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polishing a crank

General engine tech -- Drag Racing to Circle Track

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chevyart
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polishing a crank

Post by chevyart »

could one of our members tell me what grit emery cloth i would use to polish the crank journals. perhaps there is a better material for this.all info
is appreciated. thanks art
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Re: polishing a crank

Post by ProPower engines »

chevyart wrote: Mon Jan 11, 2021 5:11 pm could one of our members tell me what grit emery cloth i would use to polish the crank journals. perhaps there is a better material for this.all info
is appreciated. thanks art

If you are doing this in a lathe purpose made 320 to 400 grit belts are common for use with aa electric hand held polisher.
Crocus cloth is also a good option for doing it by hand holding the paper.

What type of crank do you have to polish??
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Re: polishing a crank

Post by rebelrouser »

If you are polishing by hand, using a shoe string, I use 400, then 600, then 1000 grit, the 1000 grit should be pulled the opposite direction of the other two grits. Then clean with soap and water, lightly oil, and clean with a rag until you get no dirt on an oily rag. I have a stand to spin cranks and a polishing machine, use 400 and then I have a band with what looks like scotchbrite that I final polish with, again rotating the opposite direction. Not sure if these methods are perfect, but they seem to give good results. And my rule is if you can catch your fingernail on a groove in the crank, it is too deep to polish, needs replaced or turned. Please any of you professional machinists chime in and tell me if I am all wet. Always willing to learn.
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Re: polishing a crank

Post by David Redszus »

Crankshaft journal surfaces should be ground and polished to a surface finish of 15 micro inches
roughness average Ra or better. Journals on highly loaded crankshafts such as diesel engines
or high performance racing engines require a finish of 10 micro inches Ra or better.

The above is a simple straight forward specification which can be measured with special equipment.
However, there is more to generating a ground and polished surface than just meeting the
roughness specification. To prevent rapid, premature wear of the crankshaft bearings and to aid
in the formation of an oil film, journal surfaces must be ground opposite to engine rotation and
polished in the direction of rotation
.

Metal removal tends to raise burrs. This is true of nearly all metal removal processes. Different
processes create different types of burrs. Grinding and polishing produces burrs that are so small
that we can't see or feel them but they are there and can damage bearings if the shaft surface is
not generated in the proper way. Rather than "burrs", let's call what results from grinding and
polishing "microscopic fuzz." This better describes what is left by these processes. This microscopic
fuzz has a grain or lay to it like the hair on a dog's back.

The direction in which a grinding wheel or polishing belt passes over the journal surface will determine
the lay of the micro fuzz.

In order to remove this fuzz from the surface, each successive operation should pass over the
journal in the opposite direction so that the fuzz will be bent over backward and removed.
Polishing in the same direction as grinding would not effectively remove this fuzz because
it would merely lay down and then spring up again. Polishing must, therefore, be done opposite
to grinding in order to improve the surface.


In order to arrive at how a shaft should be ground and polished, we must first determine the
desired end result and then work backwards to establish how to achieve it. The analogy to the
shaft passing over the bearing is like petting a dog from head to tail. A shaft polished in the
opposite direction produces abrasion to the bearing which would be like petting a dog from tail to head.

The direction of shaft rotation during polishing is not critical if a motorized belt type polisher is
used because the belt runs much faster than the shaft. If a nutcracker-type polisher is used,
then proper shaft rotation must be observed (Figure 4). Stock removal during polishing must not
exceed .0002" on the diameter.

Having determined the desired surface lay from polishing, we must next establish the proper
direction for grinding to produce a surface lay opposite to that resulting from polishing. The
proper orientation will be achieved by chucking the flywheel flange at the left side of the grinder
in the headstock). Achieving the best possible surface finish during grinding will reduce the
stock removal necessary during polishing.


The surface lay generated by grinding would cause abrasion to the bearing surfaces if left unpolished.
By polishing in the direction shown in either the figure 3 or 4, the surface lay is reversed by the
polishing operation removing fuzz created by grinding and leaving a surface lay which will not
abrade the bearing surface.

Nodular cast iron shafts are particularly difficult to grind and polish because of the structure of the iron.
Nodular iron gets its name from the nodular form of the graphite in this material. Grinding opens
graphite nodules located at the surface of the journal leaving ragged edges which will damage a
bearing. Polishing in the proper direction will remove the ragged edges from these open nodules.

All of the above is based on normal clockwise engine rotation when viewed from the front of the
engine. For crankshafts which rotate counterclockwise, such as some marine engines, the crankshaft
should be chucked at its opposite end during grinding and polishing. This is the same as viewing the
crank from the flanged end rather than the nose end in the accompanying figures.

Unlike many engine bearings available today, Clevite engine bearings utilize a superior Clevite TriMetal™
material design. Stamped "Clevite," this design incorporates the strength of a copper-lead alloy layer
on a steel back and finally, a precision electroplated white metal "babbitt" third layer. TriMetal™ is
an ideal bearing design producing good to excellent characteristics when judged for conformability,
embedability, slipperiness and fatigue resistance.

