indispensable for engine building today? Their superior hardness is a major factor because it provides outstanding tool life that far exceeds conventional abrasives. A set of metal bond PCD diamond honing stones can typically do 50 to 100 times as many cylinder bores as conventional vitrified stones before they're worn out and have to be replaced. A CBN grinding stone for resurfacing flywheels will typically last ten times as long as a conventional stone. CBN pucks in a milling machine will cut 20 to 50 times as many heads as ordinary carbide pucks. Because of this, superabrasives can provide better overall consistency and reduce down time for tooling changes, and even though they cost more initially, typically provide lower operating costs over the long run.
Superabrasives can also handle higher machining and grinding speeds - in fact, they require it! The ability to cut faster means shorter cycle times, improved productivity and profitability. High cutting speeds, though, also require equipment that is designed to operate at higher speeds. Simply switching your tooling from carbide to PCD or CBN may not cut it if your equipment lacks the horsepower or the adjustability to operate at higher spindle speeds. Rigidity also becomes more important as operating speeds increase. That's why many equipment suppliers have redesigned their equipment in recent years or introduced new resurfacing machines and honing machines that are capable of taking full advantage of the benefits provided by PCD and CBN.
Yet another benefit that results from all of this is the ability to achieve higher quality surface finishes that are smoother and meet today's OEM requirements. Many late model engines are equipped with Multi-Layer Steel (MLS) head gaskets. These gaskets reduce cylinder bore distortion and typically have much lower clamping loads than traditional head gaskets. But they also require an extremely smooth, flat surface finish on both the block and head to seal properly, typically 20 to 30 microinches Ra (roughness average) or less. Consequently, if your resurfacing equipment can't achieve these kinds of numbers, you're going to have problems rebuilding some of these late model engines.
When cast iron V8s and straight six cylinder engines were the most commonly rebuilt engines, surface finish requirements weren't as critical and conventional abrasives were adequate for the task at hand. Heads could be ground, milled or even belt sanded and be smooth enough and flat enough to seal most head gaskets. Likewise, cylinder bore finishes were not as critical because blocks were more rigid and cylinder bore distortion has less effect on ring sealing and blowby. The cylinder walls in many engines today are much thinner and require torque plate honing as well as a plateau finish to meet emissions and seal the rings.
Vitrified abrasives can certainly deliver a high quality finish, but not with the speed and consistency of metal bond diamond honing stones. With conventional abrasives, the stones wear almost as much as the metal surface in the bore as the cylinder is being honed. Stone life depends on the hardness of the abrasive, the hardness of the substrate that holds the abrasive together, the hardness of the engine block, honing speed, the load on the stones, and the amount of metal that's removed from the cylinder. Consequently, you have to constantly monitor the honing process and compensate for stone wear to keep the bores round and straight. It's a balancing act between cutting action and stone life.
With diamond honing stones, the amount of wear experienced by the stones is almost nil. Diamond is the hardest natural substance known, so it can hold a cutting edge much longer than a conventional abrasive. This means the bond that holds the diamonds can also be harder because it doesn't have to wear away as quickly to expose fresh stones on the surface. After honing hundreds of cylinder bores, the stones still look and cut like new. Conventional vitrified honing stones, by comparison, will be completely worn out after 200 to 250 cylinder bores.
Diamond honing stones aren't cheap. They may cost $500 to $700 for a set versus $15 to $35 for a set of conventional honing stones. So initially, changing to diamond requires a significant up front investment. But the payback comes over the long haul because the stones last and last (assuming you don't overstroke a cylinder bore and break them!). Breakage is a risk with any type of honing stone, so it pays to be especially careful when honing with diamond stones.
Pros & Cons of Diamond Honing
Because diamond is a harder material and wears more slowly than conventional abrasives, it cuts differently and typically requires more pressure. This increases the risk of bore distortion, especially if the wrong honing speed, stroke rate or coolant are used. But many diamond stones today use an improved bond that allows the stones to cut with less pressure than before. This reduces the risk of bore distortion with minimal change in stone life.
