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Cutting Tools and Abrasives
By Larry Carley
One of tricks of the trade of machining engine parts is choosing the right cutting tool or abrasive for that particular job. Different metals have different machinability characteristics. A cutting tool or abrasive that works well on one application may not work so well on another application.
Years ago when most engine parts were cast iron, you could usually achieve a satisfactory surface finish with almost anything that would remove metal. You could use a grinder, milling machine, broach or belt sander to resurface cylinder heads. But when aluminum heads became common, grinding gave way to milling as the most popular method for resurfacing heads. Even so, grinding is still used to resurface diesel cylinder heads that have ceramic precombustion chambers, and it also works well for resurfacing both diesel engine blocks with hard cylinder liners and aluminum passenger car engines with integral cast iron cylinder liners.
Milling has become the preferred method for resurfacing aluminum heads and blocks because aluminum tends to load up grinding wheels. Dressing the wheel often can help keep the grain in the abrasive open, and spraying the surface of the head or block with a lubricant will help reduce metal buildup on the grinding wheel. An aluminum compatible coolant can also be used to prevent heat buildup on the surface of the metal and to wash away the metal chips.
Silicon carbide grinding wheels are generally recommended for resurfacing aluminum, while silicon carbide or aluminum oxide (or a mixture of both) work well on cast iron. Aluminum oxide is usually required for grinding steel (such as diesel crankshafts, racing crankshafts and steel flywheels), while CBN (cubic boron nitride or “Borazon”) works best on cast iron parts.
One of the qualities that has made CBN so popular in grinding applications is its ability to operate at higher than normal wheel speeds: up to 6500 to 20,000 surface feet per minute (SFPM). The faster the wheel spins, the better it cuts. You can calculate the SFPM speed of a grinding wheel thusly: multiply the diameter of the wheel (in inches) by 3.1416, then divide by 12 (to convert the circumference in inches to feet), then multiply by the wheel speed in revolutions per minute. The bigger the wheel and the faster it spins, the higher the SFPM at the grinding surface.
To get the best possible surface finish when grinding, the wheel must be dressed regularly. Clues that may indicate a need to redress a grinding wheel include chatter marks on the workpiece, a change in the sound of wheel while it is grinding, an increase in the power (amps) needed to drive the wheel, or a change in the pattern of the sparks.
When a grinding wheel on a crankshaft grinder needs to be dressed, always use a coolant. A steady flow of coolant will keep the diamond dressing stone from getting too hot, which can dull the diamond or cause it to loosen in its mount. Also, dress from the center of the wheel in both directions, and do not exceed .002˝ in depth. The traverse rate of the dressing stone should be fast enough to prevent glazing, but slow enough to minimize spiral lead marks. Remember to rotate the diamond 30 to 45 degrees after each dressing to maintain a sharp cutting edge.
IDENTIFYING GRINDING WHEELS
There are a number of variables that determine the cutting properties of any given abrasive, including the type of abrasive particles used (aluminum oxide, silicon carbide, CBN or diamond), the grain or grit size of the abrasive, the type of bond material that binds the abrasive together (vitrified, resin or metal bond), the amount of bond in proportion to the abrasive (the “grade” of the wheel), and the relative spacing of the abrasive particles within the wheel matrix.
On every grinding wheel are manufacturer’s markings that indicate the type of abrasive, grit size, grade, structure and bond type. Lower grit numbers indicate coarser grit size while higher numbers indicate a finer grit size. A finer grit size provides the best finish and also allows better penetration when grinding hard metals. Coarse grit is generally better for unhardened metals and for grinding applications that require large areas of contact between the wheel and workpiece. Most automotive grinding wheels use grit sizes between 24 and 80.
Grade ratings refer to the amount of bond material in the wheel, and are designed by letters A through Z. A represents the softest end of the spectrum while Z represents the hardest wheels. Soft wheels work well on hard metals, but wear quickly. Hard wheels are better on softer metals, and generally produce a finer finish and longer life.
If the bond is too strong and holds the grit too long without letting it break away, the wheel may become dull, glazed, cut poorly and burn the metal. It may also clog up with debris. On the other hand, if the bond is too weak, the wheel will wear too quickly resulting in short wheel life.
The strength of the bond also affects the surface finish on the metal. A strong bond will produce a finer surface finish while a weaker bond will leave a rougher finish.
The structure rating of a wheel, which ranges from 0 (most dense) to 12 (open) indicates the spacing of the abrasive grains in the wheel. The structure of a given wheel tends to be standard for a given grit size and grain, but some have more open spacing to provide cooler, faster cutting action in dry grinding applications.