A surface finish of 10 microinches on cast iron can be accomplished using 320 grit silicon carbide
abrasive.
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Re: polishing a crank

Post by PackardV8 »

Interestingly, most production shops today have gone to micro-polishing machines for better dimensional control. The micropolishing does not remove a measurable amount of material. Even more interesting, they get complaints from customers that the micro-polished journals are not as pretty and shiny as the old belt-polished were.
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Re: polishing a crank

Post by jed »

Jack could you be more detailed of what you said "production shops have gone to micro-polishing machines"
Is micro-polishing done on a special dedicated machine?? Or can micro polishing be done on a regular crankshaft polishing
machine but using a extra fine polishing belt.
Could you clear this up for me.
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Re: polishing a crank

Post by ProPower engines »

PackardV8 wrote: Tue Jan 12, 2021 10:33 am Interestingly, most production shops today have gone to micro-polishing machines for better dimensional control. The micropolishing does not remove a measurable amount of material. Even more interesting, they get complaints from customers that the micro-polished journals are not as pretty and shiny as the old belt-polished were.
Your correct Jack
I have seen some high end cranks lately that are 10 or less on the smooth scale and the finish appearance looks dull kind of like when a journal is damp before cleaning.
I have called a couple times and inquired and was told when they micro polish it is not bright in most cases depending on the journal hardness. When pushed further for an explanation they compared a steel to a cast crank for surface hardness from one company and another just says its a process and it don't need to be bright and shiny just smooth :shock: :shock:
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Re: polishing a crank

Post by hoodeng »

In my apprenticeship i worked on RR C range diesels, these had the only post machining and linish cranks that we did. Post finishing was done with a leather strop not dissimilar to a barbers strop, journals were polished to mirror finish this way.

Incidentally, i have never know an engine to have so many injector pipes, when mounted horizontally in a rail car application the injector pump was mounted down by the sump necessitating six pipes up the sump to a bridge then six pipes under the exhaust to the heads, another bridge then six pipes into the injectors and not a lot were the same shape, come to think of it these engines were built with so many fasteners on everything, if it had a cover it seemingly had fifty fasteners!
The guy i did my time under on these engines i reckon was mentally effected by these things,but he was good,very good.
Cheers.
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Re: polishing a crank

Post by PackardV8 »

jed wrote: Tue Jan 12, 2021 11:57 am Jack could you be more detailed of what you said "production shops have gone to micro-polishing machines"
Is micro-polishing done on a special dedicated machine?? Or can micro polishing be done on a regular crankshaft polishing machine but using a extra fine polishing belt. Could you clear this up for me.
Micropolishing machines use a polishing tape backed by a plastic shoe instead of a belt like a belt polisher does. The tape comes in approximately 150-ft. rolls, and when used on a micro-polishing 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. The finish is incredibly smooth.
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Re: polishing a crank

Post by jed »

ProPower engines wrote: Tue Jan 12, 2021 2:54 pm
PackardV8 wrote: Tue Jan 12, 2021 10:33 am Interestingly, most production shops today have gone to micro-polishing machines for better dimensional control. The micropolishing does not remove a measurable amount of material. Even more interesting, they get complaints from customers that the micro-polished journals are not as pretty and shiny as the old belt-polished were.
Your correct Jack
I have seen some high end cranks lately that are 10 or less on the smooth scale and the finish appearance looks dull kind of like when a journal is damp before cleaning.
I have called a couple times and inquired and was told when they micro polish it is not bright in most cases depending on the journal hardness. When pushed further for an explanation they compared a steel to a cast crank for surface hardness from one company and another just says its a process and it don't need to be bright and shiny just smooth :shock: :shock:
My question is why does a crankshaft journal need to be bright and shiny to be serviceable or useable. I have read on cylinder honing threads that shops don't like to use soft honing brushes because the cylinder after soft honing is not bright and shiny.
I have wondered about this phenomena for years. then one day after talking to a fellow machinest he brought up the
aspect of "how the light is reflecting" off the surface.
I am not smart enough to under stand the physics of light reflecting off a machined surface and what makes it bright and shiny vs dull appearance. But it does offer a partial explanation, ressulation, for the question. jmo
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Re: polishing a crank

Post by hoodeng »

What we need to get back to is how we are perceiving surface finish. A couple of comments are based around ‘visual inspection’ as in ‘bright surface finish’, ‘grey/dull surface finish’ being the decider of what we have as a surface finish.

Optic and tactile surface finish tests have been around for years, first we have ‘Surface roughness comparators’ these are more commonly known as ‘scratch blocks’ we run our finger nail over the surface finish that we have, then run it over a scratch block to see which feels the same [tactile test] once we feel a similarity we read off the Ra or AA comparison and call this our surface finish. There are more technical versions of tactile surface finish testing using a stylus run over the surface to be tested.

The other is optical, where in the past we observed a surface finish closely and compared it to a known finish much like the scratch block; we looked for uniformity of finish, brightness of finish etc to make our judgement as to quality of surface finish or lowest Ra that we could observe with a magnifying glass. Now a common method of optical grading uses a ‘Profilometer’ a digitized version and far more accurate than the eye.
Meteorology testing of surface finish now uses both types of test, a skid type with diamond stylus is used as a contact test, 3D optical and laser profiling is the non-contact version of the same test.

