The finish on the cylinder walls is critical for proper piston ringlubrication and sealing. The bore geometry is also important, and mustbe round, cylindrical and straight for optimum sealing and minimumblowby.
The equipment used to hone cylinders has changed in recent years as thedemand for better surface finishes, faster cycle times and lowerproduction costs have driven the technology. Many engine builders havereplaced their manual honing equipment with programmable automatichoning machines that have load-sensing controls. These machines providethe precision and repeatability to achieve today’s higher qualitycylinder bore finishes. When the load-sensor detects a high spot in acylinder, the controls keep the hone head working in the same locationuntil the high spot is gone. The ability to vary dwell in any part ofthe cylinder bore results in a rounder, straighter bore with betteroverall bore geometry.
Some of the newest honing machines with all the bells and whistles evenhave the capability to cycle the hone head from one cylinder to thenext, and then from one bank to the next. Once the operator sets up theequipment and the numbers he wants, the machine does the rest. Hedoesn’t have to baby-sit the equipment while the machine is honing theblock.
DIAMONDS ARE FOREVER – WELL, ALMOST
Honing machines have also been upgraded to handle the latest generationof diamond honing stones. Diamond has become the material of choice forboth high volume production engine rebuilders (PERs) as well as customengine builders (CERs) and performance shops. The reason? Diamondhoning stones cut faster, last up to 50X longer and leave a much moreconsistent bore finish than conventional vitrified abrasives such assilicon carbide and aluminum oxide. A set of diamond honing stones maycost up to 20X to 30X times as much as a set of conventional honingstones, but when their much greater longevity is factored in, diamondscost less in the long run – and their consistency is much betterregardless of any cost difference. Advantages such as these have wonover more and more converts to diamond honing.
Diamond honing makes the most economic sense when an engine builder isworking within the same range of bore dimensions on a series ofengines. Because of the high initial cost of diamond stones, a customengine builder who rebuilds anything and everything that comes in thedoor may not find it economical to buy diamond stones to fit a widerange of bore sizes. But if the majority of honing work he does is onengines with bores in the four-inch range (plus or minus a quarterinch), he can probably cover most of these applications with a singleset of diamond stones.
The best estimates say that somewhere between 50 to 60 percent of allthe cylinders that are honed today by aftermarket engine builders arebeing honed with diamond stones. That’s a dramatic shift from a decadeago when diamonds where used almost exclusively by the OEM’s in theirnew engine plants and by only the biggest PERs. Nowadays, almosteverybody uses diamonds – even many of the die-hard performance enginebuilders who don’t build a lot of engines in a year’s time, but whorequire the tightest tolerances and highest quality bore finishes fortheir professional racing customers.
Diamond honing stones are available for most popular honing heads. Butto maximize the benefits of diamond honing, the honing machine must becapable of handling higher loads. That means buying new equipment ifyou are serious about upgrading to diamond.
Diamond is the hardest natural substance known, so it can hold acutting edge much longer than a conventional abrasive. This means thebond that holds the diamonds can also be harder because it doesn’t haveto wear away as quickly to expose fresh stones on the surface. Thetrade-off is that diamond cuts differently than conventional abrasivesand requires more pressure. Diamond tends to plow through a metalsurface rather than cut through it. This can generate heat anddistortion in the cylinder bore if the wrong type of equipment,pressure settings or lubrication is used in the honing process.
Diamond is also good for rough honing cylinders to oversize because itcan remove a lot of metal fast. Consequently, you can use a diamondhone in place of a boring bar. But rough honing takes more pressure andrequires more horsepower from the honing machine. Because of this,diamond stones work best in equipment that has been designed to takemaximum advantage of diamond’s cutting properties. That’s why honingmachines that have been re-engineered for diamond stones typically havemore rigid components and more powerful motors. This doesn’t mean youcan’t use diamond stones in an older hone head or an older honingmachine. But if your equipment can’t handle the higher loads, you maynot achieve the same degree of accuracy and repeatability as you couldwith equipment that has been specifically designed for diamond honing.Nor can you take advantage of the automation features that areavailable in the newest generation of honing machines.
Because of the way that diamond cuts metal, it tends to leave more tornand folded debris on the surface of the cylinder bore than aconventional abrasive. Consequently, a final finishing step is oftenrequired to remove this material and to leave a plateau finish in thebore.
OEMs have long favored a plateau finish in their cylinder bores becauseit allows the rings to seat almost instantly and extends ring life. Byshaving the peaks off the bore surface, the bearing area that supportsthe piston rings is increased without reducing the valley area that isneeded to retain oil for proper ring lubrication.