When diamond was first used for honing, it was used primarily for rough honing cylinders. The cylinders were then finish honed with conventional vitrified abrasives. With today's thinner cylinders, less metal is being removed so there is no rough honing step and diamonds are used to semi-finish the cylinders. The final step is to then plateau the cylinders with cork, a brush or a plateau honing tool.
If you're switching from conventional stones to diamond, you'll generally have to use a higher number grit to achieve the same Ra (roughness average) when finishing a cylinder. For example, if you have been using #220 grit conventional stones to finish cylinders for chrome rings, the equivalent diamond stones might be a #325 grit. If you have been using #280 grit conventional stones to hone for moly rings, the diamond equivalent might be #550 grit stones. The actual numbers will vary somewhat depending on the brand and grade of the stones.
A set of #325 grit diamond honing stones will typically produce a surface finish in the 20 to 25 Ra range, which is about right for moly-faced rings. A set of #500 grit diamond stones, by comparison, will leave a smoother finish in the 15 to 20 Ra range, which is better for performance applications. Finishing the cylinders with cork, a brush or plateau honing tool will generally improve the numbers even more. If necessary, you can get the overall Ra down to 8 to 12, with Rpk (relative peak height) numbers in the 5 to 15 range, and Rvk (relative valley depth) numbers in the 15 to 30 range.
To hone properly, diamond stones really need a honing machine that's designed from the get-go for diamonds. Such machines usually have stronger gears, a higher horsepower motor, more rigidity and programmable controls. Diamond honing requires less babysitting so it lends itself much more to automation. For portable honing equipment, though, conventional abrasives are probably a better choice because they require less pressure.
Another difference with diamond is the type of lubricant that's required. Some recommend using a mineral oil or organic oil while others say a synthetic water-based lubricant works best with diamond.
One application where diamond may not be the best choice for cylinder bore honing is on hard blocks or those with nickel/carbide hardened cylinder liners. The hard ceramic facing inside such a liner is a mixture of nickel and silicon carbide about 0.07 mm (.0025" to .003") thick. This creates a very hard, wear resistant surface that reduces friction and allows the engine to develop more horsepower. On this kind of surface fine grit CBN honing stones typically cut better and leave a smoother finish than diamond.
Resurfacing With Superabrasives
Diamond is obviously the material of choice for honing, but what about milling and resurfacing? The best material here depends on the type of metal that's being resurfaced.
As a rule, cutting bits made of PCD diamond work best when resurfacing nonferrous metals such as aluminum, while CBN is recommended for cast iron and steel. CBN is not the best choice for milling aluminum because aluminum tends to stick to CBN and leave a smeary finish. Even so, a common trick for using CBN to resurface aluminum is to spray the surface with a lubricant or wax.
But why would you use CBN on aluminum if PCD diamond works better? This compromise can help save the time and cost of changing tooling when you're switching from one type of cylinder head to another.
CBN also works well to resurface hard-to-cut heads such as cast iron diesel heads that have hard precombustion chambers. Some specially designed 1/2"-diameter cutter bits are available for these applications that will leave a nice finish without the chatter or waviness that often results from trying to cut these heads with conventional abrasives.
The reason diamond isn't recommended for surfacing cast iron and steel has nothing to do with hardness, however. It's because CBN can handle heat better than diamond. At high cutting speeds, diamond gets too hot and starts to react chemically with iron. This causes diamond to dull and lose its cutting edge. This isn't a problem if the cutting speed is kept low enough (as when honing with diamond stones). But in a high-speed milling machine with only one or two cutting bits, diamond bits get too hot. Diamond starts to revert back to graphite (a very soft form of carbon) at temperatures above 1,500 degrees F while CBN can withstand temperatures up to 2,500 degrees F. CBN also has high thermal conductivity, and dissipates heat about four times faster than silicon carbide or aluminum oxide.