When we look at surface finishes we make a lot of presumptions that are not measured unless we have already performed tests, that is, the part is round [for a journal], it is parallel, it is not waved, it is not flawed, it’s dimensions are to design and tolerance, and because it has come of a certain type of machine all these parameters have been met.

We now know from using the previously mentioned technology that all we see is not necessarily the case of a perfectly fit for purpose job. We trust our machinist to give us a finish that is to standard for the job at hand.

Even my quote earlier about leather stropping a crank is contradicted by my previous paragraphs,,,but,, it was the method of the day, that gave a finish that the manufacturer deemed applicable and worth doing in a quest for longevity of their product, that had been proven in test and field applications.

Just looking at the price range of the equipment that we can buy tells us we can buy just what we need,,or,, we can elevate our test regime to the n’th degree of space technology. We have to keep our quest in the parameters of what we need.

A crank that has been pre tested for suitability and soundness, then ground to spec on a decent grinder and to required process for that part, post polishing with a known process if reqd and final inspection using a process that a crank manufacturer would use will give a serviceable part not dissimilar to a new part.

Now, hardness and heat treatment? Another discussion, but aligned to the quest at hand.
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Re: polishing a crank

Post by BCjohnny »

If you're going to do this for a living talk to the rep or company who'll supply the consumables and get the latest skinny on the deal

If you occasionally do it manually in your garage, carefully hold the crank in a vice and use a good quality 320 strip of emery cloth about 18-24" long, knock the bite off it first by polishing it around a scrap bar or such, and strop it back and forth with a level of sympathy

Resist the temptation to try to 'polish' out all but the most minor witness, and absolutely no need to use a string or the proverbial 'shoelace'

Depending on the fillet type I rip the strip to either slightly under or slightly over the bearing width

Some people I know use oil or Kero, but as I see it it's just more mess to clean up

Don't overthink the 'shade tree' method, or try to convince yourself there's any kind of 'science' to it ..... JMO
"If an honest man is wrong, after demonstrating that he is wrong, he either stops being wrong or he stops being honest." Anon
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Re: polishing a crank

Post by David Redszus »

Don't overthink the 'shade tree' method, or try to convince yourself there's any kind of 'science' to it ..... JMO
While "overthinking" can be wasteful, "underthinking" can be harmful.
The science is always there, whether we know it or not.

With all the modern metal machining technology available, why would anyone seriously consider "shade tree" methods?
Why not ride a horse? Or a bicycle?

If you stroke a dogs hair to smooth it down, it becomes shiny. If you stroke it the opposite way, it becomes
rough and irregular. The same is true with polished metals. A close examination of the finished surface,
under a microscope, would reveal the actual surface that the bearing sees. Not so hard to do.
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Re: polishing a crank

Post by BCjohnny »

David Redszus wrote: Fri Jan 15, 2021 11:34 am
Don't overthink the 'shade tree' method, or try to convince yourself there's any kind of 'science' to it ..... JMO
While "overthinking" can be wasteful, "underthinking" can be harmful.

As stated above ...... strop it back and forth with a level of sympathy .......

The science is always there, whether we know it or not.

With all the modern metal machining technology available, why would anyone seriously consider "shade tree" methods?
Why not ride a horse? Or a bicycle?

If you stroke a dogs hair to smooth it down, it becomes shiny. If you stroke it the opposite way, it becomes
rough and irregular. The same is true with polished metals. A close examination of the finished surface,
under a microscope, would reveal the actual surface that the bearing sees. Not so hard to do.

Back in the real world, and while certainly not text book, thousands of engines win races every year using common sense rather than chasing unicorns :wink:
"If an honest man is wrong, after demonstrating that he is wrong, he either stops being wrong or he stops being honest." Anon
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Re: polishing a crank

Post by David Redszus »

BCjohnny wrote: Fri Jan 15, 2021 4:00 pm
David Redszus wrote: Fri Jan 15, 2021 11:34 am
Don't overthink the 'shade tree' method, or try to convince yourself there's any kind of 'science' to it ..... JMO
While "overthinking" can be wasteful, "underthinking" can be harmful.

As stated above ...... strop it back and forth with a level of sympathy .......

The science is always there, whether we know it or not.

With all the modern metal machining technology available, why would anyone seriously consider "shade tree" methods?
Why not ride a horse? Or a bicycle?

If you stroke a dogs hair to smooth it down, it becomes shiny. If you stroke it the opposite way, it becomes
rough and irregular. The same is true with polished metals. A close examination of the finished surface,
under a microscope, would reveal the actual surface that the bearing sees. Not so hard to do.

Back in the real world, and while certainly not text book, thousands of engines win races every year using common sense rather than chasing unicorns :wink:
In the real world, thousands lose races every year because they can't catch the unicorns.
Unicorns are so fast they are hard to see. And they are very smart. :D
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