【转帖】plating tolerance

yang686526

plating tolerance
we typically specify zinc plating according to astm b633 including the thickness of the plating. we do not specify a tolerance for the thickness and i haven't been able to find anything that indicates what "standard" tolerances are for zinc electroplating.
does anybody have any standards or experience with this you would be willing to share?
thanks!!
if you are looking for calling out the plating tol on a dwg, i suggest leaving it off. let the plater worry about the tol per the spec.
it depends on the type of plating and material.
a google search shows a lot of data on this.
i think the key here is finding a good plater.  i've had sloppy platers that leave parts in the solution until the edges smooth over.
heckler
sr. mechanical engineer
swx 2007 sp 3.0 & pro/e 2001
xp pro sp2.0 p4 3.6 ghz, 1gb ram
nvidia quadro fx 1400
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(in reference to david beckham) "he can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. apart from that, he's all right."  -- george best
here's an interesting link.  i have never used this plater but they have some good information on their website.
ctopher
what would you do if you need to manufacture a plated 3a or 3b class thread on bolt or on tnternal thread? there is no way to do it without giving the thread tolerances before plating and after plating. the pre-plated tolerances dictate the desired plating tolerances. from my experience the tolerances for zinc electorplating a tolerance of 0.003 to 0.0004 mm is common. for internal thread or holes an electrode inside the thread/hole may be necessary.
for example qq-z-325c federal standard dicatates 0.005 mm  for class iii and 0.013 for class ii. these are minimum plated thickness insure the 12/96 hours at salt spray bath. therefore, you have to give tolerances to the plating if you manufacture parts such as class 3a and 3b bolts and threads.

i've rarely seen the thickness on a drawing, and if i recal when i did it was either on very old drawings or drawings that didn't reference an industry spec.  
however, i often see 'dimensions apply after plating' as part of the plating note.
probably not much help, sorry.
kenat, probably the least qualified checker you'll ever meet...
the problem is that the astm b633 spec does not indicate a tolerance only 4 commonly used nominal thickness values. we specify a thickness because this effects salt spray resistance and the cost of plating. but how much is enough or to much?
we have some parts that have some tight tolerances so this can become very critical and chops away at the tolerances afforded to our manufacturing guys.
the link posted by ctopher seems to indicate to me that +/- .00015" on plating thickness is reasonable but i don't know if  this is actually easily produced using standard (i.e. low cost) plating methods.

we do not put plating thickness on mechanical drawings.  we use note "dimensions apply after finishing".  then our manufacturing engineers create what is called a processing drawing in which they account for finishing tolerances in the part creation.
like i mention above.....find a good plater and talk with them about their process and ask good questions.
heckler
sr. mechanical engineer
swx 2007 sp 3.0 & pro/e 2001
xp pro sp2.0 p4 3.6 ghz, 1gb ram
nvidia quadro fx 1400
      o
  _`\(,_
(_)/ (_)
(in reference to david beckham) "he can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. apart from that, he's all right."  -- george best
i agree with kenat using the 'dimensions apply after plating' note. i have tried at various companies and designs using different types of plating, it can be very difficult to meet a thickness requirement indicated on a dwg. the tolerance you call out may work with one plater, but not another. plating can be especially difficult to meet or inspect on threads.
every company i have been with, the outcome was to leave off the tol on the dwg and call out the appropriate spec with type of plating.
i suggest calling your plater and ask them what their recommendation is.
my experience.
chris
solidworks 07 4.0/pdmworks 07
autocad 06
plating thickness is a function of the part geometry and topography, among other things.
there is no way to have a "standard variation" for plating thickness without also defining a standard test coupon.
i agree with the others that the finished part has its necessary tolerances defined by the part design engineer, and that the tolerances apply after plating.
a good plater will have a decent idea of what thickness variation to expect based on geometry and topography, and will work with you to suggest changes to the unplated part so that the finished part will be what you need.
a bad plater will feed you a line of bs about "standard plating thickness tolerances".
we also utilize a "dimensions apply after finishing" note.  the main reason being the variability in processing as indicated by previous posters.  for highly toleranced parts, work to select a supplier and then help them develop the appropriate process controls for running your part.  for our more critical components, we have worked with the supplier (one who maintains excellent control on their bath chemistries) on racking locations and established standard lot sizes per run in addition to the more standard parameters such as masking locations.
finding a good supplier and keeping communications going with them has proven beneficial to the results.  the trick then becomes keeping them from being cut out as a result of someone elses cost reduction efforts (unless the alternate source proves equally as capable)
regards,