Summary for plating thickness specification Discussion
Plating thickness specification is the key issue here: Janhavi asked whether an 18 micron requirement means an average thickness of 18 microns, and whether parts measuring 17 microns or ranging from 17–30 microns should be rejected. Ted Mooney replied that the specifier must define the requirement clearly; if the drawing or purchase spec does not say “minimum thickness” or “average thickness,” a plating shop could reasonably argue that 17–30 microns still complies with an 18 micron specification. He also noted that the 17 vs. 18 micron difference is so small that measurement uncertainty becomes important, especially if the part cannot be destroyed to verify thickness. Ted suggested that a better specification would state complete visible coverage, a minimum thickness on accessible surfaces, and an absolute maximum thickness anywhere, rather than relying on an ambiguous single number.
Q. Hi
I would like to know if the coating thickness specified is 18 microns then does that mean that the average coating thickness is 18 and in that case would any component with 17 microns be rejectable. Also if the coating thickness varies from 17-30 is it rejectable too?
Are there any coating thickness standards which would specify the average coating thickness calculations?
Thanks & Regards,
Janhavi Sindhu
Plating shop employee – Pune, Maharashtra, India
August 26, 2008
A. Hi, Janhavi. While a specifier should certainly try to avail himself of the expertise of his platers, it is the specifier’s job to decide what specifications are to be met.
The specification must be clear as to what is required. If it doesn’t say “minimum thickness” or “average thickness”, I would think that a plating shop can make an excellent case that 17-30 microns is compliant with an 18 micron specification. I would also say that the difference between 17 and 18 microns is so small that the measurement method starts getting into it. How can the buyer strongly assert that the part is surely not 18 microns without destroying the part?
The usual way to write such a thickness specification might run something like this: “Complete visible plating coverage required on entire part, with 18 micron minimum thickness on any surface which can be touched with a 3/4″ dia. ball, and 30 micron absolute maximum thickness anywhere.”
Note that I am just using your numbers for the example, not suggesting that this is a good specification for the part in question. Good question, and I hope others will chime in.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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August 27, 2008
A. Remember that the tighter the tolerance is, the more that it will cost you.
Also, you get a dog bone effect in most plating where the edges are a much higher current density area, so will plate much heavier than the center. Therefore you have to get with the plater and find out what it will cost you to be very specific about the job. If you do not specify, then you get “average” thickness.
James Watts
– Navarre, Florida
August 28, 2008
A. Just to emphasise on what Ted has already stated, I am one of those people that write the specifications. A good specification will always include either a minimum average or a thickness range. It should also define where measurements should be taken, on a metric side I normally specify that it should be able to be touched by a 19 mm hemispherical end.
If the specification is unclear it is better to go back to the specifier and ask them what they really want.
Anyone rejecting 17 microns for an 18 micron minimum average probably doesn’t understand the nature of measurement. Now if they were rejecting because a particular dimension was out of tolerance I could understand it.
Brian Terry
Aerospace – Yeovil, Somerset, UK
August 29, 2008