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Polishing Hard Chrome

jcorsico

Aluminum
Joined
Feb 17, 2009
Location
Maryland
Does anyone have a recommended process for polishing hard chrome? We have four hydraulic cylinder piston shafts (40mm in diameter) that have been hard chrome plated. The current finish is 10 uin Ra. The seal supplier recommends we a 6 uin Ra finish, so we have to do some polishing.

How do you polish this stuff? We've tried various approaches, but to limited success. Five micron diamond paste applied using a wood backer worked the best, but took about 2 hours on the lathe to do a single rod. There has to be a better way!

Thank you!
Jon
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
One place I worked polished hardchrome rod using an old centreless grinder with cork wheels and SiC flour...........i actually made some replacement wheels by gluing together about a 100 thicknesses of cork mat to get the 3 ft wide wheel
 

Big Buck

Plastic
Joined
Oct 27, 2005
Location
tc mi.
Does anyone have a recommended process for polishing hard chrome? We have four hydraulic cylinder piston shafts (40mm in diameter) that have been hard chrome plated. The current finish is 10 uin Ra. The seal supplier recommends we a 6 uin Ra finish, so we have to do some polishing.

How do you polish this stuff? We've tried various approaches, but to limited success. Five micron diamond paste applied using a wood backer worked the best, but took about 2 hours on the lathe to do a single rod. There has to be a better way!

Thank you!
Jon
i've alway's found it easier & cheaper to make new rods out of hardend chrome rod with a mirror finish . When I did rework unusal piston rods , i'd grind the O D .003 to .005 under size ,chrome & regrind on a Kellenberger grinder , which would leave a 5 or 6 micro . You could polish them best you can with a emery cloth and try them out ,there should be only a small amount if weeping depending on the lenght of the stroke.
 

dian

Titanium
Joined
Feb 22, 2010
Location
ch
maybe electropolish or diamond burnish? i have seen rotating shafts polished by large, soft backed discs.

btw, you are trying for a 0.15µ finish with 5µ grit?
 

jcorsico

Aluminum
Joined
Feb 17, 2009
Location
Maryland
Thank you all.

We don't have a centerless grinder, and a Sunnen exterior hone would indeed be too expensive for this one job. Also, re-making the shafts with pre-polished rod is not cost effective in this case.

eKretz - what type of abrasive belt did you use?

John.k - cork is an interesting idea. We'll try that as a backer to apply the diamond paste. Do you remember the grit of the silicon carbide abrasive?

dian - yes, we are trying to get a 0.15µm finish (6 uin) using 5µ diamond abrasive. We tried finer grits (2µ and 0.5µ), but these didn't seem to cut fast enough. Maybe we were using the wrong speed, the wrong backer, or not enough pressure. With both the 2µ paste and the 0.5µ paste, the grinding "swarf" wasn't really swarf. It was still the color of the paste. With the 5µ paste, the swarf turns black, as I would expect.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Thank you all.

We don't have a centerless grinder, and a Sunnen exterior hone would indeed be too expensive for this one job. Also, re-making the shafts with pre-polished rod is not cost effective in this case.

eKretz - what type of abrasive belt did you use?

John.k - cork is an interesting idea. We'll try that as a backer to apply the diamond paste. Do you remember the grit of the silicon carbide abrasive?

dian - yes, we are trying to get a 0.15µm finish (6 uin) using 5µ diamond abrasive. We tried finer grits (2µ and 0.5µ), but these didn't seem to cut fast enough. Maybe we were using the wrong speed, the wrong backer, or not enough pressure. With both the 2µ paste and the 0.5µ paste, the grinding "swarf" wasn't really swarf. It was still the color of the paste. With the 5µ paste, the swarf turns black, as I would expect.

