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Powder Coat thickness tolerance on machined parts and holes

We are designing machined and sheet metal hardware from principally aluminum and stainless steel. In some cases our parts have to be powder coated. The issue is with a local machine shop who insist our tolerances are too tight on designs. I maintain that the powder coating process should be fairly repeatable so the holes can be oversized by maybe .002" and then you hit your tolerance right on. As an example we would specify a .281 diameter hole for a 1/4" bolt. Hole tolerance is ± .005". This is standard practice for a floating fastener application. Can someone with powder coating expertise weigh in and give their thoughts.

Thanks,

Joe Grant
amti - Hanahan, South Carolina
2006

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2006

Your hole tolerances are too tight for powder coating (at least in my experience).

Most powder coat thicknesses range from .005 to .030 inches. Quite a large range and difficult to hold a consistent thickness. For your bolt holes I would specify they be masked from the powder coat then you can have whatever tolerance you like on the diameter.

Unless this is a chemical resistance issue, this should be adequate. If chemistry is involved, anodization or electropolishing (depending on the metal) may be a better way to go.

Ryan Blaquiere
- Kalispell, Montana, USA

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2006

The paint thickness can be very repeatable if you give as much attention to the paint process as with everything else. A typical attitude is that it is just paint, can't the painters get it right? Here are some of the running parameters of the paint process--
grounding
air/paint ratio
part orientation to gun
distance from gun
KV setting
hopper fluidization
wear parts in gun and pump
humidity/heat in paint area
paint condition(virgin/reclaim ratio)
length/age of delivery hose
age of paint
type of paint, etc,
there are more but this gives you an idea of what you need to have control of to keep the thickness consistent for the LOP.
It's pretty standard to give the tolerance to the paint in order to avoid as much headache as possible. That's my two cents.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina