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Crank And Cam Polishing: Are You Smooth Enough?
It is more important than ever for engine builders to be as perfect, or near perfect, as possible when it comes to surface finish requirements.
By Brendan Baker
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Manufacturers are designing today's engines with tighter tolerances and less room for error. They make more power, live longer, produce less noise, vibration and friction, burn less fuel and produce lower emissions. So in light of all this, it is more important than ever for engine builders to be as perfect, or near perfect, as possible when it comes to surface finish requirements.
Crankshaft and camshaft finishes are no exception. In today's engines, rotating assemblies ride on a thin wedge of oil only .00005˝ thick in some cases. And to help reduce friction as much as possible, oil itself is much thinner today as well, so it is especially important to achieve the proper surface finish on all components in order to avoid problems down the road.
Engine builders big and small have the same need to produce a smooth surface on cranks and cams, but their respective budgets and business volumes may dictate what equipment is used to get the job done properly. Large production engine remanufacturers (PERs) can justify purchasing a micropolisher and an automated surface finish gauge while small custom engine rebuilders (CERs), by-and-large, feel they can't afford such luxuries. This is not to say that it's out of the realm of possibility for CERs to afford a micropolishing machine or a surface finish profilometer; but higher volume is generally what justifies making such a purchase.
Belt polishing is the traditional method, used by engine builders for many years, to polish crankshafts as well as camshafts. In the past, belt polishing worked well to easily produce surface finishes that were extremely close to OEM levels. Today, however, vehicle manufacturers have automated industrial polishing machines that cost many thousands of dollars and produce very smooth and consistent results, results that are increasingly more difficult for engine builders to reproduce with manual equipment. Difficult, perhaps, but not impossible.
"We achieve an extremely fine finish on the grind, before we polish." says Bob Heidbreder, Northampton Crankshaft in Cuyahoga Falls, OH. "Our method is to first dress the grinding wheel very fine and then use polishing belts for the final finish. We do all different styles of crankshafts this way and we've never had a failure or a comeback because of the finish."
Customarily with late-model crankshafts, says Heidbreder, rebuilders mic the journals and go through a two-step polishing process. "If the crankshaft is salvageable and you don't have to grind it, you polish it with a #400 grit aluminum oxide polishing belt. And then, if you need, you would have the option to micropolish the crank to achieve a finer Ra finish. Of course, this depends on the crankshaft you're working on but most want it as smooth as possible."
Aftermarket distributors recognize the demand for near-OE quality finishes and offer rebuilders a number of products. "We have a brand new belt that we introduced about a year ago," says Chris Jensen, Goodson Shop Supplies, Winona, MN. "It's a GSW micropolishing belt and it is the best thing we've found for final crank polishing. You put it on with a little crank polishing rouge and turn it approximately 10 rotations."
According to Jensen, these belts have even worked well enough for a number of NASCAR racing teams to give them a try. These teams now use them for final polishing the cranks in their race engines.
"What is so nice about this belt is that there's a serration on it so it can polish the large radius on these high performance cranks," says Jensen. "High performance cranks have a large radius for strength while many production engines have virtually no radius."
A number of other distributors offer micropolishing belts as well as portable belt polishers. Tom DeBlasis, K-Line Industries, Holland, MI, says portability brings polishing capability to the price range and user abilities of almost every rebuilder.
"We offer a couple of portable crankshaft polishers: one electric and one pneumatic, which can be used on either a crank grinding machine after you move the head away or on a rotating polishing stand," says DeBlasis. "Most guys start out with a #320 grit belt, then go to a #400 grit and then progress to a very fine micro-polishing belt for a few revolutions, depending on the application."
"A long time ago stones were used to polish cranks and cams but that technology has gone by the wayside now," says Ken Barton, QPAC, Lansing, MI. "Today, micropolishing is technically the most advanced way to achieve OEM-level surface finish on cranks and cams."
Barton says many engine builders believe in some polishing "myths," which can impact a shop's profitability. "Some rebuilders believe if you use very fine grit belts you will not remove any material. That's kind of a misconception. You always take off some metal when you polish," says Barton.
"When you are done grinding the piece with the grinder it looks like a mountain range," Barton continues. "If you put a micrometer on the piece you're going to measure from the highest peak to the highest peak. So when we take those peaks down and you remeasure it, you're going to get a different measurement. It might be just a small amount of material, but some material is being removed."
Typically that amount may be as small as .0002˝ according to Barton. One of the challenges is convincing shops that such a small amount actually makes a big difference. "Of course, you have to take material off to achieve the correct surface finish. With micropolishing you're taking off the peaks and getting down towards the valleys, and the more peaks you remove the more surface area you have," he says.
Another misconception that some users have is they think they can put a polishing belt on and go crazy with it. "Some people think making the part shiny is enough, but it probably has taper, crown and who knows what else," says Barton. "If it is within a couple of tenths the error is still there. That's why we back our machines up with a rigid shoe behind the tape. So now you have a rigid setup that won't taper or go out of round. That way we keep things round and flat. When you finish you index the tape approximately one inch and go on to the next journal."
Micropolishing machines use a polishing tape instead of a belt like a belt polisher does. The tape comes in approximately 150-ft. rolls, and when used on a micropolishing machine operators index each roll after each use. When a crank or cam is polished on one of these machines, it uses about one inch of tape per journal, so fresh abrasive is used for every polishing job, but because it's such a small amount, many users say they've saved money. "One PER reported a 50-75 percent savings in the cost of tape vs. belts," Barton says.
The type of abrasive tape you should use for grinding cranks and cams varies, according to Barton. "From our position it all depends on what is on the machine. Is it a steel, cast iron or nodular iron part? Is it a hardened steel or is it a forged part? Then once you get through the material, what kind of surface are you grinding? We've had surfaces as high as 45 Ra and as low as 15 Ra. All that determines what abrasive you use on the polishing tape. It could be 9 micron or it could be 50 micron (note: 20 micron is roughly equal to #600 grit). It just depends on how aggressive you need to be."
Barton understands that not every shop will purchase his equipment. "For shops to really see a payoff they need to rebuild about 15 cranks or engines a day. Some specialty and high performance shops that sell engines and cranks for much higher prices than average can also make their ROI."
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