They were micron grade grit belts by 3M IIRC, can't recall exactly what grit. With chrome it's not super important anyway, it's so hard that even pretty coarse grit belts don't get much bite unless they're diamond grit. I'd probably try starting at 15μ or around 800-1000 grit and see how that does.
 

boslab

Titanium
Joined
Jan 6, 2007
Location
wales.uk
Split copper lap in a wooden clothes peg shaped holder
Cerium oxide on the copper, slow, you can certainly reduce the CLA to what you need, diamond would be quicker if you have a tube of blue 5 micron, or the slurry diamond ( keep shaking the bottle!)
Mark
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Id just run the hardchrome with the finish it has.........the seal material used now is incredibly tough and wear resistant ,and I doubt a gnats fart of differences will be noticable in the wear life of the assembly.
 

dian

Titanium
Joined
Feb 22, 2010
Location
ch
i have been wondering: hard chrome is "porous" and holds oil. how does that involve roughness? wouldn a rougher surface hold more oil? if we are down to 0.1µ ra, how much oil can get traped in the asperities? or are the "pores" a different animal?
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
i have been wondering: hard chrome is "porous" and holds oil. how does that involve roughness? wouldn a rougher surface hold more oil? if we are down to 0.1µ ra, how much oil can get traped in the asperities? or are the "pores" a different animal?

As I understand it, it's not porous like the surface of a sponge. More like a pretty uniform surface with holes here and there. It is possible to purposely make chrome plating more porous, but standard plating is not like that. Once the chrome is polished or ground it is no different than any other surface in terms of roughness. I've never heard any mention of standard chrome plating "holding" oil. The stuff that has been purposely made more porous via etching or whatever it is that they do yes, but not the standard chrome plating.
 

dian

Titanium
Joined
Feb 22, 2010
Location
ch
example: piston rings are chromed for tribological reasons. they hold oil. i have to investigate how that works. if they are "nano-pores" the oil would have trouble bridging the asperities, i reckon.

my understanding also is that hard chrome is not shiny for that very reason. can it be polished to a mirror finish? (somebody on "finish.com" said no.)
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
Porous hardchrome is the result of a special process....the Van Der Horst process.........normally hardchrome is impervious .........decorative chrome is only a few thou thick normally and is porous ,and detoriates quickly in some circumstances...........I have old IH and Cat hardchromed cylinder rods that have been oxy cut for pins,and the chrome is still perfect 70 years later..........the rubbish you get today will go rusty on the machine if left unused for a few months.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
All chrome plating is "hard" chrome. The only difference between decorative chrome plating and "hard" chrome plating is the thickness. I've seen charts that show a better and less porous surface is produced by plating thicker layers at a higher amperage. And yes it certainly can be polished to a shiny finish - unless it's the type that is deliberately made more porous. Again, I have never heard anyone claim that chrome plating "holds oil" other than the stuff that's made porous on purpose.

As regards piston rings, are you sure you're not thinking of moly coated rings?
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
There are/were definitely chrome plated rings.........that would cause a disaster with a hardchromed bore........UK made Ford diesels often had hardchrome bores,and rings were never chromed.........Perkins used chrome bores ocassionally,and fitting the standard chrome rings would cause scoring in few seconds after the Engine started......Leyland also fitted hardchrome bores in some of the big motors ,like the 680 Powerplus engines...............I would think nickasil has killed hardchrome cylinder bores.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
There are/were definitely chrome plated rings.........that would cause a disaster with a hardchromed bore........UK made Ford diesels often had hardchrome bores,and rings were never chromed.........Perkins used chrome bores ocassionally,and fitting the standard chrome rings would cause scoring in few seconds after the Engine started......Leyland also fitted hardchrome bores in some of the big motors ,like the 680 Powerplus engines...............I would think nickasil has killed hardchrome cylinder bores.

Yes, I'm aware. I meant that they aren't considered to be porous or known for "holding" oil, which moly rings on the other hand, are known for.
 

jcorsico

Aluminum
Joined
Feb 17, 2009
Location
Maryland
To close the loop on this, the process that worked best for us (by a huge margin) was spinning the shaft in our lathe while simultaneously using a handheld 7" rotary buffer at high speed, like that used to polish a car. We used 2 um diamond paste, applied on top of a 3M Trizact soft foam disc in 3000 grit. The foam disc itself sat on top of two 1/2" thick soft foam backing pads. This made disc have a very soft action, that easy conformed to the shaft.

Using this process, we could polish an entire 30" long shaft from 10-11 uin Ra down to 2-3 uin Ra in less than 10 minutes. It went very fast.
 








 
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