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Essential information mandated by NADCAP
Specifications for the electro-finishing industry, whether Military, Federal, ISO, ASTM or SAE-AMS specifications, have an initial section that contains essential information mandated by NADCAP to be supplied by the purchaser to the electroplater.
More times than not, much of this information is omitted from part prints and purchasing documents which opens the door to potential poor communication and finishing shortfalls.
Please take a moment to review these common ommision and ensue that you provide those to us when submitting your quote.
Plating Thickness and Tolerance Requirements
Often a minimum/maximum thickness tolerance without a defined checkpoint cannot be realistically obtained with a traditional electroplating process. For example, a plating specification that lists a minimum plating thickness of 0.0005” and a maximum plating thickness of 0.0008” for all surfaces of an electroplated part would not be achievable on most part geometries due to the inherent variations in electrolytic plating distribution.
Plating thickness varies due to the inconsistent distribution of plating current on an electroplated part. Ionic plating current, like electrical current, takes a path of least resistance with the corners and ends of a part receiving a higher current density and therefore higher plating thickness. If the design of a component requires a very tight plating tolerance, it is important to designate a functional surface where a plating thickness check point can be established. If the plating is specified as 0.0005” minimum and 0.0008” maximum with a specific checkpoint, a plating protocol can generally be developed to consistently meet this requirement. Any specific part dimensional tolerances should be addressed in combination with a plating thickness checkpoint to allow for a combined plating thickness and part dimensional inspection protocol.
Plating Build Up on Threads
It is not uncommon for improper allowance to be made for plating buildup on threads. Since plating builds on two sides of the part and both sides of the thread land, a total of four times the nominal plating thickness should be used as a guide for the increase (male) or decrease (female) in thread pitch.
The formula below details the calculation for change in pitch diameter as a function of the plating thickness and included angle
(Note: the included angle for most common thread forms is 60-degrees):
As an example, a 0.0002” plating thickness will cause the pitch diameter of a male thread to increase by a factor of four or 0.0008” for standard 60° UNC or UNF threads. This plating buildup is even further exaggerated on the lead threads of long fasteners or shafts where a localized plating thickness at the lead thread may be as high as four to five times the nominal thickness depending on where the plating thickness checkpoint is established. In such a case, the target plating thickness of 0.0002” may be as high as 0.0008” as measured at the lead thread resulting in as high as 0.0032” increase in thread pitch.
In such applications, it is very important to discuss allowances for thread pitch build and define a plating thickness checkpoint to ensure that parts can be successfully plated and meet post-plate gauging requirements.
Condition of the Incoming Material
Hardness or Strength Requirements
Racking Contact Location
In addition, plating chemistry/solutions cannot fully wet the surface of the part beneath the rack contact points. Due to the fact that the plating coverage will be incomplete with possible visible marks at the contact points. It is important that the purchaser convey non-functional or non-critical locations on the drawing where racking is permissible. This is usually a location where the purchaser deems that rack marks are not detrimental to the functionality nor physical appearance of the part.