The color of the wheel itself depends on the type of abrasive in the wheel as well as the other ingredients in the wheel. Silicone carbide ranges in color from green to dark gray. Aluminum oxide is typically white to light gray.
Vitrified grinding wheels can be cracked by mishandling, mechanical or thermal shock. Because of the high speed at which they rotate, cracks may cause a wheel to explode. Inspect the wheel before you start the machine, and replace it if you see any cracks. You can also tap the wheel lightly on the side. A good wheel should ring. A cracked wheel won’t ring.
Always wear eye protection when grinding.
Never exceed the maximum spindle speed rating for the wheel when grinding.
Wheels should be trued and balanced when first installed, and rebalanced as needed to compensate for wear. If a wheel has gone out of balance due to uneven wear, it needs to be replaced.
All wheel and machine guards must be in place before grinding.
Do not stand in a direct line with the wheel when the equipment is first started.
When making contact between the grind wheel and part, contact should be made gently without bumping or gouging.
Grind only on the face of a straight wheel, never the side.
Never force grind so that the motor slows noticeably or the work gets hot.
Over the years, many older grinding machines have been converted to mill cylinder heads by replacing the grinding wheel with a cutter head that holds one or two cutting bits. It works, but not as well as a machine that has been designed from the ground up to be a milling machine.
The spindles and table drives in many of these older grinding machines cannot hold close enough tolerances to achieve a really smooth, flat finish if the machine is converted to mill heads. Many of these older machines are not rigid enough to hold the cutter steady so the tool bit doesn’t lift or chatter when it makes an interrupted cut. By comparison, today’s high speed milling machines have more powerful motors, heavier castings, electrically driven ball screw tables, and have tighter assembly tolerances. What’s more, these milling machines are designed to take full advantage of superabrasive cutting bits made of CBN or PCD (polycrystalline diamond).
Conventional carbide bits in a milling machine can produce a high quality finish on aluminum or cast iron, but carbide won’t hold a sharp edge as long as a CBN or PCD bit will, nor can carbide handle the cutting speeds of a superabrasive.
Diamond is the hardest material known to man and is three times harder than its conventional abrasive counterpart, silicon carbide. That’s why a diamond is used to dress a conventional grinding wheel. Diamond also has a thermal conductivity that is about six times that of silicon carbide and aluminum oxide, which permits high speed grinding speeds without excessive heat buildup in the workpiece.
CBN is second in hardness only to diamond, and is 2.5 times as hard as its conventional counterpart, aluminum oxide. CBN can withstand higher temperatures than diamond (2500 degrees F versus 1300 for diamond), and is also a good conductor of heat, with a thermal conductivity about four times higher than silicon carbide and aluminum oxide.
When CBN or PCD is used as a cutting tool to machine metal, it provides many times the tool life of a conventional abrasive. This more than offsets the higher initial price of the superabrasive in most cases. Longer tool life also improves consistency, productivity (less down time to replace worn bits) and profitability. It also allows faster cutting speeds to reduce the time it takes to refinish a part.
For example, when using a carbide insert to refinish a cast iron head, the spindle rpm required will typically be about 140 rpm for an 11-inch cutter, 120 rpm for a 13-inch cutter or 110 rpm for a 14-inch cutter.
With CBN or PCD inserts, the recommended spindle speeds are much higher: typically 1040 rpm for an 11-inch cutter, 880 rpm for a 13-inch cutter, or 720 rpm for a 14-inch cutter. If the head or block being resurfaced is harder, high silicon content alloy, the speeds need to be slowed down a bit: 690 rpm for an 11-inch cutter, 580 rpm for a 13-inch cutter or 540 rpm for a 14-inch cutter.
With a single CBN or PCD insert cutter spinning at 1,000 to 1,500 rpm, the feed rate should probably be less than two inches per minute on the final cut to achieve a surface finish in the low teens.
PCD is the best choice for milling aluminum heads and blocks, while CBN gives the best results on cast iron. Yet many shops find CBN works fine on both types of metals and eliminates the need to change tooling when resurfacing different types of heads.
The problem with using CBN to mill aluminum is that aluminum tends to stick to CBN and leave a smeared finish. As with grinding, this can be minimized by spraying a lubricant on the surface of the metal. Ordinary olive oil works well for this purpose, as does furniture polish, WD-40 or specialty lubricants for machining aluminum.
PCD is not recommended for resurfacing cast iron heads or blocks because it can get too hot at high cutting speeds, react chemically with iron and break down. CBN, on the other hand, can handle the higher temperatures and won’t break down when cutting iron or steel.