Diamond works well on aluminum because aluminum is a much softer metal than cast iron or steel. The hard particles of silicon that are usually alloyed with aluminum dull conventional abrasives. When a carbide cutting edge hits a silicon crystal in aluminum, it tends to push rather than cut. This dulls the tool and leaves a rough finish on the surface of the metal. Diamond, on the other hand, is much harder than silicon carbide, aluminum oxide or tungsten carbide, and cuts right through the silicon crystal without dulling the tool or tearing the metal. The result is a smoother finish with little wear on the tool bit.
Something else that must be considered when using CBN to resurface heads and blocks is the minimum depth of cut. CBN inserts typically have a honed edge, so the minimum depth of cut is usually limited to about .004" or .005" on cast iron. If too shallow a cut is attempted, the result can be edge deterioration, poor tool life or chipping of the insert (CBN is sometimes coated with titanium to improve tool life).
Too slow a cutting speed with CBN can also increase tool wear. To cut efficiently and cleanly, CBN needs a fast enough speed so it doesn't "rub" across the surface. A surface speed of 1,800 to 4,000 feet per minute (ft/min) is usually recommended for rough milling soft cast iron (at a depth of cut of .025"), and a surface speed of 2,000 to 5,000 ft/min for finishing cast iron.
When resurfacing aluminum heads or blocks with PCD diamond that have less than 12 percent silicon content, a surface speed of 800 to 6,000 ft/min is recommended for rough cuts, and 1,000 to 10,000 ft/min for finish cuts.
CBN has become the superabrasive of choice for grinding iron, steel and even powder metal. Several aftermarket suppliers sell CBN grinding wheels for resurfacing flywheels. The CBN wheels cost four or five times as much as a conventional aluminum oxide grinding wheel, but last up to ten times longer and deliver a much smoother finish. The wheels also have to be dressed less frequently.
CBN grinding wheels can handle higher operating speeds than ordinary vitrified abrasives, and cut most efficiently at higher speeds. Recommended surface speeds for many grinding operations with CBN are now as high as 6,500 to 9,000 surface feet per minute. Some industrial applications are using CBN at speeds as high as 20,000 to 30,000 feet per minute! The only limit on speed seems to be the structural integrity of the grinding wheel (do not exceed the maximum recommended rpm!).
Because of its excellent performance, CBN has become popular for grinding crankshafts, camshafts and many other parts that require high quality, precision finishes. Grinding can be done wet or dry, depending on the application, but many suppliers recommend using mineral oil as a lubricant rather than a water-based coolant when wet grinding with CBN. Differences in coolant can make a huge difference in the life of the grinding wheel, with the best coolants significantly extending wheel life.
Valve Guides and Seats
Diamond valve guide hones have become popular as a means of refinishing valve guides, and work well on harder guide materials such as silicon bronze and powder metal. Solid carbide valve guide reamers are another option.
For refacing valve seats, 3-angle cutters made of tungsten carbide are still the industry standard and deliver excellent results. Apparently no one is making 3-angle cutters out of PCD or CBN for the aftermarket yet, but you can buy triangular PCD and CBN bits for cutting seat pockets and finishing valve seats one angle at a time.
PCD inserts with flat or sharpened 10-degree sides, and negative or positive rake are available for use on nonferrous metals such as aluminum heads and beryllium copper valve seats (for use in performance heads equipped with titanium valves). The PCD bits provide extended life compared to carbide and ceramic bits, and can be used for both roughing and finishing operations. The recommended cutting speed for PCD is 400 to 600 meters per minute. Triangular valve seat bits are also available in CBN for cutting seats in cast iron heads, and refacing integral valve seats, hardened steel seats and powder metal seats. The recommended cutting speed for CBN is slower, and ranges from 150 to 400 meters per minute depending on the application.
The best advice for choosing the right superabrasive for a given machining operation is to ask your tool supplier what they recommend. They can tell you which materials work best where, what kind of lubricant or coolant (if any) is recommended, and what cutting speed or grinding speed will produce the best results.