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Hard Industrial Chroming of Aluminum, Brass or Steel Cylinders


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#1 Jim Allen

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Posted 25 March 2017 - 07:42 AM

The photo shows a complete chroming tank with heaters. The chrome tank is maintained at 130 deg F & is run at a current density of 3 amps/sq in. The total current required for the brass holding fixture & the .95 cu in size cylinder is 24 amps. The brass holding fixture prevents the buildup of chrome on the top & bottom of the cylinder. This buildup now takes place on the brass fixture & is removed from the brass holding fixture afterwards with a warm 50% solution of HLC acid. At a current density of 3 amps/sq in, .0012" of chrome/side/hour will be deposited. In 2 to 2.5 hours, a total of .0024"+ of chrome will be deposited on the cylinder's inner wall. This simple bath will deposit chrome with a hardness of 1,000 Vickers. Carbide tooling WILL NOT cut 900 Vickers hard chrome!

 

Jim Allen



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#2 Michael Costanzo

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Posted 25 March 2017 - 02:30 PM

you never stop amazing me with the things you do!! like a human dictionary. are you just doing this for your nelson motors or are you taking customers engines?





#3 Jim Allen

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Posted 25 March 2017 - 04:27 PM

Michael,

 

I have done this for my engines for more than 30 years & for the Nelson engines for more than 4 years. As you probably know, all the Nelson engines use an aluminum cylinder & my engines use a brass cylinder. The fixtures used are the same, however the chemicals, current density, procedures & the processes are very different. The chroming of aluminum is some what complicated & the chroming of brass or steel is relatively simple. I will describe in detail exactly what is required for an individual modeler to do this, if there is an interest. Consider the fact that any brass alloy cylinder can be chromed & re-chromed as many times as necessary!

 

Jim Allen

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#4 Mike Rappold

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Posted 25 March 2017 - 07:20 PM

Jim,

Can you further explain how you control or mask off the areas that are not supposed to be chromed? For example the port areas.

 

Mike



#5 Ray Sametz

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Posted 25 March 2017 - 07:59 PM

Again Jim, what a flashback the days in Brian's basement first stripping the OEM chrome off the new K&B and OPS sleeves. Then re-chrome and hone on his lathe. He chromed the drum on the intakes also. With RPM rods and SS bearing housings those motors won a lot of races. The only weak link was the 3 piece cases. Good times for sure. B)



#6 mike walker

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Posted 25 March 2017 - 09:20 PM

I did hard chrome and electroforming for years and by no means am I an expert. Your anode and cathode set up is awesome!

Nice, nice work



#7 Jim Allen

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Posted 26 March 2017 - 07:12 AM

Jim,

Can you further explain how you control or mask off the areas that are not supposed to be chromed? For example the port areas.

 

Mike

Mike,

 

There are several methods that can be used to mask off areas that are not supposed to be chromed. I never attempted to use tapes because I wanted only the "cylinder's bore" to be chromed. The brass holding fixture serves two purposes that tapes would not be able to do. Notice that the brass sleeve the cylinder is setting in & the brass clamp ring that sets on top are effectively making the chrome tank "think" the piece being chromed is longer in length than the cylinder itself. This is necessary to prevent any extra buildup of chrome at the top edge & bottom edge of the cylinder that would normally take place without the brass sleeve & brass clamp ring. 

 

The brass holding sleeve & the clamp ring also become the necessary electrical contact (cathode) for the cylinder. Everything is clamped together with the threaded PVC cap. This sleeve is sitting on the two stripped wire leads coming out of both sides of the holding fixture. Both leads fit "tightly" in holes drilled in the sides of the PVC cylinder. These leads must be insulated from the chroming solution in the tank because they form part of the cathode where chrome will be deposited.

 

One characteristic of any chrome tank is it's poor throwing power. This means there will be very little chrome plated on the exhaust & transfer window surfaces. However, a very small amount of chrome will be plated on the brass holding fixture areas behind these windows. Therefore, the brass holding sleeve is undercut in the areas where the exhaust & transfer windows are, to allow easy removal of the cylinder.

 

Anode size, accurate placement of the anode & any contact point must be "positive"! Lead antimony anodes are some times cast when large sizes are needed.

 

Jim Allen

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#8 Jim Allen

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Posted 26 March 2017 - 07:16 AM

Again Jim, what a flashback the days in Brian's basement first stripping the OEM chrome off the new K&B and OPS sleeves. Then re-chrome and hone on his lathe. He chromed the drum on the intakes also. With RPM rods and SS bearing housings those motors won a lot of races. The only weak link was the 3 piece cases. Good times for sure. B)

Ray,

 

Brian visited me several months ago. He was an excellent student & I greatly enjoyed mentoring him in those early days. 

 

Jim Allen



#9 Terry Keeley

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Posted 26 March 2017 - 09:36 AM

Hey Jim:

 

I really like your tank set up and sleeve fixture, I will probably copy it in some form or another when I get back to this down the road.

 

I want to try some really low taper liners with that RSA 431 piston material, I'm sure there are gains to be had there for my SAW program.

 

My first attempts 20 years ago turned out OK but I think now I'd do a lot better.  Back then I used some special 3M vinyl masking tape to mask the outside of the sleeve but your idea is much better.  Going to set up my Falcon grinder in the Super 11 lathe for finishing.

 

 

 

 

 

Any idea if the chroming bath goes "bad" over time?



