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[How2Build] Rhodium Plater


#1

Hi greetings from Ireland I was wondering if any one out there
has built a plater for Rhodium plating or if there is a web site
that would take me through the steps of making up the individual
tanks and what power supply to use what anodes etc. All ideas and
thoughts will be much appreciated.

Thanks
Derek


#2
  Hi greetings from Ireland I was wondering if any one out
there has built a plater for Rhodium plating or if there is a
web site that would take me through the steps of making up the
individual tanks and what power supply to use what anodes etc.
All ideas and thoughts will be much appreciated. 

Derek,

You need from about 3 volts to maybe 6 volts, for a full range
of abilities. But for most use, anything in that range will work,
including plain old 6 volt dry cell batteries. Does have to be
DC, or course, but good ripple filtering is not needed for
rhodium. The lower voltage ranges are mostly useful for pen
plating, or small details, to avoid “burning”. But you can get
around a fixed voltage by simply reducing the anode area, which
despite the fixed voltage, will also reduce the current density,
and the current density is the actual factor you’re trying to
control, not voltage.

rhodium is easy to plate. The solution is used cold, and a
little bit of agitation (move the piece around in the bath)
helps. The anode must be either platinum, or platinized
titanium. If you’ve rolling mill make your own with a bit of
platinum wire, rolling one end to a thin foil, and thus creating
a bit of a wad with the desired surface area (which normally
should be equal or greater to the area being plated.) The
solution is acidic (sulphuric acid based), so stainless steel
tanks are not a good idea. Use a glass beaker. Contact of the
solution with iron, copper, silver, or other metals which
dissolve in sulphuric acid should be avoided, as it contaminates
the solution, leading to spotty plating. To plate over these
metals, you have to preplate with nickel, which requires similar
setups as rhodium, though it’s happier at 3 volts than at six,
and needs a pure nickel anode. The nickel bath is also sulphate
based, usually, but contact with the other metals is less often a
problem. Unlike rhodium or most plating baths, a correctly
running nickel bath will show NO bubbling. If it’s bubbling, the
voltage is too high. (or reduce anode area) The most important
aspect to rhodium plating is that things are properly clean
before hand. Use an ultrasonic if you have one, and even better,
a steam cleaner after an ultrasonic, to be sure the item is
clean. Rinse off cleaning solution before dipping in the rhodium
bath. Avoid fingerprints… Oh, and the wire you hang the
workpiece from must also be gold or platinum. Not copper, etc.

If you actually wish to make a plating machine, the commercial
ones amount to a variac (variable transformer), driving a step
down transformer. The variac gives you 0-120 Volts AC from 120v
mains, and the step down transformer is a higher amperage (as
desired for the supply’s design capacity) low voltage one
designed to take 120 volts and reduce it to the desired voltage
at the high end of the machine. Depending on the type of
filtering you add, the end highest voltage may be a bit higher
than this spec, but don’t waste too much time worrying about it
(for reference, if you rectify 120 volts, and filter it with a
filter containing a capacitor, then the end DC voltage is about
170 volts)

The step down transformer, then is usually giving you, after the
variac controls it, a range of 0 to maybe 12 volts in most
machines, with whatever amperage rating that transformer can put
out. The step down transformer is also what isolates the output
circuit from the mains circuit, an important safety requirement.
This output is still, of course, alternating current. it’s feed
to a rectifier circuit. There are several types, by I prefer a
full wave bridge ciruit. You can build your own from four diodes
of heavy enough amperage rating (again, use diodes rated at least
twice to three times the maximum output voltage, to protect them
in case of voltage spikes) You can also find modular ready made
bridge rectifiers, and these are easier to use for amperage
ratings of this type. This gives you Direct current, but with
high ripple. This will do rhodium plating just fine, but if
you’ve gone this far, add a large capacitor to the output to give
you a ripple filtered output. Be sure the voltage rating on the
capacitor is at least twice the stated max voltage of the
stepdown transformer. After that, as big a cap as you can find
is good. One of the big electrolytic caps approaching the size
of a soda can will work great. You can often find them in
surplus for little money. Smaller will work too, but will not
filter as well at higher amperages. Install a high value
resistor across the capacitor to discharge it slowly after you
turn the machine off, again as a safety measure, so you don’t
find yourself accidentally welding thing with sparks.

After that, it’s just a matter of adding switches, a box, etc.
The easiest way to add a volt meter is to use an off the shelf
cheap digital voltmeter (the little testers, not a raw meter.)
Much more accurate than the dials. Some of these will also allow
you to read amperage if connected in series with the circuit,
instead of accross the ouputs as you do with voltage.

For more complete design specs, if there is a tandy Radio Shack
store anywhere in your part of the world, they publish a real
cheap little paperback book called 'designing power supplies",
which of course is what you’re doing. If a radio shack store
isn’t around, the nearest library should also have references you
can find…

One note. If you’re building a power supply to operate off the
mains current. please be sure you know enough about electricity
to do this safely. These are simple circuits, but parts of them,
especially the variac and full voltage sections, can be dangerous
if you don’t know what you’re doing. The parts of the circuit
downstream of the step down transformer are fairly safe (except
if you have a fully charged large capacitor and short it out
through something you didn’t want to melt…)

Hope this helps.

Peter Rowe