If the bores are not plateau finished with a final honing operation orfinishing step, the rings will do the work instead. The scouring actionof the rings will wear down the peaks in the bores, but it will alsotake a toll on the rings, shortening their ultimate service life. Themetal that is worn off by the rings will also end up in the crankcase,can can contribute to wear elsewhere inside the engine. That’s why manyengine builders today favor the plateau honing process as the finalstep in finishing a cylinder.
A plateau finish can be achieved a variety of ways by using a two orthree-step finishing process with conventional or diamond stones, bypolishing the cylinders with a cork stone, or by finishing with aplateau honing tool or brush. Stroking the bores with a flexibleabrasive brush (such as Brush Research "Flex-Hone" tool), or a plateauhoning tool with abrasive embedded in nylon bristles (such as Sunnen’sPHT tool) shears off the sharp peaks and significantly improve thesurface finish without changing the bore dimensions.
The proper plateau honing technique can generally get the surfacefinish down to 8 to 12 microinches (roughness average or Ra), withrelative peak height (RPK) numbers in the 5 to 15 range, and relativevalley depth (RVK) numbers in the 15 to 30 range. This is well withinthe ideal range for most ring manufacturers. For stock and streetperformance engines with moly rings, an average surface finish of 15 to20 Ra is typically recommended.
WHAT’S NEW IN ABRASIVES
Diamond honing stones as well as the honing machines that are designedto use them continue to evolve to meet the honing challenges posed bytoday’s engines. Special abrasives are needed to hone performanceengines that have hard liners, high nickel or silicone alloys, or hardfacings such as "Nikasil" (Nikasil is a trademarked electrodepositedoleophilic nickel matrix silicium carbide coating). The surfacehardness in a cylinder coated with Nikasil is about 90 HRc, and thethickness 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 honingthe cylinder. Nikasil retains oil well, so the bores can be honed to asuper smooth 4 to 6 microinch finish to minimize friction.
Tim Meara of Sunnen Products Company, said Sunnen is constantlydeveloping new abrasives to meet the specific needs of both PERs andCERs. Meara says by using different concentrations of diamond in thestone and changing the bond that holds it together, the cuttingcharacteristics of the abrasive can be custom-tailored to suit theneeds of almost any kind of engine or honing operation.
Dave Cox of Peterson Machine Tool said his company has just developed abrand new line of diamond abrasives to fit popular honing equipmentsuch as "AN" style honing heads. The stones come in two holders withfour stones total in the set. Cox said the new diamond stones can beused in place of conventional abrasives to take advantage of diamond’slonger tool life, and to achieve more consistent bore finishes.
Winona Van Norman is also introducing a new line of diamond abrasives,according to Britton Harper. The stones are designed to work withexisting hone heads, and will be offered in short and long lengths for2.7 to 4.1 inch bore diameters, and 3.5 to 5.5 inch bore diameters.Harper says the new stones will work well with all applications,including Nikasil and other hard alloys. Grit sizes will include 100,220 and 400. The stones can be used with conventional honing oils orwater-based synthetic coolant, though coolant is recommended for bestresults.
Mike Miller of Brush Research says his company is currently developinga new line of diamond abrasives for their popular Flex-Hone tool aswell as for plateau honing brushes. "Finer surface finishes are moreimportant than ever before with low tension rings. Silicon carbide andaluminum oxide are fine for finishing the cylinders in most engines.But for harder materials such as Nikasil, you really need diamond,"said Miller.
WHAT’S NEW IN HONING EQUIPMENT
Though honing has always been done on dedicated honing machines, BatesTechnology in Indianapolis, IN has developed honing mandrels with anISO-tapered tool holder that can be used in multi-purpose Computercontrolled numeric (CNC) machining centers. The honing tool can bestowed in the tool carousel like any other tool, and loaded in thespindle when a honing operation is required. This allows one machine todo both milling and honing, eliminating the need to move the engineblock from one machine to another. CNC honing lends itself to highvolume production applications where cycle times must be kept to aminimum. Most applications of CNC honing are at the OEM level due tothe cost of the tooling, but some aftermarket equipment suppliers arealso interested in the process.
"Most of the development work that equipment suppliers are doing todayis being done on CNC multi-purpose machining centers," said Dave Cox ofPeterson Machine Tool. Our current CNC machine can do both boring andresurfacing, but not honing. We plan to add that capability by early2008."
