Plating Allowance
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  1. #1
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    Default Plating Allowance

    Many of you guys are familiar with this every day, so I'll ask here:


    I've been getting quite a few jobs from a customer that calls for zinc plating. Normally, I've only dealt with black oxide parts. These parts often have bores held within .0005 or less.
    I don't feel comfortable with this, because I don't know how much to allow for plating, if anything. The customer doesn't call out specs BEFORE plating.

    So.....how much will zinc plating effect a tight bore like this ????


    Since we're on the subject........how much does anodize effect a tight bore like this ???



    Frank

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    Default

    In my experience it is usually between .0001 to .0002 build up on small parts. The best thing to do is call your plater and ask them what they can realistically hold for tolerance on the plating because it is up to them how your parts are finished good or not.

    Jason,

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    OK, a quick zinc plating tutorial.
    Plating thickness is a product of current density, and time. most commercial zinc plating specs, call for a thickness of 2/10ths (.0002) plus a clear or yellow chromate conversion coating. Plating will be thicker on large flat areas (high current density) and less so on recessed areas, such as holes. Think of plating as applying paint from a nearly dead can of spray paint. Close areas get a lot, far away areas not so much.

    Zinc plating is typically applied 2 ways, rack, and barrel. large pieces, castings, and weldments are typically rack plated. that is the part is mounted on a "rack" and oriented in such a way that the anode "sees" the greatest area. Problems arise when the part is too close to the anode (burning the plating from too high a current density) and too far away (causing poor or no coverage (due to low current density).

    Barrel plating is different, a process that allows a large volume of small parts to be plated at once. think nuts, and bolts. Parts rotate in a polypro drum, and current is conducted through a large flexible cable with a conducting end inside each end of the barrel, called "danglers" . The advantage of barrel work is the ability to get a lot of stuff plated in a short amount of time. uniform plating thickness, as the parts are constantly in motion, insuring overall coverage. The downside of barrel work is parts are tumbled, so critical edges will be abraded. A lot of places will spec barrel plating, to pass on the deburring to the plater.

    Back to the initial question. If your part is rack plated, the plater will have to hold the part in such a way for maximum coverage. Typically done using some holes found on the part. In these cases, the holes will show rack marks, and no plating will be at the contact point. Other holes will have coverage, but the plating thickness will vary with the depth, and diameter of the hole.

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    Default

    It depends on the exact spec, but beware, not all platers are equal. Talk to vendors about it, but if they have the bore plated too I would not be comfortable with that tolerance. If you can mask the bore then no problem.

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    Hmmmm,

    that seems like a tight tolerance to be guessing on the plating. Sure would be nice to know exactly what the customer wants before sending parts off to get plated to get back at the wrong size.

    We send our precision bored cylinders out for anodizing and we specify EXACTLY how much buildup/thickness we want. The plating company nails our tolerances every time. There has been some mistakes with them but all on our end not telling them the right stuff (not my job, I do the honing)

    So my advice, use caution in this situation or you can make a lot of scrap.

    hope this helps,
    Joey

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    Default

    Interesting topic.

    I design a ton of parts with powdercoat on them. I usually account for that on my drawings and expect them to be inspected before powdercoat.

    Another person in our business thinks that everything is inspected after the coating is applied.

  7. #7
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    Geez where are you guys getting your Zinc done that thin? I have always gotten about .001"+ Or is this pulling numbers from a book?

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    I recommending that you gain understanding of your customers plating requirements. Once that is done machine appropriately then pass these requirements onto your plater identified on the purchase order.

    Our customer's requirements are:

    Zinc Plate to ASTM B633 Class B Type III .0003" min and .0005" max.

    Plating thickness requirements vary for various reasons. We have a .0005" max because a lot of our parts are bent and crimped etc and thicker plating will cause quality issues. Thicker zinc plating also causes quality issues in general.

  9. #9
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    Quote Originally Posted by IAMATT View Post
    Geez where are you guys getting your Zinc done that thin? I have always gotten about .001"+ Or is this pulling numbers from a book?
    The .0001 to .0002 are typical but greater thicknesses are possible. The issue is that after about .0005 that the plating may become spongy.

    The major reason for the plating is to have a base coat of zinc that can be converted to zinc chromate which is where the greatest corrosion protection is. My experience is that with barrel plating, there will be about .0004 to .0006 change in the pitch diameter to external threads with "normal" plating thicknesses.
    Tom

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    I'd strip and replate before I'd let it go to scrap. Send parts with gages then your covered.


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