#10 Jim Allen

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Posted 26 March 2017 - 11:12 AM

Terry,

 

The chromic acid/sulfate bath can last indefinitely. When fixtures are removed from the bath there will be some pull out of the solution. We add more solution, however distilled water could be added to bring the level back up. The tank we are using has chromed more than 1,000 aluminum cylinders by simply adding more solution.

 

One advantage of cylindrical grinding of the cylinder compared to "back honing" with a Sunnen Honing Machine is the elimination of any bell mouth. Of course, honing is much faster & does not require a mounting fixture. Taper amounts used in any cylinder are directly tied to the cylinder's bore & the stroke of the engine. The cylinders taper will determine the amount of taper used on the piston's upper outside diameter to give the correct wear band.

 

We use only RSA 444 for pistons because there are no expansion problems after final machining.

 

JA

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#11 Terry Keeley

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Posted 27 March 2017 - 08:31 AM

Thanks again for the info Jim.

 

Can you tell us what tapers are "typical" for our brass sleeved engines?  Does the use of that RSA high silicon piston material change what taper is run in the sleeve?  What taper is used on the piston?  :rolleyes:



#12 Jim Allen

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Posted 27 March 2017 - 10:04 AM

Terry,

 

"Can you tell us what tapers are "typical" for our brass sleeved engines?" There is no "typical" taper amount that is used for a brass or aluminum cylinder engine. "The best cylinder taper amount in any particular engine, brass or aluminum cylinder, can only be determined by being able to accurately test". In "general", a larger bore, shorter stroke engine, can stand more cylinder taper than a smaller bore, longer stroke engine. The accurate measurement of what ever taper amount is being used becomes critical in this process. We have found that different alloy materials, DO NOT change the correct taper amount, but they will change the cold fit of the engine. 

 

There is another very important factor to be considered. "Keeping the cylinder, regardless of material, as round as possible at the engines operating temperature". The piston's wear band & the cylinders wear pattern will tell you if the cylinder & piston are remaining round at the engine's operating temperature. Notice how the cylinders lip is designed on the Nelson pylon racing engines & my engines. That lip is .100" thick on the .45 size engines & it is .200" thick on the .90 size engines; it is clamped between the engine's head & the crankcase. It is as large as the OD of the crankcase. Imagine the rigidity in keeping the bore round & the heat transfer that takes place in this most critical area of any high performance engine with this simple modification!

 

Jim Allen

 

Note: There is much more on the types of chemicals & procedures used to successfully chrome brass or aluminum. Many engine manufactures have given up on chroming aluminum because of the difficulties encountered (blistering, pitting, pealing off & insufficient hardness). 


Edited by Jim Allen, 01 April 2017 - 10:21 AM.


#13 Jim Allen

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Posted 28 March 2017 - 06:18 PM

Terry,

 

 "Does the use of that RSA high silicon piston material change what taper is run in the sleeve?" NO! However, the taper used at the piston's crown will change because different thermal expansion amounts are possible for different alloys The distance down from the top edge of the piston does remain the same. When taking a careful look at the difference in thermal expansion, thermal conductivity, elongation at 250* C, hardness, ultimate tensile strengths at various temperatures & fatigue strength when comparing RSA-444 & RSA-431, it can be seen why we prefer the former alloy for pistons in a Pylon racing engine that is operated at wide open throttle from the beginning of the flight until it runs out of fuel.

 

I will post some very clear photos of the wear band that should be visible when fitting a piston to a sleeve. Both the piston, after machining, & the cylinder, after honing, should be round ,within .00005", to clearly see this wear band. In the fitting of pistons to a cylinder, I use pistons that are machined in .0001" steps. Cylinders are honed to the correct taper amount, checked for roundness & are then fitted with a piston. The final step for the piston is to be de-burred; the wrist pin hole honed to size & then washed before assembly.

 

JA


Edited by Jim Allen, 28 March 2017 - 06:18 PM.


#14 Terry Keeley

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Posted 29 March 2017 - 06:24 PM

Hey Jim:

 

A fascinating topic for sure, any info you can pass along would be greatly appreciated. 

 

As you probably know I run all CMB engines from 21's to 90's and am pretty fussy about running in with proper lubrication etc.  Somehow I never seem to get a completely "clean" wear band around the piston like you're describing.  Here's a photo from the MB site that I think shows what you mean?

 

https://fiorimet.hom....nl/Running.htm

 

I've checked my CMB pistons & liners and they are really quite good, usually round to within a couple tenths.  After running in tho my pistons show a lot more wear on the exhaust side on both the piston and sleeve.  Do you think trying to hold the sleeve round in some way would help?  Maybe making a brass ring that's a tight fit to the top of the sleeve kinda like your wide flange?  In my case is the piston possibly just "outgrowing" the sleeve at temperature and therefore wearing more?

 

Hmmmm...


Edited by Terry Keeley, 30 March 2017 - 06:44 AM.


#15 Ken Retallick

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Posted 30 March 2017 - 04:36 AM

Hi Terry,

 

The reason you are not getting the nice wear band on the piston is due to the piston design.

They have a large part of the lower skirt removed opposite the exhaust port. this allows the piston to tilt towards the exhaust port.

I have seen this on many many pistons, The more the skirt is removed the worse the tilting.

 

With a full skirt the piston cannot tilt more in one direction.

Also having the pin placed higher in the piston will reduce the force tilting the piston by a little bit.

 

Here is a pic to show the difference.  On the Left is a piston out of a KRE 90 engine. Full skirt, and a nice even wear band. Also a high pin placement.  The other 3 pistons are out of various CMBs, 100, 91 Vac and a 67. All these are factory skirted, with wear on the piston running right to the top on the exhaust side.

 

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