Ed Kiebler of Rottler Mfg., says everybody wants a diamond honingmachine today because of the better bore finishes it can deliver.Rottler’s HP6A Diamond Honing Machine was the first such machine to beengineered specifically for diamond honing, and was also the first tohave automatic load sensing, which allows a constant load to beprogrammed into the unit and maintained throughout the entire honingcycle. Automatically controlling the load allows the machine tocompensate for thin areas in the block that can cause the cylinder todistort. The end result is better bore geometry and better ring sealing.
The HP6A’s list of automatic features include programmable roughing andfinishing load sensing, automatic feed-out, automatic controlled stockremoval with .0002" (.01mm) resolution, automatic plateau finishprogram, automatic lower bore short stroke or dwell finish program, andautomatic brush finish program. The machine also has infinitelyvariable stroke speed control and spindle speed control. Kiebler saidthe HP6A can hone nickel/carbide cylinders and liners with ease.
Sunnen’s CK-10 honing machine is no longer in production, having beenreplaced by the SV-10 cylinder hone several years ago. The SV-10 hastwo motors, one for the spindle and one for the stroker, and isdesigned to be used with traditional tools as well as Sunnen’sDH-series diamond hone head. The DH-series tool has four slots and canhold up to 16 stones, though it can also be used with just one or twostones per holder.
The automated controls on the SV-10 includes a full-bore profile thatshows a real time graphical display of the bore cross section. Thisallows the hone head to dwell in any area of the cylinder bore thatneeds additional honing to improve bore geometry. Variable spindlespeed and stroke speed provides the flexibility to create any requiredcrosshatch finish.
Meara says a new automated capability that will be offered on Sunnen’sSV-10 includes the ability to index the hone head from one cylinderbank to the next. The automated controls can cycle the hone from onecylinder to the next now, so adding the capability to reposition thehone head from one cylinder bank to the next on a V6 or V8 engine willsave additional time and operator input.
Sunnen also has a SV-200 vertical honing machine (which replaces theprevious CK-21 machine). The SV-200 is designed for medium to highvolume production engine rebuilders, and can be fully automated forminimal operator input. A new SV-310 servo-driven ball stroke honingmachine is also available for automated production.
Winona Van Norman’s current honing machine is the PS2V, which canaccommodate engine blocks up to 42 inches in length (big enough tohandle many diesel blocks). The hone head handles bore sizes from 2.7to 5.5 inches in diameter, and from 1.5 up to 7.0 inches with optionaltooling. The infinitely variable hone speed and stroke rate allow themachine to reproduce virtually any crosshatch finish. The PS2V usesmechanical drive systems for the power stroke and hone head, which donot slow down or slip as the load increases. Variable speed motorsdeliver power to the moving components through gear reduction boxes.
Solutions For Common Honing Problems (With Conventional Abrasives)
Courtesy of Goodson Tools & Supplies
PROBLEM: Hone is not cutting
If stones are loading, you need to apply more honing oil or lubricantto properly flush stones. Filtered oil or lubricant is preferred.
Make sure that your guide blocks are not too tight. Honing stonesshould contact the cylinder first. You should put the hone in thecylinder and then expand. The guide blocks should have about .010" to.030" clearance.
Try narrowing your stones, this will put more pressure on the abrasive.Use an old file or dressing stone to narrow the face of the hone stone.Along the entire length of the stone, take approximately 1/3 off theleading edge. This produces more effective pressure on the stonewithout changing the pressure setting of the hone.
PROBLEM: Hone is chattering
To eliminate chatter, try varying the stroke rate as you hone. Try someof the tips listed for "hone is not cutting" as these will also helpovercome chatter.
PROBLEM: Stones and guides are wearing unevenly
This is most commonly the result of light honing pressure. To remedythis problem, increase pressure and vary stroke rate momentarily.
PROBLEM: Taper in cylinders with limited access
Taper will develop while honing a cylinder that doesn’t permit equaloverstroking at both ends. There is a tendency for the hone to dwell atthe open end of the cylinder and remove too much material. Dwelling atthe web end is not a solution because it removes too much material fromthe center and causes a barrel-shaped cylinder. Goodson offers specialhoning stones that prevent taper in these cylinders. They have shorterstones that exert more pressure at the bottom and are speciallydesigned for use in short or blind holes.
PROBLEM: Bore distortion
Use torque plates when honing lightweight blocks. When the torque plateis bolted to the block, it will simulate the distortion caused by thecylinder head bolts. The end result will be rounder bores in theassembled engine with minimal distortion and blowby.