Something else to keep in mind when using CBN to resurface heads and blocks is to optimize the depth of the cut. CBN inserts typically have a honed edge, so the depth of cut must usually be at least .004 to .005˝. 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).
Though CBN and PCD bits will outlast carbide bits by a significant margin, they won’t last forever. Don’t try to cut too many heads or blocks with the same edge. If you are using a round CBN button for resurfacing, you should rotate the button about 5 degrees after 20 to 30 heads to maintain an optimum cutting surface. Rotating the button just a little bit when it starts to get noisy will expose a fresh edge and reduce the risk of chipping the button or wearing it too far. Buttons with a beveled edge can be relapped to restore the edge if they are not too badly worn. But if the button has lost too much of its edge, the only option is to replace it with a new one.
Here’s another tip to prolong the life of your cutting bits regardless of the type of material you are using: Don’t try to cut through rust or calcium deposits. Iron oxide on a cast iron head or calcium deposits in and around the water jacket openings can dull your bits and shorten their life. The bit can also pick up this debris and drag it across the surface, leaving a groove. So remove the rust and calcium first by thoroughly cleaning the parts.
Diamond has become the material of choice for honing cylinder blocks today. Diamond honing stones cut faster, last up to 50 times longer and leave a much more consistent bore finish than conventional vitrified abrasives such as silicon carbide and aluminum oxide. A set of diamond honing stones may cost a lot more than a set of conventional honing stones. But when their much greater longevity is factored in, diamonds usually cost less in the long run and their consistency is much better regardless of any cost difference.
Diamond honing makes the most economic sense when an engine builder is working within the same range of bore dimensions on a series of engines. Because of the high initial cost of diamond stones, a custom engine builder who rebuilds anything and everything that comes in the door may not find it economical to buy diamond stones for a wide range of bore sizes. But if most of the honing work you do is on engines with bores in the four-inch range (plus or minus a quarter inch), you can probably cover most of these applications with a single set of diamond stones.
The best estimates say that somewhere between 50 to 60 percent of all the cylinders that are honed today by aftermarket engine builders are being honed with diamond stones. That’s a dramatic shift from a decade ago when diamonds were used almost exclusively by the OEM’s in their new engine plants and by only the biggest PERs. Nowadays, almost everybody uses diamonds, even many of the die-hard performance engine builders. They are using diamonds for honing because it allows them to maintain tighter bore tolerances and bore geometry.
Diamond honing stones are available for most popular honing heads. But to maximize the benefits of diamond honing, the honing machine must be capable of handling higher loads. That may mean upgrading to a newer honing machine if your old honing machine wasn’t designed for diamond honing. But some of the newest diamond honing abrasives are designed to be 100% compatible with older honing machines. Check with the abrasive supplier to see what they recommend.
Monocrystalline diamond is also being used to electroplate fixed diameter honing tools, not only for OEM honing machines but also aftermarket honing machines. This application works well on harder materials such as compacted graphite diesel engine blocks and nickel-carbide hardened cylinders.
Diamond honing stones cuts differently than conventional abrasives and typically require more pressure. Diamond tends to plow through a metal surface rather than cut through it. This can generate heat and distortion in the cylinder bore if the wrong type of equipment, pressure settings or lubrication is used. It also leaves more torn and folded debris on the surface of the cylinder bore than a conventional abrasive. Consequently, a final finishing step may be required to remove this material and to leave a plateau finish in the bore. Stroking the bores with a plateau honing tool or a flexible abrasive brush shears off the sharp peaks and significantly improve the surface finish without changing the bore dimensions.
Diamond is also good for rough honing cylinders to oversize because it can remove a lot of metal fast. Consequently, you can use a diamond hone in place of a boring bar. But rough honing takes more pressure and requires more horsepower from the honing machine. Because of this, diamond stones work best in equipment that has been designed to take maximum advantage of diamond’s cutting properties.
Special abrasives are needed to hone performance engines that have hard liners, high nickel or silicone alloys, or hard facings. The surface hardness in a coated cylinder is about 90 HRc, and the thickness of the coating is only about 0.07 mm (.0025 to .003˝) thick. Consequently, you don’t want to remove a lot of material when honing the cylinder. These coatings retain oil well, so the bores can be honed to a super smooth 4 to 6 microinch finish to minimize friction. Stroking a hard-lined cylinder with a 240 grit aluminum oxide flexible brush is also a good way to restore some of crosshatch to retain oil.
The type of coolant required for honing will depend on the stones. The trend today is away from machine oils to water-based synthetic coolants. Follow the abrasive supplier’s coolant recommendations.